Bioactivity of essential oils against thrips pests

The biological activity of essential oils and their main compounds can be used to interfer with the host selection process of the western flower thrips and the onion thrips and to inhibit their feeding activity and/or their reproduction on their host plants. Plants procuce essential oils as part of their chemical defence system. The oils and/or their main compounds have the potential to modify the behaviour of insect pests and to interfer e.g. with their host selection process. The present project provides first basic data on the influence of essential oils treatments of crop plants on the host selection of two thrips pest species. In laboratory bioassays essential oils from plants within the Lamiaceae family and their major volatile compounds were applied on host plants of the western flower thrips Frankliniella occidentalis (Pergande) and the onion thrips Thrips tabaci (Lindeman). Both species occur worldwide on horticultural as well as agricultural crops in the glasshouse or in the field. The key results: - Application of essential oils and/or their volatile main compounds at 1 % and/or 0.1 % concentration on a host plant interfer with feeding and reproduction of T. tabaci and F. occidentalis. - Several of the oils/compounds are active after alighting of the thrips on treated plant parts. Only few oils/compounds repel the thrips before they alight on a treated plant. - In some cases the bioactive properties of the essential oils can be attributed to one of their pure compounds. - The essential oils of marjoram (Origanum majorana L.), rosemary (Rosmarinus officinalis L.), oregano (Origanum vulgare L.) und thyme (Thymus vulgaris L.) and the pure compounds carvacrol, thymol, 1,8-cineole und salicylaldehyde repel/deter T. tabaci. - The essential oils of marjoram, rosemary, lavender (Lavandula angustifolia L.) and mint (Mentha arvensis L.) and the pure compounds carvacrol, terpinen-4-ol, linalool and eugenol repel/deter F. occidentalis. - Generally, the oils/compounds show only low insecticidal effects to adults and larvae of both thrips species. Primarily, crops plants are protected especially during sensitive growth periods as the feeding activity of the thrips pests is inhibited. As the reproduction success is also affected consequently, numbers of larvae feeding on the plants remain low. The aspects of essential oils and/or their volatile main compounds as part of biological control or Integrated Pest Management (IPM) strategies are promising since they are natural, inexpensive to produce, generally regarded as not toxic to non-target organisms, do not persist in the environment and can be blended with each other to achieve a desired spectrum of activity.