HORTS CIENCE 25(12):1643-1646. 1990. Selection for Resistance to Diamondback Moth (Plutella xylostella) in Cabbage M.H. Dickson Department of Horticultural Sciences, New York State Agricultural Experiment Station, Cornell University, Geneva, NY 14456 A.M. Shelton and S.D. Eigenbrode Department of Entomology, New York State Agricultural Experiment Station, Cornell University, Geneva, NY 14456 Margaret L. Vamosy and Marvin Mora Panamerican School of Agriculture, Zamorano, Honduras Additional index words. insect resistance, breeding, Brassica oleracea var. capitata Abstract. Inbreds and hybrid cabbage (Brassica oleracea var. capitata) selected for resistance to diamondback moth (DBM, Plutella xylostella L.) were tested in the field in New York and Honduras for resistance. In New York, plants were inocrdated with up to 400 eggs per plant to enhance the severity of the tests. In Honduras, where natural populations of DBM were high, especially in the dry season, there were three distinct classes of susceptibility to DBM: the very susceptible controls or standard cultivars, the highly resistant glossy-leaved lines, and the intermediate selections with normal leaf bloom. Some normal-leaf hybrids were more resistant than either of their parents, which indicates the need to select for specific combining ability for high re- sistance levels. At maturity, the glossy-leaved hybrids produced marketable crops with: out aid of chemical sprays. The diamondback moth (DBM) is the ma- jor limiting factor in crucifer production in tropical areas of the world. In the Philipp- ines and Southeast Asia, Central and South America, as well as in the Caribbean Basin, the current strategy for controlling DBM with insecticides is failing because of resistance to insecticide (Talekar and Griggs, 1986). Such resistance is also becoming widepsread in the United States, as evidenced in a survey of 40 DBM populations collected and bioas- sayed in 1988–89 (Shelton and Wyman, un- published data). Because of resistance to insecticide and insecticide residues on food crops and in the environment, research is needed to develop new control strategies that are more environmentally sound and more durable. Promising approaches include the development of resistant crop cultivars (Dickson et al., 1986). In 1972 (Dickson and Eckenrode, 1975), we identified PI 234599, a glossy-leaved cauliflower, as being highly resistant in the field to the imported cabbage worm (ICW) (Pieris rapae L.) and the cabbage looper (CL) [Trichoplusia ni (Hubner)]. In 1978, we found it was, also resistant to DBM in Australia and the United States (Dickson and Eckenrode, 1980). Genetic studies showed the resistance was linked to the glossy dark-green leaf found Received for publication 1 Nov. 1989. The cost of publishing this paper was defrayed in part by the payment of page charges. Under postal regu- lations, this paper therefore must be hereby marked advertisernent solely to indicate this fact. in PI 234599. In F 2 populations, there was some variation among glossy plants for the level of resistance, but all were moderately or very resistant, while nonglossy plants were moderately resistant or very susceptible (Dickson and Eckenrode, 1980). Further- more, we observed that when glossy selec- tions were crossed with nonglossy tolerant selections, the F 1 was often more susceptible than a cross between two nonglossy lines with intermediate insect resistance. We con- cluded that this was the case because the glossy-leaf resistance masked other genes for resistance or susceptibility. In addition to the resistance in the glossy leaf, there was also evidence of a preference for oviposition (Jure et al., 1984) on glossy-leaved lines in the greenhouse, although this preference was not evident in field studies (Shelton et al., 1988). Most of the above studies were done with natural field populations of lepidopterous pests, and levels of resistance were based on number of eggs, larvae, and/or pupae ob- served or on damage levels. While these studies have been useful, it was recognized that in small-plot evaluations, small differ- ences in ovipositional preferences may cause large differences in larval counts and plant injury, but in a “nonchoice” situation, such as a commercial field, such differences may not occur (Cantelo and Sanford, 1984; Shel- ton et al., 1988). To overcome this problem, we began to inoculate field-grown plants in 1985 with known numbers of eggs of DBM, CL, or ICW and then evaluate the survivor- ship and plant injury. The threshold recom- mended for spraying fresh-market cabbage in New York state to control DBM is 10 larvae/plant. Because resistance to the three insect species was similar and because DBM was a more important pest on a worldwide basis, we inoculated plants only with eggs of DBM beginning in 1986. In this study, we report the results of screening inbreds and hybrids in the field in New York state using a procedure of placing laboratory-reared DBM eggs directly on each plant to be tested and then assessing the in- jury. These results are compared to tests conducted in Honduras using the same plant lines, but tested under severe natural infes- tations. New York trials. In 1986,52 cabbage lines that had been selected for ICW and CL re- sistance in the field were screened for DBM resistance. The selections were developed from the cross of PI 234599 cauliflower to cabbage, followed by selection, backcross- ing, recombining, and reselection. Resis- tance was determined by rating the plants for injury on a scale of 1 (no damage) to 5 (se- vere damage) ≈3 weeks after the eggs were placed on the plants. Based on these results, selections were made, inbreds increased, and hybrids developed. These F 1 hybrids were screened in the field in 1987 (two replicates) and 1988 (four replicates), both hot, dry sea- sons conducive to high DBM population. Plantings in both years consisted of a ran- domized complete-block design with 10 plants per plot. Plants were spaced in rows 0.90 m apart with a between-plant spacing of 0.45 m. An insecticide was applied during trans- planting for maggot control, but no further insecticides were applied. Three to five plants per plot were inoculated with DBM eggs. These eggs, laid on aluminum foil, were ob- tained from laboratory colonies established from insects collected annually in the field near Geneva, N. Y., and maintained on an Table 1. DBM damage scores in cabbage par- ents and hybrids in 1987 and 1988 in New York. z 1 = No damage, 5 = severe damage. y Glossy leaf. H ORTS CIENCE, VOL. 25(12), DECEMBER 1990