Behavioral Responses of the Parasitoid Wasp Lytopylus rufipes to Herbivore-Induced Plant Volatiles Released from Pear Leaves

概要

This study focuses on how the volatiles of pear shoots affect the tri-trophic interaction system among Pyrus pyrifolia var. Kosui (pear), Grapholita molesta (herbivore), and Lytopylus rufipes (parasitic wasp) pear. Since there was no practical method to rear L. rufipes, a transitional diet system was first created to establish a L. rufipes colony in our laboratory. In this diet system, a sliced apple fruit is infested with neonate larvae of G. molesta and then used for triggering the oviposition behavior of L. rufipes. After parasitism, the apple slices are placed on artificial diet (Silkmate 2M) for further rearing. Herbivore-induced plant volatiles (HIPVs) are usually used by natural enemies to search for their hosts. However, there is no information on how pear HIPVs affect insect communities in pear orchards. A 4-arm olfactometer was used to observe the behavioral responses of L. rufipes to different odor sources. By comparing odors of intact and host-infested shoots of pear, L. rufipes females were found to prefer the host-infested shoots to intact ones. This result indicates that L. rufipes females can evaluate infestations through volatile cues.

To clarify the HIPVs that attract L. rufipes females, odors of both intact and hostinfested shoots were collected and identified by gas chromatography-mass spectrometry (GC-MS). Totally, 5 major components were identified, including (Z)-3- hexenyl acetate (H), (E)-β-ocimene (O), linalool (L), (E)-4,8-dimethyl-1,3,7- nonatriene (D), and (E, E)-β-farnesene (F). When I compared individual components were compared against solvent controls, only O and D were preferred by L. rufipes females. Further bioassays were conducted to understand the perception of L. rufipes females for different combinations of these volatiles. Results showed that L. rufipes females responded differently to volatile blends. Female wasps recognized the induced volatile F when other components were present. Furthermore, the quinary blend (HOLDF) was most preferred by L. rufipes females than other volatile blends. These results indicate that L. rufipes females need the whole volatile blend rather than blends of fewer components or individual chemicals.

In order to understand if the pear HIPVs affected the behavior of G. molesta in the field, the oviposition preference bioassays were performed with an acrylic cylinder with a dual choice design. As a result, G. molesta females preferred to lay eggs near the pear shoots rather than the leaves. The volatile emissions from shoots was higher than leaves, indicating that female moths can recognize different part of host plant via volatile cues. Moreover, G. molesta females laid more eggs near hostinfested shoots than intact shoots, suggesting that female moths may be attracted by HIPVs released from infested shoots. Subsequent bioassays between the quinary blend and solvent control demonstrated that female moths preferred to lay eggs in volatile treated areas rather than control areas.

This research provides basic information on the tri-trophic interaction system in pear orchards. However, some questions still remain such as the seasonal fluctuation of volatile emission from pear plants and the attraction of female moths to HIPVs from infested plant that should be considered for practical applications. Also, the quinary volatile blend should be tested in a pear orchard.