Frequency distribution of larval codling moth, Cydia pomonella L., aggregations on trees in Unmanaged apple orchards of the Pacific Northwest.

The codling moth, Cydia pomonella L. (Lepidoptera: Tortricidae), is a key pest of pome fruits in North America. After locating a pupation site, larvae spin a cocoon from which aggregation pheromone disseminates that attracts conspecific larvae. In two unmanaged apple orchards in Wenatchee and Yakima, Washington State, we systematically surveyed cracks and crevices of tree trunks for cocooning C. pomonella larvae. Aggregates of larvae were found significantly more often than solitary larvae. The number of cocooning larvae in aggregates (=group size) was inversely correlated with the frequency occurrence of that group size. Group size ranged between 2 and 20 cocoons. Height above ground had no effect on location of aggregates. In orchard 1, the cardinal direction of the tree trunk had no effect on location of aggregations, but in orchard 2, aggregations were located significantly most often on the south side of trunks. The mean ratio of males and females in aggregations was 1.08:1 and 1.04:1 in orchards 1 and 2, respectively. Moreover, the number of males in aggregates did not significantly differ from that of females. Our data support the conclusion that larvae seek pupation sites not by chance but in large part in response to pheromone signal and microhabitat cues. The probability of aggregates forming is likely proportional to the population density of C. pomonella.