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Several methods were investigated to identify insecticide resistance in commercially available Phytosciulus persimilis Athias-Henriot and Amblyseius cucumuris Oudemans from 3 different companies supplying this product to growers in Canada. Cypermethrin (cymbush) was the most toxic to the mites of the insecticides tested. Imidacloprid (Admire) was the least harmful of the 5 insecticides/acaricides tested. Relative toxicity was measured in the slide dip tests wherein 5 insecticides were tested against all three commercial strains of both species of mites. The greatest relative toxicity was seen with mites from company B: comparing the LC50's, cypermethrin was 2429 times more toxic to P. persimilis than was imidaloprid, and >4000 times more toxic to A. cucnmeris than was imidacloprid. The leaf dip method tested residual toxicity of residues remaining on the leaf disks while the potter spray tower method combined both direct contact and residual toxicity, which is reflected by intermediate toxicities between those of the leaf dip and slide dip results. Of the three commercial strains of both species of mites, those mites from company B were the most tolerant to imidacloprid and dicofol (kelthane) compared to company A and company C. In the slide dip test with P. persimilis, mites from company B are 4.7 and 5.7 times more tolerant to imidacloprid than comapnay C and company A mites, respectively. As well, in the slide dip test with P. persimilis, mites from company B are 5.3 times more tolerant to imidacloprid than company C mites. On the other hand, mites from company B appeared to be the most susceptible to cypermethrin, but it was a small difference. There was little difference between the commercial strains in their response to pyridaben and thiamethoxam (actara). Of the two mite species tested, P. persimilis was more susceptible to the insecticides tested by slide dip than A. cucumeris in most cases or not significantly different. In leaf dip tests with dicofol and pyridaben against both species, the result appeared to be the opposite, that is, A. cucumeris was more susceptible, particularly to dicofol. In conclusion, we have identified that imidacloprid is a preferable candidate for use in an Integrated Pest Management (IPM) program because of its low toxicity to predatory mites when imidacloprid is being used against other pests. Dicofol and pyridaben are acaricides that should be considered as products for selection of resistance in mites. |
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