HORTSCIENCE 25(3):362. 1990. Effect of Plant Age on Survival of Diamondback Moth on Two Cabbage Genotypes S.D. Eigenbrode and A.M. Shelton Department of Entomology, New York State Agricultural Experiment Knowledge of age-dependent expression of resistance is critical for evaluation of breeding lines and for effective use of resis- tant cultivars in pest management. Resis- tance to the Lepidoptera pest complex in some Brassica has been shown to increase with plant’ age (Dickson and Eckenrode, 1980). Our studies of insect resistance now focus on the diamondback moth [Plutella xylo- stella (L.)], the most serious pest of cole crops worldwide (Talekar and Griggs, 1986). Resistance to this insect in glossy Brassica genotypes, descended from cauliflower P1234599, is due to reduced larval survival (Lin et al., 1983; Eigenbrode et al., 1990). The objective of this study was to measure survival of diamondback moth larvae throughout crop development in NY acces- sion 2518, a glossy-leafed resistant inbred cabbage line, and the susceptible hybrid ‘Round Up’. Resistance was measured by inoculating the fourth expanded leaf of test plants with 100 to 200 diamondback moth eggs (de- pending on plant size) and determining per- cent of larvae surviving 8 to 10 days later (fourth instar (Eigenbrode et al., 1990). Two experimental designs produced plants for in- oculation 14, 28, 42, 56, and 70 days after transplanting. In the first experiment (Single Cohort), 45 eight-week-old seedlings of each line were transplanted from Speedling-type flats into the field on 6 June 1988. Six to nine plants from each line were inoculated on 20 June, 5 and 18 July, and 1 and 15 Aug. In the second experiment (Single In- oculation), six to nine g-week-old seedlings from each line were transplanted on 6 and 20 June, 5 and 18 July, and 1 Aug. All plants in this design were inoculated on 15 Aug. Received for publication 10 May 1989. We thank R. Burgess for technical assistance and M.H. Dickson and M.T. Vittum for reviewing early drafts of the manuscript. The cost of publishing this pa- per was defrayed in part by the payment of page charges. Under postal regulations, this paper therefore must be hereby marked advertisement solely to indicate this fact. Station, Geneva, NY 14456 Additional index words. Brassica oleracea, Plutella xylostella The percentage of larvae surviving on ‘Round Up’ was significantly greater (P £ 0.05) than on NY 2518, with the exception of 28 days after transplant in the Single In- oculation (Table 1). In the Single Cohort ex- periment, the percentage of larvae surviving on ‘Round Up’ was lowest at 14 days after transplant, significantly higher at 56 days after transplant, and significantly higher yet at 28, 42, and 70 days after transplant. In the Sin- gle Inoculation experiment, the percentage of larvae surviving on ‘Round Up’ was low- est in the 14- and 28-day periods. In both experiments, larval survival on NY 2518 was consistently low, never higher than 10.3%. In the Single Inoculation experiment, rel- ative resistance generally increased with plant age, although the effect was not significant. This pattern was mostly due to increased lar- val survival with increased plant maturity on ‘Round Up’. In the Single Cohort experi- ment, environmental effects on larval sur- vival in the different inoculations may have obscured plant age effects on relative resis- tance. Resistance in younger ‘Round Up’ and NY 2518 plants may be due to the quality or size of the plants, or both. Transplanted seed- lings, which have relatively hard leaves, may resist feeding until new growth occurs. Also, smaller plants may provide the larvae fewer refuges from rain, desiccation, and other en- vironmental causes of mortality. As a result, diamondback moth larvae may survive poorly on plants of both genotypes soon after trans- plant, while later in crop development only the resistance mechanisms of the glossy plants continue to operate. Regardless of the mech- anisms involved, our study shows that NY 2518 is highly resistant to diamondback moth, due to reduced larval survival, from 2 weeks after transplant. This result is consistent with Dickson and Eckenrode’s (1980) report that a related glossy cabbage had consistently low injury from the Lepidoptera pest complex at 33 and 66 days after transplant. Our result suggests that cultivars resistant to diamond- back moth from transplant to harvest can be derived from NY 2518 or related glossy lines. Literature Cited Dickson, M.H. and C.J. Eckenrode. 1980. Breed- ing for resistance in cabbage and cauliflower to cabbage looper, imported cabbageworm and diamondback moth. J. Amer. Soc. Hort. Sci. 105:782-785. Eigenbrode, S.D., A.M. Shelton, and M.H. Dick- son. 1990. Two types of resistance to the dia- mondback moth in cabbage. Environ. Entomol. (In press.) Lin, J., C.J. Eckenrode, & M.H. Dickson. 1983. Variation in Brassica oleracea resistance to dia- mondback moth (Lepidoptera: Plutellidae). J. Econ. Entomol. 76:1423-1427. Talekar, N.S. and T.D. Griggs. 1986. Diamond- back moth management. Proc. First Intl. Wkshp. Asian Veg. Res. and Devel. Center, Shanhua, Taiwan. Table 1. Percentage of larvae surviving on cabbage cultivar Round Up and diamondback moth-resistant glossy-leafed cabbage NY 2518, and relative resistance of NY 2518 (survival on NY 2518 expressed as a percentage of that on ‘Round Up’) at five plant ages and two inoculation schemes. z ‘Comparisons, using Student’s t test, between ‘Round Up’ and NY 2518 at each point after transplanting in each inoculation scheme were significant (P ≤ 0.001), except 28 days in the Single Inoculation (P = 0.067). Means in each row with the same letter are not significantly different (P ≤ 0.05, Fisher’s protected LSD). NS = nonsignificant. 362 HORTSCIENCE, VOL. 25(3), MARCH 1990 View publication stats View publication stats