Differential spider mite population densities were created in commercial corn fields in the San Joaquin Valley using various rates of propargite and abamectin. Effects on silage production and grain production were evaluated. Results from 2000 and 2001 studies showed that the within plant distribution of spider mites followed a ?bell-shaped? curve with the highest mite densities on the middle leaves (leaf positions 3 to 7) and the lowest mite densities on the newest leaves and oldest leaves. The 4th leaf in 2000 showed a positive linear relationship between percentage of infested leaves and the cumulative mite-days, indicating that this leaf may be used as a presence/absence sampling method for mite population estimation. Sufficient correlation was not seen in 2001 to support this relationship. Overall, different mite densities were created in plots using miticide treatments. There was a significant trend for lower mite densities in the plots treated with high compared to low miticide concentrations. Hand-infested plots in 2001 resulted in mite densities greater than the untreated control. Despite the different mite populations achieved among plots, no significant differences were found among yields for silage in Yolo County location. A weak linear relationship was found with increased mite populations resulting in decreased grain yields. It is expected that mite populations have to reach higher densities than those observed during 2001 to obtain more significant negative effects on yields. Mite densities in the Kern and Tulare County locations were significantly higher than those in Yolo County. Similar studies are ongoing in 2002.
Species 1: Acari Tetranychidae Tetranychus urticae (twospotted spider mite)
Keywords: population dynamics, binomial sampling
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