Feeding and Growth Responses of Laboratory and Field Strains of Velvetbean Caterpillars (Lepidoptera: Noctuidae) to Food Nutrient Level and Allelochemicals

The performance of Anticarsia gemmatalis (Hubner) larvae from a laboratory colony in existence for > 100 generations was compared in several experiments with that of larvae from the field (first generation). In general, field-strain larvae exhibited prolonged development and a lower biomass-relative growth rate (RGR) when fed an artificial diet because of their slower biomass-relative consumption rate (RCR), compared with laboratory-strain larvae. In contrast, laboratory-strain larvae grew more slowly than field-strain larvae when both were fed foliage of the wild legume Indigofera hirsuta L., a larval foodplant of A. gemmatalis in the field. This slower growth rate was associated with a reduced efficiency of conversion of digested food to biomass (ECD); the RCR of laboratory- and field-strain larvae fed foliage did not differ significantly. When fed artificial diets with progressively reduced nutrient levels (as a percentage of fresh mass, fm), larvae of both strains compensated Similarly by consuming more food (fm) at a faster rate (RCRfrn). Nonetheless, total nutrient intake, biomass gain, pupal dry mass (percent frn), and pupal lipid content (percent dry mass, dm) for both strains declined on the diet with the lowest nutrient level. Nutrient utilization efficiencies also changed on the most diluted diet; for both strains, approximate digestibility of nutrients (ADnu) increased and ECDnu decreased. When fed artificial diet containing either the phenylcoumarin isoflavonoid, coumestrol (an ‘evolutionarily familiar’ allelochemical to A. gemmatalis ), or the methylxanthine alkaloid, caffeine (an ‘evolutionarily novel’ compound to this species), significantly higher mortality occurred among field-strain larvae. Whether this difference was caused by their inherently greater sensitivity to these allelochemicals compared with the laboratory-strain larvae, or to the synergistic effect of the additional stress imposed on them from consuming the artificial diet, was not determined. For field-strain larvae that survived on allelochemical-amended diets, differences in their consumption and utilization of food, compared with laboratory-strain larvae, were those associated with consuming artificial diet and did not reflect inters train differences in the effect of the allelochemicals. Coumestrol had no deleterious effect on larvae of either strain surviving to the prepupal stage, even though it was tested at a concentration (0.5% frn) ≈ 10-fold greater than it naturally occurs in foliage. In contrast, caffeine, added to the diet at a naturally occurring concentration (0.1% frn), prolonged development and reduced biomass gain, RGR, ECD, and pupal lipid (% dm) of larvae from both strains. Pupal dry mass and lipid content were generally higher in the field strain. These results suggest that the use of insects from the laboratory colony to study feeding responses to diet nutrient level is justified (in terms of the similarity of responses to the field strain), but that conclusions concerning their sensitivity to allelochemicals should be applied to field populations with caution. Use of an artificial diet to evaluate effects of xenobiotics on field-strain insects may give misleading results if the insects develop poorly on the control diet.