The pharmacology of Microbracon venom.

1. Wasps of the genus Microbracon generally paralyse larvae of Lepidoptera. The wasp larva is exophagous on the paralysed host. The venom acts slowly and causes a flaccid paralysis. The venom of M. hebetor is extremely active in Lepidoptera, less active in honeybee workers, much less active in locusts and mealworms and probably inactive in non-insects. 2. In Lepidoptera and in locusts the venom presynaptically blocks the excitatory, but not the inhibitory, neuromuscular transmission. M. hebetor venom contains two high mol. wt toxins. A-MTX and B-MTX, with essentially the same action on neuromuscular transmission as the crude venom. Differences in dose-response curves of the two toxins, as well as differences in sensitivity to the two toxins between insect species are described. In wax moth larvae both A-MTX and B-MTX cause a slow and transient paralysis with a maximal effect after about 20 hr. The rate of recovery is dose-dependently decreased by the toxins.