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The major ecdysteroids of last-stage Oncopeltus fasciatus larvae were determined by HPLC/RIA analysis. An ecdysteroid other than ecdysone or 20-hydroxyecdysone, probably makisterone A, was the major ecdysteroid in hemolymph and in whole body extracts of 4- to 5-day-old larvae, when hemolymph ecdysteroid titers were at their peak. Little, if any, ecdysone or 20-hydroxyecdysone was detected in the hemolymph. Bioassay of the relative activities of ecdysone, 20-hydroxyecdysone, and makisterone A in last-stage larvae revealed that makisterone A was ten times more active than 20-hydroxyecdysone, and that ecdysone was inactive at doses as high as 1 mg/ml, its limit of solubility in 10% ethanol. Thus, it is suggested that makisterone A is the molting hormone in Oncopeltus. The implications of this discovery are discussed.

The ecdysteroid titers of insect growth regulator (IGR)-treated and untreated large milkweed bug nymphs, Oncopeltus fasciatus (Dallas) (Heteroptera: Lygaeidae), were determined by radioimmunoassay. All of the IGRs tested affected the ecdysteroid levels and the ecdysis of both sexes in some way. AI3-63604, a juvenile hormone mimic, accelerated ecdysteroid production and its subsequent decline, shortened the stadium, and induced a supernumerary nymphal molt. Diflubenzuron had no effect on the onset of ecdysteroid production but slightly retarded the decline in ecdysteroid titers before adult ecdysis, and the ecdysis was incomplete. Ecdysteroid production was delayed and erratic in nymphs treated with azadirachtin and AI3-63967, but the effect on ecdysis was different for each compound. Azadirachtin caused incomplete adult ecdysis, whereas AI3-63967 completely prevented any attempts at ecdysis. The large peak of exdysteroid activity associated with apolysis was absent in starved insects, and they made no attempt to ecdyse.