Whereas it is generally agreed that modern agriculture needs a sustainability overhaul, the best trajectory to sustainable production is less clear and progress in sustainable innovation is slow. Today, control of pests and diseases still relies heavily on cover sprays. Innovations are sorely needed that selectively target pests and reduce or eliminate cover sprays, minimising the impact on an already dwindling insect community2,24. Odor-based methods offer this perspective through selectively attracting or confusing target insects. Lures laced with attractants, phagostimulants and small amounts of insecticides can selectively target pest species, while their specificity avoids bycatches from the food web. Unfortunately, bycatches are not consistently reported, which makes evaluation of lures in terms of sustainable control of pests difficult. Although lures have been reported for numerous pest insects, they may be broadly attractive and similarly to insecticides, impact non-target species.
In our study, we empirically evaluated the effect of ratio, release rates and composition of a lure16 on capture of L. botrana and analysed the concurrent effect on specificity. The lures attracted both male and female L. botrana, and could be used to support pheromone-based intervention methods. Aiming to further increase the effectiveness of the lure, we found that such increases may come at the expense of specificity of the lure. Increased release rates and blend complexity strongly decreased specificity, while not always increasing attractiveness to L. botrana. There thus appears to be a tradeoff between attractiveness and specificity, and the ‘most attractive’ in terms of total catch may not necessarily be the ‘most attractive’ in terms of specificity and thus sustainability.
A two-component blend alone can selectively attract L. botrana
Based on previous work with AA and 2-PET, where considerable numbers of L. botrana were caught16, we assessed whether the lure’s attractiveness could be further enhanced by changing the release rate and ratio of AA and 2-PET. Both 2-PET and AA appeared to be necessary for capturing L. botrana. The two compounds synergize with each other, reminiscent of components in a pheromone blend, where frequently small amounts are necessary and sufficient to increase attraction25. However, in another study in Hungary, 2-PET did not synergize with AA25, although the authors used a much higher dose of AA (3000 mg instead of 500 mg) and another dispensing technique, making the results hard to compare with our study. It is rather surprising that a lure consisting of so few and such generic fermentation volatiles can be so selective. Acetic acid is a common fermentation volatile and indeed a constituent of lures for diverse insect taxa, including flies, moths, lacewings and wasps26,27,28,29,30,31. 2-PET is another rather general microbial volatile. It indicates the breakdown of phenylalanine and thus a protein source, with similar or derived compounds attracting various insect taxa25,29,32,33. That a combination of these two can be selective, may indicate that even though insects commonly rely on fermentation volatiles for adult feeding, the olfactory circuitry of different species key into different components in orientation. This is supported by recent work on tephritid fruit flies34, where an ecological niche-driven divergence in the detection of fruit volatiles was measured, in spite of these sources generically being attractive to all species tested.
Besides, differential tuning to fermentation volatiles, the high selectivity of the 2-component blend to L. botrana (Fig. 6) may also result from its dominant presence in the vineyard, whereas selectively would be much lower in situations where this is not the case. The fact that in early season catches (1st and 2nd flight) selectivity was dramatically lower, underlines this. Claims about a lure’s selectivity thus need verification throughout the flight season and possibly in different geographical areas.
A higher load of AA increased capture rates of L. botrana while higher 2-PET loads increased capture rates of lacewings. This underlines that research should not solely focus on increasing capture rates of the target insect species, but carefully balance ratio, load and composition to reduce bycatches.
Other fermentation volatiles lacked synergy, and decreased specificity of AA and 2-PET
As a blend consisting of only AA and 2-PET is far removed from a fermentation volatile mimic, we reasoned that addition of other fermenting volatiles could perhaps synergize the 2-component blend. Numerous reports have shown fermentation-based blends with quite different constituents, though often containing AA, ethyl acetate and primary alcohols, as being attractive to other insect taxa26,35. Among moth species, leafrollers have received considerable attention with studies on Archips, Cydia, Pandemis, Spilonota, Epiphyas and Choristoneura spp.36,37. Attractants comprised both constitutive plant compounds such as pear ester and induced volatiles released upon feeding damage by leafroller larvae, such as 2-PET, benzyl alcohol and benzyl cyanide. However, plant volatiles only significantly attracted when combined with AA36,38.
Surprisingly, however, in our study none of the fermentation volatiles (as identified in16) increased catches of L. botrana. A number of reasons could underlie this. As we only tested a single load and ratio, we cannot exclude that other doses and ratios would have induced increased capture rates. Furthermore, the release rates and strong synergistic effect of 2-PET on AA may have obscured additive effects of the additional fermentation volatiles. Finally, the release rates of the compounds from the vials may have differed considerably, something that was not verified in this study. Future work could expand on the current by evaluating these factors.
Although the additional fermentation volatiles did not increase L. botrana captures, they did significantly increase attraction of other insect taxa, among which other pests: adding alcohols attracts Musca spp.; adding major fermentation compounds attracted Tephritidae in the first flight and Drosophila spp. in the last. The highest catches for all species were observed with a complex blend of 13 compounds. We also confirm a synergy between AA and 3-methyl-1-butanol for Hypsopygia costalis (Lepidoptera: Pyralidae28,39;). Apparently, lures can be designed from generic fermentation volatiles, that, depending on their composition and release rate, can be selective for certain insect taxa. These results only further underline the significance of the synergy between AA and 2-PET for L. botrana specifically.
We suggest that future studies such as this one should always carefully analyze blend ratio, composition and release rates to optimize not only attractiveness for the target pest, but also selectivity to avoid non-target species.
Source: Ecology - nature.com
