Factors affecting the response of thrips to an olfactory cue

Abstract

Some thrips (Order Thysanoptera) are key pests of many greenhouse and outdoor crops worldwide. Biological attributes such as polyphagy, rapid reproduction, cryptic behaviour and insecticide resistance make them particularly difficult to manage. Because a large number of insects utilize plant volatiles in host finding, there is growing interest in exploiting the olfactory cues that insects use to find their hosts, including the use of semiochemicals as attractants in pest management. However, our understanding of the mechanisms eliciting the response of thrips to semiochemicals is still only rudimentary and a greater understanding of their behavioural response to olfactory cues is needed to optimize the use of semiochemicals.

An ecological approach has been taken to investigate the reasons why the use of a known thrips attractant, methyl isonicotinate (MI), in combination with coloured traps works better in some situations than in others that appear very similar. The approach investigated the influence of selected intrinsic and extrinsic factors on the response of the worldwide polyphagous pest western flower thrips (Frankliniella occidentalis (Pergande)). Several studies undertaken previously using MI-based kairomone lures have shown large variations in trapping efficacy both within and between different trials. Most notable were results from southeast Spain where there appeared to be an overall lack of effect. Specific objectives include examination of the release kinetics and release rates of MI from passive dispensers, the effect of different release rates of MI on thrips response in greenhouse environments, the attraction of WFT to MI under controlled laboratory conditions, and further investigations of the lack of responsiveness of Spanish WFT to MI to determine whether there are intraspecific differences among WFT populations.

Two main results can be drawn from the research in this thesis. First, the results showed that intrinsic factors such as phenotype play a much larger role than previously expected in the responses of thrips to an olfactory cue. Y-tube olfactometer assays testing MI under controlled conditions using populations collected from different countries showed that WFT differ in their response to an olfactory cue depending on which region of the world they are from. The work in this thesis confirmed previous experiments that WFT collected from southeast Spain did not respond to MI. Significantly more WFT collected from New Zealand (73%), The Netherlands (72%), Kenya (70%) and France (61%) preferred the MI laden arm in the Y-tube olfactometer over the clean arm. However, Spanish WFT populations did not show any significant preference for the MI laden arm. Additional Y-tube olfactometer studies undertaken to further explore this non-response, using different doses and concentrations, and an alternative thrips kairomone lure, showed the same result: no positive behavioural response was found for the WFT population collected in Spain. This suggests the development of a different behavioural phenotype of WFT in Spain has occurred. Additional experiments using electroantennogram (EAG) technique showed that Spanish WFT can detect MI at least at the peripheral olfactory reception level, and suggest that the non-response is due to failure in the olfactory signal pathway of the central nervous system. While the work undertaken in this study suggests that underlying genetic variation between populations is the basis of the difference, the reason for this non-response is still to be determined.

Second, the study identified that several extrinsic factors influence the level of response in populations of WFT that showed a significant response to MI. While the dispensers were used for releasing MI at a constant rate (zero-order kinetics), temperature was shown to influence the release rate strongly. Increasing the temperature from 15 to 25°C, resulted in a 2.5-fold increase in the release rate of MI for a commercially available sachet. Airflow was also shown to affect the release rate. Considering the critical role release rates of kairomones play in insect trap responses, it is reasonable to infer that differences in release rates as a result of choice of passive dispenser and extrinsic factors, such as temperature and air flow, could account for some of the variation previously experienced in field and greenhouse trials using MI to trap thrips. This was confirmed in a greenhouse study showing the response thrips to MI was at least partially positively dose related. The greenhouse trial also highlighted other areas believed to play a role on the responsiveness of thrips such as odour saturation of the air surrounding traps used in the greenhouse.

The fundamental information on the intrinsic and extrinsic factors affecting the behavioural response of thrips to odours obtained from this research can be used to design better tools for monitoring and trapping thrips pests using semiochemicals in New Zealand and worldwide.