Implications of changes in arthropod distribution following chemical application
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The influence of pesticide application on the within-field distribution of arthropods was investigated forTetranychus urticae, the twospotted spider mite, on strawberries. Analyses of dispersions based onGreen's coefficient,Iwao's regression of mean crowding on the mean, andTaylor's power law all indicated that mite populations were highly aggregated initially. As densities increased, more of the avialable niches were filled, leading to a less clumped dispersion. However, pesticide applications causing greater than 99.9% mortality acted in a nearly density independant fashion and, although the originating populations were similar in number, did not produce dispersions equivalent to the initial migrants. As a result, ignoring these changes by developing sampling plans based on dispersion indices which generated a single slope for an entire data set, led to statistical errors that invalidated the sampling programs. In order to accurately reflect the field biology of the spidermites, sampling plans for pre and post-treatment populations were substantially different.
The impact of such changes in dispersion were graphically demonstrated using both sequential and binomial sampling techniques. Both methods showed that fewer samples were necessary to estimate densities at a given precision level for post-treatment populations. Also, these techniques indicated that post-treatment populations had similar, but significantly different, dispersions. The implications of changes in pre and post-treatment dispersions, as well as problems associated with inconsistant dispersions following pesticide use, are discussed.
KeywordsSampling Plan Active Stage Pesticide Application Precision Level Binomial Sampling
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