WITHIN-PLANT DISPERSAL OF BANKS GRASS MITE (ACARI, TETRANYCHIDAE) ON CORN

We studied mite dispersal as a possible causative factor in the within-plant distribution of Banks grass mite (BGM), Oligonychus pratensis (Banks), on corn, Zea mays. Dispersal was measured by the redistribution of adult females among leaves along the vertical axis of corn plants. Groups of adult BGM females were placed on leaves at different heights on the plants and mite distributions were observed after 3 days. Initial position of infestation, mite density, and plant growth stage were evaluated for their influence on mite redistribution. On average, adult female BGM moved up the plant slightly when placed on leaves 1 and 2 (lowest green leaves), moved down the plant when placed on leaves 5-9, and remained on leaves 3 and 4 when placed on those leaves. Growth stage of the corn did not change the pattern of BGM redistribution. Initial density did not affect redistribution but it did affect the number of BGM found after 3 days; however, percentage recovery was relatively low regardless of the leaf on which mites were started. Recovery percentages at the 25 and 50 density levels were significantly lower than at the 100 level in 1986 and 1987. No density effects were found in 1988 when density levels were 50, 100, and 200. Overall, the results suggest that site selection by adult female mites colonizing a plant establishes the pattern of distribution. Mite distribution may be related to differences in leaf age, microclimate, or chemistry at different heights on the plant.