Evaluation of juvenoid insect growth regulators and a new non-steroidal ecdysone agonist on the American grasshopper, Schistocerca americana (Drury), and the desert locust, Schistocerca gregaria (Forskal) (Orthoptera: Acrididae).

Abstract

Three aromatic juvenoids (fenoxycarb, JH-286 and a new pyrazole analog) were evaluated for morphogenetic, chromotropic, morphometric and prothoracicotropic activities on the American grasshopper, Schistocerca americana (Drury), and their activities compared with those of commercial methoprene and synthetic JH-III. Fenoxycarb, the most active insect growth regulator examined, was found to induce a significant mortality in developing nymphs and caused severe morphogenetic damage during ecdysis to the adult. Juvenoid treatment were also observed to promote some overaged nymphs and to induce chromotropic changes in both the cuticle and hemolymph pigmentation. Except for a reduced larvicidal action, similar levels of activity were obtained when fenoxycarb was tested on the desert locust, Schistocerca gregaria (Forskal). The chromoproteins responsible for the green coloration characteristic of the hemolymph of S. americana and S. gregaria were isolated and characterized. Hemolymph from last instar nymphs and adults of S. americana was characterized by the presence of yellow lipophorin (Lp), a green chromoprotein (G-P), and two cyanoproteins (Cp-I and Cp-II). Treatment with fenoxycarb enhanced the concentration of the two cyanoproteins in nymphs and adults, and induced the appearance of a third cyanoprotein (Cp-F) in nymphs. Hemolymph from last instar nymphs of S. gregaria was characterized by the presence of yellow lipophorin (Lp) and two blue cyanoproteins (HCp and LCp), whereas only Lp and LCp were present in adults. Treatment with fenoxycarb increased the concentration of both cyanoproteins in nymphs, but only of LCp in adults. The non-steroidal ecdysone-agonist 2RH-5849 (1,2-dibenzoyl-1-tert-buthyl-hydrazine) was found to be an effective neuropoison when injected on S. americana. Treated grasshoppers became immediately hyperactive, followed by loss of coordination, paralysis and eventually death. We discovered that this insect growth regulator also induced bilateral autotomy of the methatoracic legs. However, no evidence of ecdysonergic or morphogenetic activities were found. Synergism studies with neurotoxins of known mode of action suggested that RH-5849 has a mechanism of action similar to that of 4-amino pyridine, which blocks potassium channels.