SEASONAL ABUNDANCE OF DIAMONDBACK MOTH (LEPIDOPTERA, YPONOMEUTIDAE) ON CRAMBE ABYSSINICA

Little formation is available on seasonal abundance of diamondback mot h, Plutella xylostella (L.), in the northern Great Plains. This study was conducted in Fargo, ND, in 1993, 1994, and 1995 to examine the sea sonal population changes of P. xylostella in the crop host, crambe, Cr ambe abyssinica Hochst. ex R. E. Fries, and the weed host, field penny cress, Thlaspi arvense L. The abundance of adults was measured using p heromone traps and by flusing them from vegetation. The 1st peak in ad ult density was observed in the weed host and the subsequent peaks in the crop were recorded at approximate to 300 DD (base 7.5 degrees C) i ntervals. In 1993, these peak flights occurred in early to mid-June an d July; in 1994 in early to mid-June, mid-July, and mid- to late Augus t; and in 1995 at end of June, mid-July, early to mid-August, and earl y September. The sex ratio based on larval collections was approximate to 1:1. There was a strong relationship between the egg density and v isual counts of flushed adults. From the comparison of phenology of th e crop host and P. xylostella, the 1st-generation larvae may be injuri ous if the crop is planted early and no weeds are available to oviposi ting females near the crop. The larvae present in the 2nd half of July may potentially injure the ripening crop.