Effects of Dietary Soybean Trypsin Inhibitors on Detection of Resistance to Pyrethroid and Spinosad Insecticides in Helicoverpa armigera Hübner (Lepidoptera: Noctuidae)

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During the last 50 years, worldwide use of synthetic insecticides to control insect pests has led to both insecticide resistance and environmental problems (Roush and Tabashnik, 1990). Helicoverpa armigera (Hubner) is arguably Australia’s most important agricultural pest and insecticide resistance remains an enduring threat. Throughout its history, Australian H. armigera has been shown to evolve a range of mechanisms to confer resistance to insecticides (Gunning et al., 1991; Gunning et al., 1996a; Gunning et al. 1996b). Therefore it is vital that any resistance monitoring programme must not only be able to detect resistance at low frequencies but also be able to detect resistance resulting from any number of potential mechanisms. Pyrethroid and spinosad resistance in Australia are known to be caused by sequestration and or hydrolysis of non-specific esterases (Gunning et al., 1996b; Gunning et al., 2007; Gunning & Balfe 2002). Soybean based diets are common artificial diets for Heliothine species. To avoid chronic exposure to artificially high levels of protease inhibitors in artificial insect diets, it is usual to heat-treat legume components to denature the inhibitors (Shorey & Hale, 1965; Teakle and Jensen, 1985). However, not all laboratories follow this regime, and while some laboratories heat-treat legume components of diets, other laboratories involved in Heliothine rearing, use or have used, raw soybean flour. There are also a variety of procedures used to heat treat soybean trypsin inhibitors in artificial diet preparation, which may result in varying degrees of protease inhibitor degradation. Leguminous seeds, such as those from soybean, are protected against herbivores, including insects, by anti-nutritional defence proteinstrypsin inhibitors, which are found in the seeds and raw flour made from the seeds. Levels of these plant protease inhibitors are high in legume seeds, comprising 1-5% of total protein (Macintosh et al., 1990; Sastry et al., 1987) Trypsin inhibitors of the Kunitz type (Kunnitz, 1945a; Kunnitz, 1945b) are single chain polypeptides (~ 20 kDa) that act on target serine proteases in the gut forming a 1:1 complex. Perhaps the best known of these inhibitors is Kunitz soybean trypsin inhibitor (KSTI).