Differential Sterol Utilization by Larvae of Sitophilus oryzae and Sitophilus granarius1,2

Abstract

The utilization of dietary sterols by the symbiotic larvae of the rice weevil, Sitophilus oryzae (L.), and the asymbiotic larvae of the granary weevil, S. granarius (L.), was compared. Larvae of both species developed on diets containing 0.005% (wt/wt) of cholesterol; however, the rate of development increased as the concentration of cholesterol was increased to 0.1%. Survival to adult was independent of the cholesterol concentration once the diets contained a threshold amount, ≤0.005% for S. granarius and between 0.005–0.01% for S. oryzae. Larvae of S. oryzae utilized I²-sitosterol, ergosterol, cholesterol, cholesterol acetate, and 7-dehydrocholesterol equally well and growth on diets containing cholestanol was delayed only slightly. Larvae of S. granarius utilized β-sitosterol equally as well as cholesterol. However, growth on diets containing cholesterol acetate, ergosterol, and 7-dehydrocholesterol was delayed by ca. 3 days. Growth of S. granarius on diets which contained cholestanol was much slower and only 63% reached the adult stage. Attempts to determine whether aposymbiotic larvae of S. oryzae could utilize cholestanol were inconclusive, but the mycetomal microorganisms in S. oryzae apparently can modify different sterols to the benefit of the host insects; in particular, they can assist in the utilization of the fully saturated sterol, cholestanol, a sterol which the asymbiotic larvae of S. granarius can develop on only poorly.