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Grasshoppers are perhaps the primary native herbivores in western United States and cause severe damage to rangelands. The effects of these fairly small herbivores are commonly misapprehended or remain ignored on the native vegetation, predominantly in case of non-economic grasshopper species (Hewitt and Onsager 1982), unless they reach outbreak densities. Several researchers (Anderson 1961, Hewitt et al. 1976, Hewitt 1977, 1978, Hardman and Smoliak 1982, Quinn et al. 1993) have acknowledged the damage potential of grasshoppers and their competition with livestock for the forage.
There are only about 2 % of grasshopper species that occur in high numbers out of total grasshopper species in United States (Lockwood 2001; Hewitt and Onsager, 1983, Brust et al. 2008) and among these 2% of species, one or more species is present in almost every major range ecosystem (Hewitt and Onsager 1982). Estimates of more than 100 grasshopper species have been found in Nebraska (Skinner 2000; USDA-APHIS 2006; Brust et al. 2008). Around 66% area of western Nebraska receive low annual precipitation (Dow 1932) and thus, remain largely as rangeland (Veneman et al. 2004). Due to less precipitation, the western dry part of Nebraska is not suitable for growing most of the row crops. Western rangeland accounts for an area of about 262 million hectares (USDA 1978) and grasshoppers are frequently found in sufficient numbers to cause forage losses in 17 of the 22 western states. Several grasshopper outbreaks have been reported in Nebraska (Hauke, 1953; Lockwood 2001). Most species pose a threat to grasses and forbs and cause annual losses of 21-23% in range vegetation (Hewitt and Onsager, 1983).
Grasshoppers occur in all types of rangelands. Their densities are higher in areas where annual precipitation does not exceed 60 cm (Hewitt and Onsager, 1983). Diet preference, developmental stage, densities of each grasshopper species and species composition are important factors to determine the damage to rangelands by grasshoppers. During high temperature periods, grasshoppers consume more foliage due to their increasing developmental rate (Langford 1930). Grasshoppers do not always consume all the forage during feeding rather they supplement to further loss and damage in terms of cutting the leaves but not consuming them (Skinner, 2000).
Several biotic and abiotic factors are involved in insect herbivory. Some of these factors are water content of the plants, contaminated, damaged plants and availability of different nutrients like nitrogen and proteins. These stressors play an important role in insects for their increased or decreased feeding and also in their growth rates (White 1976, 1984; Rhoades 1979; Crawley 1983; Lewis 1984). These effects may be attributed to the change in the concentration of amino acids, sugars and other chemicals of the plants (Schoonhoven et al. 2005). Water stress is worth mentioning because of the outbreaks caused by some insects (White 1976) during drought conditions. Many reports on the insect herbivory with relation to water stress have focused mostly on the development and survival of insects while less information is available on insect feeding in relation to water status of plants (Haile 2001). Warm and dry weather periods usually results in the outbreak of insect pests in rangeland. The reasons of outbreak are still unknown. Probably the drought stress have a negative effect on the resistance mechanism of plants while at the same time drought stress also increases the nutritional value for insects (Koricheva et al. 1998). Increased level of sugars and nitrogen in plant foliage is the result of drought.
The northern rangelands of the western United States commonly have C3 grasses whereas; the southwestern rangeland is mostly dominated with warm season (C4) grasses. Thus, these rangelands differ in the economics involved in grasshopper control due to differences in plant species, weather patterns, and herbivory season. The available threshold models (Torell et al. 1987, Berry et al. 1991) being used are based on data mainly from northern rangelands. The Nebraska sandhills grassland has both C3 and C4 grasses. Heidorn and Joern (1984) found the preference for C3 plants when A. deorum was offered a choice to feed both on C3 and C4 in the field. However due to abundance of C4 grasses in grassland, A. deorum had also been found feeding largely on C4 grasses.
The whitewhiskered grasshopper, Ageneotettix deorum (Scudder), is a widely distributed and economically important pest species that can reach high densities in mixed and bunchgrass prairies of western United States (Pfadt 1994). In Nebraska, this species is usually the most abundant species in the Nebraska Sandhills (Brust et al. 2008). There is no clear evidence of host plant preference in A. deorum. However, it may consume a variety of grass species available in abundance. Important host plants include blue grama [Bouteloua gracilisÂ (Willd. ex Kunth) Lag. ex Griffiths], which is dominant in shortgrass regions, western wheatgrass [Pascopyrum smithiiÂ A. Love], Kentucky bluegrass (Poa pratensis L.), and needleandthread [HesperostipaÂ comataÂ (Trin. & Rupr.) Barkworth] (Mulkern 1967, Pfadt and Lavigne, 1982; Brust et al. 2008). However, laboratory studies also show its preference toward some grasses over native grasses. In addition, the whitewhiskered grasshopper also feeds on clipped and fallen leaves, seeds, and dead insects (Pfadt 1994).
