DIAMONDBACK MOTH (LEPIDOPTERA, PLUTELLIDAE) CONTAMINATION OF CABBAGE TRANSPLANTS AND THE POTENTIAL FOR INSECTICIDE RESISTANCE PROBLEMS

Samples collected from 1989 to 1992 document that the diamondback moth , Plutella xylostella (L.) was introduced into New York in early sprin g on cabbage, Brassica oleracea capitata (L.) transplants grown in the southern United States. During 1989, transplant shipments from five t ransplant companies in Florida, Georgia and Maryland were sampled for P. xylostella. In 1989, average seasonal infestations per transplant c ompany ranged from 1.3 to 3.5 P. xylostella per 100 transplants. Durin g June, when the majority of transplants arrived in New York, P. xylos tella infestations were as high as 12.8 insects per 100 transplants on an individual shipment. Infestations by cabbage looper, Trichoplusia ni (Hubner), 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 stan dard susceptible field population, the P. xylostella which were collec ted from transplants demonstrated moderate to high (> than 100-fold in one case) levels of resistance to permethrin or methomyl. In 1990, av erage seasonal infestations per transplant company varied from 0.3 to 12.0 P. xylostella per 100 plants, but an individual shipment from Flo rida had 30.4 P. xylostella per 100 transplants. A population of P. xy lostella collected in 1990 from Florida transplants had >200-fold resi stance to methomyl. Despite intensive treatments, a New York grower wh o used the transplants with high contamination of resistant P. xyloste lla was unable to achieve acceptable control in his field. Samples col lected from 1989 to 1992 from a transplant grower in Maryland indicate that better management in the field can reduce contamination levels t o < 0.5%. The introduction of P. xylostella, especially those resistan t to insecticides, on transplants poses a serious threat to cabbage gr owers and interregional management strategies should be adopted.