| Varroa destructor is an obligate ectoparasite of honey bees that shifted from its original host Apis cerana to the new host A.mellifera,in the first half of the twentieth century.This host shift has resulted in a significant threat to the health and survival of A.mellifera colonies worldwide.Currently,there are no green and efficient acaricides available for the control of V.destructor.Chemical signals play a crucial role in all aspects of the V.destructor life cycle,including host finding.However,our knowledge of the role of host-derived chemical signals in the host-finding behavior of V.destructor is still very limited.In this study,we systematically profiled the cuticular volatiles of both worker and drone larvae of the two honey bee species using solid phase micro-extraction coupled with gas chromatography-mass spectrometry(SPME-GC-MS).We then utilized electrophysiological recording and a Y-tube olfactometer bioassay to study the potential roles of the selected compounds.The main results obtained here are as follows:1.In total,63 cuticular volatiles of honeybee larvae were identified in this study,including 17 in worker larvae of A.cerana cerana(Acc),20 in drone larvae of Acc,and 36 in both worker and drone larvae of A.mellifera ligustica(Aml).Among these cuticular volatiles were four aliphatic esters:ethyl myristate(EM),methyl palmitate(MP),ethyl palmitate(EP)and ethyl oleate(EO),which were commonly detected in all four types of larval hosts.Additonally,the honey bee tail-swing pheromone component(tricosane)was detected in Aml worker and drone larvae,and the hunger pheromone component(ocimene)was only detected in Aml worker larvae.2.To compare the difference in content of each component among worker/drone larvae from different bee species,we found that Aml drone larvae displayed a significantly higher level of EM than Acc drone larvae,while the level of EM was not significantly different between the two types of worker bee larvae.The levels of MP in Aml worker were significantly higher than those in Acc worker larvae,whereas MP did not significantly differ between the two types of drone larvae.Acc worker larvae displayed significantly higher levels of EP and EO than Aml worker larvae,while the levels of EP and EO did not significantly differ between two types of drone larvae.3.To compare the difference in content of each component between the worker and drone larvae of the same bee species,we found that the levels of EM were significantly higher in Aml drone larvae than in their worker larvae,while there was no significant difference in Acc.The levels of MP were significantly higher in Aml drone larvae than in their worker larvae,and were significantly higher in Acc drone larvae than in their worker larvae.The levels of EP were significantly higher in Aml drone larvae than in their worker larvae,while there was no difference between worker and drone larvae in Acc.The levels of EO were significantly higher in Aml drone larvae than in their worker larvae,while there was no difference between worker larvae and drone larvae in Acc.4.The electrophysiological response of Varroa olfactory organ to four common fatty acid esters and two pheromone components was analyzed using electrophysiological recording technology.The experimental results showed that(1)EM: V.destructor responded to doses of 0.1 μg and 1 μg,but not responed to 100 μg.(2)MP: V.destructor responded to doses of 0.1μg,10 μg,100 μg and 1000 μg,but not to 1 μg.(3)EP: V.destructor responded to doses of 1 μg,10 μg,100 μg and 1000 μg,but not to 0.1 μg.(4)EO: V.destructor responded to doses of 1 μg,100 μg and 1000 μg,but not to doses of 0.1 μg and 10 μg.(5): V.destructor did not respond to tricosane and ocimene at five different doses.5.The Y-tube olfactometer bioassay was used to detect the behavioral choice of V.destructor regarding four common fatty acid esters and two pheromone components.The experimental results showed that(1)EM: V.destructor was attracted at a dosage of 10 μg but was repelled the parasites at 1000 μg.(2)MP: V.destructor had no preference at any of the five dosages.(3)EP: V.destructor was repelled at dosages of 10 μg and 100 μg.(4)EO: V.destructor was repelled at dosages of 0.1 μg,10 μg,100 μg and1000 μg.(5): tricosane and ocimene repelled V.destructor at a dosage of1000 μg.In conclusion,this study identified and compared the cuticular volatiles of Acc and Aml larvae.We found that A.m.ligustica larvae have richer cuticular volatiles than Acc larvae.Additionally,EM was identified for the first time in this study and was found to attract V.destructor at moderate concentrations,but repell them at high dosages.Therefore,cuticular volatiles like EM may provide candidate targets for the study of chemical signal exchange mechanisms between V.destructor and its hosts,as well as a new strategy for the development of safe and efficient V.destructor treatment measures. |
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