Among many of the geophilous grasshoppers, the mottled sand grasshopper, Spharagemon collare (Thomas), has been documented to feed on ground litter and clipped leaves (Pfadt, 1994). In Nebraska, this species is frequently found in sandy areas of west where it mostly feeds on grasses, forbs and sedges (Joern 1982, Brust et al, 2008). This grasshopper species is of less economic importance in rangelands but it can cause damage in the disturbed areas of wheat and other grain crops (Pfadt 1994).
Insects can consume native vegetation in large quantities during severe outbreaks. The actual forage consumption by insects is sometimes much less than the total injury to forage plants. Many insects repeatedly clip the plant leaves, stems, and blades but consume only a small portion of these clippings and thus, results in an additional loss. For instance, grasshoppers consume about six times less foliage than they actually clip. Several researchers (Hewitt 1977, 1978; Mulkern et al. 1969; Putnam 1962) found that a single grasshopper/square meter can cut about 14-17 kg/ha of available forage. Similarly, Hardman and Smoliak (1982) estimated a loss of 16-60% of the total production by 10 grasshoppers/square meter in their entire life.
Little bluestem [SchizachyriumÂ scopariumÂ (Michx.) Nash.], a warm season grass is most conspicuous in Great Plains. In the prairies of Nebraska, this is one of the prominent species of intermediate height. Its growth period starts with few leaves in spring but then it grows rapidly. Buffalograss [Buchoe dactyloids (Nutt.) Engelm.] is also a warm season, nutritious (National Academy of Science 1971) and native shortgrass forage in shortgrass and mixed-grass prairies (Johnson and Nichols 1970). Its growth generally starts in late spring and continues through the whole summer months (Hoover at al 1948).
We studied the clipping behavior among the different species of grasshoppers on little bluestem and buffalograss to quantify the amount of clipping generated by each species. Furthermore, we also tested this phenomenon with different levels of moisture within the plants to observe the effect of water/ or moisture in generating the amount of clippings.
We hypothesized that rangeland grasshoppers do not clip the vegetation and that water level of plants has no effect on the amount of clipped grasses.
Materials and Methods
Grasshoppers, A. deorum and S. collare were collected from the sandhill areas of western Nebraska around 11 Km, northwest of Anselmo in Custer county. We collected these species in late July and early August during the summer of 2010. Arphia simplex Scudder was used in this study as a supplemental species having less economic importance. This species was collected from the Cottonmill Park and Recreation Area on the Oldfather Prairie Reserve about 2.4 Km west of Kearney, NE.Â We used aerial nets at all locations for collection. These grasshoppers were then transported to the Eco-Physiology lab of University of Nebraska at Kearney. Each species was stored separately in plastic bags with plants to feed for two days at room temperature of about 25 oC.
Samples of grass, little bluestem were collected from the Sandhill area from Custer county similar to collection sites of A. deorum and S. collare while the buffalograss was collected from Cottonmill Park and Recreation Area. The grasses were dig up along with their roots using shovel and were kept in plastic pots. These pots were supplied with suffienct water to prevent the dehydration. Only those clumps of grasses were dig up which have healthy, greenish appearance and of similar size.
For the clipping study, sections of approximately 12 cm length were prepared for each grass and weighed near to 500 mg. These sections were then planted in a small plastic cups filled with sand. These cuttings were held upright through the holes of covering lid to prevent them from falling down during grasshopper feeding. Three replicates of each grasshopper species were made on each grass species. The grass held pots were put into 1 gallon size plastic pot covered with mesh cloth to allow the movement of grasshopper. One grasshopper was released in each mesh covered pot to allow feeding and observe the clippings. Clippings were collected at an interval of 12 h for each grasshopper for 3 days. Sand filled pots were kept moist throughout the experiment to keep the grasses fresh.
