Insects can increase their efficiency of acquiring suitable resources over time as a result of learning. Learning is difficult to demonstrate because improved performance with experience often may result from maturation of neuromuscular systems rather than from learning (Papaj and Prokopy 1989).Although an unambiguous definition of learning has eluded ethologists, a simple definition involves any repeatable and gradual improvement in behavior resulting from experience (Papaj and Prokopy 1989, Shettleworth 1984). From an ecological viewpoint, learning increases the flexibility of responses to variation in resource availability
and may be most adaptive when short-term variation is low but long-term variation is high (Stephens 1995).
Learning by insects has been appreciated less widely than has learning by vertebrates, but a number of studies over the past half century have demonstrated learning by various insect groups (cf., Cunningham et al. 1998, Daly et al. 2001, Drukker et al. 2000, Gong et al. 1998, J. Gould and Towne 1988, A. Lewis 1986, Meller and Davis 1996, Papaj and Lewis 1993, Raubenheimer and Tucker 1997, Schneirla 1953, von Frisch 1967, Wehner 2003). Schneirla (1953) was among the first to report that ants can improve their ability to find food in a maze. However, the ants learned more slowly and applied experience less efficiently to new situations than did rats. Learning is best developed in the social and parasitic Hymenoptera and in some other predaceous insects. Nonetheless, learning also has been demonstrated in phytophagous species representing six orders (R. Chapman and Bernays 1989, Papaj and Prokopy 1989). Several types of learning by insects have been identified: habituation, imprinting, associative learning, observational learning, and even cognition.
Habituation is the loss of responsiveness to an unimportant stimulus as a result of continued exposure. Habituation may be the mechanism that induces parasitoids to emigrate from patches that are depleted of unparasitized hosts (Papaj and Prokopy 1989). Although host odors are still present, a wasp is no longer responsive to these odors. Habituation to deterrent chemicals in the host plant may be a mechanism underlying eventual acceptance of less suitable host plants by some insects (Papaj and Prokopy 1989).
Imprinting is the acceptance of a particular stimulus in a situation in which the organism has an innate tendency to respond. Parasitic wasps may imprint on host or plant stimuli at the site of adult emergence. Odors from host frass or the host's food plant present at the emergence site offer important information used by the emerging wasp during subsequent foraging (W. Lewis and Tumlinson 1988). A number of studies have demonstrated that if the parasitoid is removed from its cocoon or reared on an artificial diet, it may be unable to learn the odor of its host or its host's food plant and hence be unable to locate hosts (Godfray 1994).
Associative learning is the linking of one stimulus with another, based on a discerned relationship between the stimuli. Most commonly, the presence of food is associated with cues consistently associated with food. Godfray (1994) summarized a number of examples of associative learning among parasitic Hymenoptera. Information gathered during searching contributes to increased efficiency of host discovery (W. Lewis and Tumlinson 1988). Searching wasps learn to associate host insects with plant odors, including odors not induced by herbivory (Fukushima et al. 2002). Subsequently, they preferentially search similar microhabitats (Godfray 1994, Steidle 1998). However, exposure to new hosts or hosts in novel habitats can lead to increased responsiveness to the new cues. Bjorksten and Hoffmann (1998) reported that such learned stimuli can be retained (remembered) for at least 5 days.
Bisch-Knaden and Wehner (2003) demonstrated that desert ants, Cataglyphis fortis, learned to associate local foraging trail vectors with individual cylindrical landmarks during homebound runs but not during outbound runs. However, ants returning to the nest initially reverse the outbound vector, then start a systematic search for the nest, indicating that these ants cannot learn separate inbound and outbound vectors that are not 180-degree reversals and that recalibration during homebound runs is dominated by the outbround vector (Wehner et al. 2002). Ants are thus able to reach the nest along the shortest route and later return to the food source by 180-degree vector reversal.
Classical conditioning involves substitution of one stimulus for another. Laboratory studies have demonstrated classical conditioning in parasitic wasps. These insects respond to empty food trays after learning to associate food trays with hosts or respond to novel odors after learning to associate them with provision of hosts (Godfray 1994).
Operant conditioning, or trial-and-error learning, is associative learning in which an animal learns to associate its behavior with reward or punishment and then tends to repeat or avoid that behavior accordingly. Association of ingested food with postingestion malaise often results in subsequent avoidance of that food (R. Chapman and Bernays 1989, Papaj and Prokopy 1989). For example, laboratory experiments by Stamp (1992) and Traugott and Stamp (1996) demonstrated that predatory wasps initially attack caterpillars that sequester plant
Honey bees can remember how to approach specific flowers in relation to the time of day. Bees trained to land at different positions (+) of an artificial flower at different times in the morning subsequently preferred to land on the petal on which they were trained during the same part of the morning. From J. Gould and Towne (1988).
defenses, but after a few days they will reject unpalatable prey. Honey bees, trained to approach a particular flower from different directions at different times of day, will subsequently approach other flowers from the direction appropriate to the time of day at which rewards were provided during training (Fig. 3.15). Fry and Wehner (2002) and Horridge (2003) found that honey bees can distinguish pattern and landmark orientations and are able to return to food resources even when associated landmark orientation is altered. A. Lewis (1986) reported that cabbage white butterflies became more efficient at obtaining floral rewards by selectively foraging on a particular floral type based on experience. Such floral fidelity can increase pollination efficiency (see Chapter 13). However, improved nectar foraging on larval food plants may increase the likelihood that females will use the same plant for nectar foraging and oviposition (Cunningham et al. 1998,1999).
