Combining natural enemies and selective pesticides in IPM programmes of exotic pests: the Tuta absoluta (Lepidoptera: Gelechiidae) case

Abstract

The recent invasion of the Mediterranean basin by the South American tomato pinworm, Tuta absoluta (Lepidoptera: Gelechiidae), has lead to a swift increase in insecticide applications in tomato crops. Such increase may compromise the tomato Integrated Pest Management (IPM) packages that were sustainable and commonly used by farmers. My work aimed at providing key bases for including indigenous biocontrol agents of T. absoluta in IPM programs on tomato in Europe. A survey carried out in 3 Italian regions demonstrated that T. absoluta was rapidly used as host by indigenous parasitoids. The recovered species were mainly generalist idiobiont parasitoids (mainly belonging to the Eulophidae and Braconidae families) which showed relatively low levels of parasitism when attacking the invasive moth. Among them, only few were identified as potential biocontrol agents of the pest. More specifically, Bracon nigricans (Hymenoptera: Braconidae), an idiobiont, synovigenic, gregarious ectoparasitoid, showed some potential for inclusion in tomato IPM programmes. In a preliminary assessment of possible compatibility of biocontrol agents with pesticides used for IPM in tomato crops, dustable sulphur (mineral fungicide/acaricide) was showed to be moderately harmful (as a fresh residue) for a generalist predator (Nesidiocoris tenuis [Hemiptera: Miridae]) commonly used on tomato for biological control purposes. In a second step, we assessed the risks of 14 pesticides, commonly used in tomato crops, on a generalist predator model: Orius laevigatus (Hemiptera: Anthocoridae). We demonstrated that the pesticides greatly differed in their toxicity, both in terms of lethal and sublethal effects, as well as in their persistence after initial application on plants. Among the tested biopesticides only Bacillus thuringiensis proved to be harmless; whereas spinosad, emamectin benzoate and metaflumizone were moderately harmful until 7d after the treatment and abamectin was the most noxious and persistent and was classified as harmful up to 14 days after the treatment. While the other tested pesticides (including the synthetic ones) were slightly harmful or harmless. Further risk assessments of biopesticides showed that most of them have low acute toxicity (i.e. lethal effect) on B. nigricans. By contrast, spinosad proved strong acute toxicity on the tested wasps (100% of mortality). All biopesticides, except Bt, caused multiple sublethal effects, notably reductions in parasitism rate on T. absoluta, fertility, longevity and also a male-biased sex-ratio of the progeny. In addition, demographic models developed by integrating possible sublethal effects of pesticides on B. nigricans life history traits, showed that biopesticides would strongly impact long-term population growth of the wasp. A thorough review of articles assessing side effects of spinosyns on beneficial arthropods further hinted the potential multiple negative sublethal effects of this type of products in IPM packages. Taken as a whole, my research showed that indigenous biocontrol agents could play a key role in the control of T. absoluta in Europe and that various pesticides should be of use only with high caution in tomato IPM programmes.