In a parallel experiment, clipping behavior of two grasshoppers, A. deorum and S. collare, was also observed with relation to different levels of grass moisture. We maintained 3 moisture levels, high, medium and low for both grasses. We used an oven dry method to calculate the moisture level of grasses prior to the start of the experiment. We measured the level of moisture within plants after applying different amount of water for three days. From this we calculated the desired amount of water to apply to attain and maintain these moisture levels for 3 days. We prepared10 replicates of each grasshopper species at each level of high, medium and low moisture of buffalograss and little bluestem grass. The clippings were collected after every 12 hours and weighed. The data were analyzed using R statistical packages (R developmental Core team 2008).
For the preliminary clipping study, neither grasshopper species nor the grass species revealed any significant effect on the amount of clipping (P = 0.192 and 0.191). Similarly the effect of different grasshoppers do not depend on what grass is present There is not a statistically significant interaction between grass and grasshopper (P = 0.202). Among the three grasshopper species, S. collare generated the most clippings for little bluestem, with A. deorum producing somewhat fewer. For buffalograss, A. simplex clipped the most while S. collare produced less. Ageneotettix deorum generated the least amount of clippings for buffalograss (Table 1). Although there were no statistically significant effect of both insect and grass, these results show that the phenomenon of clipping does occur in certain species of grasshoppers.
Further investigating clipping behavior of grasshopper species in relation to moisture level within the plants, there is a statistically significant interaction between grasshopper, grass and moisture level (P = <0.001) which indicates that the effect of one factor is not consistent at all combinations of the two other factors (Table 3). The effect of grasshopper Ã- grass interaction depends on what level of moisture level is present. There is a significant grasshopper Ã- grass interaction at level high of moisture level (P = 0.007) and at medium level of moisture (P = 0.001). We observed A. deorum clipped the most (23.853 mg/day) for buffalograss with a medium level of moisture but, least amount in case of little bluestem. Both grasshopper species showed a very similar trend of clipping in low moisture of both grass species. At high moisture, S. collare clipped the most buffalograss (19.193 mg/day) but the least for little bluestem (Figure 3).
A greater amount of above ground biomass in grasslands is being utilized by grasshoppers (Hewitt 1977, 1978). The unusual losses to plants by grasshoppers are mostly greater than the usual losses due to herbivory (Joern 1989). Direct losses to foliage have been documented by several studies. Hinkle (1938) and Pfadt (1949) used cages to measure the losses caused by rangeland grasshopper. Majority of studies (Pfadt 1949, Nerney 1960, Putnam 1962) measured only yield losses. There is few literature available that mentioned the related damage (grasshopper cut the foliage but do not consume) but no literature is available quantifying these clippings that were generated by grasshoppers. According to the model (Mitchell and Pfadt 1974), the possible fate of forage plants grazed by grasshoppers may result utilizing all ingested foliage for its respiration, growth and reproduction while feces as end product or grasshoppers only clip vegetation and do not consume the clippings and ultimately resulting in the litter. The amount of destroyed foliage increases with the increasing maturity of grasshoppers (Hewitt 1978).
We studied and quantified grasshopper species belonging to subfamilies gomphocerinae and oedipodinae for their related damage. Although we could not find any significant effect of either insect or grass on the amount of clipping generated but we found that all three species, A. deorum, A. simplex and S. collare generated clippings which were not consumed by these grasshoppers and that the amount of clipping varied with grasshopper as well as by plant species. But when we quantified clippings at different level of moisture of grasses, a significant interaction effect of grasshopper, grass and moisture of grass was found.
Agenotettix deorum and S. collare feeds on several species of grasses (Criddle 1933, Brust et al. 2008) and abundantly found in sandhills of Nebraska. In our study we found that A. deorum clipped more buffalograss at all moisture levels with maximum amount at moderate level of moisture. The A. deorum during first four instars occasionally crawl onto the plants and prefer feeding on dry material found on ground. Usually this grasshopper moves back to ground after feeding on plants (Anderson and Wright 1952). The first four instars of A. deorum prefer staying on ground during feeding and extending hind legs as far as to reach the plants without much movement. This might be one of the possible reasons in clipping more buffalograss being less tall than little bluestem and easily accessible for A. deorum. After fourth instar, this species is mostly found on plants and eat green vegetation and also continue eating more dry material found on ground. Even the adults were also found feeding on grass seeds (Anderson and Wright 1952).
