Insecticide resistance management in Australian cotton pests Bemisia tabaci and Helicoverpa armigera conferta

Abstract

Evolution of resistance in pest insects is a consequence of natural selection caused by pesticides, which poses significant challenges to agricultural productivity. This thesis investigates the molecular mechanisms of resistance using Bemisia tabaci MEAM1 (silverleaf whitefly) and Helicoverpa armigera conferta (cotton bollworm) as focal species, with a focus on evaluating the effectiveness of molecular screening techniques when used alongside traditional bioassays. Amplicon-based high-throughput DNA sequencing was used to analyse B. tabaci species composition and assess resistance allele distribution for insecticides such as pyrethroids and organophosphates (chapters 2 and 3). In Australia, MEAM1 is the only invasive B. tabaci species detected within the ‘tabaci’ species complex, with the second invasive ‘MED’ species, has remained undetected in Australia. Frequencies of resistance alleles for different classes of insecticides were investigated using high-throughput DNA sequencing. Organophosphate resistance alleles were highly prevalent (>99%), whereas pyrethroid (0.1% ≤ frequency ≤ 97.5%) and spirotetramat (28.8% ≤ frequency ≤ 98.7%) resistance varied by region. In chapter 4, transcriptomic and gene expression analyses using RNAseq and RT-qPCR investigated neonicotinoid resistance mechanisms. CYP6CM1 over-expression was detected in a B. tabaci MEAM1 resistant laboratory population and associated with neonicotinoid resistance, which was further validated in field populations. I also explored resistance mechanisms in H. armigera conferta, a major cotton pest in Australia primarily controlled by transgenic cotton expressing Bacillus thuringiensis (Bt) toxin proteins. In chapter 5, target-site resistance was validated using a genetically modified strain with a cadherin (HaCad1) gene knockout. Dose-response bioassays confirmed resistance to Cry1 proteins (Cry1Ac, Cry1A.105 and Cry1F), and transcriptomic analysis suggested that molecular detoxification pathways may also contribute to Cry1 resistance.

Date of Award	2025
Original language	English
Supervisor	Tom Walsh (Supervisor) & Michael FRESE (Supervisor)