Relationship among Sampling Methods in Density Estimates of Pear Psylla (Homoptera: Psyllidae): Implications of Sex, Reproductive Maturity, and Sampling Location

Abstract

Correlations among three sampling methods in counts of summerform pear psylla, Cacopsylla pyricola Foerster, were determined. Counts obtained using yellow sticky traps, beat trays, and open-ended organdy bags were linearly and positively related. Factors that affected correlations among methods included sex, height in the tree canopy from which the sample was obtained, and reproductive maturity of the insects. For a given beat tray count or bag count, sticky trap catches were larger for males than females, were larger in the upper canopy than in the lower canopy, and were reduced during sampling intervals in which most females were reproductively immature. These results suggest that males were more active than females, that psylla in the upper canopy were more active than those in the lower canopy, and that reproductively immature psylla were inactive comparatively. Absolute densities (psylla per leaf) were estimated by dropping openended organdy bags over pear shoots. This method proved to be time consuming, and therefore a regression equation that transformed beat tray counts into numbers of psylla per leaf was fitted; the model explained 49.3% of the variation. Sex ratios varied with sampling method and height in the canopy. Sticky trap counts were more male-biased than the other two sampling methods. Male bias was larger in the upper canopy than in the lower canopy. Percentage of female psylla that were reproductively mature varied with sampling method and height in the canopy. Females collected with sticky traps and in the upper canopy were often older than those collected with bags or in the lower canopy. Wing size of males and females did not differ between canopy heights. Wing size decreased between subsequent generations.