Attraction of adult Harmonia axyridis to volatiles of the insectary plant Cnidium monnieri

Plant volatiles mediate the interactions within plant-insect communities and influence the distribution and behavior of insects. Natural enemies often utilize the volatiles that released from plants to locate the profitable patches for foraging. The lady beetle Harmonia axyridis (Pallas) is a dominant generalist predator of various pests in agriculture fields, yet little is known about the chemical cues which the predator employs to locate its prey. In this study, we tested the attractiveness of volatiles emanating from healthy and aphid-infested insectary plant Cnidium monnieri (L.) Cuss. to H. axyridis adults in a Y-tube olfactometer. Next, using coupled gas chromatography - electroantennogram detection (GC-EAD) and gas chromatography - mass spectrometry (GC-MS) identified the volatile components which elicited electrophysiological activity in H. axyridis adults. Lastly, Y-tube olfactometer tests in laboratory and field traps were used to verify the effectiveness of the volatile components. In olfactometer trials, H. axyridis adults preferred healthy and aphid-infested plants versus control. In GC-MS and GC-EAD trials, two antenna-active components (1,2-diethylbenzene and p-diethylbenzene) were detected among the volatile blends emanating from healthy and aphid-infested plants; and in ensuing behavioral assays, H. axyridis adults were attracted to two synthetic volatile compounds. Under field conditions, the number of H. axyridis adults as recorded in traps for two synthetic volatile components at three different concentrations (100 mg/ml, 10 mg/ml, and 1 mg/ml) were significantly higher than those for mineral oil (control). These findings could provide an effective means for the cultivation of insectary plants to develop natural enemy attractants and support the biological control in the field.