Unique pathways of sterol metabolism in the Mexican bean beetle, a plant‐feeding insect

Abstract

Radiolabeled sterols,¹⁴C‐cholesterol,¹⁴C‐cholestanol,³H‐stigmasterol,³H‐stigmastanol, and³H‐sitosterol, were fed to larvae of the Mexican bean beetle,Epilachna varivestis Mulsant, by coating soybean leaves. Free sterol and sterol ester fractions from treated insects were isolated and analyzed, and in each case nearly 30% or more of total radiolabeled sterols retained by the insect were found in the sterol ester fraction after 8 days. Δ⁵ sterols were readily reduced to stanols, and C₂₉‐stanols thus produced were dealkylated to cholestanol. Significant amounts of labeled lathosterol were formed from Δ⁵‐C₂₉ sterols; little, if any, radiolabeled cholesterol was detected in insects fed either of the labeled Δ⁵‐phytosterols, stigmasterol or sitosterol. Sterol metabolism of this insect thus differs considerably from that found for most phytophagous insects.