Effects of dust caused by annual crops adjacent to apple orchards on biological control of mites

Five apple orchards exposed to varying amounts of dust were used. In each orchard 25 evenly spaced trees were identified as sample trees. One leaf was taken from the periphery and one from the inside of each tree. Adult phytophagous and predatory mites were counted on these leaves, which were placed in a paper bag and transported to the laboratory where all stages were counted microscopically. One leaf was taken from each tree for determining the amount of dust. They were placed individually in paper bags. In the laboratory the dust was washed off and filtered through pre-weighed filter paper, which was dried and re-weighed. The difference in weight before and after filtering represented the weight of dust on the leaves. The area of each leaf was determined, enabling estimation of the amount of dust in terms of g m^-2. The season with the least dust had the most number of mites. Therefore, seasons during which there is a lot of dust do not necessarily result in mite outbreaks. There was no relationship between the amount of dust on leaves from different orchards and mite numbers, suggesting that dusty orchards do not necessarily harbour high mite population levels. Of the 15 correlations between the amount of dust on individual trees and the number of mites on these trees, two were marginally significant, and one was highly significant, but the correlation was negative. Therefore, trees with a lot of dust do not always harbour more mites than trees with less dust.