Abstract
Agricultural practices often involve tank-mixing and co-application of insecticides with fungicides to control crop pests. However, natural methods relying on biological control agents such as hymenopteran parasitoids have been shown to be highly effective in suppressing crop pest populations. The current body of insecticide risk assessment data accounting for fungicide co-application is very small, the present study being the first to examine this in a parasitoid wasp. Through low-dose exposure to dry residues of the neonicotinoid insecticide thiacloprid, we examined its mortal and knockdown effect on Aphelinus abdominalis when co-applied with increasing doses of the fungicide tebuconazole. Both of these acute effects of thiacloprid were synergised (toxicity increased to a greater-than-additive effect) by tebuconazole, resulting in significant mortality from low-dose co-applications of tebuconazole, and significant knockdown even without co-applied tebuconazole, the effect increasing as tebuconazole concentration increased. We show the highly toxic effect that a low dose of thiacloprid imposes on A. abdominalis populations, and a synergistic toxicity when co-applied with low doses of tebuconazole. Our work suggests a need for updating pesticide risk assessment methods, accounting for pesticide mixtures, in order to make these risk assessments more field relevant.
Highlights
Insects contribute to several ecosystem services that are indispensable to agriculture [1], one of which is biological control of crop pests
When exposing A. abdominalis to all 8 treatments in our examination of thiacloprid at one tenth manufacturer’s recommended dose (MRD) combined with increasing doses of tebuconazole (N = 240 per treatment), we observed tebuconazole’s synergising effect on thiacloprid, with regard to mortality at 24 h for combinatory treatments containing tebuconazole at one twentieth MRD, one tenth MRD, one half MRD and MRD (Fig 2a)
Using the parasitoid wasp A. abdominalis exposed to field-realistic doses of dry pesticide residues, our study provides evidence of an insecticide’s acute lethal and sublethal effect synergised by co-application of a fungicide, the two compounds being commonly tank-mixed for simultaneous use in a variety of agroecosystems [28]
Summary
Insects contribute to several ecosystem services that are indispensable to agriculture [1], one of which is biological control of crop pests. Parasitoid wasps in particular can be very effective at suppressing insect pest populations in agroecosystems [2,3,4,5,6,7,8,9,10]. In conventional agriculture, farmers apply pesticides to manage crop pests, often routinely without regard for pest. Effect of thiacloprid synergised by tebuconazole in a parasitoid role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. There was no additional external funding received for this study
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