Survey and evaluation of native and released predators of the Banks grass mite (Acari: Tetranychidae) in corn and surrounding vegetation.

Abstract

Surveys were made in southwestern Kansas to identify and establish relative abundances of predaceous arthropod species associated with the Banks grass mite (BGM), Oligonychus pratensis (Banks), in field corn and in the surrounding vegetation. A two-year experiment also was done to determine whether the release in the spring of commercially pro duced predatory mites (Acari: Phytoseiidae) in grasses adjacent to corn field borders would enhance BGM suppression over that in corn fields where no predators were released. In 1996, the release of Neoseiulus fallacis (Garman), Neoseiulus californicus (McGregor), and Ga landromus occidentalis Nesbitt in roughly equal proportions at a ratio of 1 predator per 24 BGM did not significantly reduce pest mites compared to control fields. Releases in 1997 of only N. californicus at a 1:1 predator to prey ratio also did not have a significant impact on BGM populations. Berlese samples in corn and in alternate hosts 2 weeks after release failed to recover N. californicus or G. occidentalis in experimental fields, and neither predator oc curred naturally in control (nonrelease) fields. Neoseiulus fallacis was recovered in both re lease and control fields in 1996 and 1997, but could not be distinguished from native popu lations. In addition to N. fallacis, four other species of naturally occurring phytoseiids were collected: Neoseiulus comitatus (DeLeon), Neoseiulus setulus (Fox), Proprioseiopsis ovatus Garman, and Amblyseiella setosa Muma. All five phytoseiids were recovered in corn and in alternate host vegetation. However, N. fallacis was the most abundant phytoseiid in corn and its population densities were significantly correlated with BGM populations. The insidious flower bug, Orius insidiosus Say (Hemiptera: Anthocoridae), was also a relatively abundant predator in corn, but its populations were not significantly correlated with those of the BGM. Several other species of predators suspected to attack BGM are reported. The Banks grass mite (BGM), Oligonychus pratensis (Banks), is an economically important pest in field corn, Zea mays L., and sorghum, Sorghum bicolor (L.), throughout the semi-arid High Plains of North America (Ehler, 1974; Owens et al., 1976). Banks grass mite populations generally overwinter in winter wheat and other grassy alternate hosts around the fields (Holtzer et al., 1984). In spring or early sum mer these spider mites disperse into field corn from the adjacent host plants (Holtzer et al., 1984), often with the aid of wind currents (Brandenburg and Kennedy, 1982; Margolies, 1987). In late summer, spider mite populations can reach damaging lev els in corn and miticides must be used to prevent or reduce economic losses (Archer and Bynum, 1990). Naturally occurring predators of the BGM have been documented in field corn. In Texas, surveys in corn and sorghum, as well as Rhodesgrass, revealed multiple species of predators associated with the BGM (Dean, 1957; Pickett and Gilstrap, 1984). Included among these was the phytoseiid mite, Typhlodromus (-Neoseiulus) fallacis (Garman), which occurs throughout the Great Plains and is believed to be an important predator of spider mites in Kansas (Bell, 1972; Dick, 1987), Colorado (Logan et al., 1983), and Nebraska (Berry, 1988). 1 Present address: City of New Orleans Mosquito and Termite Control Board, 6601 Stars and Stripes Blvd., New Orleans, Louisiana 70126. This content downloaded from 207.46.13.158 on Sun, 10 Apr 2016 04:17:42 UTC All use subject to http://about.jstor.org/terms VOLUME 73, ISSUE 2 113 Current pest management programs for spider mites in field corn rely heavily on conservation of predators such as phytoseiid mites. This goal can be accomplished by delaying insecticide applications or by using insecticides that have a minimal im pact on spider mite predators. However, because spider mite predators are not al ways present early enough or in sufficient numbers to be effective, supplemental re leases of phytoseiids represent an option that could lead to more reliable mite suppression. In fact, biological control of spider mites through augmentative releases of commercially produced phytoseiids has had some success in other cropping systems. In southern California, Neoseiulus californicus (McGregor), Phytoseiulus per sim ilis Athias-Henriot, and Galandromus occidentalis Nesbitt were mass released on strawberry beds where phytoseiids were able to suppress spider mite populations (Oatman et al., 1977). Augmentative releases were cost-effective in this fruit crop and this practice has been successful in commercial pest management (McMurtry, 1991). In contrast, colonization of the same predator species complex on spider mites in field corn in western Texas has been more variable (Gilstrap et al., 1977; Pickett and Gilstrap, 1986a). Of the three non-native phytoseiids released, only N. califor nicus and P. persimilis were recovered. In addition, despite some evidence of spider mite suppression, the cost of such large-scale releases was shown to be too high to be feasible in commercial field corn (Pickett and Gilstrap, 1986a). An alternative strategy that might be more cost-effective would be to make inoc ulative, early-season releases of phytoseiids into the vegetation near corn field bor ders. Because this vegetation serves as an overwintering habitat for BGM, it could supply prey on which predator populations might develop. Provided that spider mite populations were not overexploited, both predators and prey could later move into field corn. A similar technique was used successfully by Corbett et al. (1991), who established populations of G. occidentalis in alfalfa strips adjacent to cotton and recorded lower populations of the twospotted spider mite, Tetranychus urticae Koch in cotton where alfalfa was absent. Therefore, our primary objective was to experi mentally evaluate whether early-season releases of phytoseiid mites in border vege tation would have an impact on BGM populations in field corn. Our second objec tive was to determine whether or not released predators could be recovered. Because the naturally occurring complex of BGM predators has not been studied in south western Kansas, a third objective was to survey the relative and seasonal abundance of resident phytoseiid mites and other predators in corn and grassy border vegetation.

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