Transgenic And Non Transgenic Maize Agronomic Performance Biology Essay

Published:

This essay has been submitted by a student. This is not an example of the work written by our professional essay writers.

Field experiments were conducted at three sites in Nebraska Mead, Clay Center, and Concord for one cropping season to evaluate the agronomic performance, and to assess the effects of different transgenic and non-transgenic maize, with and without insecticide applications on adult target insects, and selected non-target arthropods specifically the key herbivores, predators, parasitoids, and saprovores using yellow sticky cards. The treatments were: a) a Cry1Ab x CP4 EPSPS maize, b) a CP4 EPSPS maize-1, c) a CP4 EPSPS maize-2, d) a CP4 EPSPS maize plus an insecticide application to control the first generation Ostrinia nubilalis (Hubner), e) a CP4 EPSPS maize plus an insecticide application to control second generation O. nubilalis, f) a Cry1Ab x Cry3Bb1 x CP4 EPSPS maize, and g) a conventional maize.

Among the transgenic maize, Cry1Ab x Cry3Bb1 x CP4 EPSPS maize and Cry1Ab x CP4 EPSPS maize had slight agronomic differences on ear height, stalk lodging, dropped ears, seed moisture and grain yield. Significant agronomic variations were observed among the sites at Mead, Clay Center and Concord in Nebraska, and little or negligible interaction effects on arthropod populations.

A total of 165,358 arthropods were identified and enumerated from the yellow sticky cards in four maize growth sampling periods. Arthropod population abundance was not significantly impacted by Cry1Ab x Cry3Bb1 x CP4 EPSPS maize and Cry1Ab x CP4 EPSPS maize on the target insect pests, and non-target arthropods such as herbivores, predators, parasitoids and saprovores. Orius insidiosus and Scymnus spp. were abundant on transgenic maize, and significantly affected by insecticide applications. Significant variations were recorded on field adult mean counts of target and non-target arthropods among sites using yellow sticky cards in three sites in Nebraska (Mead, Clay Center, and Concord). Results of the study revealed that transgenic maize has no significant impact on the non-target arthropods.

Keywords: European corn borer; corn rootworm; non-target arthropods; genetically modified plants; Orius insidiosus; Scymnus spp.; transgenic maize; Cry1Ab maize, Cry1Ab x CP4 EPSPS maize, Cry1Ab x Cry3Bb1 x CP4 EPSPS maize; CP4 EPSPS maize; yellow sticky cards

1. Introduction

Maize (Zea mays L.) is the most important cereal with a total world production of 695 million metric tonnes in 2006 (FAOSTAT, 2007), and cultivated in a wide range of environmental conditions (Paliwal, 2000; Martin et al., 2006). In the USA, there were 37.88 million hectares (ha) planted to maize with the top 5 major producing states including Iowa (5.75 million ha), Illinois (3.34 million ha), Nebraska (3.80 million ha), Minnesota (3.39 million ha), and Indiana (2.63 million ha) (USDA-NASS, 2008).

With the use of modern biotechnology, maize is genetically improved to express genes for insect resistance, herbicide tolerance, disease resistance, tolerance to abiotic stress, male sterility for hybrid seed production, enhanced yield potential and quality traits (Armstrong et al., 2000). Globally, the major transgenic maize producers are the USA, Argentina, Canada, South Africa, Uruguay, Philippines, and Spain with greater than 100,000 hectares. In the USA, transgenic maize accounts for 63% of the "stacked products" expressing both insect resistance and herbicide tolerance (James, 2008).

The use of Bacillus thuringiensis (Bt) in transgenic maize poses public concerns on their potential impact to non-target organisms (Conner et al., 2003; O'Collaghan et al., 2005; Romeis et al., 2006). Non-target organisms are selected on a range of criteria (e.g. abundance in the field, ease of handling in the laboratory, taxonomic certainty, value of agro-ecosystem, endangered status), and those for which toxicity is demonstrated are subjected to more detailed investigations (e.g. higher tiered testing) (Cowgill et al., 2003; Dutton et al., 2003; Frederici, 2003; Schmitz et al. 2003). Safety risk assessments were reviewed based on the changes over time of non-target species and biodiversity, gene flow and evolution of resistance (Andow et al., 2006) as well as a tier-based approach (Romeis et al., 2006; Rose, 2007; Romeis et al., 2008). A conceptual model was also reviewed on ecological versus eco-toxicological methods for assessing the environmental risks of transgenic crops (Raybould 2007). The OECD (2007) published a consensus document on the safety of transgenic plants expressing Bt-derived insect control proteins.

Transgenic maize containing the Cry1Ab toxin is grown throughout North America (Daly and Buntin, 2005; Ostlie et al., 2008), and maize containing Cry3Bb1 toxin x CP4 EPSPS maize is now commercially available in the USA (USEPA, 2005). Maize with Bt Cry1Ab toxin expresses resistance to Lepidoptera, particularly European corn borer [Ostrinia nubilalis (Hubner)] (USDA-APHIS, 1996), while Bt Cry3Bb1 toxin has resistance to feeding damage caused by the corn rootworm complex (Diabrotica spp.) (USDA-APHIS, 2002).

Several studies evaluated the potential impact of transgenic maize containing Cry1Ab and Cry3Bb1 Bt toxins on non-target arthropods. Most of the laboratory and ecological studies on transgenic maize have not detected consistent significant effects on the non-target arthropods (Sims 1995; Donegan et al., 1996; Dogan et al., 1996; Yu et al., 1997; Orr and Landis, 1997; Pilcher et al., 1997; Riddick and Barbosa, 1998; Riddick et al., 1998; Lozzia et al., 1998; Lozzia, 1999; Zwahlen, et al., 2000; Al-deeb et al., 2001; Duan et al., 2002; Tommasini et al., 2002; Lundgren and Wiedenmann, 2002 and 2005; Federici, 2003; Jasinski et. al., 2003; Bhatti et al., 2005; Daly and Buntin, 2005; De la Poza et al., 2005; Divey, 2005; Head et al., 2005; Lopez et al., 2005; Mullin et al., 2005; Oyediran et al., 2005; Pilcher et al., 2005; Zwahlen and Andow, 2005; Ahmad et al., 2006; Eckert et al., 2006; Ludy and Lang, 2006; Obrist et al., 2006; Torres and Ruberson 2006; Fernandez et al., 2007; Marvier et al., 2007; Rose and Dively, 2007; Duan et al., 2008; and Wolfenger et al., 2008).

