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

Abstract

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.