Marta L. DEL CAMPO COURATIER & J. Alan. A. RENWICK.
Boyce Thomson Institute at Cornell University, Tower Road,
Ithaca NY 14853, USA
The host range of Manduca sexta (Lepidoptera: Sphyngidae) is restricted in nature to plants in the family Solanaceae. However, naive hatchling larvae often accept foliage from a wide spectrum of non-host plants, and they may even complete their development on them. On the other hand, experienced larvae that have fed on solanaceous plants refuse to feed on these non-host plants. This phenomenon may be due to an induced response to stimulants present in solanaceous plants, or to an increased sensitivity to deterrents in the non-host plants. Experiments were designed to explore the role of chemical constituents of various plants in this phenomenon. Fourth instar larvae reared on tomato (Lycopersicon esculentum), potato (Solanum tuberosum), tobacco (Nicotiana tabacum), the non-host cow pea (Vigna sinensis), and wheat germ artificial diet were tested for their acceptance or preference when offered a new diet in both choice and no-choice situations. Deterrent activity of cowpea foliage extracts was tested using potato leaf discs as a substrate. Similarly, stimulant activity was tested by applying potato foliage extracts to cowpea leaf discs. In both cases, larvae were offered untreated leaf discs as alternative choices. In no-choice situations, larvae were transferred from one diet to another. Cowpea extracts did not deter feeding by larvae that were reared on any of the diets. However, the acceptance of cowpea leaf discs treated with potato extracts was strongly affected by the larval dietary experience. Larvae reared on any of the solanaceous plants (potato, tomato or tobacco) strongly preferred the treated discs, while larvae reared on cowpea or wheat germ diet did not discriminate between treated and control discs. Moreover, in no-choice situations solanaceous foliage was readily accepted by larvae from all diets, whereas cowpea foliage acceptability was strongly affected by the larval dietary experience. Those larvae reared on solanaceous foliage did not feed on cowpea, even after 24 hours, while those larvae reared on cowpea or wheat germ diet readily accepted it. The reported deterrent effect of cowpea is questionable, since the extracts were not capable of reducing consumption of a preferred host. Our results suggest that addiction to host chemical constituents is the mechanism for induced preference for Solanaceae in M. sexta larvae. This fixation on host plant chemistry has adaptive significance. Addicted larvae are likely to stay and actively feed on the solanaceous plant that its mother has chosen. However, if a larva falls off or runs out of foliage, it would be capable of quickly identifying a suitable host, thus reducing the time of exposure to environmental hazards and avoidinz the selection of a toxic or unsuitable non-host.