Head capsule width as an instar indicator for larvae of the cranberry fruitworm (Lepidoptera: Pyralidae) in southeastern New Brunswick.

The objective of this study was to determine the number of instars of the cranberry fruitworm Acrobasis vaccinii Riley in southeastern New Brunswick based on the distribution of head capsule widths from field and laboratory observations. In 2000, head capsules from field samples were measured across their widest point, and the results were plotted against observed frequencies. The data from field samples suggested that A. vaccinii exhibited five instars in 2000. In 2001, larvae were reared in the laboratory until the final molt, and head capsules were counted and measured. The results were also plotted against observed frequencies. None of the laboratory specimens exhibited more than five instars, supporting the results of the previous year. Various factors are invoked to explain the difference between these results and those of a previous study conducted 50 yr earlier and 200 km away, in which six instars and larger head capsules were reported.