Ontogenetic and Environmentally Induced Changes in Cuticular Hydrocarbons of Oryzaephilus surinamensis (Coleoptera: Cucujidae)

Abstract

The influence of short-term environmental challenges such as desiccation and temperature extremes on the cuticular hydrocarbon composition of insects is not well understood. We report here the effects of chilling (4°C), exposure to moderate relative humidity (≈30%), and exposure to dusts of silica gel, diatomaceous earth, and freeze-dried, ice-nucleating bacteria ( Pseudomonas syringae ) on the cuticular hydrocarbons of larvae and adults of the sawtoothed grain beetle, Oryzaephilus surinamensis (L.), a major pest of stored grain. Larvae and adult beetles have some hydrocarbons in common (particularly n-alkanes and small quantities of internal methyl branched alkanes), but they differ substantially in alkene composition. Unstressed larvae have ≈4% of Z-(9)-alkenes (C25 to C31) and 1% of a homologous series of Z,Z-(6,9)-dienes of the same chain lengths. Adult beetles, however, in unstressed situations have none of the larval dienes on their cuticle, and possess only ≈3% of the Z-(9)-monoenes. When the adult beetles were environmentally stressed for 24 h, they released up to 3% of phenotypically larval dienes onto their cuticle. In addition, desiccation stresses resulted in adults substantially increasing the quantity of monoenes on their cuticle (up to 5% of the total), whereas low temperature caused them to reduce the quantity of monoenes. Larvae exposed to similar stresses did not increase the quantity of dienes on their cuticle, but rather decreased their alkenes and increased the abundance of alkanes. The release of the dienes by the adults was shown not to be from new biosynthesis, but rather to result from release of stored dienes. The physiological and ecological ramifications of these changes in cuticular hydrocarbon profile by the various life stages of the sawtoothed grain beetle in response to environmental stresses are discussed.