Soybean cultivar resistance to defoliating insects

Utilization of soybean, Glycine max (L). Merr., cultivars with moderate levels of insect resistance can increase grower profits by reducing the use of insecticides and reduce the risk of insecticidal residues in the human food chain. Research was conducted to determine soybean cultivar variation for resistance to defoliation by corn earworm (CEW), Heliothis zea (Boddie); soybean looper (SBL), Pseudoplusia includens (Walker); velvetbean caterpillar (VBC), Anticarsia gemmatalis (Hubner); and beet armyworm (BAW), Spodoptera exigua (Hubner). Fifty-six Maturity Group (MG) V, VI, VII, and VIII genotypes, consisting of 46 cultivars and 10 insect-resistant checks, were evaluated in the greenhouse and field. Greenhouse experiments were conducted by infesting 14-d-old plants with neonate larvae of CEW, SBL, or VBC. