Abstract

    Grasshopper foraging and feeding behavior is a complex behavior, comprised of many different demands and constraints on an individual that ultimately determine its actions. My goal is to outline the research that defines the major demands and constraints and how these affect grasshopper foraging and feeding behavior. In this way, I hope to present a synthetic overview that will be a useful resource to those interested in grasshopper foraging and feeding behavior.

    Much of the behavioral research on grasshoppers has focused on foraging and feeding habits. Grasshoppers are often categorized as grass-feeders, forb-feeders, or mixed-feeders. Grasshoppers can also be thought of as generalists (polyphagous) or specialists (monophagous), depending on whether they feed on many different items or focus on a few specific items. What, when, where, and especially how grasshoppers eat are the questions I hope to elucidate.

    Grasshoppers mostly eat vegetation, but some also scavenge on grasshopper feces and even on dead grasshoppers of the same species. Distance between food sources affects what a grasshopper will eat, possibly limiting a generalist to a specialist diet. Nutritional quality, secondary chemicals, physical properties of plants, and the presence of plant pathogens may deter grasshoppers from feeding on certain plants. Abiotic factors in the environment, competition, and predation also shape the foraging and feeding behavior of grasshoppers. Grasshoppers modify their foraging and feeding behavior accordingly in order to minimize costs and maximize benefits associated with foraging and feeding.

Introduction

    Getting food is one of the most basic needs common to all animals. Because all animals are heterotrophic, they must gain energy from an outside source by ingesting matter and transforming it into energy. If reproductive success is the ultimate destination of every animal, then food is the fuel that allows the animal to reach its destination. Consequently, there is a predominant demand on animals to find food and eat it. Underlying this need for food are many constraints, such as predation, that put limits on an animal's feeding. Foraging behavior is characterized by the actions and patterns by which an animal finds food, determined by certain demands and constraints on the animal. Feeding behavior is characterized by what, when, or where an animal eats.

    Grasshopper foraging and feeding behavior is determined by several demands and constraints (Fig. 1). Grasshopper demands for food include the need for nutrients, minerals, and water (Chapman 1990; Bernays and Simpson 1990; Joern and Behmer 1998). Ultimately, of course, these needs allow the grasshopper to survive and reproduce. Constraints on grasshopper foraging and feeding include food availability, environmental conditions, competition, and predation (Rothley et al. 1997; Lockwood et al. 1996; Woods et al. 1997; Bernays et al. 1997).

    Grasshoppers cope with demands and constraints by altering their behavior to gain maximum benefits. Non-overlapping life history, microhabitat preference, mode of foraging, diet specialization, and feeding periodicity are some ways in which grasshoppers can partition their food resources (Fig. 1) (Lockwood et al. 1996). Grasshopper species are constantly evolving and learning to behave in a way that is best-suited to gaining the most reproductive success. Thus, if a certain grasshopper species comes under heaviest predation from its primary predator at noon, it may evolve to change its foraging pattern to late afternoon. In addition, at the behavioral level, grasshoppers may learn to avoid foraging at certain times of the day.
 

Discussion

Demands

    Why does a grasshopper eat? Ultimately, it is to gain energy that will eventually allow it to mate and reproduce successfully. But proximately, there are a number of small stimuli and behaviors that result in the individual grasshopper feeding. If all exterior constraints on a grasshopper were alleviated, there would still be a need to moderate intake of food. Simpson (1990) describes some of the factors determining feeding in grasshoppers. Time since the previous meal, and size and nutritional quality of the previous meal are some of the primary factors determining feeding. These factors may be monitored by the grasshopper via stretch receptors in the gut, nutrient feedbacks, and blood osmolality. Grasshoppers may also have short-term endogenous cycles that tell the grasshoppers when to begin behaviors associated with foraging and feeding, regardless of the previous meal. Many of these behaviors, such as locomotion, may occur without the grasshopper actually initiating feeding. As many as eight cycles may pass before a grasshopper feeds. Defecation may also induce feeding, although it may simply be  correlated with the onset of feeding.

