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Switchgrass (Panicum virgatum L.), native to North America is an imminent source of biomass that can adapt a variety of environmental conditions and geographic distribution (Sanderson et al. 1996, Parrish and Fike 2005). In contrast to annual crops, switchgrass is more valuable to decrease pesticide application rates, particularly in reducing soil erosion and water runoff (Vaughan et al. 1989, Sanderson et al. 1996). It has also been recognized to be useful not only for wildlife but also in maintaining stream banks and buffers (Sanderson et al. 1996, Parrish and Fike 2005). Despite the fact that switchgrass is not hydrophilic and can even be grown on soils with moderate fertility, it could be a suitable alternative crop in the areas facing regular droughts to offset the yield loss and production costs of the traditional crops.
Over the years, switchgrass might get its status as emerging bioenergy crop (Sanderson et al. 2004). The curiosity to use switchgrass for biomass energy feed-stock has raised several questions about its management (Mclaughlin and Walsh 1998). Switchgrass and other warm season grasses depend largely on effective use of water and nitrogen for improved production (Epstein et al. 1996). Warm season grasses have higher photosynthetic rates at high temperatures (Waller and Lewis 1979), efficient use of nitrogen (Waller and Lewis 1979) and phosphorus (Morris et al. 1982).
Comparatively, monoculture of any crop on larger scale than on small scale may possibly lead towards limited insect diversity (Bourn and Thomas 2002). In contrast, polyculture has better alternative food sources and natural enemies of the herbivores (Root 1973). Hence, the plantation with limited plant diversity or monoculture may lead towards insect outbreaks (Andow 1991). Weeds, on the other hand also compete with switchgrass for available resources (Parrish and Fike 2005) during the establishment year. Very few insect studies for switchgrass has been conducted (Parrish and Fike 2005) as much of the work on switchgrass had focused more on improving biomass yield and weed control (Sanderson et al. 1996, Parrish and Fike 2005). Planting switchgrass on large scale could result in a suitable habitat for many insects (Parrish and Fike 2005).
More than 100 species of short-horned grasshoppers have been reported in Nebraska (Brust et al. 2008). Among many species of grasshoppers, some have attained the status of being serious pests in rangeland (Mulkern et al. 1969). Most of the grasshopper species feed on a variety of plants (Joern 1983). The general perception that grasshoppers feed on whatever the plant species is available has basically been dismissed by studies of host species choice. Uvarov (1977) and Joern (1979) in grasshoppersâ€™ foregut studies convincingly demonstrated the food preference for the available plant species. Similarly, grasshoppers also have the ability to fulfill its specific nutritional requirements from specific plant (Behmer and Joern 1993). In contrast, grasshopper selection among individual plants and plant tissues of a single species has not been well studied. Though the effect of the plant quality on host choice by a number of generalist grasshoppers and locusts has rarely been directly examined but it is likely that it may have a strong influence both within and among plant species. Few reports propose existence of preference for damaged (Anderson and Wright 1952), diseased (Bernays et al. 1977), dead (Gangwere 1961, Uvarov 1977), wilted (Kaufmann 1968, Ueckert and Hansen 1971), or succulent tissue (Chapman 1957, Uvarov 1977).
Grasshopper, locust and other orthopteran insects are commonly found in grassland ecosystems. The available food plants and extremely localized environment are particularly very important factors in determining the grasshopper species composition (Joern and Lawlor 1981) Commonly found species in Unites States include Melanoplus femurrubrum (DeGeer), Melanoplus bivittatus (Say), Phoetaliotes nebrascensis (Thomas), Eritettix simplex (Thomas), Aulocara elliotti (Thomas), Phlibostroma quadrimaculatum (Thomas), Psoloessa deliculata and Trachyrhachys aspersa (Welch et al. 1991, Craig et al. 1999) and high densities of grasshoppers can cause a significant effect on the economics of forage production. An approximate of dozen grasshopper species which are economically important for crops and forages in western U.S. (Pfadt 2002, Brust et al. 2008).
