Abstract
We investigated the role that the ratio and concentration of ubiquitous plant volatiles play in providing host specificity for the diet specialist grape berry moth Paralobesia viteana (Clemens) in the process of locating its primary host plant Vitis sp. In the first flight tunnel experiment, using a previously identified attractive blend with seven common but essential components (“optimized blend”), we found that doubling the amount of six compounds singly [(E)- & (Z)-linalool oxides, nonanal, decanal, β-caryophyllene, or germacrene-D], while keeping the concentration of other compounds constant, significantly reduced female attraction (average 76% full and 59% partial upwind flight reduction) to the synthetic blends. However, doubling (E)-4,8-dimethyl 1,3,7-nonatriene had no effect on female response. In the second experiment, we manipulated the volatile profile more naturally by exposing clonal grapevines to Japanese beetle feeding. In the flight tunnel, foliar damage significantly reduced female landing on grape shoots by 72% and full upwind flight by 24%. The reduction was associated with two changes: (1) more than a two-fold increase in total amount of the seven essential volatile compounds, and (2) changes in their relative ratios. Compared to the optimized blend, synthetic blends mimicking the volatile ratio emitted by damaged grapevines resulted in an average of 87% and 32% reduction in full and partial upwind orientation, respectively, and the level of reduction was similar at both high and low doses. Taken together, these results demonstrate that the specificity of a ubiquitous volatile blend is determined, in part, by the ratio of key volatile compounds for this diet specialist. However, P. viteana was also able to accommodate significant variation in the ratio of some compounds as well as the concentration of the overall mixture. Such plasticity may be critical for phytophagous insects to successfully eavesdrop on variable host plant volatile signals.
Highlights
Understanding the mechanisms used by herbivorous insects to recognize and find suitable hosts has ecological, evolutionary and economic significance as herbivorous insects play vital roles in ecosystem functioning by mediating transfer of energy and nutrients [1], while competing with humans for food [2]
Since the doubling of single compounds may not be representative of the variation observed in nature, during the summer of 2009 we modified host plant volatile profiles by inflicting Japanese beetle damage on clonal V. riparia grapevines
It is possible that one day of feeding may not be enough to elicit maximal inducible responses of volatiles from grape shoots as shown in some other systems [39] and the natural variation in emission of some plant volatiles, such as green leaf volatiles, can be much higher than the variation observed in this study [40], the changes in ratio of key volatile constituents, within the relatively narrow range tested in our study, resulted in up to 87% reduction in female full upwind flight in the flight tunnel
Summary
Understanding the mechanisms used by herbivorous insects to recognize and find suitable hosts has ecological, evolutionary and economic significance as herbivorous insects play vital roles in ecosystem functioning by mediating transfer of energy and nutrients [1], while competing with humans for food [2]. Volatiles play a critical role in the evolution of host use by phytophagous arthropods [3] and the ability to efficiently eavesdrop on the host plant from a distance [4,5] has contributed to the success of these organisms [6]. Complicating matters further, volatile emissions from plants are not static but greatly vary due to a number of biotic and abiotic factors [9,10,11,12]. This raises the question of how phytophagous insects extract host specific information from the mixture of common plant volatiles that vary in both time and space. Phytophagous insects are not the intended receiver of the host volatiles, and may have been selected for the ability to eavesdrop on the volatiles for their own advantage, whether to colonize a suitable host or avoid an unsuitable one [20,21]
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