Phenology and Infestation Patterns of the Cottonwood Twig Borer (Lepidoptera: Tortricidae) in Iowa

Cottonwood twig borer, Gypsonoma haimbachiana (Lepidoptera: Totricidae), phenology and infestation patterns on Populus spp. were examined over a 2-year period in Iowa. Weekly sampling of infested shoots during the host growing season verified the existence of five instars. Head capsule size increased nonlinearly from the first to the fifth instar and corresponded to a concomitant geometric increase in the volume of larval feeding galleries. The sampling indicated that the cottonwood twig borer had two generations per year in Iowa. Corresponding with the two generations, two peaks of larval abundance were observed; one in the second week of June and the other in the first week of August. Greater volume of feeding galleries occurred in the early season generation compared with the late season generation. Sampling of infested shoots revealed that more than 80% of infested terminals contained only one active attack (freshly bored hole in tree terminal with frass present); more than 88% of feeding galleries contained only one larva; and more than 80% of the larvae were found in the first active attack nearest the terminal apex. These data were compared with results published on the phenology and attack patterns of the cottonwood twig borer in the southern United States.