The biological control of the Banks grass mite, Oligonychus pratensis (Banks), infesting commercial field corn in the Texas High Plains

Abstract

Field studies were conducted on the Banks grass mite (BGM), Oligonychus pratensis (Banks), infesting commercial field corn in the Texas High Plains. The population dynamics and spatial distribution of BGM and its associated natural enemies were studied at three study sites, from 1981 to 1983. Additionally, the suppressive capabilities of two exotic predacious site species were examined in laboratory and field experiments. Although BGM was the most abundant spider mite in early summer, the two-spotted spider mite (TSM), Tetranychus urticae Koch, often reached relatively high densities in August. Low summer rainfall favored BGM populations over TSM (P [less than or equal to] 0.017). BGM was the first species during the season to infest corn (June). TSM usually invaded fields in mid-July and the combined spider mite densities increased exponentially through August. Spider mite populations were aggregated at all locations on most sampling dates. Taylor's Power Law index "b" varied from 1.3 to 1.6. Green's coefficient varied from 0.0 to 0.94 and indicated that populations became more random through the growing season. A relationship was demonstrated between mean number of spider mites per plant and the proportion of spider mite infested plants. The proportion and density of spider mites infesting corn increased together in a curvilinear fashion indicating that binomial sampling could be used to estimate spider mite population size. Several species of predacious arthropods were found associated with spider mite infestations. Orius insidiosus (Say) (Hemiptera: Anthocoridae) and Dictyna consulta Gertsch and Ivie (Araneida: Dictynidae) were the most abundant predators collected in the early growing season. O. insidiosus and Feltiella macgregori (Felt) (Diptera: Cecidomyiidae) were most abundant during the latter part of the growing season. The augmentative biological control of BGM was investigated using two predacious species of Phytoseiidae exotic to Texas, Phytoseiulus persimilis Athias-Henriot and Amblyseius californicus (McGregor). Both species reduced growth rates of BGM populations under controlled laboratory conditions (P [less than or equal to] 0.05). Field releases of these phytoseiids in 1982 suppressed spider mite population growth, but the reduction was not statistically significant in most fields. Phytoseiids were released earlier during the 1983 season and significantly reduced spider mite population growth in release plots (P [less than or equal to] 0.05)...