The major non-target arthropods in transgenic maize agro-ecosystems have been separated into functional groups including saprovores, herbivores, predators, and predators (Dively, 2005). The potential impact of transgenic maize to the populations of non-target arthropod species are both direct and indirect (Daly and Buntin, 2005). The detrimental impacts vary widely because of differing levels of sensitivity among various species present in the maize ecosystem (Wolfenberger and Phifer, 2000).

Many ecological studies of the effects of transgenic maize on arthropod predators have studied the minute pirate bug [Orius insidiosus (Say)], and other key predators like coccinellids [Scymnus sp., Coleomegilla maculata (DeGeer), Coccinella septempunctata (Linneaus), Harmonia axyridis Pallas, and Hippodamia convergens (Guerin Meneville)] and green lacewings Chrysoperla carnea (Stephen) (Al-deeb et al., 2001; Tommasini et al., 2002; Jasinki et al., 2003; Musser et al., 2004; Daly and Buntin, 2005; Dively, 2005; Oyediran et al., 2005; Pilcher et al., 2005; Ahmad et al., 2006; Obrist et al., 2006; Fernandez et al., 2007; Rose and Dively, 2007). In the Midwest, O. insidiosus is one of the important generalist predators consistently reported in maize, soybeans, and natural ecosystems (Wright, 2004; Brosius et al., 2006). Some of the other non-target arthropods reported include corn leafhoppers, planthoppers, aphids, thrips, flea beetle, spider mites and many others.

Ecological studies to assess the impact of transgenic maize on the non-target arthropods are limited in Nebraska. This study is designed to evaluate the agronomic performance, and to assess the effects of different transgenic and non-transgenic maize, with and without insecticide applications on adult target insects, and selected non-target arthropods specifically the key herbivores, predators, parasitoids, and saprovores using yellow sticky cards.

2. Materials and methods

2.1. Site location and description

The experiments were conducted during 2007 at three geographically different University of Nebraska-Lincoln experimental research stations. Specifically, the experimental field areas were located at the Agricultural Research and Development Center near Mead (N41o11.07' WO96 o27.263'), the South Central Agricultural Laboratory near Clay Center (N40 o34.216' WO98 o07.958') and the Northeast Research and Extension Center-Haskell Agricultural Laboratory near Concord (N42 o23.037' WO96 o57.193'). Soil types were Sharpsburg silty clay loam, Kennebec silty clay loam and Butler/Crete silt loam, respectively. All locations were previously planted with soybeans.

2.2. Agronomic practices

Experiments were planted in a no-till maize system on 5/10/07, 5/11/07 and 5/15/07 at Mead, Clay Center, and Concord, respectively. Nutrient management, irrigation, and herbicide application were conducted for each treatment based on the normal agronomic requirements of each specific site.

2.3. Experimental design and treatments

The experiment design was a randomized complete block design with four replications. The treatments were: a) a Cry1Ab x CP4 EPSPS maize, b) a CP4 EPSPS maize-1, c) a CP4 EPSPS maize-2, d) a CP4 EPSPS maize plus an insecticide application for the control of first generation O. nubilalis, e) a CP4 EPSPS maize plus an insecticide application for the control of second generation O. nubilalis, f) a Cry1Ab x Cry3Bb1 x CP4 EPSPS maize, and g) a conventional maize.

In the CP4 EPSPS maize plus an insecticide application for the control of first (1st) generation O. nubilalis, permethrin (Pounce 1.5G®) insecticide was applied at the recommended rate of 12 oz. rate/1000 row ft band at Mead (7/3/07), Clay Center (7/4/07), and Concord (7/9/07) using an improvised jar shaker applicator uniformly applied at whorl corn stage (V9-12). Bifenthrin (Capture 2 EC®) insecticide was sprayed in a formulation of 6.66 ml for every 2 gallons (7,571 ml) of water using a carbon-gated sprayer for the control of second (2nd) generation O. nubilalis at Mead (8/2/2007), Clay Center (8/3/2007), and Concord (8/15/2007 for replications 3 and 4, and 8/16/2007 for replications 1 and 2).

Each plot measured 60 square meters and consisted of 10 meters with 8 rows in each plot with ~400 plants per plot (~50 plants per row). Border rows and alleyways measured approximately 3 meters between plots, and were planted with conventional maize. The treatments were randomized in each block and site.

2.4. Data collection and sampling techniques

2.4.1. Agronomic data

The agronomic data were gathered from rows 2 and 3 of each plot from vegetative growth stages (V3-V5, V7-10, V11-V13, V14-VT growth stages), 30-35, 53-58, 60-65 and 75-80 days after planting (DAP), and reproductive growth stages (R1, R1-3 and R5 growth stages) at 80-85, 90-95 and 120-125 DAP. Maize vegetative and reproductive growth stages were identified based on the descriptions of Ritchie et al. (2005). The number of maize leaf collars during the vegetative stages of 10 randomly selected plants were counted at 30-35 DAP, 53-58 DAP, 60-65 DAP and 75-80 DAP. The height of each plant was measured from the ground level to the highest part of 10 randomly selected plants at 30-35 DAP, 60-65 DAP, 90-95 DAP, and 120-125 DAP in centimeters (cm). Ear height was measured from the ground level to the highest part of 10 randomly selected maize ears at 90-95 DAP in centimeters. The number of tillered plants was counted from rows 2 and 3 taken at 60-65 DAP. The number of plants lodged were counted at 120-125 DAP, and dropped ears prior to harvest in rows 2 and 3. Stalk lodging referred to the 30 degrees angle lodge of stalk from the ground level. The number of standing plants was counted prior to harvest (140-145 DAP). Grain moisture content was calculated from randomly selected seeds for each treatment using moisture meter after harvest (140-145 DAP). Readings were recorded in percent (%). All of the maize ears within each 3.048m/row for each rows 2 and 3 of each plot were harvested, husked, shelled, and weighed after harvesting (140-145 DAP) for the grain yield. The data on grain yield were expressed in kilograms (kg) per hectare.

