E. A. Bernays

Host-plant selection by phytophagous insects

Preface. Acknowledgements. Introduction. 1. Patterns of host-plant use. 2. Chemicals in plants. 3. Sensory systems. 4. Behavior: the process of host-plant selection. 5. Behavior: the impact of ecology and physiology. 6. Effects of experience. 7. Genetic variation in host selection. 8. Evolution of host range. Glossary. Taxonomic index. Subject index.

On the Evolution of Host Specificity in Phytophagous Arthropods

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.

Plant tannins and insect herbivores: an appraisal

Abstract. 1. The bioassays with tannins and insects, and the ecological studies on insects implicating tannins, are summarized and discussed.

Dietary Mixing in a Generalist Herbivore: Tests of Two Hypotheses

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: (1) 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 distinguish between the two hypotheses. Experiments using artificial foods with complementary nutrients demonstrated that dietary mixing improves growth while the amount ingested is decreased. Nutritionally identical foods with differing plant secondary compounds led to different growth rates, but insects able to mix generally grew at intermediate rates. Experiments with domestic or wild plant mixtures gave results generally closer to the pattern found with complementary nutrients than the pattern found with differing poisons, but both hypotheses are supported to some extent.

Control of Food Intake

Publisher Summary The regulation of food intake has been investigated in a number of insects, most notably blowflies and locusts. Results have tended to suggest that the mechanisms involved vary considerably between groups, between species within groups, and even within a species under different conditions. This chapter provides an overall synthesis of feeding in the insects. It emphasizes on the blowflies and locusts but where possible data from other insect groups have been incorporated. The chapter focuses on the regulation of the quantity of food ingested: what initiates feeding, what determines how much is eaten in one meal and what determines the frequency with which meals are taken. In a normally feeding insect, the sensilla that initiate feeding become effective in sequence; tarsi, palps, cibarial cavity. Sinoir suggests that continuous feeding requires stimulation of all the groups of chemoreceptors within the cibarial cavity, as well as the palp tips, and gives a description of probable stages of chemoreception at the beginning of a meal. Ablation experiments have also suggested that the major groups of chemoreceptors are all involved in the regulation of ingestion, since in most cases their removal was shown to decrease amounts eaten.

INSECT HERBIVORES: DIFFERENT REASONS FOR BEING A GENERALIST

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.

