Abstract
The tomato leafminer, Tuta absoluta, now a major pest of tomato crops worldwide, is primarily controlled using chemical insecticides. Recently, high levels of resistance to the insecticide spinosad have been described in T. absoluta populations in Brazil. Selection of a resistant field-collected strain led to very high levels of resistance to spinosad and cross-resistance to spinetoram, but not to other insecticides that target the nicotinic acetylcholine receptor (nAChR). In this study the mechanisms underlying resistance to spinosad were investigated using toxicological, biochemical and molecular approaches. Inhibition of metabolic enzymes using synergists and biochemical assessment of detoxification enzyme activity provided little evidence of metabolic resistance in the selected strain. Cloning and sequencing of the nAChR α6 subunit from T. absoluta, the spinosad target-site, from susceptible and spinosad-resistant strains were done to investigate the role of a target-site mechanism in resistance. A single nucleotide change was identified in exon 9 of the α6 subunit of the resistant strain, resulting in the replacement of the glycine (G) residue at position 275 observed in susceptible T. absoluta strains with a glutamic acid (E). A high-throughput DNA-based diagnostic assay was developed and used to assess the prevalence of the G275E mutation in 17 field populations collected from different geographical regions of Brazil. The resistant allele was found at low frequency, and in the heterozygous form, in seven of these populations but at much higher frequency and in the homozygous form in a population collected in the Iraquara municipality. The frequency of the mutation was significantly correlated with the mortality of these populations in discriminating dose bioassays. In summary our results provide evidence that the G275E mutation is an important mechanism of resistance to spinosyns in T. absoluta, and may be used as a marker for resistance monitoring in field populations.
Highlights
The tomato leafminer, Tuta absoluta is a global threat to tomato production because of its great damage potential, short life cycle and high reproductive capacity [1,2,3]
The indiscriminate use of insecticides in Brazil for the control of T. absoluta has resulted in the rapid emergence of resistant populations [10, 11, 27]
As a result resistance to this compound has been described in T. absoluta, with a recent study describing high levels of spinosad resistance in certain T. absoluta populations from Brazil [27]
Summary
The tomato leafminer, Tuta absoluta is a global threat to tomato production because of its great damage potential, short life cycle and high reproductive capacity [1,2,3]. T. absoluta originated in South America and was introduced to Brazil sometime between. – 1980, after which it quickly spread nationwide causing serious damage to tomato crops [4, 5]. This pest arrived in Europe in 2006 and subsequently spread to. The intensive use of organophosphates, carbamates, pyrethroids, benzoylureas, avermectin, indoxacarb and diamides has led to reports of resistance in T. absoluta populations in Brazil, Chile, Argentina, Greece and Italy [3, 7,8,9,10,11,12,13].
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