2.4.2. Arthropod data

Arthropods were collected using yellow sticky cards. The sample unit consisted of two yellow sticky cards measuring 23 by 28 cm and sticky on one side only (Pherocon® AM, Trécé Inc., Adair, OK) (Musser et al., 2004; Pilcher et al., 2005). Two wooden stakes (2.5 by 2.1 by 244 cm) were positioned between rows 5, 6, and 7 of each plot at 23-28 DAP. The cards were folded and clipped with 2 binder clips around the wooden stake facing the maize rows just above maize whorl tips at the V2-3 growth stages (23-28 DAP) and V7-10 growth stages (53-58 DAP) while just above the maize ears at the R1-R2 growth stages (83-88 DAP) and R5 growth stage (113-118 DAP). Yellow sticky cards were collected for approximately 7 days at the V3-V5 growth stages (30-35 DAP), V11-R13 growth stages (60-65 DAP), R1-R3 growth stages (90-95 DAP), and R5 growth stage (120-125 DAP), sealed in a plastic bag, and brought to the Department of Entomology Laboratory at the University of Nebraska-Lincoln for enumeration. Voucher arthropod specimens were properly preserved using 85 percent alcohol, and some representative arthropod specimens were pinned and labeled. Arthropods were identified based on diagnostic key references, field guides, data bases, and on-line information (Tree of Life Web Project, 1995; CABI, 2002; Triplehorn and Johnson, 2005; BugGuide.Net, 2006; Zipcodezoo, 2004-2008; Brands, 1989-2007; Eaton and Kaufman, 2007; Evans, 2007). All arthropods in the sticky cards were counted with the aid of a dissecting microscope. Entomology graduate students who assisted in identification and enumeration of the specific arthropods were properly trained and randomly counter checked.

The adult counts from the 2 yellow sticky cards were pooled, and mean arthropod adult counts per card per day was used for the analysis. The adult counts of the specific arthropods were group into target insect pests and non-target arthropods.

Target insect pest of Cry1Ab toxin is Ostrinia nubilalis (Hübner), while for Cry3Bb1 toxin includes Western corn rootworm (Diabrotica virgifera virgifera LeConte), Southern corn rootworm (Diabrotica undecimpunctata howardii Barber), and Northern corn rootworm (Diabrotica barberi Smith and Lawrence).

The non-target arthropods were grouped into herbivores, arthropod predators, parasitoids, and saprovores. Specifically, herbivores are the following: corn earworm [Helicoverpa zea (Boddie)], corn leafhoppers, (primarily Dalbulus maidis DeLong and Wolcott), corn planthoppers (Anotia bonnetii Kirby), aphids [primarily Rhopalosiphum padi (Linnaeus)], thrips (primarily Frankliniella spp.), corn flea beetle (Chaetocnema pulicaria Melsheimer), corn blotch leafminer (primarily Agromyza sp.), whiteflies [primarily Bemisia argentifolii Bellows and Perring and Bemisia tabaci (Gennadius)], and spider mites (primarily Tetranychus urticae Koch, and Tetranychus turkestani Ugarov and Nikolski). Arthropod predators are the following: minute pirate bug [Orius indisiosus (Say)], scymnus lady beetle (Sycmnus sp.), twelve-spotted lady beetle [Coleomegilla maculata (DeGeer)], seven-spotted lady beetle [Coccinella septempunctata (Linneaus)], multicolored Asian lady beetle (Harmonia axyridis Pallas), convergent lady beetle [Hippodamia convergens (Guerin Meneville)], green lacewings [Chrysoperla carnea (Stephen)], syrphid fly (Eristalis sp.), long-legged flies (family Dolichopodidae), and spiders [primarily nursery web spiders (family Pisauridae), lynx spiders (family Oxyopidae), and long-jawed spiders (family Tetragnathidae)]. Parasitoids includes the following: tachinid fly (primarily Lydella thompsoni Gherting), chalcids (superfamily Chalcidoidea), ichneumonid wasps (family Ichneumonidae), and braconid wasps (family Braconidae). Lastly, saprovores are the following: house flies (family Muscidae), fungus gnat (family Mycetophilidae), and picture-winged flies (family Ulidiidae).

2.5. Data analysis

The data were analyzed by analysis of variance (ANOVA) using SAS's PROC MIXED (SAS 2003). The type 3 tests of fixed effects were used in the analysis (treatment, site, and treatment x site). The values of P<0.05 were considered significant, and values of P<0.0001 were considered highly significant. The treatment and site effects had significant differences. No significant differences were revealed on the treatment x site effects, and these were not presented in the results and discussions. Mean separations were done by using least significant differences (LSD).

3. Results

3. 1. Agronomic performance

Among the agronomic growth parameters, significant differences were revealed on the collar counts at 60-65 DAP, ear height, stalk lodging, dropped ears, standing plants, seed moisture content after harvest and grain yield, while significant variations were revealed among the sites at Mead, Clay Center and Concord in Nebraska (Table 1).

Cry1Ab x CP4 EPSPS maize and Cry1Ab x Cry3Bb1 x CP4 EPSPS maize had no significant differences for mean collar counts at all the sampling periods compared to CP4 EPSPS maize and conventional maize. Among all treatments, there were no significant differences in height measurements. With regards to mean tiller counts, there were no significant differences among the treatments. Cry1Ab x Cry3Bb1 x CP4 EPSPS maize had a significantly the tallest mean ear height of 150 cm comparable to Cry1Ab x CP4 EPSPS maize of 149 cm at 90-95 DAP. Cry1Ab x Cry3Bb1 x CP4 EPSPS maize obtained significantly fewer stalk lodging of 0.67 stalk lodge and 1.08 dropped ears, comparable to Cry1Ab x CP4 EPSPS maize, and Cry1Ab x CP4 EPSPS maize applied with insecticide to control the first generation of O. nubilalis. The highest mean counts of stalk lodging were recorded for the CP4 EPSPS maize and conventional maize. At 140 DAP, Cry1Ab x Cry3Bb1 x CP4 EPSPS maize recorded higher standing plants comparable to Cry1Ab x CP4 EPSPS maize. Cry1Ab x CP4 EPSPS maize had the highest percentage of seed moisture (16.77%), and conventional maize was the lowest seed moisture (15.74%) after harvest. Based on the computed grain yield from 3.048m/row for each rows 2 and 3 of each plot, Cry1Ab x Cry3Bb1 x CP4 EPSPS maize had the highest mean grain yield of 9,604 kg/ha, which was comparable to Cry1Ab x CP4 EPSPS maize mean with a 9,183 kg/ha. CP4 EPSPS maize with and without insecticide applications, and conventional maize were not significantly different in grain yield ranging from 8,604 kg/ha to 8,849 kg/ha.

