The Role of Honey Bee Derived Aliphatic Esters in the Host-Finding Behavior of Varroa destructor

<i>Varroa destructor</i> is an obligate ectoparasite of honey bees and shifted from its original host <i>Apis cerana</i> to the new host <i>Apis mellifera</i> in the first half of the twentieth century. The host shift has resulted in a great threat to the health and survival of <i>A. mellifera</i> colonies worldwide. Chemical signals play a crucial role in all aspects of the <i>Varroa</i> life cycle, including host finding. However, the chemical cues that affect the host finding behavior of <i>Varroa</i> mites are still not fully understood. In this study, we systematically profiled the headspace volatiles of both worker and drone larvae of the two honey bee species by using solid phase micro-extraction coupled to gas chromatography-mass spectrometry (SPME-GC-MS), and then used electrophysiological recording and Y-tube olfactometer bioassay to study the potential roles of the selected compounds. The chemical profiling showed that there were four aliphatic esters, ethyl myristate (EM), methyl palmitate (MP), ethyl palmitate (EP), and ethyl oleate (EO) commonly detected from all four types of larval hosts. Among them, EM was a new substance identified from honey bee headspace volatiles. Results from electrophysiological recordings indicated that all the aliphatic esters could elicit significant responses of <i>Varroa</i> pit organs on its forelegs. Moreover, behavioral analyses revealed that EM could significantly attract <i>V. destructor</i> at a medium dosage (10 µg), while MP had no observable effect on the mites and both EP and EO were able to repel the parasites. Our findings suggest an important role of host-derived aliphatic esters in <i>Varroa</i> host finding, and provide new chemicals for <i>Varroa</i> monitoring and control.