Besides orthopteran insects, Stem borers also carry severe threat particularly to grasses and in general for graminaceaous plants (White et al. 2005). Likewise, grass loopers (Mocis spp.) and fall armyworm (Spodoptera frugiperda (J.E. Smith) are major pests in different grass species, and may contribute towards the economic losses of the end users (Meagher et al. 2007). Numerous researchers have identified and reported insects feeding on switchgrass include; thrips (Gottwald and Adam 1998), the yellow sugar cane aphid and grasshopper, but not as preferred hosts (Kindler and Dalrymple 1999, Parrish and Fike 2005). The carabid beetles have also been found in switchgrass fields (Ward and Ward 2001). Holguin (2010) also studied the switchgrass for insect dynamics and their effect on the yield.
Grasshoppers are very common herbivore being responsible for removing a great portion of above ground biomass (Hewitt 1977). Despite the fact that the physical and chemical defense system does exist in plants but specialist grasshoppers have developed the ability to resist these defenses. Plant productivity has been greatly influenced by insect herbivory (Crawley 1983) especially when grasshopper densities are high. However, the net effect of herbivory has been assumed to be positive (Dyer et al. 1982) while negative under most (Belsky 1986). Presently, the influence of insect herbivores in most grassland especially on dominant grasses is not clearly understood. Parrish and Fike (2005) reported few insects in switchgrass and Vogel (2004) found the potential for negative effect of grasshoppers on switchgrass biomass production.
The differential grasshopper, Melanoplus differentialis (Thomas) is commonly found in North America. This grasshopper is primarily a forb feeder but can also feed on grasses while, it could also be locally monophagous. Although its populations to east and west of the areas between the Rocky Mountains and Mississippi River are spotty and discontinuous but within these landmarks, it is found to be in large numbers (Pfadt 1994). The differential grasshopper is a severe pest of several crops including corn, alfalfa, cotton and fruit trees. The adults are strong flyers having ability to move upwind for food. The preference of this species for wilted or damaged sunflower is possibly attributed towards the chemical changes in wilted tissues of sunflower (Pfadt 1994). The differential grasshopper is well adapted to warmer climates in south due to its tolerance to high temperatures that results in the more frequent and majority of outbreaks of this grasshopper species (Pfadt 1994).
Handful information is possible about food requirement of insects and plant defense if measured with their performance on preferred and non-preferred plants. Numerous studies of different insects, butterflies (Wolfson 1980), leaf-cutting ants (Waller 1982), and beetles (Hughes et al. 1982) revealed the fact that even within the same species, plants may differ in their acceptance to different insect species. Lewis (1984) observed that Melanoplus differentialis preferred diseased or wilted over healthy tissue of wild sunflower (Helianthus annuus L.) and other plant species (Lewis 1979).
In this study, we aimed to quantify the amount and feeding preference of Melanoplus femurrubrum (DeGeer), M. differentialis, Arphia xanthoptera (Burmeister), Erritettix simplex (Scudder) and Psoloessa delicatula (Scudder) for switchgrass cultivars (Kanlow and Shawnee) and big bluestem. Further, we also quantified the preference of two grasshopper species, M. femurrubrum, M. differentialis for their herbivory on upland cultivar (Shawnee) to test whether or not these grasshopper species restrict their feeding to dry/ or wilted leaves of switchgrass.
Materials and Methods
Feeding preference to Turgid and Wilted switchgrass
We quantified the amount of both, wilted and turgid switchgrass biomass before and after serving to grasshoppers. Adults of M. femurrubrum and M. differentialis were collected in late August of 2011 using sweep net from the surrounding fields of University of Nebraska-Lincoln, NE. The grasshoppers were transferred to plastic bags and kept in the greenhouse of University of Nebraska at room temperature of approximately 25 oC prior to starting the trials. These adult grasshoppers were not offered any food for two days before the experiment.
Two days before allowing grasshoppers to feed, samples of switchgrass cultivar shawnee were collected from the Agricultural Research and Development Centre (ARDC) Mead, NE about 50 Km North of Lincoln, NE. Healthy and green plants of approximately similar size were dig up along with roots using a shovel and transferred to plastic pots. These pots were then transferred to greenhouse and maintained with sufficient water to prevent dehydration. The pots used for wilted condition were not given the water two days before experiment.