At 30-35 DAP, maize in Clay Center site revealed highest mean collar counts among the sites, while Concord at 53-58 DAP (10 collars), 60-65 DAP (13 collars), and 75-80 DAP (17 collars). With regards to plant height, Clay Center recorded significantly tallest plants at 30-35 DAP (62.20 cm) and 60-65 DAP (245.10 cm), while Mead at 80-85 DAP (294.41 cm), and 120-125 DAP (286.70 cm). Maize in Mead site revealed fewer mean tiller counts (1 tiller), smaller ear height measurement (144.13 cm), fewer stalk lodging (1.79 stalk lodge), and higher dropped ears (5.25). Among the sites, Concord recorded significantly highest standing plants (49 plants) at 140-145 DAP, and percentage of seed moisture content (17.79%). Clay Center had the highest computed mean grain yield of 8,851 kg/ha followed by Mead (8,570 kg/ha), and Concord (8,270 kg/ha).

3.2. Arthropod population

3.2.1 Summary of arthropods

A total of 165,358 arthropod were counted in increasing trend among the sampling periods at 30-35 DAP (34,956 arthropods), 60-65 DAP (39,223 arthropods), 90-95 DAP (43,132 arthropods), and 120-125 DAP (48,047 arthropods) using yellow sticky card (Table 2).

Cry1Ab x Cry3Bb1 x CP4 EPSPS maize had the lowest significant adult counts of 11,940 arthropods at 90-95 DAP, which was comparable to Cry1Ab x CP4 EPSPS maize (12,691 arthropods). The number of arthropods trapped in the yellow sticky cards increased from 30-35 DAP to 120-125 DAP. The highest significant adult counts of arthropods in the yellow sticky cards were in Concord at 30-35 DAP (18,082 arthropods), Mead at 60-65 DAP (15,681 arthropods), Clay Center at 90-95 DAP (19,508 arthropods), and Mead at 120-125 DAP (18,641 arthropods)

3.2.2. Target insect pests of Cry1Ab and Cry3Bb1toxins

Cry1Ab x CP4 EPSPS maize, and CP4 EPSPS maize with and without insecticide applications had no significant differences in the population of O. nubilalis at all sampling periods (Table 3). Additionally, Cry1Ab x Cry3Bb1 x CP4 EPSPS maize and conventional maize showed no significant differences on the adult counts of O. nubilalis. CP4 EPSPS maize applied with permethrin (Pounce 1.5 G®) to control the first generation of O. nubilalis, and bifenthrin (Capture 2 EC®) insecticides to control the second generation of O. nubilalis had no significant differences compared to other treatments. However, there were significantly highest mean adult counts of O. nubilalis among the sites at 60-65 DAP in Mead (0.0128 O. nubilalis/card/day), and 120-125 DAP in Concord (0.0204 O. nubilalis/card/day).

Cry1Ab x Cry3Bb1 x CP4 EPSPS maize had no significant effects in the mean adult counts of D. v. virgifera, D. undecimpunctata howardi, and D. barberi as well as on Cry1Ab x CP4 EPSPS maize, and CP4 EPSPS maize with, and without insecticide applications. Significant variations were recorded on the mean adult counts of D. v. virgifera among the sites. The highest mean adult counts were recorded at Concord (0.0306 D. v. virgifera/card/day) at 30-35 DAP, and Clay Center at 60-65 DAP, 90-95 DAP, and 120-125 DAP with 0.1199, 1.3929, and 0.3572 D. v. virgifera/card, respectively. Mead recorded the highest mean adult counts of 0.0332 D. undecimpunctata howardi/card/day at 60-65 DAP, while Concord had the lowest mean adult counts of 0.0281 D. undecimpunctata howardi/card/day at 90-95 DAP. Mead recorded the highest mean adult counts of 0.0816 D. barberi per yellow sticky card per day at 90-95 DAP, while Mead and Concord recorded significantly comparable mean adult counts of 0.0255, and 0.0408 D. barberi/card/day at 120-125 DAP, respectively.

3.2.3. Non-target arthropods

3.2.3.1. Herbivores

Cry1Ab x CP4 EPSPS maize and Cry1Ab x Cry3Bb1 x CP4 EPSPS maize had insignificant effects on the mean adult herbivore counts of H. zea, D. maidis (primary species), A. bonneti, R. padi (primary species), Frankliniella spp. (primary species), C. pulicaria (primary species), Agromyza sp. (primary species), Bemisia spp. (primarily B. argentifolii and B. tabaci), and Tetranychus spp. (primarily T. urticae, and T. turkestani) in all sampling periods, while the mean adult counts in most of these non-target herbivore populations had significant variations in each of the site (Table 4).

The mean adult counts of H. zea revealed no significant differences. Cry1Ab x Cry3Bb1 x CP4 EPSPS maize, CP4 EPSPS maize, and conventional maize had no significant effects on mean adult counts of D. maidis at all sampling periods. There was a decreasing trend of D. maidis mean adult counts per yellow sticky card per day from 30-35 DAP to 120-125 DAP. With regards to adult counts of A. bonneti, results revealed no significant differences among treatment means. Cry1Ab x CP4 EPSPS maize had significantly the fewest mean adult counts of R. padi at all sampling periods comparable to Cry1Ab x Cry3Bb1 x CP4 EPSPS maize. Cry1Ab x CP4 EPSPS maize had the lowest mean adult counts of 8.3 Frankliniella spp./card/day at 60-65 DAP, which was comparable to conventional maize (8.5 thrips/ card/day) and Cry1Ab x Cry3Bb1 x CP4 EPSPS maize (8.7 Frankliniella spp./card/day). Among treatments, conventional maize had the lowest mean adult counts of 0.0357 C. pulicaria/card/day at 60-65 DAP. At 90-95 DAP, Cry1Ab x Cry3Bb1 x CP4 EPSPS maize had the lowest mean of 2.1548 Agromyza sp./card/day, which was comparable to the Cry1Ab x CP4 EPSPS maize (2.4702 Agromyza sp./card/day). Cry1Ab x Cry3Bb1 x CP4 EPSPS maize had significantly fewest mean adult counts of 3.4881 Bemisia spp./card/day comparable to the conventional maize (3.4405 Bemisia spp./card/day), and Cry1Ab x CP4 EPSPS maize (3.8571 Bemisia spp./card/day) at 90-95 DAP. Among treatments, there were no significant effects on mean adult counts of Tetranychus spp. in all sampling periods.

