The variety of resources and their physical and biochemical properties, including defensive mechanisms, is too great in any ecosystem for any species to exploit all possible resources. The particular physiological and behavioral adaptations of insects to obtain sufficient nutrients and avoid toxic or undigestable materials determine their feeding preferences, i.e., which resources they can or will exploit. Potential resources vary widely in nutritional value. Animal tissues have higher nutritional value than do plant tissues because of the preponderance of indigestible cellulose in plant tissues. Nutritional quality of foliage is higher than that of root tissue. Nutritional value varies between bark, sapwood and heartwood tissues (Hodges et al. 1968, Schowalter et al. 1998). In fact, exploitation of sapwood requires mutualistic interaction with fungi or bacteria, or other adaptations, to acquire sufficient nutrients from a resource that is largely indigestible cellulose (Ayres et al. 2000, see Chapter 8). Insects specialized to exploit particular physical and chemical conditions often lose their ability to exploit other resources. Even species that feed on a wide variety of resource types (e.g., host species) are limited in the range of resources they can exploit. For example, the variety of plant species (representing many plant families) eaten by gypsy moth share primarily phenolic defenses; plants with terpenoid or alkaloid defenses usually are not exploited (J. Miller and Hansen 1989).
Particular compounds can be effective defenses against nonadapted herbivores and, at the same time, be phagostimulants for adapted herbivores (Shonle and Bergelson 2000). For example, Tallamy et al. (1997) reported that cucur-bitacins (bitter triterpenes characterizing the Cucurbitaceae) deter feeding and oviposition by nonadapted mandibulate insect herbivores but stimulate feeding by haustellate insect herbivores.
Malcolm (1992) identified three types of consumers with respect to a chemically defended prey species. Excluded predators cannot feed on the chemically defended prey, whereas included predators can feed on the chemically defended prey with no ill effect. Peripheral predators experience growth loss, etc., when fed chemically defended prey as a result of the effects of the defensive chemicals on predator physiology or on the nutritional quality of the prey. The effectiveness of peripheral predators on prey differing in chemical defense may be a key to understanding the ecology and evolution of predator-prey interactions. Feeding preferences generally depend on three integrated factors: resource quality, susceptibility, and acceptability.
Resource quality, as described in the preceding text, represents the net nutritional value of the resource as determined by the nutrients available to the insect less the energy and resources needed to detoxify or avoid defenses. Just as production of defensive compounds is expensive for the host in terms of energy and resources, production of detoxification enzymes or development of avoidance mechanisms is expensive in terms of energy, resources, time searching, and exposure to predators. Some of the nutrients in the food must be allocated to production of detoxification enzymes or to energy expended in searching for more suitable food. Although diversion of dietary N to production of detoxification enzymes should be reduced if N is limiting, Lindroth et al. (1991) found little change in detoxification enzyme activity in response to nutrient deficiencies in gypsy moth larvae. However, defenses can have beneficial side effects for the consumer. M. Hunter and Schultz (1993) reported that phenolic defenses in oak leaves reduced susceptibility of gypsy moth larvae to nuclear polyhedrosis virus.
Resource susceptibility represents the physiological condition of the host, whether for herbivores or predators. Injury or adverse environmental conditions can stress organisms and impair their ability to defend themselves. Initially, stress may prevent expression of induced defenses, an added cost, or may prevent production of constitutive defenses but allow induction of defenses, as needed. Nitrogen limitation may prevent production of nitrogenous defenses but increase production of nonnitrogenous defenses. Reduced production of biochemical defenses reduces the cost of detoxification or avoidance to the predator. Hence, specialist species can allocate more energy and resources to growth and reproduction, and generalists may be able to expand their host range as biochemical barriers are removed.
Resource acceptability represents the willingness of the insect to feed, given the probability of finding more suitable resources or in view of other tradeoffs. Most insects have relatively limited time and energy resources to spend searching for food. Hence, marginally suitable resources may become sufficiently profitable when the probability of finding more suitable resources is low, such as in diverse communities composed primarily of nonhosts. Courtney (1985, 1986) reported that oviposition by a pierid butterfly, Anthocharis car-damines, among several potential host plant species was inversely related to the suitability of those plant species for larval development and survival (Fig. 3.10). The more suitable host plant species were relatively rare and inconspicuous compared to the less suitable host species. Hence, butterfly fitness was maximized by laying eggs on the most conspicuous (apparent) plants, thereby ensuring reproduction, rather than by risking reproductive failure during continued search for more suitable hosts. Nevertheless, insects forced to feed on less suitable resources show reduced growth and survival rates (Bozer et al. 1996, Courtney 1985,1986).
