The effects of quantity and quality of diet nitrogen on the growth, efficiency of food utilization, nitrogen budget, and metabolic rate of fifth-instar Spodoptera eridania larvae (Lepidoptera: Noctuidae)

Abstract

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...

The Effects of Climate Warming on Plant-Herbivore Interactions

“…Protein repair and cellular maintenance occupy a larger share of the overall N budget at high temperatures, yielding less excess N available for growth after accounting for basic cellular processes . Indeed, respiration and metabolic rates were highest for caterpillars reared on low-N diets arising from the greater difficulty of processing diets rich in non-labile carbohydrates (Martin & Van't Hof 1988;Karowe & Martin 1989). However, increased cellular respiration rates do not generally translate into increased metabolic N turnover.…”

“…We created three diets of varying quality by replacing casein in the standard lepidopteran diet with non-digestible cellulose to maintain constant water content. This is a standard method of generating artificial diets, and previous work demonstrated that N assimilation efficiency does not vary with casein concentrations in this species (Karowe & Martin 1989). We determined diet N concentrations of each diet via elemental analysis using 8 replicate subsamples from each diet (FlashEA 1112 Series CHN analyzer, Thermo Electron Corporation; Table 3.1).…”

“…Insects package metabolized N into uric acid and excrete it along with undigested material in a single frass pellet. We determined uric acid concentrations in the frass of each caterpillar using a modified spectrophotometric analysis (Bhattacharya & Waldbauer 1969;Martin & Van't Hof 1988;Karowe & Martin 1989). For each sample, we combined ~ 5 mg dry frass powder from each sample with 1 mL 0.6% lithium carbonate solution in 1.5 mL microcentrifuge tubes.…”

“…Spodoptera exigua growth and performance are jointly controlled by both diet quality and temperature (Lee & Roh 2010). The physiological basis for such temperature-diet interactions remains unknown, despite the fact that metabolic N demand of its congener, S. eridania, has previously been described in detail (Karowe & Martin 1989). We used laboratory feeding assays to test three hypotheses.…”

“…Finally, a greater portion of frass N excretion at high temperatures would be comprised of N bound in uric acid at high temperatures. Increased production of uric acid, which contains metabolically processed N, often indicates more rapid higher N metabolism and turnover (Martin & Van't Hof 1988;Karowe & Martin 1989). Our results demonstrate that rising temperatures do induce N limitation in an ectothermic herbivore by increasing metabolic N demand, and that S. exigua offsets this increase metabolic demand by drastically increasing consumption rates of low-quality diets.…”