Grasses with the C 3 photosynthetic pathway are commonly considered to be more nutritious host plants than C 4 grasses, but the nutritional quality of C 3 grasses is also more greatly impacted by elevated atmospheric CO 2 than is that of C 4 grasses; C 3 grasses produce greater amounts of nonstructural carbohydrates and have greater declines in their nitrogen content than do C 4 grasses under elevated CO 2 . Will C 3 grasses remain nutritionally superior to C 4 grasses under elevated CO 2 levels? We addressed this question by determining whether levels of protein in C 3 grasses decline to similar levels as in C 4 grasses, and whether total carbohydrate : protein ratios become similar in C 3 and C 4 grasses under elevated CO 2 . In addition, we tested the hypothesis that, among the nonstructural carbohydrates in C 3 grasses, levels of fructan respond most strongly to elevated CO 2 . Five C 3 and five C 4 grass species were grown from seed in outdoor opentop chambers at ambient (370 ppm) or elevated (740 ppm) CO 2 for 2 months. As expected, a significant increase in sugars, starch and fructan in the C 3 grasses under elevated CO 2 was associated with a significant reduction in their protein levels, while protein levels in most C 4 grasses were little affected by elevated CO 2 . However, this differential response of the two types of grasses was insufficient to reduce protein in C 3 grasses to the levels in C 4 grasses. Although levels of fructan in the C 3 grasses tripled under elevated CO 2 , the amounts produced remained relatively low, both in absolute terms and as a fraction of the total nonstructural carbohydrates in the C 3 grasses. We conclude that C 3 grasses will generally remain more nutritious than C 4 grasses at elevated CO 2 concentrations, having higher levels of protein, nonstructural carbohydrates, and water, but lower levels of fiber and toughness, and lower total carbohydrate : protein ratios than C 4 grasses.

Abstract-Relative growth rate and relative nitrogen accumulation rate for fifth-instar Spodoptera eridania larvae vary less than 20% on artificial diets in which protein content varies more than 250%, due to compensatory adjustments in consumption rate and changes in efficiencies of food and nitrogen utilization. The substitution of zein for two-thirds of the casein in a diet containing 26.0% protein results in a 25% decrease in both relative growth and nitrogen accumulation rates, due to reduced values of relative consumption rate, relative nitrogen consumption rate, approximate digestibility, approximate digestibility of nitrogen and efficiency of conversion of digested nitrogen. Although larval growth rate is relatively independent of diet nitrogen, larval composition is highly dependent upon both nitrogen quantity and quality. As diet nitrogen increases, larval nitrogen content increases and fat content decreases. Larvae on diets containing a mixture of casein and zein produce 3.53.9 times as much uric acid and respire at rates significantly higher than larvae on diets containing the same total amount of protein but lacking zein. Thus, there is a measurable metabolic cost associated with processing low quality protein. We conclude, however, that elevated metabolic rates of larvae on diets containing nutritionally unbalanced protein are not the cause of reduced growth rates. On these diets, growth is limited by an amino acid present in limiting quantities, and the elevated metabolic rate is due, at least in part, to increased synthesis of uric acid.Finally, we discuss the possibility that variation in amino acid profiles across host plant species might be a factor favouring specialization in insect herbivores.Key Word Index: Southern armyworm, Spodoptera eriaimia, growth, nutritional indices, food utilization, nitrogen budget, uric acid production, respiration rate, metabolic cost INTRODUCHONThe nutritional quality of a protein is a function ofThe importance of dietary nitrogen as a factor influits amino acid composition.The critical importance encing both the fitness of individual herbivorous of the amino acid composition of the diet to growth insects and the dynamics of insect populations has and reproduction in insects has been elegantly docubeen extensively documented (Slansky and Feeny, mented in studies of several species of aphids (Dadd and Krieger, 1968;Retnakaran and Beck, 1968; van 1977;McNeil1 and Southwood, 1978;Mattson, 1980;Emden, 1972; Carter and cole, lg77), adult Lee er al., 1983;Scriber, 1984). According to Scriber (1984), the literature contains "at least 115 different mosquitoes (Briegel, 1985), and larvae of the comstudies in which insect damage, growth, fecundity, or mercial silkworm moth Bombyx mori (Horie and numbers increased with increased plant nitrogen". In Inokuchi, 1978;Horie and Watanabe, 1983).the same review, however, Scriber notes that "at least Schroeder and Malmer (1980) (Rodriguez et 1970;Watson and Creaser, 1975; Yeoh and Watson, al., 1970;Hare, 1983;Horie and Watanab...

Feeding efficiencies of ultimate instar larvae of two polyphagous tree-feeding Lepidoptera, Malacosoma disstria (Lasiocampidae) and Orgyia leucostigma (Liparidae), were measured on artificial diets containing from 0% to 8% tannic acid. Relative growth rate (RGR) of O. leucostigma was not affected by up to 8% tannic acid, suggesting that O. leucostigma has evolved an effective counteradaptation to hydrolyzable tannins. In contrast, as little as 0.5% tannic acid caused a significant reduction in RGR of M. disstria, due both to reduced efficiency of conversion of digested food (ECD) and reduced relative consumption rate (RCR), and caused a significant increase in mortality during the pupal stage. Moreover, when reared from hatching on tannin-containing diets, no M. disstria larvae survived past the fourth instar.Although tannins are commonly referred to as "digestibility-reducing substances", tannic acid did not reduce the ability of M. disstria or O. leucostigma larvae to digest either the whole diet or nitrogen contained in the diet. For M. disstria, tannic acid acts as a toxin and a feeding deterrent, but not as a digestibility-reducing substance. Growing evidence that tannins commonly act as toxins warrants a reassessment of their role in anti-herbivore chemistry.

The evolution of host plant range is of both theoretical and economic importance, since present associations between insect herbivores and their host plants are largely attributable to past host shifts and host range expansions. Although the evolutionary lability of host plant range among herbivorous insects has long been recognized (Dethier, 1954; Ehrlich and Raven, 1964), the study of host range evolution among herbivorous insects remains essentially a post hoc science. However, as argued by Gould (1978, 1983), this need not be the case. For herbivorous insects with relatively immobile larvae, such as many Lepidoptera, colonization of a potential host plant may be precluded by the failure of females to recognize the potential host as a suitable oviposition site. However, if at least some females accept the potential host for oviposition, and if such variation is heritable, then the course oflocal host range evolution might reasonably be anticipated from knowledge of (1) the initial relative fitness of the herbivore on potential and existing

Extensive sympatry is currently arising between the common sulfur butterfly, Colias philodice Latreille (Lepidoptera: Pieridae) and a potential leguminous host plant, Coronilla varia (L.). In laboratory trials, larval surviviorship and growth were higher on the primary host, Medicago sativa (L.), than on the nonhost C. varia. However, because females reared from C. varia were on average more fecund than females reared from M. sativa, fitness on C. varia (approximately as survivorship times fecundity) was commensurate with fitness on M. sativa. Thus, it is predicted that selection would favor oviposition on C. varia, if such behavior were to arise. In addition, significant among-family variation exists for several measures of larval performance on both C. varia and M. sativa, indicating that C. philodice can potentially respond to selection for increased performance on each species. Moreover, larval performance was significantly positively correlated across these species, suggesting that selection for increased performance on each species will facilitate, not constrain, evolution of increased performance on the other. It is concluded that behavioral rather than physiological barriers currently account for the absence of C. philodice from C. varia and that, if such barriers are overcome, C. philodice will expand its host plant range to include C. varia.