Increasing the attractiveness of corn plants to parasitoids with the use of plant enhancers

Chemical plant strengtheners find increasing use in agriculture to enhance resistance against pathogens in crop plants. In an earlier study, it was found that treatment with one such resistance elicitor, BTH (Benzo-(1, 2, 3)-thiadiazole-7-carbothioic acid S-methyl ester), increases the attractiveness of maize plants to a parasitic wasp. This surprising additional benefit of treating plants with BTH prompted us to conduct a series of olfactometer tests in order to find out if BTH and another commercially available plant strengthener, Laminarin, generally increase the attractiveness of maize to three important parasitic wasps, Cotesia marginventris, Campoletis sonorensis and Microplitis rufiventris. In virtually each case, plants that were sprayed with the plant strengtheners and subsequently damaged by Spodoptera littoralis caterpillars or artificially triggered to release herbivore-induced plant volatiles were more attractive to the parasitoids than plants that were only sprayed with water. The elicitors alone or plants that were sprayed but not induced to release volatiles were not attractive to the wasps. Interestingly, plants treated with the plant strengtheners did not show any increase in volatile emissions with the possible exception of trans-ocimene. On the contrary, treatment resulted induction in most compounds, most notably indole, a compound that has been reported to interfere with parasitoid attraction. On the same trend, (E)-beta-caryophllene was significantly suppressed in BTH and Laminarin treatments. Else, both Beta-Bergrgamotene and (E)-beta-farnesene have the same tendency in suppression in Laminarin treatment but not for both control and BTH treatments. Expression profiles of marker genes confirmed that BTH and Laminarin induced typical pathogenesis related genes and that BTH suppressed a key gene involved in indole synthesis. The results confirm the notion that, as yet undetectable and unidentified compounds, are of major importance for parasitoid attraction and that these attractants may be masked by some of the major compounds in the volatile blends. This study confirms that elicitors of pathogen resistance are compatible with the biological control of insect pests and may even help to improve it.nrpages: 115status: publishe