Food quality affects the amount of food required to obtain sufficient nutrition for growth and reproduction, and the energy and nutrients required for detoxification and digestion (see Chapter 3). Insects feeding on hosts with lower levels of defensive compounds invest fewer energy and nutrient resources in detoxification enzymes or continued searching behavior than do insects feeding on better defended hosts. Herbivores process much indigestible plant material, especially cellulose, whereas predators process animal material that generally is more similar to their own tissues. Accordingly, we might expect higher assimilation efficiencies for predators than for herbivores (G. Turner 1970). Although indigestible and toxic compounds in plant tissues reduce assimilation efficiency for herbi vores (Scriber and Slansky 1981), toxins sequestered or produced by prey also reduce assimilation efficiency of predators. However, few studies have addressed the effect of toxic prey on assimilation efficiency of predators (L. Dyer 1995, Stamp et al. 1997, Stephens and Krebs 1986).
Insects may ingest relatively more food to obtain sufficient nutrients or energy to offset the costs of detoxification or avoidance of plant defensive chemicals. Among herbivores, species that feed on mature tree leaves have relative growth rates that are generally half the values for species that feed on forbs because tree leaves are poor food resources compared to forbs (Scriber and Slansky 1981).Although specialists might be expected to feed more efficiently on their hosts than do generalists, Futuyma and Wasserman (1980) reported that a specialist (the eastern tent caterpillar, Malacosoma americana) had no greater assimilation or growth efficiencies than did a generalist (the forest tent caterpillar, M. disstria). Some wood-boring insects may require long periods (several years to decades) of larval feeding to concentrate nutrients (especially N and P) sufficient to complete development.
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