Feeding essential oils and 2-heptanone in sugar syrup and liquid protein diets to honey bees (Apis mellifera L.) as potential Varroa mite (Varroa destructor) controls

Abstract

SummaryEssential oils were fed to honey bees to determine whether the components were absorbed into bee larvae. The oils were added in either sucrose syrup (origanum and 2-heptanone) or in a liquid protein diet (origanum oil, cinnamon oil, thymol, and 2-heptanone), because sugar and protein sources are differentially utilized in food fed to larvae of different ages. The volatiles emitted by isolated larvae from oil-supplemented colonies were sampled at three different ages (Day 4, Day 6 and Day 9) by Solid Phase Micro-Extraction (SPME) and analyzed by Gas Chromatography-Mass Spectrometry (GC-MS). The only oil volatiles recovered in larvae were the origanum oil components carvacrol and thymol; neither 2-heptanone nor the cinnamon oil components were detected in any larvae. For larvae from colonies fed oil- supplemented sugar syrup, carvacrol volatiles were emitted at higher amounts in younger larvae (Day 4) than in older larvae (Day 9). In contrast, carvacrol and thymol volatiles were detected only in older larvae (Day 6 and Day 9) in colonies reared on oil-supplemented liquid protein diet. Carvacrol was also detected in the cocoons of Day 9 larvae from colonies fed oil-supplemented liquid protein diet, but not oil-supplemented sugar syrup diet. We believe that the age-related differences in oil incorporation by bee larvae reflect the relative importance of supplemental sugars and protein as food sources for bee larvae of these ages. Supplementation in a liquid protein diet represents a more efficacious route for the incorporation of origanum oils in fifth instar bee larvae targeted for invasion by Varroa destructor mites.