Submitted to: Journal of Neuroscience
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: April 1, 1998
Publication Date: N/A
Interpretive Summary: Many insect pests are parasitized by wasps or flies which cause developmental arrest in their respective hosts. Edovum comstockii is an external wasp parasite which attacks the larval stage of the bollworm, fall armyworm, and cabbage looper. Its venom prevents molting in the larval stage which is parasitized. Interestingly, Edovum venom can also induce developmental arrest in insect pests which are not normally parasitized in nature. These include the European corn borer, southwestern corn borer, diamondback moth, and tobacco budworm. Thus, the potent and versatile Edovum venom is a potential biocontrol agent which could be utilized to control a variety of insect pest species. To capitalize on the venom's ability to inhibit molting, it is necessary to understand its mechanism(s) of action. Results of our investigations show that the action of the venom is multifaceted. In the European corn borer, the venom can inhibit the production/release of molting hormone and/or prevent shedding of the old cuticle, depending upon the stage envenomated. In addition, the venom can alter the metabolism of molting hormone to produce inactive analogues. Our findings show that Euplectrus venom could be a powerful weapon for controlling corn borers and provide necessary information regarding its mode(s) of action.
Technical Abstract: Euplectrus comstockii Howard (Hymenoptera: Eulophidae), is an ectoparasitic, gregarious wasp which parasitizes the larval stage of several important lepidopteran pests. Parasitization of both natural and unnatural hosts prevents molting in the parasitized instar. Here we report the effect of wasp venom on the European corn borer (unnatural host), an important pest of corn and other vegetables. Venom injected into O. nubilalis 5th instars, inhibited growth, development and molting of the injected larvae. The observed effect on molting was dose and age dependent. When 3rd, 4th and 5th instar O. nubilalis were envenomated by adult wasps, the larvae also were developmentally arrested and failed to undergo a molt. However, 3rd and 4th instars underwent apolysis (separation of the epidermis from the old cuticle) and produced new cuticle. Fifth instars did not. A premolt hemolymph ecdysteroid peak was not observed in these experimental 5th instars, but injections of 20-hydroxyecdysone induced apolysis and new cuticle formation. Envenomated 4th instars exhibited a premolt hemolymph ecdysteroid peak. HPLC/RIA revealed that 20-hydroxyecdysone was present in the hemolymph of these pharate 5th instars. Thus, in the European corn borer, the mode of action of the venom depended upon the instar parasitized. Our results support the presence of a regulatory mechanism in day-3 5th instars which influences the ability of the brain-prothoracic gland axis to function in the production of ecdysteroid.