The competence and necessity of chemosensory organs in food selection by Manduca sexta (Lepidoptera, Sphingidae) were studied by measuring food preferences of 5th instar larvae having different complements of chemosensory organs remaining after microsurgery. The following plant species were used in 2-choice food preference tests: the hosts Lycopersicon esculentum , Datura innoxia, Solanum pseudocapsicum, the acceptable non-host Raphanum sativus, and the unacceptable non-host, Pelargonium hortorum.
Antennal competence varied from competent (S. pseudocapsicum and P. hortorum) to partly competent (D. innoxia, L. esculentum) to incompetent (R. sativus). Antennae are partly needed (R. sativus) or not at all for normal (unimpaired) food selection. The maxillary palps are competent for L. esculentum and S. pseudocapsicum, partly competent for D. innoxia, and incompetent for the other plant species tested. Thus, antennae and maxillary palps may each be competent to mediate normal food selection for a particular plant species. However, their necessity in eliciting this behavior could not be demonstrated, except for S. pseudocapsicum where both are needed. Therefore, these chemosensory organs seem to play a minor role, if at all, in food selection by M. sexta larvae for the plants tested.
Interestingly, larvae having only maxillary palps remaining prefer four normally rejected plant species: R. sativus, Vigna sinensis, P. hortorum and Vinca rosea. Apparently, information from the maxillary palps may contain a qualitatively different message from that sent by the other chemosensory organs. Perhaps, for some plant species the maxillary palps inform M. sexta larvae only about ubiquitous plant-like characteristics.