In Nebraska, the mean adult counts of H. zea revealed no significant variations among three sites. Concord had recorded highest mean counts of 8 D. maidis/card/day at 30-35 DAP, while Mead at 60-65 DAP, 90-95 DAP and 120-125 DAP. Highest mean adult counts of A. bonnetii were recorded in Clay Center at 60-65 DAP (0.0536 A. bonnetii/card/day), and in Concord at 120-125 DAP (1.8546 A. bonnetii/card/day). In all maize growth sampling periods, significant variations were revealed among the sites with the highest R. padi mean adult counts in Concord at 30-35 DAP (0.6454 R. padi/card/day) and 60-65 DAP (0.3240 R. padi/card/day), while Mead at 90-95 DAP (2.4388 R. padi/card/day) and 120-125 DAP (2.9337 R. padi/card/day). Concord had the highest mean adult counts of 17 Frankliniella spp./card/day at 30-35 DAP. Clay Center had the highest mean adult counts of Frankliniella spp. at 90-95 DAP and 120-125 DAP. Mead had significantly the highest mean adult counts at 60-65 DAP (0.1454 C. pulicaria/card/day, 90-95 DAP (0.5561 C. pulicaria/card/day), and 120-125 DAP (0.3699 C. pulicaria/card/day). Consequently, Mead had the highest mean adult counts of 6.1582 Agromyza sp/ card/day at 60-65 DAP, Clay Center at 90-95 DAP with 5.1224 Agromyza sp./card/day, and Mead again at 120-125 DAP with 4.0638 Agromyza sp./card/day. At 30-35 DAP, Concord had the highest mean adult counts at 0.0370 Bemisia spp./card/day, Mead at 60-65 DAP (0.9184 Bemisia spp./card/day), and Clay Center at both 90-95 DAP (11.7806 Bemisia spp./card/ day) and 120-125 DAP (2.8163 Bemisia spp./card/day). Among the sites, Concord obtained the highest mean adult counts of 0.2551 Tetranychus spp./card/day at 30-35 DAP.

3.2.3.2. Arthropod predators

Cry1Ab x Cry3Bb1 x CP4 EPSPS maize, Cry1Ab x CP4 EPSPS maize, and CP4 EPSPS maize had no significant effects on the mean adult counts per sticky card day of O. insidiosus, Scymnus sp, C. maculata, C. septempunctata, H. axyridis, H. convergens, C. carnea, Eristalis sp., long-legged flies (family Dolichopodidae), and spiders (primarily in the family Pisauridae, Oxyopidae and Tetragnathidae) compared to conventional maize (Table 5). However, significant variations were recorded among the sites on the mean adult counts per sticky card day of some arthropod predators in different sampling periods.

O. insidiosus and Scymnus sp. were the two important arthropod predators recorded with significant effects on mean adult counts per sticky card per day. At the 30-35 DAP, there were no significant differences on Cry1Ab x CP4 EPSPS maize, and Cry1Ab x Cry3Bb1 x CP4 EPSPS maize compared to CP4 EPSPS maize with insecticide applications and conventional maize on the mean adult counts of O. insidiosus. CP4 EPSPS maize plus an insecticide application for the control of 1st generation O. nubilalis had the fewest significant mean adult counts of 0.4643, and 0.4583 O. insidiosus/card/day at 60-65 DAP, and 90-95 DAP, respectively. There were no significant differences on mean adult count of O. insidiosus among Cry1Ab x CP4 EPSPS maize and Cry1Ab x Cry3Bb1 x CP4 EPSPS maize compared to CP4 EPSPS maize with insecticide applications, and conventional maize at 120-125 DAP. Conventional maize had the highest significant mean adult counts of 1.4345 Scymnus sp./card/day at 90-95 DAP, which was comparable to the mean adult counts of Cry1Ab x Cry3Bb1 x CP4 EPSPS maize (1.2143 Scymnus sp./card/day), Cry1Ab x CP4 EPSPS maize (1.1667 Scymnus sp./card/day), and CP4 EPSPS maize with 1.1667, and 1.1131 Scymnus sp./card/day, respectively.

Significant variations on O. insidiosus adult counts were recorded among the sites. In all sampling periods, Clay Center had the highest O. insidiosus mean adult counts at 30-35 DAP (0.0459 O. insidiosus/card/day), Clay Center at 60-65 DAP (0.3112 O. insidiosus/card/day), and 90-95 DAP (1.0077 O. insidiosus/card/day). Mead had the highest significant mean adult counts of 1.6760 O. insidiosus/card/day, which was comparable to Concord (1.1429 O. insidiosus/card/day) at 120-125 DAP. Concord recorded the highest mean adult counts of 0.1046 Scymnus sp/card/day at 30-35 DAP, while Clay Center at 60-65 DAP (0.3316 Scymnus sp/card/day), 90-95 DAP (1.4311 Scymnus sp/card/day), and 120-125 DAP (2.5893 Scymnus sp/card/day). Mead had the highest significant mean adult count at 30-35 DAP, 90-95 DAP and 120-125 DAP with 0.0238, 0.2066 and 0.5076 C. maculate/card/day, respectively. No significant variations on adult counts of C. septempunctata, H. axyridis among the sites at all sampling periods of maize. Concord had the highest mean adult counts of 0.0638 H. convergens per yellow sticky card per day at 30-35 DAP, which was comparable to mean adult counts in Clay Center (0.0434 H. convergens/card/day). At 120-125 DAP, there were significant variations for the mean adult counts of C. carnea among the sites, with Mead having the highest mean adult counts of 0.6684 C. carnea/card/day. Peak abundance of C. carnea adults was observed at 120-125 DAP. Clay Center had the highest mean adult counts of 0.4515 Eristalis sp./card/day at 30-35 DAP, while Concord at 90-95 DAP (0.2730 Eristalis sp./card/day), and 120-125 DAP (0.1454 Eristalis sp./card/day). Mead had the highest significant variations with 0.0944 long-legged flies/card/day at 30-35 DAP, and 0.17806 long-legged flies/card/day at 60-65 DAP. Concord had the highest variations mean adult counts of 1.1531 long-legged flies/card/day at 90-95 DAP, while Mead had the highest at 120-125 DAP with 0.9592 long-legged flies/card/ day. Among the sites, significant variations were observed among the adult spiders for each sampling periods.

