Eleven populations of diamondback moth, Plutella xylostella (L.), were collected in 1990 from Brassica plants in six states of the United States and in Indonesia and tested for their responses to two formulations of Bacillus thuringiensis subsp. kurstaki (Javelin we and DipeI2X), permethrin, and methomyI. Populations from Florida that had been treated extensively over several years with these insecticides displayed significantly higher LC50s. In 1992, field tests in geographically separate areas in Florida and laboratory assays of populations from those fields indicated control failures and resistance to products containing B. thuringiensis subsp. kurstaki and low levels of resistance to a product containing B. thuringiensis subsp. aizawai (XenTari). These B. thuringiensis subspp. differ in the number of toxins produced, but whether resistance to them is a result of cross-resistance or independent selection was not determined. We documented significant differences between the response of resistant and susceptible populations to two products containing B. thuringiensis subsp. kurstaki, thus suggesting that the products actually differed in the number or amounts of toxins. In laboratory bioassays of three products containing B. thuringiensis suhsp. aizawai and two products containing B. thuringiensis subsp. kurstaki, the variation in response (as determined by resistance ratios) varied by 321-to 461-fold for B. thuringiensis subsp. kurstaki and by 3-to 4.1-fold for B. thuringiensis subsp. aizawai. These studies indicate increasing resistance problems caused by intensive use of any B. tlmringiensis product. We conclude that if B. thuringiensis is to remain a durable insecticide in parts of the world where resistance does not already occur, other tactics such as biological control, host-free periods, plant resistance, and cultural controls must he incorporated into the management programs.

Rape seedlings, Brassica napus L., and a wheat germ-based artificial diet were compared as media for rearing diamondback moth (DBM), Plutella xylostella (L.), for six generations. Mean pupal weight and total number of eggs laid per female were always greater when larvae were reared on artificial diet; however, percentage of eggs hatching was usually greater and development time usually shorter when larvae were reared on rape seedlings. High larval survivorship (>70%) could be obtained on either media. Larvae which were reared on artificial diet were consistently more susceptible to the insecticides methomyl and permethrin, indicating potential problems in using artificial diet for insecticide studies. When larvae were reared on either medium for six generations and then transferred to cabbage, larval survivorship was nearly equal, indicating that either method could be used for artificially incoculating plants for host plant resistance studies. Although it was easier and cheaper to rear DBM on artificial diet, recommendations for using one rearing method over the other must be based on the ultimate use of the colony.

Samples collected from 1989 to 1992 document that the diamondback moth, Plutella xylostella (L.) was introduced into New York in early spring on cabbage, Brassica oleracea capitata (L.), transplants grown in the southern United States. During 1989, transplant shipments from five transplant companies in Florida, Georgia and Maryland were sampled for P. xylostella. In 1989, average seasonal infestations per transplant company ranged from 1.3 to 3.5 P. xylostella per 100 transplants. During June, when the majority of transplants arrived in New York, P. xylostella infestations were as high as 12.8 insects per 100 transplants on an individual shipment. Infestations by cabbage looper, Trichoplusia ni (Hübner), imported cabbageworm, Artogeia (=Pieris) rapae (L.), and cabbage webworm, Hellula rogatalis (Hulst), on an individual shipment were as high as 19.7 insects per 100 transplants. Compared with a standard susceptible field population, the P. xylostella which were collected from transplants demonstrated moderate to high (> than 100-fold in one case) levels of resistance to permethrin or methomyl. In 1990, average seasonal infestations per transplant company varied from 0.3 to 12.0 P. xylostella per 100 plants, but an individual shipment from Florida had 30.4 P. xylostella per 100 transplants. A population of P. xylostella collected in 1990 from Florida transplants had >200-fold resistance to methomyl. Despite intensive treatments, a New York grower who used the transplants with high contamination of resistant P. xylostella was unable to achieve acceptable control in his field. Samples collected from 1989 to 1992 from a transplant grower in Maryland indicate that better management in the field can reduce contamination levels to < 0.5%. The introduction of P. xylostella, especially those resistant to insecticides, on transplants poses a serious threat to cabbage growers and interregional management strategies should be adopted.