    Grasshoppers are primarily herbivores, although many will scavenge on arthropod cadavers and grasshopper feces, given the opportunity (Woods et al. 1997; O'Neill et al. 1997). Most grasshoppers are polyphagous, meaning they feed on more than a few species of plants, although some are monophagous (Lockwood et al. 1996; Chapman 1990). For polyphagous grasshoppers, growth rate is generally increased by mixed feeding (Bernays et al. 1997). This diet mixing helps to round out a diet that would otherwise be lacking in certain nutrients (Bernays and Simpson 1990). Monophagous grasshoppers may receive certain benefits that outweigh the costs of slower growth.

    How does a grasshopper choose what to eat? From a distance, grasshoppers receive visual and olfactory cues from a food source (Chapman 1990). When the grasshopper is in contact with a plant, it may be stimulated to bite it by taste and smell. Upon biting the plant, nutrient chemicals as well as secondary chemicals are sensed. Based on the nutrient quality of the plant, the water content of the plant, the secondary chemicals within the plant, the physical properties of the plant (hairiness, hardness), and the internal state of the grasshopper (as outlined above), the grasshopper will make a decision whether and for how long to continue eating.

Constraints

Plant Availability

    Plant availability contributes to what, where, and how frequently grasshoppers will eat. If there are no food plants, grasshoppers cannot eat. If food plants are abundant, grasshoppers may spend less time foraging and travel shorter distances between foods, thus reducing the total energy spent foraging (Chambers et al. 1996; Chapman 1990).  Plants may also be effectively unavailable to grasshoppers if they do not provide suitable nutrients, contain defensive secondary compounds, or are physically unable to be eaten. Chapman (1990) notes that leaf pubescence, hardness, and size can often deter grasshoppers from feeding. Lopez et al. (1995) found that an endophytic fungus on the host plant of the redlegged grasshopper did not deter the grasshopper from feeding. However, there may be some plant pathogens that render host plants unsuitable for grasshopper feeding.  Some plants may provide certain nutrients but lack others. Bernays et al. (1997) found that generalist grasshoppers raised on a mixture of kale and cotton grew faster than grasshoppers that fed on only one of the plants. However, there was a cost associated with how far away the two plant species were placed from each other. Beyond a certain distance, grasshoppers would not leave one plant to feed on another, possibly because the increased time and distance would have resulted in more energy used than nutrients gained. Chambers et al. (1996) found that, in two cryptic species of polyphagous grasshoppers, feeding time was concentrated on a single plant for longer periods of time, contrary to other studies in which polyphagous grasshoppers have had relatively short feeding bouts on many different plants. Cryptic species may further realize the benefits of their camouflage by remaining relatively motionless on one plant, at the cost of gaining certain nutrients that may be unavailable on that plant. Chambers et al. also found that successive feeds on the same plant become shorter, whereas feeding on a new plant was longer, regardless of whether the plant had increased nutritional value. Bernays et al. (1992) suggest that polyphagous grasshoppers may be programmed to choose a novel flavor, thus insuring that different sources of nutrients are tested.

    Secondary compounds in plants can be stimulants, deterrents, have mixed effects, or have no effects at all on grasshopper feeding (Chapman 1990). Bernays et al. (1995) found that secondary compounds in one plant can affect grasshopper foraging on other plants in complex ways. If deterrent secondary compounds are present in one plant, grasshoppers switch plants less frequently. Less frequent switching may be the result of the higher costs associated with the chance that switching plants may result in landing on a plant with deterrent secondary compounds, and then having to switch plants again before a suitable meal can be found.

Environmental Conditions

    Temperature, humidity, wind conditions, rain, and other abiotic environmental conditions can put constraints on when and where a grasshopper forages. Simpson (1990) states that thermoregulation, along with other factors, is an important determinant of when a grasshopper feeds. At certain times of the day in the warm season, grasshoppers may need to minimize their exposure to the sun to avoid becoming overheated. This serves to limit grasshopper foraging and feeding both spatially and temporally. Conversely, if it is too cold, grasshoppers cannot move about freely. Temperatures in the intermediate range may determine how quickly a grasshopper forages, with colder temperatures resulting in longer foraging times.

    Lockwood et al. (1996) examined temporal food partitioning in grasshoppers. They found that nocturnal feeding is common in grasshoppers. Nocturnal feeding in some species may help them to avoid predation and competition with other grasshoppers, but it also may be important on very warm days when feeding time is limited in daylight hours.

