Mechanisms for Exploiting Variable Resources

In a classic paper that stimulated much subsequent research on factors affecting herbivory, Hairston et al. (1960) argued that herbivore populations are not limited by food supply because vegetation is normally abundant, and herbivores, when numerous, are able to deplete plant resources. We now know, as described in the preceding text, that plant resources are not equally suitable or acceptable and that herbivore populations often are limited by availability of suitable food. Herbivore populations are regulated by a combination of factors, as discussed in Chapter 6, including dietary toxins. At the same time, insects are capable of feeding on defended hosts. Feeding preferences reflect one mechanism for avoiding defenses. However, insects exhibit a variety of mechanisms for detoxifying, avoiding, or circumventing host defenses.

Herbivorous insects produce a variety of catalytic enzymes, in particular those associated with cytochrome P-450, to detoxify plant or prey defenses (Feyereisen 1999, Karban and Agrawal 2002). Some insects produce salivary enzymes that minimize the effectiveness of plant defenses. Salivary enzymes, such as glucose oxidase applied to feeding surfaces by caterpillars, may inhibit activation of induced defenses (Felton and Eichenseer 1999). Saliva of Heteroptera and Homoptera gels into a sheath that separates the insect's stylet from plant cells, perhaps reducing induced plant responses (Felton and Eichenseer 1999).

Digestive enzymes responsible for detoxification usually are microsomal monooxygenases, glutathione S-transferases, and carboxylesterases (Hung et al. 1990) that fragment defensive compounds into inert molecules. Microsomal monooxygenases are a general-purpose detoxification system in most herbivores and have higher activity in generalist species, compared to specialist species or sap-sucking species (Hung et al. 1990). More specific digestive enzymes also are produced by some species. Detoxification enzymes can be induced in response to exposure to plant toxins (Karban and Agrawal 2002). For example, caterpillars feeding on diets containing proteinase inhibitors showed reduced function of particular proteinases but responded by producing other proteinases that were relatively insensitive to dietary proteinase inhibitors (Broadway 1995,1997). The compounds produced through detoxification pathways may be used to meet the insect's nutritional needs (Bernays and Woodhead 1982), as in the case of the sawfly, Gilpinia hercyniae, which detoxifies and uses the phenolics from its conifer host (Schöpf et al. 1982).

The ability to detoxify plant defenses may predispose many insects to detoxify synthetic insecticides (Feyereisen 1999, Plapp 1976). At least 500 arthropod species are resistant to major insecticides used against them, primarily through a limited number of resistance mechanisms that confer cross-resistance to plant defenses and structurally related toxicants and, in some cases, to chemically unrelated compounds (Soderlund and Bloomquist 1990). Le Goff et al. (2003) reported that several cytochrome P-450 genes code for detoxification of DDT (dichlorodiphenyltrichloroethane), imidacloprid, and malathion.

Gut pH is a factor affecting the chelation of nitrogenous compounds by tannins. Some insect species are adapted to digest food at high gut pH to inhibit chelation. The insect thus is relatively unaffected by high tannin contents of its food. Examples include the gypsy moth, feeding on oak, Quercus spp., and chrysomelid beetles, Paropsis atomaria, feeding on Eucalyptus spp. (Feeny 1969, Fox and Macauley 1977).

Sequestration and excretion are alternative means of avoiding the effects of host toxins that cannot be detoxified. Sequestered toxins are transported quickly to specialized storage tissues (the exoskeleton or protected pouches), whereas remaining toxins are transported to the Malphigian tubules for elimination. Sequestered toxins become part of the insect's own defensive strategy (Blum 1981,1992, Conner et al. 2000).

Several mechanisms are used to avoid or circumvent host defensive chemicals. Life history phenology of many species is synchronized with periods of most favorable host nutritional chemistry (Feeny 1970, Varley and Gradwell 1970). Diapause can be an important mechanism for surviving periods of adverse host conditions, as well as adverse climatic conditions. In fact, diapause during certain seasons may reflect seasonal patterns of resource availability more than abiotic conditions. For example, many tropical herbivores become dormant during the dry season when their host plants cease production of foliage or fruit and become active again when production of foliage and fruit resumes in the wet season. Diapause can be prolonged in cases of unpredictable availability of food resources, as in the case of insects feeding on seeds of trees that produce seed crops irregularly. Turgeon et al. (1994) reported that 70 species of Diptera, Lepidoptera, and Hymenoptera that feed on conifer cones or seeds can remain in diapause for as long as 7 years. In other words, insect populations often have considerable capacity to survive long periods of unsuitable resource conditions through diapause.

Some herbivores sever the petiole or major leaf veins to inhibit translocation of induced defenses during feeding (Becerra 1994, Karban and Agrawal 2002). Sawflies (Diprionidae) sever the resin canals of their conifer hosts or feed gregariously to consume foliage before defenses can be induced (McCullough and Wagner 1993). Species feeding on plants with photooxidant defenses often feed at night or inside rolled leaves to avoid sunlight (Berenbaum 1987, Karban and Agrawal 2002).

Several aphids and gall-formers have been shown to stimulate plant accumulation of nutrients in colonized tissues. For example, Koyama et al. (2004) reported that the amount of amino acids exuding from leaves galled by the aphid Sorbaphis chaetosiphon was five times that from ungalled leaves. Furthermore, galls retained high amino acid concentrations throughout April, whereas amino acid concentrations declined rapidly during this period in ungalled leaves. Koyama et al. (2004) also compared growth and reproduction of another aphid, Rhopalosiphum insertum, which can displace gall aphids or colonize ungalled leaves. Aphid growth and reproduction were significantly higher for colonies experimentally established in galls, compared to colonies established on ungalled leaves, indicating a positive effect of gall formation.

Some insects vector plant pathogens that inhibit host defense or induce favorable nutritional conditions in plant hosts. However, not all insects that vector plant pathogens benefit from host infection (Kluth et al. 2002).

Many predaceous insects use their venoms primarily for subduing prey and secondarily for defense. Venoms produced by predaceous Heteroptera, Diptera, Neuroptera, Coleoptera, and Hymenoptera function to paralyze or kill prey (Schmidt 1982), thereby minimizing injury to the predator during prey capture. The carabid beetle, Promecognathus, a specialist predator on Harpaphe spp. and other polydesmid millipedes, avoids the cyanogenic secretions of its prey by quickly biting through the ventral nerve cord at the neck, inducing paralysis (G. Parsons et al. 1991). Nevertheless, host defenses increase handling time and risk of injury and mortality for the consumer (Becerra 1994).

Diversion of limited resources to detoxification enzymes or efforts to circumvent or avoid defenses all involve metabolic costs (Karban and Agrawal 2002, Kessler and Baldwin 2002). Lindroth et al. (1991) evaluated the effect of several specific nutrient deficiencies on detoxification enzyme activity in the gypsy moth. They found that larvae on a low-protein diet showed compensatory feeding behavior (although not enough to offset reduced protein intake). Soluble esterase and carbonyl reductase activities increased in response to protein deficiency but decreased in response to vitamin deficiency. Polysubstrate monooxy-genase and glutathione transferase activities showed no significant response. Furthermore, Carrière et al. (2001b) reported that pink bollworm, Pectinophora gosypiella, resistance to transgenic (Bt) cotton was associated with reduced per centage emergence from diapause, compared to nonresistant bollworm, indicating fitness costs of developing resistance strategies.

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