Inhibition of spiracle and crochet formation and juvenile hormone activity of isothiocyanate derivatives in the tobacco hornworm, Manduca sexta

The structure-activity relationship of various aliphatic, alicyclic, and aromatic mono- and bisisothiocyanates affecting the growth and development of the tobacco hornworm, Manduca sexta, larva was studied. While topical application of short-chain aliphatic monoisothiocyanates to thirdinstar M. sexta larvae did not affect larval development, aromatic mono- and diisothiocyanates were toxic within the dose range tested. Short-chain alkylene and alicyclic 1,3- or 1,4-bisisothiocyanate derivatives, however, prevented the formation of abdominal spiracles and of crochets on the abdominal prolegs at molts following treatment. Their ED₅₀ values for causing spiracular abnormalities ranged from 25 to 208 nmol/larva. Moreover, long-chain mono- or bisisothiocyanates with structural resemblance to known juvenoids had juvenile hormone activity in the black M. sexta assay (their ED₅₀ range from 0.12 to 4.8 nmol/larva) and, in some cases, the formation of supernumerary sixth-instar larvae was observed. Surprisingly, nonjuvenoidlike bisisothiocyanates derived from α-amino acids ornithine and lysine also showed charateristic juvenile hormone effect in this test system (the ED₅₀ values are 0.11 and 0.34 nmol/larva, respectively). The structural requirements for the two different types of activities as well as hypotheses about the possible mode of actions are also discussed.