Competition

    Grasshoppers must compete both intra- and interspecifically for food. The ability to find food quickly and defend food are important competitive strategies. But grasshopper species that are not quick or strong must come up with alternative strategies in order to compete for food. These grasshoppers may feed at different times of day or on different types of food, or may simply have different seasonal life cycles than their superior competitors (Lockwood et al. 1996).

    Woods et al. (1997) determined that larger grasshoppers generally found grasshopper cadavers (a food resource) more quickly and were able to defend them better than smaller grasshoppers. O'Neill et al. (1994) also found that larger grasshoppers were more efficient foragers, but that higher soil surface temperatures meant longer foraging times. O'Neill et al. (1993) found that body size was the most important factor in finding and keeping food, while speed of attraction to food, and thermal tolerance were secondary factors. O'Neill et al. (1993) also found that grasshopper carnivory peaks when mature females are present in the population. Belovsky et al. (1996) determined that females prefer males with superior forgaging tactics, demonstrating that foraging ability involves competition for mates as well as for food.

Predation

    Predation is one of the most important constraints on grasshopper foraging and feeding. There have been many studies focusing on the pressure of predation on grasshoppers and how it affects foraging and feeding. In the presence of predators, grasshoppers must alter their foraging behavior to find a balance between starvation and direct mortality due to predation (Rothley et al. 1997). This balance was termed the "optimal mean gain" by McNamara and Houston (1987).

    Schmitz et al. (1997) suggest that trophic cascades can occur by direct population-level effects (direct mortality from predation) and indirect behavior-level effects (altered behavior in the presence of predators). Schmitz et al. manipulated predator spiders to render them incapable of predation, and found that starvation remained a major cause of mortality among grasshoppers. This indicates that grasshoppers modify their foraging behavior in the presence of predators regardless of whether the predators can actually eat them. Thus, the modified behavior of the grasshoppers results in a trophic cascade in which vegetation fares better in the presence of predators. Earlier, Schmitz et al. (1994) had also shown that a decrease in predator spiders led to an increase in grasshopper biomass and a decrease in edible plant biomass. Chase (1998) found a similar trophic cascade for predator lizards, grasshoppers, and vegetation, but did not determine whether this trophic cascade was a result of direct or indirect effects.

 Schultz (1981) determined that, for a sexually dimorphic grasshopper, antipredator behavior differed with age and sex. Earlier instars were much more likely to jump to the ground when disturbed. Smaller males were also more likely to jump when disturbed, whereas females usually stayed in place. Schultz suggests that the larger size of females makes them less susceptible to predation because of the longer handling time associated with feeding on them.

    Cryptic grasshoppers may utilize wholly different feeding and foraging strategies in the face of predation. Chambers et al. (1996) showed that two species of cryptic grasshoppers were more likely to remain on one plant for a longer period. For cryptic grasshoppers, this behavior may help them avoid visual predators because of the grasshoppers' limited locomotion.

Conclusion

    The interaction of the demand for food and the constraints of plant availability, environmental conditions, competition, and predation serve to create a complex set of foraging and feeding behaviors for grasshoppers. The individual grasshopper must calculate whether it's too hot to forage, whether the risk from predation is too great, and whether to go to battle with another grasshopper over a food resource, all at the same time. Intermixed with these factors, the grasshopper must determine what is a good food source, and how much and how often to eat in order to avoid starvation. McNamara and Houston (1987) called the balance between starvation and predation the optimal mean gain for an organism. This might better be thought of as the balance between starvation and all the constraints facing an organism.

    By synthesizing the research on grasshopper foraging and feeding behavior, we may begin to be able to predict how certain grasshoppers will behave under certain circumstances. First, though, it is important to recognize the characteristics that separate grasshoppers into different functional groups. Is the grasshopper polyphagous or monophagous? Is it grass-feeding, forb-feeding, or mixed-feeding? Is it cryptic or not? What are the size, age, sex, and other physical characteristics of the grasshopper? The complexity of grasshopper foraging and feeding behavior is enormous, and it is difficult to determine which characteristics of the grasshoppers contribute to certain behaviors. Further research in the area of grasshopper foraging and feeding behavior will make prediction a more refined science.
 
 

Literature Cited

Belovsky, G.E., J.B. Slade, and J.M. Chase. 1996. Mating strategies based on foraging ability: an experiment with grasshoppers. Behavioral Ecology 7: 438-444.