Feeding by lepidopteran larvae is dangerous

Continuous observations were made on natural populations It is difficult to obtain direct measures of predation on small animals such as insect herbivores. Instead, disappearance has near Berkeley, California. The caterpillars were mostly first and second instars occurring in groups near the tips of branches. sometimes been used as a rough measure of predation, or, more reliably, populations of insects inside and outside Observations lasted from dawn to dusk, approximately 12 h. Each of 298 caterpillars was recorded resting, moving or exclosures may be compared over time. In this way, many studies have deduced the importance of predation as a mortality feeding. A total of 3800 ‘caterpillar hours’ of observations was made. One or two groups of individuals could be observed factor for insect herbivores such as lepidopteran larvae. For example, Feenyet al. (1985) showed that predation was the closely at one time from a chair taken into the field and placed beside the shrub. largest source of mortality ofPapilio polyxenes larvae. Invertebrates were most important for smaller larvae while Although U. reversalisis aposematic (Bernays & Montllor, 1989) and distasteful to ants and wasps (Montllor et al., 1991), vertebrates became important for larger ones. Much can also be deduced after natural selection has operated, about the other invertebrate predators kill up to 20% of larvae in the first two or three instars. At this stage in the life history, the larvae evolutionary importance of predators by the avoidance strategies employed by caterpillars (Heinrich, 1993) are gregarious and spin loose silken shelters between leaves or even over a single leaf. After hatching from the eggs, which are However important predation of lepidopterans may be, relatively few people have spent time observing larvae for long deposited in groups of variable size, the young larvae can feed foradayorsowithin theshelter theymake.Here, theyskeletonize enough to see many predation events, except where populations of both herbivores and predators were unusually large. the leaves. After that time, they enlarge the shelter somewhat, but individuals must make regular forays to feed on other leaves. Examples of data that demonstrate the danger of movement include the finding that movement increased the risk of ant The forays consist of movements of several centimetres, a feeding bout of several minutes, after which they return to the predation on a geometrid caterpillar (Bergelson & Lawton, 1988). Similarly, caterpillar movement was shown to increase shelter. In the shelter they rest and moult. During the period of this study, the major predator was a predation by a pentatomid bug (Marston et al., 1978) and a crab spider (den Boer, 1971). So, while it may be assumed species of anthocorid bug that resided on the broom plants and presumably fed upon various small arthropods and their eggs. that movement is generally dangerous, less is known about the risk of actually feeding. This study is a quantification of the Occasional predation events were observed involving ants, crab spiders and paper wasps ( Vespulasp.). Caterpillars appeared to risk of predation by two predators, posed by feeding in two caterpillar species. remain in their shelters overnight, and gradually emerged singly some time after temperatures rose above 15 °C. During the day, individuals continued to emerge, feed, and then return to Methods and Results shelter; activity dropped off after sunset. The predatory bugs most commonly made their attacks when caterpillars moved All data were obtained by continuous observation of individual out of their shelters and began feeding. Sometimes, when a caterpillars of two species, over periods lasting at least 6 h. predator approached, a caterpillar was able to spin down from One study involved early larval stages of the pyralid, Uresiphita the leaf on a thread of silk, and dangle there for a while. Some reversalis, feeding on its most common host in northern minutes later, when it crawled back onto the leaf, the predator California,Genista monspessulanus (broom). The second study was often gone. involved the sphingid,Manduca sexta(tobacco hornworm), Caterpillars spent most of their time resting in the shelter feeding on cultivated plants of Nicotiana tabacum(tobacco). (Table 1). During observations anthocorid bugs made a total of fifteen kills of U. reversalislarvae. One kill occurred during locomotion, twelve during feeding and two when the caterpillars Correspondence: E. A. Bernays Department of Entomology were resting. No generalization can be made about the risks University of Arizona, Tucson, AZ 85721, U.S.A. E-mail: of locomotion because predation events then were so few, but schistos@ag.arizona.edu

SENSORY CAPABILITIES, INFORMATION PROCESSING, AND RESOURCE SPECIALIZATION

Numerous hypotheses have been proposed to account for the widespread occurrence of specialized behavior, especially with regard to food and host-related resource utilization. None of these hypotheses has been widely accepted, although most are recognized as being important for particular taxa or in certain circumstances. We discuss features of neural function as primary proximate mechanisms involved in resource specialization, which may underlie many of the other hypotheses. Diet and host breadth may be associated with the potential informational complexity of an organisms environment. The processing of complex information ultimately entails costs associated with decision time, relative efficiency of food or oviposition site-selection behavior, and concomitant exposure to potential mortality factors. Sensory focusing, through paying attention, experiential processes, or canalized sensory input decreases these costs. Increased efficiency of host finding, recognition, and discrimination can be expected as a result of a reduced probability of information overload. Such efficiencies should, in addition, decrease exposure to natural enemies.

Self-Medication as Adaptive Plasticity: Increased Ingestion of Plant Toxins by Parasitized Caterpillars

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.

Diet-Induced Head Allometry Among Foliage-Chewing Insects and Its Importance for Graminivores

Individuals of the grass-feeding caterpillar of Pseudaletia unipuncta, reared from hatching on hard grass, had head masses twice as great as those of caterpillars fed soft artificial diet, even though the larvae reached the same body mass. Larvae reared on soft wheat seedlings had intermediate head masses. Thus muscular effort increases muscular development in an insect, which in turn has a dramatic morphogenetic effect on head size. Size differences in the head capsules, with the correlated differences in mandibular power, have a direct effect on the ability of the insects to ingest hard foods rapidly: larger heads are adaptive for dealing with hard grasses.