Individually caged grasshoppers were offered a two-way choice between adjacent dry and turgid leaves in the green house for 3 days. Leaf sections of approximately 15cm long were weighed neat to 1000 mg. and placed into a small plastic pot. The turgid sections were supplied with water to keep the water level for tissue while no water was supplied for wilted sections. Both turgid and wilted grasses were kept together within a plastic pot covered with mesh cloth. Eight replicates of each grasshopper species on each plant condition were made. These pots were placed in greenhouse at 25 oC for three days to allow grasshopper species to feed, and then removed to weigh the plants. Any clipping that had fallen to the bottom of container were identified by texture and appearance and were weighed and included in the totals of mass remaining after feeding. For mass change associated with the water uptake or loss, four pots, two for each condition were prepared in the same manner as experimental groups, and except that no grasshopper was placed in. Plants were weighed before placing them in the pots and after three days. Gains in mass were interpreted as water uptake by the plants. Two way analysis of variance test and pairwise multiple comparison test (Holm-Sidak method) were performed using SigmaPlot (Systat Software, San Jose, CA).
Feeding preference among Shawnee, Kanlow and Big bluestem
In this experiment, we quantified the feeding preference of five grasshopper species belonging to subfamilies Melanoplinae, Gomphocerinae and Oedipodinae. The grasshopper species M. femurrubrum, M. differentialis, A. xanthoptera, E.simplex and P.delicatula were fed on Shawnee (upland cultivar), Kanlow (lowland cultivar) switchgrass along with big bluestem. These grasses followed the same protocol of preparing sections of 15 cm length mentioned in the materials and methods of turgid vs. wilted study except that these grasses were supplied with enough water throughout experiment to keep them fresh and healthy. Each grass species was weighed near to 1000 mg. and were placed in a small plastic pot. Single grasshopper was offered a choice to feed on three types of weighed grasses covered with mesh cloth. There were 24 replicates of M. femurrubrum, seven of M. differentialis, six of A. xanthoptera and ten for each of E. simplex and P. delicatula on each grass. Grasshopper species were allowed to feed on these grasses for three days. A total of six pots were also prepared, two for each grass species but without grasshopper to serve as control. At the end we quantified the amount of feeding for each grass and grasshopper species. Two way analysis of variance test and pairwise multiple comparison test (Holm-Sidak method) were performed using SigmaPlot (Systat Software, San Jose, CA).
Feeding preference to Turgid and Wilted switchgrass: The difference in the mean values among the grasshopper was greater than would be expected by chance after allowing for effects of difference in plant condition. There was a statistically significant difference (P = 0.034). To isolate which group differs from others, a multiple comparison test was used. The difference in the mean values among the plant condition was not great and statistically non-significant. The interaction effect of grasshopper and plant condition was also statistically non-significant (P = 0.109). Melanoplus differentialis and M. femurrubrum differ significantly in their feeding while plant condition showed statistically marginal difference (Table 6). Melanoplus differentialis fed more both on turgid as well as wilted switchgrass and there was not much difference found in case of M. femurrubrum feeding on wilted and turgid (Figure 1).
Feeding preference among Shawnee, Kanlow and Big bluestem: We found statistically significant interaction effect of grasshopper and grass (P = <0.001). The mean amount of consumption per day (234.048 mg) by M. differentialis was greater in Shawnee while in Kanlow it consumed about 139.524 mg. Among all grasshopper species, E. simplex had the minimum consumption of Shawnee (12.733 mg/day) but this grasshopper consumed maximum on big bluestem. Other grasshopper species in this study fall in between these two ranges (Table 2). We fond statistically significant differences in pairwise comparison of M. differentialis with M. femurrubrum, P. delicatula, E. simplex and A. xanthoptera (P <0.01). All other pairwise comparison of grasshopper species yielded non-significant results. When we grouped grasshopper species to their respective subfamilies and analyzed the data, the Analysis of variance showed significant interaction effect of subfamily and grass (Table 3). The subfamily Melanoplinae had the maximum amount of mean consumption of Shawnee (97.548 mg/day) and Kanlow (55.054 mg/day) while Gomphocerinae consumed about 46.950 mg/day of big bluestem. Oedipodinae consumed more Shawnee (52.722 mg) and minimum amount of big bluestem (Table 4).
