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.

Evaluations of the efficacy and persistence of Beauveria bassiana (Balsamo) Vuillemin for control of the diamondback moth, Plutella xylostella (L.),on cnlcifers were done in growth chambers, the greenhouse, and the field. In growth chamber studies done at 21 or 26°C and 60 or 90%RH, neither temperature nor humidity affected the survival oflarvae, but treatment with fungal spores always provided significantly greater mortality than the control. In 2 grecnhouse trials, 1 application of B. bassiana spores suspended in water or oil significantly reduced larval populations compared to controls. In a field trial, both treatment and insect stage at treatment (2nd and 3rd to 4th instars) significantly affected larval survival. The fungus, formulated as a wettable powder at 2 rates and as an emulsifiable suspension at a high rate, provided significant reductions in larval counts. Two applications of the wettable powder at the higher rate resulted in lower larval counts than did a single application. Two applications (but not 1) of wettable powder at the lower rate resulted in significantly lower counts of 2nd instars but not 3rd to 4th instars. Both treatment and time affected both the persistence of viable B. bassiana spores on leaves and the retention of efficacy of treated leaves for P.xylostella larvae. These trials indicate the potential for including B. bassiana in an overall management program for P. xylostella.

Entomopathogenic nematodes-Heterorhabditisbacteriophora Poinar (Oswego strain), Steinemema carpocapsae (Weiser) (NYOOI strain), Steinemema carpocapsae (25 strain), Steinemema feltiae Filipjev (=Neoaplectana carpocapsae Weiser) (369 strain), Steinemel1la feltiae (27 strain), and Steinemel1la riobravus Cabanillas and Poinar (355 strain)-were examined for pathogenicity against cabbage maggot, Delia radicum (L.), larvae in the greenhouse and field. Applications (per plant) of 3,000 and 4,000 infective juveniles of S. feltiae (369 strain), 30,000 infective juveniles of H. bacteriophora (Oswego strain), and 300 and 30,000 infective juveniles of S.feltiae (27 strain) reduced the number of D. radicum that developed to pupae on potted cabbage plants. H. bacteriophora (Oswego) at applications of 3,000 and 30,000 infective juveniles per plant and S.feltiae (27 strain) at applications of 30,000 (but not 3,000) infective juveniles per plant significantly reduced root damage caused by larvae of D. radicum. Logarithmically increased dosages between 100 and 100,000 infective juveniles per plant of S. feltiae (27 strain) linearly reduced the number of D. radicul1l pupae that developed on potted cabbage plants and the damage caused to the roots by D. radicul1llarvae. Root and stem dry weights of cabbage plants infested with D. radicum were significantly greater for plants inoculated with 100,000 infective juveniles of S. feltiae (27 strain) than for plants not inoculated with nematodes. Nematode inoculation did not prevent significant losses in root or stem dry weights at dosages less than 100,000 infective juveniles per plant. Soil surface applications of 100,000 and 200,000 infective juveniles per plant of S. feltiae (27 strain) were more effective than subsurface applications in preventing damage by natural or augmented populations of D. radicum larvae on cabbage in the field. However, mortality rates of wax moth larvae exposed to soil samples treated with S. feltiae (27 strain) suggested that this nematode showed greater persistence when applied beneath rather than on the soil surface. KEY WORDS

Control of Thrips tabaci Lindeman was evaluated using insecticides and host plant resistance. In 7 insecticide trials conducted from 1987 to 1996, using a total of 17 different insecticides, insecticide applications reduced thrips damage in only 2 of the trials. In 8 plant resistance trials conducted from 1985 to 1993 using >300 varieties or inbreds, some plant lines significantly reduced thrips injury in each trial. A 1996 trial testing variety and insecticide together found that both factors significantly affected thrips damage, although their interaction was not significant. Insecticide treatments in the 1996experiment were unnecessary for the more tolerant varieties but helped reduce damage to acceptable levels in many of the moderately susceptible cultivars. In the highly susceptible varieties, we concluded that even the frequent application of insecticides was not sufficient to keep thrips damage at acceptable levels. Planting tolerant varieties, however, is a reliable way to keep thrips damage at low levels, even without insecticides.