Insects face an evolutionary choice between maximizing the range of resources exploited (generalists) or maximizing the efficiency of exploiting a particular resource (specialists). Generalists maximize the range of resources exploited through generalized detoxification or avoidance mechanisms, such as broad-spectrum microsomal monooxygenases, but sacrifice efficiency in exploiting any particular resource because unique biochemicals reduce digestion or survival (Bowers and Puttick 1988). Generalists may benefit from a mixed diet through dilution of any one host's defensive compounds (Bernays et al. 1994) or increase their efficiency by exploiting stressed hosts that have
Cp Ca Br Ap Bv Hm
Cp Ca Br Ap Bv Hm
Persistence Short Intermed Long
Tradeoff between plant suitability for larval survival (top) and efficiency of oviposition site selection by adult pierids, Anthocharis cardamines, as indicated by the ratio of eggs per host species (middle) and plant apparency (i.e., floral surface area and longevity [bottom]). Searching females preferentially oviposit on the most conspicuous plants, although these are not the most suitable food plants for their larvae. Cp, Cardamine pratensis; Ca, C. amara; Br, Brassica rapa;Ap, Allaria petiolate; Br, Barbarea vulgaris; and Hm, Hesperis matronalis. From Courtney (1985) with permission from Oikos.
sacrificed production of defenses. Specialists maximize the efficiency of exploiting a particular host through more specific detoxification or avoidance strategies, minimizing the effect of more of the host's constitutive and induced defenses, but sacrifice ability to feed on other species with different defenses (Bowers and Puttick 1988).
Some generalists that occur over large geographic areas may be more specialized at the local level. Parry and Goyer (2004) demonstrated that forest tent caterpillar, Malacosoma disstria, is a composite of regionally specialized populations rather than an extreme generalist. In a reciprocal transplant experiment, tent caterpillars from Louisiana and Michigan, in the United States, and Manitoba, Canada, were reared on the variety of hosts exploited by northern and southern populations. Tent caterpillars from northern populations showed greatest growth and survival on trembling aspen, Populus tremuloides, and red oak, Quercus rubra, both northern host species, and poorest growth and survival on water tupelo, Nyssa aquatica, a southern host species. Tent caterpillars from southern populations showed greatest growth and survival on water tupelo and poorest growth and survival on sugar maple, Acer saccharum, a northern host species.
Specialists and generalists contribute to host population dynamics and to community structure and function in different ways, as described in Chapters 6 and 8. Generalists usually exploit more abundant host species and may reduce competition among hosts, but do not effectively target rapidly increasing host populations. By contrast, specialists focus on particular host species and control host population growth more effectively but must be able to discover sparsely distributed hosts.
Searching insects initially identify acceptable hosts. They then select particular host tissues based on nutritional value. Nutritional quality can vary even within tissues. For example, insects may target particular portions of leaves, based on gradients in the ratio among amino acids along the leaf blade (Haglund 1980, K. Parsons and de la Cruz 1980), and particular heights on tree boles, based on gradients in ratios among amino acids and carbohydrates (Hodges et al. 1968).
Many insects feed on different resources at different stages of development. Most Lepidoptera feed on plant foliage, stems, or roots as larvae, but many feed on nectar as adults. Some cerambycid beetles feed in wood as larvae but feed on pollen or nectar as adults. A number of Diptera and Hymenoptera have preda-ceous or parasitic larvae but feed on pollen or nectar resources as adults. Many aphids alternate generations between two host plant species (Dixon 1985). Clearly, these changes in food resources require changes in digestive abilities between life stages. Furthermore, population survival requires the presence of all necessary resources at an appropriate landscape scale.
The primacy of resource exploitation for development and survival places strong selective pressure on insects to adapt to changing host quality. This has led to the so-called "evolutionary arms race," in which herbivory selects for new plant defenses and the new plant defenses select for insect countermeasures. This process has driven reciprocal speciation in both plants and insects, with examples of cladograms of plant species and associated insect species mirroring each other (Becerra 1997). The long exposure of insects to a wide variety of host (especially plant) toxins has led to flexible detoxification and other mechanisms to circumvent those defenses, especially among generalists. This ability to detoxify various plant defenses predisposes many herbivorous species to detoxify synthetic insecticides.
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