There was no mean weigh loss of any grass observed in control treatments. The mean water uptake for Shawnee was 75 mg, Kanlow 60 mg and Big bluestem 12 mg. All three grasses ranged from approximately 1% to 7% in their water uptake. We found statistically significant (P â°¤0.05) value for the mass gain in control treatments. Thus, we subtracted the mean uptake of water for each grass at the end of experiment when we weigh the grasses after three days of feeding. In this way we calculated the actual feeding of each grasshopper species on each grass to eliminate the effect of water uptake. Tukey HSD test showed no significant difference of water uptake between switchgrass cultivars, Shawnee and Kanlow but these both were statistically different from big bluestem.
The grasshoppers do not do as well on native grasses as on cultivars. This may be true where grasshoppers are pest in cropland (Joern 1989). Grasshopper feeding is also associated with the vicinity and characteristics of other plants (Root 1973). Pickford (1963) found better growth and development of Camnula pellucida (Scudder) when offered wheat as food when compared with major mixed prairie plant species. The grasshoppers, M. differentialis and M. femurrubrum have the ability to damage a variety of crops including soybean, corn and alfalfa. In Nebraska, M. differentialis have been found seriously damaging corn crop. Some grasshoppers do not prefer certain grasses due to limited availability of those grasses but rather focus on common grasses (Landa and Rabinowitz 1983). Possibility of rare grasses may be due to the higher herbivory rates or may be due to their high palatability.
Our results showed that differential grasshopper, M. differentialis which is inhabitant of tall herbaceous vegetation preferred switchgrass cultivars over big bluestem. The M. differentialis consumed more switchgrass cultivar shawnee than other grass species offered. As M. differentialis is commonly found along road sides and in croplands (Pfadt 1994) and switchgrass is being grown on larger scale as monoculture, the preference of M. differentialis to shawnee cultivar may attributed to the change in food choice due to availability of switchgrass at larger scale. Melanoplus femurrubrum and A. xanthoptera also preferred Shawnee cultivar over big bluestem and Kanlow. While, E. simplex was the only species that preferred big bluestem over switchgrass cultivars. We tested all warm season native grasses where we offered each grasshopper all three grasses to feed. M. differentialis is not important in rangelands but it has known to cause severe damage in corn in Nebraska (Brust et al. 2008). Similarly M. femurrubrum is very common grasshopper found in Nebraska. Although this species is not the serious pest in rangelands but it has the potential for the pest status in crops.
In grasshoppers, species specific differences are found in regard to plant use (Mulkern 1967) that lead towards the generalist or exceptionally specialist in feeding. The members of the subfamily Melanoplinae have broader diet breadth relative to Oedipodinae which are mostly grass feeders with low average diet breadth. However, grasshoppers are selective irrespective of broader dietary plant range (Mulkern 1967). The higher consumption rates of Melanoplinae on Shawnee may indicate a new host for feeding or it may be a change in diet due to available food options.
In general, Insect herbivores prefer C3 plants over C4 plants for their feeding (Caswell et al. 1973). Warm season plants have proteins and carbohydrates which are embedded in thick wall cells that are not easily broken down in the gut to be readily available for the insect herbivores while few C3 cells also remain unbroken (Caswell and Reed 1976). Hence, hypothesized that chewing insects might prefer more nutritive C3 plants. For example, Caged Ageneotettix deorum (Scudder) when offered a choice to feed between C3 and C4 grasses from Nebraska Sandhills preferred C3 grasses. However, a contradictory trend was observed when tested the natural population of A. deorum that had largely C4 plants in their diet (Heidorn and Joern 1984). This may be the result of more C4 grass available for feeding in the field. Similarly, Arphia conspersa Scudder also had more C4 as preferred food due to high percentage of available C4 plants (Boutton et al. 1980). Thus, plant use by grasshopper depends more on structural qualities of leaves and its availability.