Bernays, E.A. and S.J. Simpson. 1990. Nutrition. In Biology of Grasshoppers. Ed. by R.F. Chapman and A. Joern. John Wiley and Sons. New York. Pp. 105-127.

Bernays, E.A., K. Bright, J.J. Howard, D. Raubenheimer, and D. Champagne. 1992. Variety is the spice of life: frequent switching between foods in the polyphagous grasshopper Taeniopoda eques Burmeister (Orthoptera: Acrididae). Animal Behaviour 44: 721-731.

Bernays, E.A., N. Gonzalez, J. Angel, and K.L. Bright. 1995. Food mixing by a generalist grasshopper: secondary compounds structure the pattern of feeding. Journal of Insect Behavior 8: 161-180.

Bernays, E.A., J.E. Angel, and M. Augner. 1997. Foraging by a generalist grasshopper: the distance between food resources influences diet mixing and growth rate (Orthoptera: Acrididae). Journal of Insect Behavior 10: 829-840.

Chambers, P., G. Sword, J.E. Angel, S. Behmer, and E.A. Bernays. 1996. Foraging by generalist grasshoppers: two different strategies. Animal Behaviour 52: 155-165.

Chapman, R.F. 1990. Food Selection. In Biology of Grasshoppers. Ed. by R.F. Chapman and A. Joern. John Wiley and Sons. New York. Pp. 39-72.

Chase, J.M. 1998. Central-place forager effects on food web dynamics and spatial pattern in northern California meadows. Ecology 79: 1236-1245.

Joern, A. and S.T. Behmer. 1998. Impact of diet quality on demographic attributes in adult grasshoppers and the nitrogen limitation hypothesis. Ecological Entomology 23: 174-184.

Lockwood, J.A., J.M. Struttmann, and C.J. Miller. 1996. Temporal patterns in feeding of grasshoppers (Orthoptera: Acrididae): importance of nocturnal feeding. Environmental Entomology 25: 570-581.

Lopez, J.E., S.H. Faeth, and M. Miller. 1995. Effects of endophytic fungi on herbivory by redlegged grasshoppers (Orthoptera: Acrididae) on Arizona fescue. Environmental Entomology 24: 1576-1580.

McNamara, J.M. and A.I. Houston. 1987. Starvation and predation as factors limiting population size. Ecology 68: 1515-1519.

O'Neill, K.M., S. Woods, D. Streett, and R.P. O'Neill. 1993. Aggressive interactions and feeding success of scavenging grasshoppers (Orthoptera: Acrididae). Environmental Entomology 22: 751-758.

O'Neill, K.M., D. Streett, and R.P. O'Neill. 1994. Scavenging behavior of grasshoppers (Orthoptera: Acrididae): feeding and thermal responses to newly available resources. Environmental Entomology 23: 1260-1268.

O'Neill, K.M., S.A. Woods, and D.A. Streett. 1997. Grasshopper (Orthoptera: Acrididae) foraging on grasshopper feces: observational and rubidium-labeling studies. Environmental Entomology 26: 1224-1231.

Rothley, K.D., O.J. Schmitz, and J.L. Cohon. 1997. Foraging to balance conflicting demands: novel insights from grasshoppers under predation risk. Behavioral Ecology 8: 551-559.

Schmitz, O.J. 1994. Resource edibility and trophic exploitation in an old-field food web. Proceedings of the National Academy of Science USA 91: 5364-5367.

Schmitz, O.J. 1998. Direct and inderect effects of predation and predation risk in old-field interaction webs. American Naturalist 151: 327-342.

Schmitz, O.J., A. P. Beckerman, and K.M. O'Brien. 1997. Behaviorally mediated trophic cascades: effects of predation fisk on food web interactions. Ecology 78: 1388-1399.

Schultz, J.C. 1981. Adaptive changes in antipredator behavior of a grasshopper during development. Evolution 35: 175-179.

Simpson, S.J. 1990. In Biology of Grasshoppers. Ed. by R.F. Chapman and A. Joern. John Wiley and Sons. New York. Pp. 73-103.

Woods, S.A., K.M. O'Neill, and D.A. Streett. 1997. Scavenging behavior of rangeland grasshoppers (Orthoptera: Acrididae): rubidium-label studies. Environmental Entomology 26: 789-796.