Dependence on host constituents controlling food acceptance by Manduca sexta larvae

The host range of Manduca sexta L. (Lepidoptera: Sphingidae) is restricted in nature to plants in the family Solanaceae. However, naive hatchling larvae often accept and continue their development on foliage from a wide spectrum of unrelated plants. In contrast, solanaceous‐experienced larvae refuse to feed on other plants. Experiments were designed to explore the role of constituents of various plants in this behavioral phenomenon. Fourth instar larvae reared on solanaceous hosts: tomato ( Lycopersicon esculentum Mill.), potato ( Solanum tuberosum L.), or tobacco ( Nicotiana tabacum L.), and the leguminous non‐host cowpea ( Vigna sinensis Savi), or on artificial diet based on wheat germ were tested for their acceptance or preference when offered a new diet in both choice and no‐choice situations. Under no‐choice conditions, acceptability of cowpea foliage was strongly affected by the larval dietary experience. Most larvae reared on solanaceous foliage did not feed, whereas those larvae reared on non‐solanaceous food readily accepted cowpea. Moreover, solanaceous foliage, leaf discs and extracts were readily accepted by larvae regardless of their dietary experience. Larvae reared on any of the solanaceous plants strongly preferred cowpea discs that were treated with solanaceous extract, while larvae reared on non‐solanaceous diets did not discriminate between treated and control discs. Assays of cowpea extracts as well as acceptance of cowpea discs treated with solanaceous extract indicated a lack of detectable deterrent in cowpea. Our results suggest that the mechanism for induced host specificity in M. sexta larvae involves development of a dependence on solanaceous chemical constituents. We argue that this dependence on host plant chemistry has adaptive significance.