Evaluating plant volatiles for monitoring natural enemies in apple, pear and walnut orchards

The ability to estimate natural enemy abundance is crucial to the integration of biological control into IPM programs. Traditional sampling approaches for natural enemies are few and most are inefficient, but recent studies suggest attraction of natural enemies to plant volatiles may be a useful proxy for direct sampling. We evaluated various combinations of herbivore-induced plant volatiles and floral volatiles as monitoring tools for natural enemies found in apple, pear, and walnut orchards in California, Oregon, and Washington. In 2010 we used a full factorial experimental designs to evaluate lures for all combinations of acetic acid (AA), acetophenone (AP), phenylacetaldehyde (PAA) and 2-phenylethanol (PE). Of nine natural enemy taxa analyzed, we found syrphid flies responded strongly to PE, but combining AA with PE attenuated trap catch and combining PAA to PE eliminated the activity of PE. Chrysoperla spp. (Chrysopidae) responded strongly to most of the individual compounds and the various interactions between the components allowed multiple ways to achieve roughly the same trap catch. All of the hymenopteran taxa collected responded strongly to PAA, and PAA containing lures were nearly always a component of the top eight lures. A smaller factorial experiment testing all possible combinations of AA, PAA and methyl salicylate (MS) showed that single component AA or MS lures were generally not significantly different than the controls for all taxa tested, but for the hymenopteran taxa, traps baited with MS+PAA performed the best or were not significantly different than the best performing lure. A 2011 trial was conducted to test the influence of the addition of AA and/or MS on previously tested lures. Combining AA or MS with other lures, improved the capture of Chrysoperla spp.; Scaeva pyrastri (L.) (Syrphidae) capture was enhanced when MS was used with PE; and PE was attractive to the three syrphid flies, Chrysoperla spp., and the parasitoid Aphelinus mali (Haldeman) (Aphelinidae). The differential responses to various blends exhibited among taxa show that combinations of plant volatiles can be chosen to increase specificity of attraction to a few taxa or increase the number of species attracted. This flexibility should add to the general value and breadth of use of plant volatile monitoring lures.