Variation in Food Quality

Food quality varies widely among resource types. Plant material has relatively low nutritional quality because N usually occurs at low concentrations and most plant material is composed of carbohydrates in the form of indigestible cellulose and lignin.Woody tissues are particularly low in labile resources readily available to insects or other animals. Plant detrital resources may be impoverished in important nutrients as a result of weathering, leaching, or plant resorption prior to shedding senescent tissues.

Individual plants differ in their nutritional quality for a number of reasons, including soil fertility. Ohmart et al. (1985) reported that Eucalyptus blakelyi subjected to different N fertilization levels significantly affected fecundity of Paropsis atomaria, a chrysomelid beetle. An increase in foliar N from 1.5% to 4.0% increased the number of eggs laid by 500% and the rate of egg production by 400%. Similarly, Blumberg et al. (1997) reported that arthropod abundances were higher in plots receiving inorganic N (granular ammonium nitrate, rye grass cover crop) than in plots receiving organic N (crimson clover, Trifolium incarna-tum, cover crop). However, the effects of plant fertilization experiments have been inconsistent, perhaps reflecting differences among plant species in their allocation of N to nutritive versus nonnutritive compounds or differences in plant or insect responses to other factors (Kyto et al. 1996, G. Waring and Cobb 1992).

The nutritional value of plant resources frequently changes seasonally and ontogenically. Filip et al. (1995) reported that the foliage of many tropical trees has higher nitrogen and water content early in the wet season than late in the wet season. R. Lawrence et al. (1997) caged several cohorts of western spruce budworm, Choristoneura occidentals, larvae on white spruce at different phenological stages of the host. Cohorts that began feeding 3-4 weeks before budbreak and completed larval development prior to the end of shoot elongation developed significantly faster and showed significantly greater survival rate and adult mass than did cohorts caged later (Fig. 3.1). These results indicate that the phenological window of opportunity for this insect was sharply defined by the period of shoot elongation, during which foliar nitrogen, phosphorus, potassium, copper, sugars, and water were higher than in mature needles.

Food resources often are defended in ways that limit their utilization by consumers. Physical defenses include spines, toughened exterior layers, and other barriers. Spines and hairs can inhibit attachment or penetration by small insects or interfere with ingestion by larger organisms. These structures often are associated with glands that augment the defense by delivering toxins. Some plants entrap phytophagous insects in adhesives (R. Gibson and Pickett 1983) and may obtain nutrients from insects trapped in this way (Simons 1981).Toughened exteriors include lignified epidermis of foliage and bark of woody plants and heavily armored exoskeletons of arthropods. Bark is a particularly effective barrier to

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2 3 4 5 6 Spruce budworm cohorts

Larval and pupal survival, adult dry mass, and development time from 2nd instar through adult for eight cohorts of spruce budworm caged on white spruce in 1985. The first six cohorts were started at weekly intervals beginning on Julian date 113 (April 23) for cohort 1. Cohort 7 started on Julian date 176 (June 25), and cohort 8 started on Julian date 204 (July 23). Each cohort remained on the tree through completion of larval development, 6-7 weeks. Budbreak occurred during Julian dates 118-136, and shoot elongation occurred during Julian dates 118-170. From R. Lawrence et al. (1997) by permission from the Entomological Society of Canada.

penetration by most organisms (Ausmus 1977), but lignin also reduces ability of many insects to use toughened foliage (e.g., Scriber and Slansky 1981). The viscous oleoresin (pitch) produced by conifers and some hardwoods can push insects out of plant tissues (Fig. 3.2).

Many plant and animal species are protected by interactions with other organisms, especially ants or endophytic fungi (see Chapter 8). A number of plant species provide food sources or habitable structures (domatia) suitable for colonies of ants or predaceous mites (e.g., Fischer et al. 2002, Huxley and Cutler 1991). Cecropia trees, Cecropia spp., in the tropics are one of the best-known plants protected by aggressive ants, Azteca spp., housed in its hollow stems (Rickson 1977). Central American acacias, Acacia spp, also are defended against

Insect Chimical Deffense
| The wound response of conifers constitutes a physical-chemical defense against invasion by insects and pathogens. The oleoresin, or pitch, flowing from severed resin ducts hinders penetration of the bark.

herbivores by colonies of aggressive ants, Pseudomyrmex spp., housed in swollen thorns (Janzen 1966). Many species of plants produce extrafloral nectaries or food bodies that attract ants for protection (Fischer et al. 2002). Some plants protect themselves from insect herbivores by emitting chemical signals that attract parasitic wasps (Kessler and Baldwin 2001,Turlings et al. 1993,1995). G. Carroll (1988), Clay et al. (1993), and D.Wilson and Faeth (2001) have reported reduced herbivory by insects as a result of foliar infection by endophytic fungi.

Both plants and insects produce a remarkable range of compounds that have been the source of important pharmaceuticals or industrial compounds as well as effective defenses. These "secondary plant compounds" function as toxins or feeding deterrents, killing insects or slowing development rates, which may or may not increase exposure and effect of predators and parasites (Lill and Marquis 2001). Biochemical interactions between herbivores and their host plants and between predators and their prey have been one of the most stimulating areas of ecological and evolutionary research since the 1970s. Major points affecting ecological processes are summarized in the next section. Readers desiring additional information are referred to Bernays (1989), Bernays and Chapman (1994), K. Brown and Trigo (1995), Coley and Barone (1996), P. Edwards (1989), Harborne (1994), Hedin (1983), Kessler and Baldwin (2002), Rosenthal and Berenbaum (1991,1992), and Rosenthal and Janzen (1979).

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