Effect of Plant Age on Survival of Diamondback Moth on Two Cabbage Genotypes

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. Knowledge of age-dependent expression of resistance is critical for evaluation of breeding lines and for effective use of resistant cultivars in pest management. Resistance 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 xylostella (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 accession 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 (depending on plant size) and determining percent of larvae surviving 8 to 10 days later (fourth instar (Eigenbrode et al., 1990). Two experimental designs produced plants for inoculation 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 Inoculation), 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.