Absence of phytosterol dealkylation and identification of the major ecdysteroid as makisterone A in Dysdercus fasciatus (Heteroptera, Pyrrhocoridae)

The absence of phytosterol dealkylation in the cotton stainer bug, Dysdercus fasciatus, has been established and the major ecdysteroid in the fifth‐stage larvae identified. The demonstration that the free and esterified sterols in D fasciatus consisted of 95–96% sitosterol and 4–5% campesterol, a similar composition to the cottonseed diet, together with the lack of conversion of [14C]sitosterol into cholesterol, establishes that phytosterol dealkylation does not occur in this insect species. The ecdysteroid titer determined by radioimmunoassay in the fifth instar of D fasciatus shows a distinct peak at day 6, the instar lasting for 7 days. Makisterone A was purified by HPLC from insects at a time of high ecdysteroid titer and identified as a major component by both fast atom bombardment and electron impact mass spectrometry. Gas‐liquid chromatography/mass spectrometry (selected ion monitoring) confirmed the occurrence of makisterone A and revealed the presence of two unidentified compounds. One of these occurs in a similar amount to makisterone A and may be 26‐hydroxymakisterone A, whereas only a minute amount of the other compound, which may be 20‐deoxymakisterone A, was present; further identification of the latter compounds is necessary. C27 ecdysteroids (eg, ecdysone and 20‐hydroxyecdysone) and C29 ecdysteroids (eg, podecdysone A) were undetectable. The specificity of the enzymes of ecdysteroid biosynthesis is discussed.