Graduate Thesis Or Dissertation
 

Effects of early season immigration on acaricide resistance of Tetranychus urticae Koch on strawberry in the California Pajaro and Salinas valleys

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  • In the Pajaro Valley near Watsonville, California, twospotted spider mite (Tetranychus urticae Koch) populations were surveyed in 1983 and '84 to detect the influence of immigrants from nursery plants or surrounding vegetation on subsequent resistance development to cyhexatin and formetanate in strawberry fields. Susceptible mites (cyhexatin LC50 = 0.0039 % a.i.; formetanate LC50 = 0.0018 % a.i.) from strawberry nurseries initially colonize newly planted fruiting fields, having survived in diapause on transplants in cold storage (average 0.068 mites/plant) and subsequent transplanting (average 0.0051 mites/plant) in fruiting fields. At some sites these mites are subsequently outnumbered by immigration of resistant mites (average cyhexatin LC50 = 0.064 % a.i.; average formetanate LC50 = 0.086 % a.i.) which overwinter on surrounding vegetation. At other sites, however, susceptible mites appear to predominate until the first acaricide sprays are applied. Immigration of mites from January through March was documented under four different alternate host plant settings surrounding individual fields. At three sites resistance was influenced by heavy immigration of resistant mites from surrounding vegetation in 1984 which resulted in average LC50's of 0.077 % a.i. and 0.037 % a.i. for cyhexatin and formetanate for all sites, respectively, by mid-March. At one isolated 1984 site without significant immigration of resistant mites, LC50's of only 0.0035 % a.i. and 0.0074 % a.i. occurred for cyhexatin and formetanate, respectively by the same time period. At high immigration sites (i.e. Lewis, Bachan and Eaton), average peak densities of active mites were 6.48 mites per plant near adjacent heavily mite-infested vegetation and 0.136 mites per plant opposite from mite-infested vegetation. At one site not experiencing high immigration rates (Smith), peak active mite densities were only 0.0312 mites per plant near adjacent lightly mite-infested weeds and 0.000 mites per plant away from mite-infested weeds. In 1983 heavy winter rainfall (8.5-fold higher than same period in 1984) slowed resistance development in T. urticae at strawberry sites by reducing survival of overwintering or early immigrant populations. Alterations of some crop production practices to improve management of immigrant T. urticae and their subsequent effects on acaricide resistance management are discussed.
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