Seasonal Comparison of Lipid Composition and Metabolism in Parasitised and Non-Parasitised Clover Root Weevil (Sitona lepidus)

Since its arrival in New Zealand the clover root weevil (Sitona lepidus (syn. flavescens) (Coleoptera: Curculionidae) (CRW) has caused serious damage to New Zealand’s agricultural sector. The introduction of the biocontrol agent, Microctonus aethiopoides has caused a significant reduction in the population of CRWs. During their research on the CRW, AgResearch scientists discovered that the abdominal fat body and lipids present in the haemolymph in adult CRW varied with season, sex, insect age and parasitism. Parasitism has been reported to change the lipid composition of other species of insects. The purpose of the present study was to compare the chemical composition of the lipids present in parasitised and non-parasitised CRW adults, and determine how these lipids change with physiological state and parasitoid development. The investigation into how M. aethiopoides initiates these changes was extended to examine the roles of juvenile hormones and teratocytes in lipid regulation. A one-step method of extraction and derivatisation was created to determine the fatty acid profile of CRWs. This method gave higher recovery percentages and higher reproducibility than the traditional two-step methods that were trialled, and enabled individual CRWs to be analysed. The fatty acid profile of the CRW was similar to that of other insects reported containing mainly the 16 carbon saturated and monounsaturated fatty acids and the 18 carbon saturated, monounsaturated and polyunsaturated fatty acids. The one-step method was also used to track differences in the fatty acid profiles of individual CRWs with differing sex, age, parasitism and physiological state. The fatty acid profiles of male and female CRWs were similar, with only significant differences between the concentrations of the 16:1 and 18:0 fatty acids. Due to variation between individual samples in the entire sample set, no obvious correlations were found between physiological state and fatty acid composition, or distinctions between state of parasitism and fatty acid profile. Basic statistical methods were utilised initially, however, the complexity of the data set required multivariate analysis. PCA, LDA and QDA were utilised but again no correlations or distinctions were found when the whole sample set was analysed. Results from the Gisborne subset, collected from the same location and at the same time, had reduced variation between individuals, and this allowed some distinctions to be made between parasitised and non-parasitised samples. Teratocytes are cells that have dissociated from the serosal membrane that occur in the haemolymph of CRWs that have been parasitised by M. aethiopoides. The fatty acid composition of these cells was investigated using the one-step method and a MALDI-TOF spectrometry method, which detects triacylcglycerols. The fatty acid profile of teratocytes was not significantly different to that of the CRW. Juvenile hormones (JHs) control postembryonic development and adult reproduction. They are present in all insects and JH III is the most common of the six possible JHs. The LC-MS method of Miyazaki et al was modified and this allowed for the determination of JH III within samples of 50 CRW adults. The comparison between parasitised and non-parasitised samples found that parasitised samples had significantly higher levels of JH III than did their non-parasitised counterparts