The Effects of Synthetic Pyresroids on Spider Mite Resurgences

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Mechanisms underlying spider mite population resurgences were tested in eggplant fields which were sprayed periodically with each of three Synthetic Pyrethroids (SP). The study showed that regardless of the miticidal effects of the SP's, peak densities of the spider mite, Tetranychus Kanzawai, in the SP Plots were more than 10 times higher than those in plots treated with other pesticide (s) and those in fields that were not sprayed. The mite populations in the SP plots exihibited 2 to 4 distinct peaks throughout the study period.The dominant predators in this field were the phytoseiid mite, Amblyseius longispinosus and the anthcorid bug, Orius sp. In the SP plots, the densities of these predators remained much lower and increaesd much less in response to increases in mite density. If the phytoseiid mites were artificially removed from the SP plots leaving only anthcorid predators, the spider mite populations still reached peak densities which were almost as high as those when the phytoseiids were present. These clearly indicated that the mite outbreaks occurred because of the adiverse effects of the SP sprays on the anthcorid predators.The SP sprays enhanced spider mite dispersal from host plants as a result of their repellent action on the mites. However, this did not change significantly the spatial distribution patterns of the mite populations and never resulted in an increase in the mite populations. This rendered the hypothesis that pyrethroids-induced dispersal will be the important factor responsible for mite population resurgences.