Azadirachtin potentiates the action of ecdysteroid agonist RH-2485 in Spodoptera littoralis

Edysteroid agonist RH-2485 induces an immediate and fatal molt in Spodopter a littoralis when added to the diet of the 2nd and 4th instar larvae at 1 p pm, and to that of the 6th instar larvae at 0.001 ppm concentration. Ten ti mes lower doses fed to the larvae continuously allow an apparently normal l arval development that is terminated by a supernumerary larval molt. The ot her effects of RH-2485 include death during metamorphosis and impaired fert ility of emerged adults. The number of progeny is reduced even with low RH- 2485 doses that do not interfere with moltings; e.g., insects fed 0.0001 pp m since the 2nd, 4th, and 6th instar produce 72%, 62%, and 22%, respectivel y, less progeny than the controls. Feeding larvae with 10-1000 ppm Suneem o il (containing about 0.1-10 ppm azadirachtin) causes, in a stage- and dose- dependent manner, a cessation or reduction of feeding, delay of molts, deat h of larvae and pupae, and sterility of emerged adults; with 10 ppm Suneem oil, the number of progeny is reduced by 20-32%. Presence of Suneem oil in the diet does not influence the potential of RH-2485 to induce a prompt mol t, but it increases ten times the potency to elicit a supernumerary larval molt. Certain combinations of RH-2485 with Suneem oil provoke up to 3 extra larval molts. Lethal developmental derangements and sterility are more fre quent, and the response of larvae off different age is more uniform, when S uneem oil and RH-2485 are combined than when each of these agents is admini stered alone. (C) 2000 Elsevier Science Ltd. All rights reserved.