Insects are unable to distinguish between palatable and unpalatable food plants unless they chew with their mandibles or touching by their maxillary palpi. This phenomenon is also true for grasshoppers (Kaufmann 1968). The age of grasshoppers and physical structure, mechanical strength of plants are very important in determining the plant parts first to be eaten (Kaufmann 1968).
The mechanism of cutting leaves but not consuming might involve certain adaptations in grasshoppers. In summer less rain and hot periods turns the green vegetation to dry and brown (Kaufmann 1968). Thus grasshopper tends to adapt these severe conditions by bringing change in their feeding behavior and clipping the grass blades to feed later when there are less fresh and green plants available. The western parts of Nebraska receive less rainfall and A. deorum and S. collare are abundantly present in these parts of state. So these species might be involved in preparing for the adverse conditions when there is less green vegetation and food available to feed. Both, A. deorum and S. collare have been found feeding on dead and fallen leaves so it is probable that these both species clip vegetation to lower the water level of plant or to prepare themselves to feed on fallen leaves. We did not test later consideration in our experiments but we can assume that grasshoppers somehow are involved with lowering the water status of plant.
The Amount of water in the fresh weight of foliage ranges from 45% to 95%. The water contents in the food of certain insects can predict about the nutrients. For instance, a number of larvae in Lepidoptera can grow better on the plants that have sufficient amount of water (Schoonhoven et al. 2005). The acridids on the other hand have the ability to accept lesser amount of water in their host plants and many plant species facing water stress are edible to grasshoppers while only a few species are inedible due to water stress (Bernays and Lewis 1986).
This phenomenon of clipping among grasshopper species might also point toward several other reasons. The biotic and abiotic stressors play an important role in increased or decreased insect herbivory and subsequent damage. Thus, water, diseased and damage plants and different nutrients greatly influence insect herbivory. Particularly, the likelihood of insect herbivory in the arid areas is strongly affected by lack of water as the water status in plants has a strong effect on the food choice (Lewis 1979). Drought can alter the chemical defense and nutritional contents of plants and can allow increased herbivory that results in damage of plants (Haglund 1980).
Besides their effect on feeding, these factors also influence the developmental rates of grasshoppers (Lewis 1984), while these effects may be attributed towards the change in the concentration of amino acids, sugars and other chemicals of the plants. Water stress is valuable to discuss because of the outbreaks caused by some insects (White 1976) during drought conditions. Water plays important role in plant growth and is a source of mineral transportation within the plant parts. There is mutual variation in the amount of water and nitrogen. A greater amount of water (80-90%) and nitrogen (5-6%) contents are found in grasses and herbs while woody plants have lesser amount of water (60%) and nitrogen 2% (Slansky 1993). Fresh or newer leaves contain more amounts of water and amino acids compared to matured or older leaves.
Many reports on the insect herbivory with relation to water stress focused mostly on the development and survival of insects but less information is available on insect feeding in relation to water status of plants (Haile 2001). A number of sap feeders and other herbivores are susceptible to water stress (Huberty and Denno 2004) but in other cases, water stress may contribute beneficially in increasing the herbivore populations. Relatively low water requirement is another important need of grasshopper species. Water and other environmental stressors may be involved bringing changes in the physiological conditions of plants. The water stress being generated by these grasshoppers probably leads towards the weekend resistance of plants and this stress also cause to increase the nutritional value of plants for insects (Koricheva et al. 1998) and may increase the available nitrogen and sugars in leaves (White 1984, Louda 1986). The feeding choice by grasshoppers based on nitrogen in the leaves is ambiguous and that grasshoppers may not have the ability to detect very low level of nitrogen contents in leaves (Joern 1989). Similarly, the water stress in plants also initiate increased accumulation of free amino acids (Mattson 1979) due to reduced amino acid metabolism and protein synthesis (Hsiao 1973). These amino acids, especially proline act as feeding stimulant for insect herbivores and increase plant susceptibility for insect damage. The outbreak of insects in situations of drought or water stress is somewhat a result of more availability of nitrogen during drought which has a role in enhanced growth and development of phytophagous insects (White 1969, 1993).
Usually, majority of herbivorous insects prefer food with high amount of water. However, grasshoppers' requirement of water is less which is about an average of 60% of their food. Behavioral regulation is important in herbivorous insects so that they can fulfill their water requirement by selective feeding. For instance, locust after feeding on drier plants for certain period of time ate more plants with higher water contents to fulfill the deficiency of water (Roessingh et al. 1985).