3.2.3.3. Parasitiods

Cry1Ab x CP4 EPSPS maize and Cry1Ab x Cry3Bb1 x CP4 EPSPS maize had no significant differences on mean adult counts of tachinid fly (primarily L. thompsoni), chalcids (superfamily Chalcidoidea), ichneumonid wasps (family Ichneumonidae), and braconid wasps (family Braconidae) compared to CP4 EPSPS maize with and without insecticide applications, and conventional maize, while significant variations were recorded among the sites (Table 6).

Mead had recorded the highest mean adult counts of tachinid fly at 60-65 DAP (0.0536 tachinids/card/day) and 90-95 DAP (0.0434 tachinids/card/day), while Clay Center at 120-125 DAP (0.2271 tachinids/card/day). In the super family Chalcidoidea, the majority of the families counted in the yellow sticky cards were in the family Chalcididae, Trichogrammatidae, Pteromalidae, Aphelinidae, and Mymaridae. Concord recorded the highest mean adult counts of 1.0332 chalcids/card/day at 30-35 DAP, while Clay Center obtained the highest mean adult counts of 4.5893, and 6.9413 chalcids/card/day at 60-65 DAP and 90-95 DAP, respectively. Significantly comparable chalcids mean adult counts were recorded at Mead (8.3495 chalcids/card/day) and Concord (7.9464 chalcids/card/day) at 120-125 DAP. Eriborus terebrans (Gravenhorst) was the most frequent ichneumonid wasps counted. Mead had the highest mean adult counts of 1.6480, 0.6888, and 0.7985 ichneumonid wasps/card/day at 30-35 DAP, 60-65 DAP and 90-95 DAP, respectively. Macrocentrus grandii Goidanich was among the braconid wasps species counted and identified. Concord had significantly highest mean adult counts of 0.1760 braconid wasps/card/day at 90-95 DAP, which was comparable to Clay Center (0.1428 braconid wasps/card/day). The peak presence of braconid wasps adults were recorded at 120-125 DAP. Mead had the highest significant mean adult counts of 3.6964 braconid wasps/card/day.

3.2.3.4. Saprovores

House flies (family Muscidae), fungus gnat (family Mycetophilidae), and picture-winged flies (family Ulidiidae) mean adult counts revealed no significant differences among Cry1Ab x Cry3Bb1 x CP4 EPSPS maize and Cry1Ab x CP4 EPSPS maize compared to conventional maize, while significant variations were recorded on the saprovore mean adult counts among the sites (Table 7).

Cry1Ab x Cry3Bb1 x CP4 EPSPS maize had lowest mean adult counts of 2.2738 house flies/card/day, which was comparable to Cry1Ab x CP4 EPSPS maize with 2.4404 house flies/card/day, and conventional maize with 3.1310 house flies/card/day at 120-125 DAP. There were no significant differences among the Cry1Ab x CP4 EPSPS maize, and Cry1Ab x Cry3Bb1 x CP4 EPSPS maize compared to CP4 EPSPS maize with insecticide applications, and conventional maize on the mean adult counts of fungus gnat and picture flies at all sampling periods.

There were significant variations on house fly adult counts among sites. Concord had the highest significant mean adult counts of 2.0281 house flies/card/day. Mead had the highest significant mean adult counts with 1.9566 house flies/card/day at 60-65 DAP, while Clay Center had the highest adult counts of 1.4668 house flies/card/day at 90-95 DAP, which was comparable to Mead with 1.3546 house flies/card/day. Moreover, Concord had the highest mean adult counts of 3.7245 house flies/card/day among the sites, which is comparable to Clay Center with 3.2041 house flies/card/day at 120-125 DAP. Clay Center had the highest mean adult counts of 8.0893 fungus gnats/card/day at 30-35 DAP, Concord at 60-65 DAP (2.5176 fungus gnats/card/day), and Clay Center at 120-125 DAP (5.1173 fungus gnats/card/day). The highest mean adult counts of picture-winged flies were recorded at Mead in an increasing trend from 0.5740 fungus gnats/card/day at 30-35 DAP to 2.4592 fungus gnats/card/day at 120-125 DAP.

4. Discussion

4.1. Agronomic performance

In this study, Cry1Ab x Cry3Bb1 x CP4 EPSPS maize had the highest grain yield after harvest comparable to Cry1Ab x CP4 EPSPS maize. Dillehay et al. (2004) reported that Bt hybrids produced higher yields, but also with higher grain moisture content at harvest and lower test weight than isoline and test hybrids. Interestingly, the same result trend in the study were observed on Cry1Ab x CP4 EPSPS maize had the highest seed moisture content at harvest compared to Cry1Ab x Cry3Bb1 x CP4 EPSPS maize, CP4 EPSPS maize and conventional maize.