Nutritional quality of the host plants may have a role in the distribution and feeding preference of the herbivore. The nutritional value of host plant species and grasshopper growth and reproduction are directly proportional (Mulkern 1967). The amount and quality of food are also have marked effects on reproductive potential of the herbivores (Pickford 1963). The preference of generalist grasshoppers to switchgrass cultivars could involve the leaf anatomy and nutritive value. The Shawnee had an excellent forage quality and also withstand water shortage. The early growth stages in switchgrass are more nutritive but its nutritive values drops rapidly after the seed head emergence. There is a seasonal variation of the nutrients found in big bluestem and switchgrass. With the passage of growing season, a significant decrease in the protein occurs. In Nebraska, switchgrass have higher crude protein than big bluestem. In early June, switchgrass crude protein contents were found to be 17.5% and 11.4% in late June. During mid-July it further decreased to 8.4% (Newell 1968). Furthermore, Shawnee is not as coarse and taller as lowland cultivar Kanlow which have thick stems, broad and greener leaves (Casler 2005). During the mid-June in Nebraska, the big bluestem had the crude protein around 14.4% and 10.6% in late June (Newell and Moline 1978) while there was further decrease in subsequent time period in protein content of big bluestem.Â
Grasshopper species vary in manipulating the host plants (Gangwere 1961, Mulkern 1967). These variations for the host use also prevail even in coexisting species. There are grasshopper species which are specialist in their feeding while others behave as generalists. There is a remarkable separation is found between forb and grass feeders in that species rarely include both grasses and forbs in their diet and chose one of them to feed (Mulkern 1967, Otte and Joern 1977).
Many of secondary toxic chemicals in a number plant species are a barrier or serve as deterrent for grasshopper herbivory (Bernays et al. 1974). Generally the grasses have limited chemical defenses (Bernays 1977). Grasshopper species vary in their susceptibility to plant defenses. For instance, secondary chemical, tannins (Capinera et al. 1983) had a negative effect on Schistocerca migratoria while in case of generalist Schistocerca gregaria, tannins had not effect (Bernays et al. 1980). Thus, variation, pattern and occurrence of secondary chemicals are important and need further investigation especially in grasses that might be helpful in determining the preferential feeding. Cool season grasses had higher tannin levels than warm season perennial grasses (Capinera et al. 1983).
Big bluestem and switchgrass has the C4 photosynthetic system and as a result complete most of their growth during the summer months (Gould and Shaw 1983). Big bluestem can be grown as monoculture but for grazing they are often grown in mixtures. Switchgrass mature later in the growing season than big bluestem, thus switchgrass typically has higher quality when harvested on the same date. The preference of Melanoplinae and Oedipodinae might be due to the better food quality of switchgrass.
Seasonal variation and factors sometimes alter the chemical composition and nutritional value of host plants thus, results in switching of insect from one plant species to other. A change in grass maturity and succulence could result in preference for local grasshopper feeding (Chu and Knutson 1970). Chu and Knutson (1970) tested the preference of a number of grasshoppers to different grasses and found that adult P. nebrassecnisis preferred switchgrass over big bluestem while M. differentialis also preferred switchgrass to feed than others. With increasing level of grass maturity, the level of nutrients decrease while the forage quality and palatabity has been found much higher before the seed head develop. This might be the reason that grasshopper co-evolved certain behavrial changes to prefer switchgrass due to greater amount of available nutrients at a particular stage of switchgrass growth. The role of temperature could not be ignored both in plants and insects. It is not evident what role actually temperature play in the preference but the preference of food sometimes changes with the change in temperature. Whether it affects the chemical composition of plants or it has an effect on insect behavior, is unknown.
Presence and absence of predators also affects the food selection behavior in herbivores. For instance, red legged grasshopper mostly feeds on grasses but in the presence of spiders they prefer to feed on herbs. This change in behavior is rather more habitat selection than shift in food preference. Sometimes, Insects prefer feeding on less nutritive food in the absence of natural enemies on that plant. These plants reflect more survival rates relative to plants with more nutrition because more nutritional contents increase the chances of more natural enemies (Mulatu et al. 2004, Ohsaki and Sato 1994, Singer et al. 2004). Insects show difference in host preference when introduced in different areas of their distribution. There has been observed inter-population variation in all major insect taxa.
The behavior of generalist and specialist grasshoppers is also very important for their preferred food. Grasshopper, Shistocerca emarginata (Scudder) is generally a polyphagous, though some of its populations are clearly monophagous (Chapman 1990). Thus, monophagy and polyphagy in this grasshopper is probably not the result of only differences in available food to different populations, but genetic differences also play a role. Although, polyphagous herbivores had a broad range of available food but Bernays (2001) argued the behavioral actions that generalist herbivores make more errors than specialists. Specialist herbivore has limited options to decide while generalists have broader range thus more errors in decision making. Regional differences in host-plant preference often reflect adaptations to local conditions. Local factors such as the presence of a competitor for food may exert a selection pressure resulting in host plant specialization.