Bacterial volatiles from mealybug honeydew exhibit kairomonal activity toward solitary endoparasitoid Anagyrus dactylopii

Anagyrus dactylopii (Howard) (Hymenoptera: Encyrtidae) is a solitary koinobiont endoparasitoid of grapevine mealybugs and has been reported to parasitize 70% of mealybug populations naturally. In the present study, we have isolated and identified by employing a 16S rRNA technique, a total of ten cultivable bacteria from the honeydew of two species of grapevine mealybugs, Maconellicoccus hirsutus and Nipaecoccus viridis . In the honeydew of M. hirsutus, seven bacteria were found, which included Micrococcus luteus , Kocuria rosea , Bacillus aquimaris , Exiguobacterium aquaticum , Staphylococcus pasteuri , Bacillus oceanisediminis and Bacillus flexus . From the honeydew of N. viridis , three bacteria, viz. Bacillus firmus , Microbacterium testaceum and Pesudomonas oryzihabitans, could be recovered. Further, we have collected the headspace from 72 h active cultures of these honeydew-associated bacteria by sorption over adsorbent trap made up of 0.2 g porapak using dynamic headspace sampling technique with pull and push system and tested the behavioral responses of mated females of A. dactylopii to these volatiles in a Y-tube olfactometer. The mated females positively responded to volatiles from five bacteria, which included only two bacteria associated with the honeydew of M. hirsutus, i.e., K. rosea , and S. pasteuri and all the three bacteria from honeydew of N. viridis . This indicates an ability of A . dactylopii to exploit cues associated with the mealybug honeydew for host location. The GC–MS analysis of bacterial headspace revealed six volatile organic compounds (VOCs), viz. propanoic acid, 2-methyl-2,2-dimethyl-1-(2-hydroxy-1-methylethyl) propyl ester; tetradecane; dodecane; 10-heptadecen-8-ynoic acid, methyl ester; 10,13-octadecadiynoic acid, methyl ester and limonen-6-ol, pivalate. Interestingly, limonen-6-ol, pivalate has been found as a common constituent of volatiles from all the three bacteria that were isolated from honeydew of N. viridis . The possible exploitation of bacterial VOCs in enhancing populations of A. dactylopii and promoting conservation biological control of mealybugs in grape vineyards is discussed.