Distribution of egg and larval populations of cranberry fruitworm (Lepidoptera: Pyralidae) and cherry fruitworm (Lepidoptera: Tortricidae) in highbush blueberries

Abstract

The spatial distribution of immature stages of the cranberry fruitworm, Acrobasis vacinii Riley, and cherry fruitworm, Grapholitha packardii Zeller was studied in Michigan blueberry farms. Single blueberry plants or individual clusters of fruit were compared as sampling units. Distributions of eggs and larvae at each sampling date were described by fitting data to either Poisson (random) or negative binomial (aggregated) distributions, and by calculating parameters of Taylor's power law. Additionally, two methods were used to calculate optimal sample sizes for use in future pest sampling. In one approach, Taylor's power law parameters were used to compute optimal sample sizes needed to estimate populations at two fixed-precision levels, 10 or 20%. In another method, the minimum number of samples required to collect at least one insect in 95% of samples was calculated. Results based on Taylor's power law parameters suggest that prohibitively large sample sizes would be required for even 20% precision, whereas the other method required substantially fewer samples and may thus be of more practical value in a pest monitoring program. All insect populations varied between aggregated and random distributions over the season, but A. vaccinii eggs and larvae were more often aggregated than G. packardii. Analysis of within-field distribution of fruitworm populations showed that A. vaccinii eggs were significantly more abundant in blueberries closer to woods when populations were at their peak. The distribution of eggs suggests that adjacent wooded habitats, which often contain wild hosts of this insect, may provide a source for individuals that colonize commercial fields.