Oleic acid emitted from frozen Trogoderma spp. larvae causes conspecific behavioral aversion

Accumulating evidence in the literature suggests that oleic acid functions as a necromone across widely divergent insect taxa. The prevelance of this phenomenon has not been fully explored, and its application to pest management remains underdeveloped. Khapra beetle (KB), Trogoderma granarium, is a pest of stored grains, with larvae that can enter facultative diapause and remain cryptic in warehouses. Here, we examine how death affects oleic acid content of Trogoderma spp. cuticular extracts, and whether the compound causes a behavioral response. To assess the generalizability of patterns, many experiments were repeated with warehouse beetle (WB), Trogoderma variabile, and larger cabinet beetle (LCB), Trogoderma inclusum. Extracts of larvae that were first killed by being frozen had greater oleic acid content than those derived from live insects. Two-choice behavioral assays compared responses of solvent controls to these extracts, at both low (~ 2 µg) and high (68–131 µg) oleic acid content. The natural extracts also contained cuticular hydrocarbons and other unidentified chemicals. High oleic acid in the extracts repelled the larvae of all three species. Lower levels of oleic acid did not affect KB and LCB movement, but were attractive to WB. We also performed the assay using a large range of doses of oleic acid alone. At the lower doses, oleic acid had no effect on movement, but it became strongly repellant at higher doses, beginning at 100 µg. These results indicate that necromones may be an overlooked aspect of stored product insect biology, which if further researched could improve pest management.