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Seasonal occurrence of a fungal pathogen, Neozygites adjarica (Entomophthorales: Neozygitaceae), infecting banks grass mites, Oligonychus pratensis, and twospotted spider mites, Tetranychus urticae (Acari: Tetranychidae), in field corn.

1995; Kansas (Central States) Entomological Society; Volume: 68; Issue: 4 Linguagem: Inglês

1937-2353

G. L. Dick, Lawrent L. Buschman,

Insect and Pesticide Research

The seasonal occurrence of epizootics caused by the fungus Neozygites ad jarica (Tsintsadze & Vartapetov) were studied in populations of Banks grass mites, Oli gonychus pratensis (Banks), and twospotted spider mites, Tetranychus urticae Koch, on field corn, Zea mays L. The first evidence of fungal infection was 5 August, but the major epizootics occurred 14 August to 8 September. The fungal epizootics followed periods of 8-10 h per day of ambient relative humidity above 80%. N. adjarica epizootics occurred in the four test fields, but it was not clear whether or not the epizootic was the major agent of spider mite mortality. Predatory arthropods, miticide applications and corn maturity also appeared to be important spider mite mortality factors. N. adjarica epizootics were erratic and occurred too late in the season to prevent spider mite damage in corn. Awareness of conditions likely to foster fungal epizootics in spider mites will help us understand spider mite population dynamics. Fungal epizootics are sometimes observed in spider mite populations (Acari: Tetranychidae) on field corn, Zea mays L., in the western Great Plains of North America. Banks grass mites, Oligonychus pratensis (Banks), and twospotted spider mites, Tetranychus urticae Koch, are important pests of field corn in this region and frequently cause economic damage to field corn (Mock et al., 1981 ; Sloderbeck et al., 1988). Very little information is available on the pathogen causing these epizootics, or on the role of these epizootics in the population dynamics of spider mites in corn. Dick et al., (1992) described the morphology of the fungal pathogen that attacks spider mites of corn in this region. They identified the pathogen as Neozygites adjarica (Tsintsadze & Vartapetov). Other species o? Neozygites (=Entomoph thora, =Triplosporium) have been reported to infect the Banks grass mite in Texas (Pickett and Gilstrap, 1986) and the twospotted spider mite in other areas of North America (Carrier and Canerday, 1968; Carner, 1976; Brandenburg and Kennedy, 1981; Smitley et al., 1986a, b; Klubertanz et al., 1991). Initiation of Neozygites spp. infections during the cropping season is thought to be associated with high mite populations and favorable weather conditions, specifically, extended periods of high relative humidity (above 90%) (Wilding, 1981; Brandenburg and Kennedy, 1982; Smitley, 1986a). The fungus is thought to overwinter either as thick-walled resting spores (Nemoto and Aoki, 1975; Carner, 1976; Bitton et al., 1979) or as hyphal bodies in mummified mites (Ken neth et al., 1972). However, Brandenburg and Kennedy (1981) demonstrated 1 1716 Pinecrest Av., Garden City, KS 67846. 2 Corresponding author. Accepted for publication 18 June 1995. This content downloaded from 207.46.13.71 on Fri, 21 Oct 2016 04:54:40 UTC All use subject to http://about.jstor.org/terms 426 JOURNAL OF THE KANSAS ENTOMOLOGICAL SOCIETY transmission of N.floridana Weiser & Muma inoculum from year to year in active overwintering populations of twospotted spider mite. Kenneth et al., (1972) did not observe resting spores of N. floridana in twospotted spider mites, but dem onstrated that hyphal bodies in mummified mites could sporulate up to 11 months after mite death. They suggested that the fungus overwintered as hyphal bodies in mummified mites, hidden in protected microhabitats. The field ecology of N. adjarica in Banks grass mites has not been reported. The objectives of this study were to determine the seasonal occurrence of N. adjarica epizootics in populations of Banks grass mite and twospotted spider mite in field corn and to examine some of the environmental factors associated with epizootics of this pathogen. Materials and Methods Commercial corn fields in southwest Kansas were surveyed in May and June 1985 to locate fields with early-season spider mite infestations. Four of these fields are included in this report. The fields were furrow irrigated and subject to normal farming practices, including miticide applications. The Finney County west field was treated 23 July with propargite at 0.76 kg Al/ha and 14 August with carbofuran at 0.34 kg Al/ha. The Stanton County field was treated 13 July with propargite at 0.76 kg Al/ha and 4 August with carbofuran at 0.34 kg Al/ha plus methidathion 0.45 kg Al/ha. The Haskell County field was treated 24 June with propargite at 0.76 kg Al/ha and 11 July with methidathion at 0.91 kg Al/ha. The Finney County east field was treated 15 August with dimethoate at 0.23 kg Al/ha. Relative humidity (RH) and rainfall were recorded at 1-h intervals by an au tomated weather station at the Southwest Research-Extension Center, Garden City, Kansas. The number of hours with ambient RH above 80% was equivalent to the number of hours within-canopy RH above 90% (Dick, 1987). The Finney County west, the Stanton County, the Haskell County and the Finney County east fields were 24, 66, 31 and 8 km from the weather station at Garden City, respectively. In each field, two study sites (one in Finney County East), ca. 2 x 20 m, were identified at least 7 m away from field margins. Weekly arthropod sampling started in early July. Four corn plants were collected systematically (every ninth plant along the row) within each study site. The first two plants were examined im mediately for larger arthropods associated with spider mites, including lady beetles and Onus spp., and specimens were collected for species determination. Plants three and four were prepared for processing with a leaf brushing machine. All green leaves on one side of each plant (two half-plants) were removed, cut into lengths to fit 3.8-1 plastic freezer bags, and transported to the laboratory in an ice chest. Leaf samples were processed immediately or refrigerated at 4-6?C for no more than 24 hours before processing. In the laboratory, the leaf sections were removed from the plastic bag, air-dried briefly to eliminate condensation, and processed with a modified (one brush removed) leaf brushing machine (Henderson and McBurnie, 1943; Dick, 1987). The glass collection disks were coated with a thin layer of Annul 535? (DeSoto, Inc.) solution (10% by volume) in 99% ethanol. The collection disks were centered over a 200-section equal area disk (Morgan et al., 1955) and examined with a dissecting microscope. Numbers of small arthropods, including live spider mites, This content downloaded from 207.46.13.71 on Fri, 21 Oct 2016 04:54:40 UTC All use subject to http://about.jstor.org/terms VOLUME 68, NUMBER 4 427 dead spider mites, predator mites, and thrips, were recorded using disk areas of 1/10 (low mite numbers) or 1/40 (high mite numbers). Numbers of arthropods were converted to numbers per whole plant for presentation. Subsamples of 25 50 live and 25-50 dead spider mites were removed systematically, mounted in Hoyer's mounting medium, and examined microscopically for mite species iden tification and N. adjarica infection. Spider mites were considered infected if they contained hyphal bodies, resting spores or had capilliconidia attached directly to the integument. While attached capilliconidia do not automatically result in in fection we considered them more detectable than hyphal bodies and, therefore, a better indicator of infection levels. The early stages of hyphal bodies were difficult to detect. Capilliconidea appear to be the primary agent of inoculation for this group of pathogens (Carner and Canderday, 1968). Voucher specimens of Banks grass mite cadavers infected with N. adjarica are preserved at the Herbarium ARSEF, USDA-ARS-PPRU, U.S. Plant, Soil, and Nutrition Laboratory, Ithaca, New York (% R. A. Humber).