Monitoring the susceptibility of citrus rust mite (Acari : Eriophyidae) populations to abamectin

A citrus leaf disk bioassay was developed to monitor the susceptibility of citrus rust mite, Phylloroptruta oleivora (Ashmead), populations to abamect in. Disks from leaves of several citrus cultivars were equally suitable bio assay substrates, and there was no difference in mortality when mites were sprayed directly or exposed to dry abamectin residue. The concentration-res ponse relationship was determined at intervals over 2 yr for a reference po pulation of citrus rust mites that had been maintained in culture and never exposed to acaricides. Three diagnostic concentrations of abamectin were s elected based on the response of the reference population and were used to test the susceptibility of 15 populations of mites from commercial citrus g roves. Comparisons with the reference population showed reduced levels of s usceptibility in some populations. Populations of citrus rust mites from 6 commercial groves were sprayed twice in 1997 with combinations of acaricide s designed to exert different intensities of selection pressure from abamec tin. None of these populations showed a change in their response to abamect in in pre- and postspray bioassays, although their susceptibility was usual ly less than that of mites from the susceptible reference population. Biwee kly counts of rust mites on fruit in these 6 groves suggested that, relativ e to groves which received no abamectin or 1 abamectin spray. mite control was not adversely affected in the groves sprayed twice with abamectin. The bioassay method is discussed in relation to factors that affect the interpr etation of results from its use, and factors that may affect the developmen t of resistance to abamectin in citrus rust mite populations are presented. This study has provided baseline data with which the results of ongoing te sts of the response of citrus rust mite populations to abamectin can be com pared.