Insects are capable of complex associative learning. Raubenheimer and Tucker (1997) trained locust, Locusta migratoria, nymphs to distinguish between food containers, differing in color, with synthetic diet deficient in either protein or carbohydrate. The locusts were forced to feed from both containers in the arena to obtain a balanced diet. The nymphs subsequently were deprived of either protein or carbohydrate and tested for ability to acquire the deficient nutrient. Locusts significantly more frequently selected food containers of the color previously associated with the deficient nutrient, regardless of color or whether the nutrient was protein or carbohydrate. Wackers et al. (2002) demon o J
strated that parasitoid wasps, Microplitis croceipes, could learn multiple tasks representing feeding and reproduction. Stach et al. (2004) found that honey bees can learn multiple conditioning patterns and generalize their response to novel stimuli based on linkage among conditioned stimuli.
Observational learning occurs when animals gather information and modify their behavior in response to observation of other individuals. Observational learning is epitomized by social bees that communicate the location of rich floral resources to other members of the colony through the "bee dance" (F. Dyer 2002, J. Gould and Towne 1988, von Frisch 1967). Movements of this dance, oriented with reference to the sun, inform other foragers of the direction and distance to a food source.
Cognition, characterized by awareness, memory, and judgment, can be demonstrated by application of information gathered during previous experiences to performance in novel situations. This basic form of thinking is widely associated with higher vertebrates. However, J. Gould (1986) demonstrated that honey bees are capable of constructing cognitive maps of their foraging area. Bees were trained to forage at either of two widely separated sites, then captured at the hive and transported in the dark to an unfamiliar site, the same distance from the hive but in a different direction, within a complex foraging area (open areas interspersed with forest). If released bees were disoriented or could not accommodate a sudden change in landmarks, they should fly in random directions. If they have only route-specific landmark memory and were familiar with a foraging route to their release point, they should be able to return to the hive and from there fly to their intended destination (site to which they had been trained). Only if bees are capable of constructing true cognitive maps should they be able to fly from the release point directly to their intended destination. J. Gould (1986) found that all bees flew directly to their intended destinations. Although some studies indicate limits to large-scale cognitive mapping by bees (Dukas and Real 1993, Menzel et al. 1998), substantial evidence indicates that honey bees construct and maintain at least local metric spatial representation, referenced to the time of day and to landmarks and line angles to floral resources (J. Gould 1985,1986, J. Gould and Towne 1988). Wei et al. (2002) further demonstrated that honey bees intensively examine the area around a food source through "learning flights." Bees turn back and face the direction of the food source and surrounding landmarks, then circle around, before returning to the hive. The duration of learning flights increases with the sugar concentration of food and the visual complexity of the surrounding landmarks and is longer following initial discovery of food than during subsequent reorientation. These results indicate that bees adjust learning effort in response to the need for visual information. Such advanced learning greatly facilitates the efficiency with which resources can be acquired.
Insects, as do all organisms, must acquire energy and material resources to synthesize the organic molecules necessary for life processes of maintenance, growth, and reproduction. Dietary requirements reflect the size and life stage of the insect and the quality of food resources. Insects exhibit a variety of physiological and behavioral strategies for finding, evaluating, and exploiting potential resources.
Defensive chemistry of plants and insects affects their quality as food and is a basis for host choice by herbivorous and entomophagous insects, respectively. Nutritional value of resources varies among host species, among tissues of a single organism, and even within tissues of a particular type. Production of defensive chemicals is expensive in terms of energy and nutrient resources and may be sacrificed during unfavorable periods (such as during water or nutrient shortages or following disturbances) to meet more immediate metabolic needs. Such hosts become more vulnerable to predation. Insect adaptations to detoxify or otherwise circumvent host defenses determine host choice and range of host species exploited. Generalists exploit a relatively broad range of host species but exploit each host species rather inefficiently, whereas specialists are more efficient in exploiting a single or a few related hosts that produce similar chemical defenses.
Chemicals also communicate the availability of food and provide powerful cues that influence insect foraging behavior. Insects are capable of detecting food resources over considerable distances. Perception of chemical cues that indicate availability of hosts is influenced by concentration gradients in air or water, environmental factors that affect downwind or downstream dispersion of the chemical, and sensitivity to particular odors. Orientation to food resources over shorter distances is affected by visual cues (such as color or pattern) and acoustic cues (such as stridulation). Once an insect finds a potential resource, it engages in tasting or other sampling behaviors that permit evaluation of resource acceptability.
Efficiency of resource acquisition may improve over time as a result of learning. Although much of insect behavior may be innate, learning has been documented for many insects. Ability to learn among insects ranges from simple habituation to continuous unimportant stimuli, to widespread associative learning among both phytophagous and predaceous species, to observational learning and even cognitive ability. Learning represents the most flexible means of responding to environmental variation and allows many insects to adjust to changing environments during short lifetimes.
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