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We argue that generalist natural enemies of herbivorous insects provide a major selection pressure for restricted host plant range. The significance of plant chemistry is discussed in terms of regulating behavior, while the chemical coevolutionary theories are considered to be of limited value.

This review points out the problem of processing multiple sensory inputs and provides evidence that generalists suffer a disadvantage compared with specialists with respect to efficiency of host plant choice and discrimination. The specialists' mechanisms for improved efficiency are discussed as well as some of the processes that may be selected to increase processing efficiency in generalists. The fitness consequences of differences in efficiency of specialists and generalists are pointed out. One of the major disadvantages for generalists is the increase in vulnerability to ecological risks, especially risks imposed by various natural enemies. Efficiency-related factors are indicated as previously underestimated elements that could influence host affiliations including diet breadth and changes in host plant use.

It has been demonstrated in several taxa that generalists grow better when they ingest a mixed diet than when they are restricted to just one or two items, but there are few cases that provide definitive evidence for how this benefit is achieved. Two hypotheses are addressed concerning the possible benefits of feeding on a variety of foods: (l) mixing foods increases the quality of the overall diet by improving the nutrient balance and (2) mixing improves the diet due to dilution of any one plant secondary compound. The generalist grasshopper Schistocerca americana was used in a series of experiments to distinguish between the two hypotheses.Experiments using artificial foods with complementary nutrients demonstrated that ~ieta'!' mixing iJ?pr~ves. growth while the amount ingested is decreased. Nutritionally ~denttcal foods ":tth dtffenng plant secondary compounds led to different growth rates, but msects able to mtx generally grew at intermediate rates. Experiments with domestic or wild plan~ mixtures gave results generally closer to the pattern found with complementary nutnents than the pattern found with differing poisons, but both hypotheses are supported to some extent.

Self-medication is a specific therapeutic behavioral change in response to disease or parasitism. The empirical literature on self-medication has so far focused entirely on identifying cases of self-medication in which particular behaviors are linked to therapeutic outcomes. In this study, we frame self-medication in the broader realm of adaptive plasticity, which provides several testable predictions for verifying self-medication and advancing its conceptual significance. First, self-medication behavior should improve the fitness of animals infected by parasites or pathogens. Second, self-medication behavior in the absence of infection should decrease fitness. Third, infection should induce self-medication behavior. The few rigorous studies of self-medication in non-human animals have not used this theoretical framework and thus have not tested fitness costs of self-medication in the absence of disease or parasitism. Here we use manipulative experiments to test these predictions with the foraging behavior of woolly bear caterpillars (Grammia incorrupta; Lepidoptera: Arctiidae) in response to their lethal endoparasites (tachinid flies). Our experiments show that the ingestion of plant toxins called pyrrolizidine alkaloids improves the survival of parasitized caterpillars by conferring resistance against tachinid flies. Consistent with theoretical prediction, excessive ingestion of these toxins reduces the survival of unparasitized caterpillars. Parasitized caterpillars are more likely than unparasitized caterpillars to specifically ingest large amounts of pyrrolizidine alkaloids. This case challenges the conventional view that self-medication behavior is restricted to animals with advanced cognitive abilities, such as primates, and empowers the science of self-medication by placing it in the domain of adaptive plasticity theory.

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Two very different factors favor generalist herbivores over specialist herbivores: greater resource availability, which should be universally beneficial, and the possibility of mixing foods to improve nutrient balance or to reduce exposure to high levels of particular allelochemicals. In this work, four species of Lepidoptera and three species of Hemiptera were fed single or mixed foods to test the hypothesis that individual generalists would benefit from mixing their diets. In all cases, insects were provided with one of three different host plants, or a mixture of all three. For most species, several different experiments were performed. Survivorship, gain in mass, and fecundity were used as measures of performance. In no case was there evidence that mixtures were better than single foods. It is argued that, in these taxa, the value of being a generalist lies largely with versatility for use of different hosts, rather than for dietary mixing. The contrast with Orthoptera, in which individuals commonly obtain substantial benefit from dietary mixing, is also discussed.

No abstract