Costs of Inducible Defense: Protein Limitation, Growth, and Detoxification in Parsnip Webworms

Abstract

Although xanthotoxin, a linear furanocoumarin, is toxic to a wide variety of lepidopterans, the parsnip webworm, Depressaria pastinacella (Lepidoptera: Oecophoridae), is restricted to feeding on only two genera of Apiaceae, both of which contain large quantities of this toxin. The cost of cytochrome P450—mediated detoxification of xanthotoxin by this oligophagous species was examined in two artificial diet experiments. In the first experiment, growth rates, silkspinning rates, and detoxification capacity of larvae were measured after they had consumed diets ranging in protein content from 0 to 100% of the amount required for maximum growth. As protein became progressively more limited, growth rates declined progressively while silkspinning rates and detoxification capacity remained unchanged; only when dietary protein was completely absent did silkspinning rates and xanthotoxin metabolism rates decline. These findings suggest that xanthotoxin detoxification capacity is maintained at the expense of growth. In the second experiment, growth rates, silkspinning rates, and xanthotoxin metabolism were measured in larvae that consumed diets with or without xanthotoxin in the presence or absence of protein. As in the previous experiment, the absence of protein led to significant reductions in growth, silkspinning, and detoxification. The presence of xanthotoxin in the diet resulted in an increase in the rates of xanthotoxin metabolism and silkspinning but did not have any effect on growth rates. Inasmuch as there were no interactions between protein and xanthotoxin treatments, induction of xanthotoxin metabolism by xanthotoxin was not affected by protein limitation. The fact that xanthotoxin metabolism is induced almost threefold in the absence of dietary protein, conditions that result in reduction of growth rates by almost 80%, is also consistent with the greater importance of detoxification capacity relative to growth over the short term. Due to its highly oligophagous habits, the parsnip webworm is unlikely to encounter host plants tissue entirely lacking furanocoumarins; circumstances under which the inability to respond to the presence of xanthotoxin in the diet by increasing rates of its metabolism may never exist. Thus, maintaining high and inducible levels of cytochrome P450—mediated xanthotoxin metabolism may be regarded as one of the costs associated with the highly specialized diet of D. pastinacella.