Lower mean counts of maize stalk lodge and dropped ears were recorded in Cry1Ab x Cry3Bb1 x CP4 EPSPS maize, Cry1Ab x CP4 EPSPS maize, and CP4 EPSPS maize applied with insecticide (Pounce 1.5G®) for the control of first generation O. nubilalis, compared to conventional maize. In correlation, Graeber et al. (1999) reported that grain yield, stalk lodging and test weight of the transgenic hybrids were not affected by first and second generation of O. nubilalis infestation, and transgenic hybrids performed the same as their non-transgenic counterparts in the absence of O. nubilalis pressure. Yield of Bt hybrids was 4 to 8% greater than standard hybrids when inoculated with O. nubilalis, and yield of Bt hybrids was 8% less than standard hybrids when an insecticide was applied (Lauer et al., 1999). The use of either Bt hybrids or whorl-applied permethrin granules resulted in significant yield advantage in only 1 out of 3 year of performance testing (Catangui and Berg, 2002). Additionally, (Catangui, 2003) reported that Bt maize hybrids had significantly higher yields than the untreated non-Bt isolines in 1997 and 1998 when O. nubilalis pressure was high.

4.2 Arthropod population

4.2.1. Effects on target insect pests of Cry1Ab and Cry3Bb1 toxins

O. nubilalis is a lepidopteran insect pest considered as the most damaging pest of maize throughout the United States and Canada, hence Bt Cry1Ab maize was commercially available to control the pest (USDA-APHIS, 2002; Ostlie et al., 2008). Adult O. nubilalis were monitored only using the yellow sticky cards for both generations with insignificant mean adult counts among the transgenic and non-transgenic, with and without insecticide applications. The highest mean adult counts of O. nubilalis were recorded on conventional maize at R5 growth stage for the second generation. Number of larvae and stalk tunnel damage was not measured because of low incidence of larval damage. Because adult counts of O. nubilalis populations were so low, damage was not detected. Indeed, the potential benefits of Bt maize can only compared to non-transgenic maize (isoline or conventional maize) when there is high insect pressure (Ostlie et al., 2008).

The corn rootworm complex (D. v. virgifera, D. undecimpunctata howardi, and D. barberi) had no significant differences for mean adult counts among the Cry1Ab x CP4 EPSPS maize, Cry1Ab x Cry3Bb1 x CP4 EPSPS maize, CP4 EPSPS maize with and without insecticide applications, and conventional maize. However, there were significant variations in the mean adult counts Diabrotica species complex among the sites. Diabrotica adult species were observed feeding on the maize tassel, and maize silks particularly on the conventional and transgenic isolines at R1-R3 growth stages. Destructive root damage assessments were not performed because the mean adult counts were very low (<1 adult/card/day) at V11-V13 growth stages, and root damages were none-observable or negligible.

4. 2. 2. Non-target arthropods

4.2.2.1. Herbivores

A number of studies found no adverse or inconsistent effects of Bt maize on non-target herbivores (Daly and Buntin, 2005; Dively, 2005; Eckert et al., 2006; Rose and Dively, 2007).

Dively (2005) provided a broad scale assessment of the abundance and taxa richness of the arthropod community including non-target herbivores potentially exposed to VIP3A and cry1Ab expressed in Bt maize, he concluded that biodiversity and community level-responses were not significantly affected by expression of the stacked VIP3A and Cry1Ab proteins. Daly and Buntin (2005) reported that thrips numbers on Bt and non-Bt maize were not significantly different in three trials, but thrips were more abundant on Bt plants than non-Bt plants. Eckert et al. (2006) reported the abundance of the aphid R. padi was significantly higher in the insecticide treated than in the Bt maize or control plots. Rose and Dively (2007) reported that biodiversity and community-level responses were not significantly affected by the transgenic hybrid.

4.2.2.2. Arthropod predators

Prasifka et al. (2005) suggested not to use small plots (width <9 m) for ecological studies of transgenic crops. In this study, the experimental plot size measured 60 square meters (10 x 6 m). Regardless of the sampling field methods, results of this study supports that Bt maize had no significant impact on O. insidiosus populations of other previous ecological field studies of O. insidiosus in Bt maize ecosystems (Pilcher et al., 1997; Al-deeb et al., 2001; Tommasini et al., 2002; Jasinski et al., 2003; Musser et al., 2004; Daly and Buntin, 2005; De la Poza et al., 2005; Dively, 2005; Oyediran et al., 2005; Pilcher et al., 2005; Ahmad et al., 2006; Obrist et al., 2006; Fernandez et al., 2007; and Rose and Dively, 2007).

O. insidiosus is a generalist predator of thrips (Riudavets, 1995), aphids (Fox et al. 2004, Rudledge et al., 2004, Brosius et al., 2006), whiteflies, spider mites and eggs of other insects (Reid, 1991; van Lenteren et al., 1997; Funderburk et al., 2000). Obrist et al. (2006) reported the effect of Bt toxin on some herbivores (aphids, thrips, leafhoppers) were negligible to spider mites, and can be transferred to predators like Orius spp. For this study, number of adult thrips, aphids, whiteflies and spider mites were measured using yellow sticky cards at all sampling growth stages. Adult thrips were present at all sampling growth stages with peak counts from the V11-V13 growth stages to R1-R3 growth stages. Aphids were most abundant at R1-R3 growth stages and R5 growth stages. Whitefly adults were recorded at all the sampling dates with the peak adult counts at the R1-R3 growth stages, while the spider mites adult counts were proportionately distributed over all samplings. Cry1Ab x CP4 EPSPS maize and Cry3Bb1 x CP4 EPSPS maize had the lowest mean adult counts of thrips, aphids, whiteflies, and spider mites. Al-deeb et al. (2001) reported that no significant differences occurred in mortality of O. insidiosus nymphs when they fed on Bt or non-Bt silk one day and on H. zea eggs next day. They also reported that the number of O. insidiosus adults and nymphs in fields of Bt maize and non-Bt maize did not differ significantly in most cases.

In this study, Cry1Ab x CP4 EPSPS maize and Cry1Ab x Cry3Bb1 x CP4 EPSPS maize did not impact abundance of Scymnus spp., C. maculata, C. septempunctata, H. axyridis, and H. convergens. These findings support previous studies results that look at transgenic crop effects on coccinellids in different geographic locations (Pilcher et al., 1997; Lundgren and Wiedenmann, 2002; Wold et al., 2002; Musser et al., 2004; Daly and Buntin, 2005; De la Poza et al., 2005; Dively, 2005; Head, 2005; Pilcher et al., 2005; Ahmad et al., 2006; Eckert et al., 2006; Fernandez et al., 2007).

