Observation and Manipulation of Thrips Behaviour

The western flower thrips, Frankliniella occidentalis (Pergande) (Thysanoptera: Thripidae) and the onion thrips, Thrips tabaci Lindeman, are main pests that cause severe losses in almost all protected crops in Europe. Moreover, T. tabaci is a serious problem in field cultures of various vegetable crops. The scientific objective of the proposed study is to elucidate behavioural aspects in the decision process of host selection and acceptance of F. occidentalis and T. tabaci, which can be manipulated by application of repellent and/or deterrent secondary plant compounds on their host plants. Firstly, a model catalogue of all behavioural patterns and activities both thrips species on different host plants is compiled. Thrips behaviour is recorded for several hours by means of a transparent observation chamber and a video equipment. Based on the analysis of the videotaping appropriate categories of all behavioural elements are created and a special software for computing of observational data is configured. Secondly, in three different bioassays specific steps in the host selection and acceptance behaviour of F. occidentalis and T. tabaci are manipulated. Selected plant compounds, which have been identified to be biologically active against both thrips species in a preceding project, are applied on their host plants and thrips responses are observed and recorded. In short-term observational assays using leaf discs, the effect of the selected plant compounds on the residence time of F. occidentalis and T. tabaci on the respective host plant is investigated. A choice test over a medium-term observation period will elucidate the influence of the plant compounds on the settling preference of F. occidentalis and T. tabaci for treated or untreated host plants. Long-term observations over a 24-hour photoperiod will show differences in frequency and/or duration of behavioural patterns of both species on treated compared to untreated plants. In addition, the feeding damage and the oviposition rate of each individual thrips is determined. The computer-assisted analysis of the videotaping allows an exact evaluation of the effects of the secondary plant compounds with regard to their future utilisation as crop protection agents. This research can contribute to the enhancement of current biological control measures and/or form novel components of Integrated Pest Management (IPM) strategies.

Secondary plant compounds acting as signal chemicals manipulate behavioural patterns of western flower thrips and onion thrips when applied on crop plants, and may be used in future behavioural pest control strategies. Secondary plant compounds may act as chemical signals, mediating interactions between insects and plants. The scientific objective of the project was to elucidate behavioural patterns particularly related to the host selection process of the two Thysanopteran pest species Frankliniella occidentalis, the western flower thrips, and Thrips tabaci, the onion thrips, which can be manipulated by application of repellent and/deterrent secondary plant compounds on their host plants. Both species occur world-wide on horticultural and agricultural crops in the greenhouse and in the field. The key findings: - Generally, the model catalogue of behavioural patterns of F. occidentalis and T. tabaci compiled in this project allowed a comparison of behaviours of the two species on their common host plant cucumber: differences were only minimal. The comparison of thrips behaviours on different host plants revealed that the plant species has a strong influence on frequency and/or duration of behavioural patterns. Both thrips species showed similar responses in preferring bean (F. occidentalis) or leek (T. tabaci) over cucumber. - Altogether we found either two plant compounds that modified the behaviours of either of the two thrips species when applied on a host plant: carvacrol and methyl salicylate (F. occidentalis), eugenol and linalool (T. tabaci). For example, carvacrol incited the western flower thrips to increased restlessness, i.e. increased exploration interrupted by many short sequences of inactivity on a treated plant. In a thrips control strategy the integration of such a compound could enhance the uptake of contact insecticides or spores of fungal insect pathogens. - When they had an alternative, thrips avoided to stay in contact with a plant surface treated with either of these four plant compounds. Moreover, these compounds proved to inhibit efficiently feeding and egg-laying of thrips on a treated plant. For this reason alone the incidence of high infestation levels and thus, economic damage can be prevented. - Carvacrol or eugenol application resulted also in initial rejection by F. occidentalis or T. tabaci, respectively, of a treated plant surface upon direct contact. Such increased leaving rates might reduce thrips colonisation on treated crop plants. These effects of, secondary compounds may be used in various strategies to contribute to the enhancement of current biological control measures and/or form novel components of Integrated Pest Management (IPM) strategies.