Combating insecticide resistance in the tomato leafminer, Tuta absoluta.

Abstract

BACKGROUND: The tomato leafminer, Tuta absoluta is a damaging pest of tomato crops worldwide. In the UK, T. absoluta is controlled using an integrated pest management (IPM) strategy that includes the pesticides spinosad and chlorantraniliprole, the biocontrol agent Macrolophus pygmaeus and pheromone-based mating disruption. Some growers have reported a loss of efficacy of this technology. There are concerns that T. absoluta may have evolved resistance to these applied chemistries as well as undergone adaptations in its capacity to reproduce asexually. In this thesis I investigate whether pesticide resistance is present in UK populations and identify the molecular mechanisms for this resistance. I will also investigate the capacity T. absoluta to reproduce asexually through parthenogenesis in the absence of males. RESULTS: I demonstrate that UK populations of T. absoluta are highly resistant to spinosad and identify two novel mechanisms by which resistance has evolved. Analysis of messenger RNA encoding the target site of spinosad, the nicotinic acetylcholine receptor (nAChR) α6 subunit, revealed resistant strains lack exon 4 resulting in a highly truncated protein. In a second resistant strain the deletion of three amino acids is detected in the transmembrane domain of the nAChR - predicted to be the binding site of spinosad. I identify low levels of tolerance to chlorantraniliprole in UK populations and show this resistance can be selected for to produce highly resistant populations. Analysis of the target site of chlorantraniliprole, the ryanodine receptor, identified amino acid substitution G4903V that has been strongly linked to diamide resistance in a range of lepidopteran species including T. absoluta. With regards asexual reproduction, I observed a small but significant increase in the rate of asexual reproduction. This allows persistence of the pest in the presence of the mating disruptor, Isonet T. Marked differences in several other life history traits associated with reproduction were also observed in these populations including increased longevity further allowing T. absoluta’s persistence within the crop. CONCLUSION: My findings show that the evolution of resistance has rendered spinosad redundant at most sites in the UK. The mechanisms identified are unique to UK populations and so have likely evolved under selection in the UK. Chlorantraniliprole remains effective, however our findings of resistance at low frequency suggest that continued use of this pesticide must be monitored carefully. The low overall occurrence of asexual reproduction observed in this study is unlikely to result in loss of efficacy of mating disruption as reproductive rate remained low. However, the observed changes in longevity and egg laying may allow T. absoluta to persist for longer within the crop, and, together with the increased frequency of parthenogenesis, may reflect selection from the use of Isonet T. Thus, regular monitoring of the reproductive capacity of UK populations should be conducted, along with continual assessment of resistance allele frequencies of pesticides to inform resistance management strategies.