Cry1Ab x CP4 EPSPS maize and Cry1Ab x Cry3Bb1 x CP4 EPSPS maize had no significant differences on the mean adult counts of C. carnea among all sampling growth stages. This result agrees with other studies (Jasinski et al., 2003; Pilcher et al., 2005). Larvae were observed at the silking and reproductive stages of maize and peak C. carnea adult populations were at R5 growth stage.

Population of syrphid flies (family Syrphidae) primarily Eristalis sp. and long-legged flies (family Dolichopodidae) had no significant differences at all samplings. Results showed that Cry1Ab x CP4 EPSPS maize and Cry1Ab x Cry3Bb1 x CP4 EPSPS maize had no effects on the populations of syrphid flies and long-legged flies. However, population size of these dipteran predators significantly varied in populations at different sampling growth stages in Mead, Clay Center and Concord. Dively (2005) reported that Eristalis sp. in the family Syrphidae and long-legged flies in the family Dolichopodidae using sticky cards. Bt hybrid containing VIP3A x Cry1Ab showed no differences in abundance compared to an isogenic control.

The most frequently reported spider species (order Araneae) were nursery web spiders (family Pisauridae) at the seedling stages, and lynx spiders (family Oxyopidae) and long-jawed spiders (family Tetragnathidae) at the vegetative and reproductive stages. Cry1Ab x CP4 EPSPS maize and Cry1Ab x Cry3Bb1 x CP4 EPSPS maize had no significant effects on the mean adult counts of spider at any sampling growth stages. However, there were significant variations in population abundance in locations. These agree with those of De la Poza et al. (2005), Dively (2005), Ludy and Lang (2006), and Ferdandez et al. (2007).

4.2.2.3. Parasitiods

Dively (2005) reported that parasitoid populations not impacted by transgenic field maize. The most frequent parasitoids captured were Mymaridae (34.6%), Trichogrammatidae (11.1%), Scelionidae (10.6%), Encyrtidae (8.2%), Eulophidae (7.4%), Braconidae (7.3%), Tachinidae (6.6%), Ceraphronidae (5.8%), and Pteromalidae (5.3%). In 2007, Rose and Dively reported egg and larval parasitoids in the families Scelionidae, Myrmaridae, Trichogrammatidae, Pteromalidae, Encyrtidae, Braconidae, Ceraphronidae, and Aphelinidae comprised 91% of all the hymenopterans recorded, and averaged 1.4 parasitoids per sticky card. Fernandes et al. (2007) in an investigation of the short-term effects of Bt maize on non-target arthropods in Brazil reported that there were no differences in egg parasitism rates of H. zea by Trichogramma sp. (Hymenoptera: Trichogrammatidae). Wolfenbarger et al. (2008) conducted a meta-analysis on the abundance of non-target arthropods in Bt and non-Bt crops. As expected, fewer specialist parasitoids of target pest occurred in Bt maize fields compared to unsprayed non-Bt controls, but no significant reduction were detected for other parasitoids.

4.2.2.4. Saprovores

Rose and Dively (2007) reported a study on the effects of insecticide-treated and lepidopteran-active Bt transgenic sweet corn on the abundance and diversity of arthropods. In their study, they reported that frit flies, picture-winged flies, and humpbacked flies had the most abundant mean densities of saprovores ranging from 9 to 50 flies per card, and generally concluded that biodiversity and community level responses were not significantly affected by the transgenic hybrid.

5. Conclusions

Among the transgenic maize, Cry1Ab x Cry3Bb1 x CP4 EPSPS maize and Cry1Ab x CP4 EPSPS maize had slight agronomic differences on ear height, stalk lodging, dropped ears, seed moisture, and grain yield. However, significant agronomic variations were observed across the three sites in Nebraska and little or negligible interaction effects on arthropod populations.

Arthropod population abundance were not significantly impacted by Cry1Ab x Cry3Bb1 x CP4 EPSPS maize and Cry1Ab x CP4 EPSPS on the target insect pests, and non-target arthropods such as herbivores, predators, parasitoids and saprovores. O. insidiosus and Scymnus spp. were abundant on transgenic maize and significantly affected by insecticide applications.

Significant variations were recorded on field adult mean counts of target and non-target arthropods among sites using yellow sticky cards in three sites in Nebraska (Mead, Clay Center, and Concord).

Results of the study supports and strengthens the previous findings on the transgenic maize field performance and ecological studies that transgenic maize has higher yield compared to conventional maize, and transgenic maize has no significant impact on the non-target arthropods.

Important key non-target arthropods identified on this study are suggested for future research.

Acknowledgement

We thank Bill McCormick, Terry Devries, Gerald Echtenkamp, Karl Bruer, Rosana Serikawa, Erica Lindroth, Khanoporn Tangtrakulwanich, and summer student technicians for assistance during the corn growing seasons. Technical assistance of Dr. Gary Brewer is highly appreciated. We also thank the Fulbright-Philippine Agriculture Scholarship Program, Department of Agriculture-Bureau of Plant Industry, Monsanto Company, and University of Nebraska-Lincoln for financial assistance.

Writing Services

Essay Writing
Service

Find out how the very best essay writing service can help you accomplish more and achieve higher marks today.

Assignment Writing Service

From complicated assignments to tricky tasks, our experts can tackle virtually any question thrown at them.

Dissertation Writing Service

A dissertation (also known as a thesis or research project) is probably the most important piece of work for any student! From full dissertations to individual chapters, we’re on hand to support you.

Coursework Writing Service

Our expert qualified writers can help you get your coursework right first time, every time.

Dissertation Proposal Service

The first step to completing a dissertation is to create a proposal that talks about what you wish to do. Our experts can design suitable methodologies - perfect to help you get started with a dissertation.

Report Writing
Service

Reports for any audience. Perfectly structured, professionally written, and tailored to suit your exact requirements.

Essay Skeleton Answer Service

If you’re just looking for some help to get started on an essay, our outline service provides you with a perfect essay plan.

Marking & Proofreading Service

Not sure if your work is hitting the mark? Struggling to get feedback from your lecturer? Our premium marking service was created just for you - get the feedback you deserve now.

Exam Revision
Service

Exams can be one of the most stressful experiences you’ll ever have! Revision is key, and we’re here to help. With custom created revision notes and exam answers, you’ll never feel underprepared again.