Natural pest management - genetic variation in volatile production in cabbage : Breeding for enhanced attraction of natural enemies

Abstract

Alternatives to pesticides, which have detrimental effects on humans, nature, and beneficial insects, are urgently needed. The use of biological pest control, i.e. natural enemies of pest insects that locate the pests on the plants and kill them, is a nature-friendly way to suppress pest survival and population development. Some plants release volatile organic compounds in response to attack by herbivores, that result in increased attraction of natural enemies. However, increasing indirect defense mechanisms in crop plants has received relatively little attention in breeding programs. In this report we explore the possibility of increasing the attraction of natural enemies by crop plants through breeding. For that, we investigated if natural variation for attraction is present in a crop and its wild relatives, in this case cabbage. In addition, we studied the variation in emission of VOCs in response to herbivore feeding. We used accessions of Brassica oleracea, including domesticated hybrids, landraces, and wild accessions, and accessions of the wild relatives B. incana, B. villosa and B. montana. Using a choice assay, we studied attraction of natural enemy Cotesia vestalis to plants that are fed on by Plutella xylostella. We compared each accession to the known attractive landrace Christmas Drumhead. We found that there is not one (wild) species, that is more attractive than the other species. In fact, we identified attractive accessions and non-attractive accessions in all studied species. In general, wild accessions and landraces seemed to be more attractive than domesticated hybrids. In addition, we measured herbivore-induced volatiles in selected accessions, and find considerable variation in VOCs between species. Relatively little variation was found between attractive and non-attractive accessions of the same species, suggesting that the attraction could be due to only a few volatiles. Attractive accessions were found to emit more alcohols and aldehydes. We conclude that there is variation for both attraction of natural enemies and volatile emission, that could potentially be used for breeding plants with enhanced attraction. Further studies should explore how enhanced attraction relates to the success of natural enemies, to investigate if it would indeed lead to a successful control of pest insects. Identified attractive and non-attractive accessions of the same species could be used to start breeding programs.