Attracting Scelionidae egg parasitoids to enhance stink bug egg parasitisation in soybean crops using methyl salicylate and (E,E)-α-farnesene

Plant volatile organic compounds (VOCs) play a crucial role in mediating interactions between plants, herbivores, and natural enemies. Methyl salicylate and (E,E)-α-farnesene are two such VOCs that have been shown to influence the behaviour of herbivores and natural enemies. Methyl salicylate has been extensively studied for its potential in herbivore and natural enemy management, and (E,E)-α-farnesene is a common sesquiterpene found in over 100 plant species and insect pheromones. Previous research showed that both methyl salicylate and (E,E)-α-farnesene are produced as herbivore-induced plant volatiles (HIPVs) by soybean plants in response to feeding, and feeding plus oviposition, damage caused by the brown stink bug, Euschistus heros. These HIPVs act as synomones, influencing the foraging behaviour of the egg parasitoid, Telenomus podisi. The objective of this study was to evaluate whether application of methyl salicylate and (E,E)-α-farnesene in soybean fields could enhance the parasitism of stink bug eggs and indirectly increase soybean production. A comprehensive three-season field experiment was conducted in the Central-West region of Brazil, using two widely grown soybean cultivars, BRS 7580 and BRS 7880. The following treatments were evaluated: control areas (without methyl salicylate (MeSa) and (E,E)-α-farnesene (EFar)); areas treated with either MeSa or EFar; areas treated with a binary mixture of these compounds (1:0.5 of EFar + MeSa). Laboratory experiments showed that BRS 7580 and BRS 7880 produced similar qualitative blends of VOCs, with methyl salicylate, (E,E)-α-farnesene, and (Z)-3-hexenyl acetate being produced by both cultivars but with significant quantitative differences; BRS 7880 produced higher total amount of VOCs compared to BRS 7580. This difference in the amount of VOCs produced between the cultivars did not affect the attraction of the egg parasitoid T. podisi to stink bug-damaged plants in behaviour bioassays. In field experiments, application of synthetic methyl salicylate and (E,E)-α-farnesene did not significantly affect stink bug abundance across the three seasons evaluated. Nonetheless, a substantial increase in egg parasitism was observed in all areas where methyl salicylate and (E,E)-α-farnesene were deployed, with parasitism rates ranging from 50-80%, whereas only a 10% parasitism rate was exhibited in control areas. In addition, areas treated with synthetic compounds attracted a greater number of Scelionidae parasitoids compared to control areas. Despite the enhancement in parasitism in areas where methyl salicylate and (E,E)-α-farnesene were deployed, this did not translate into a higher yield of viable seeds.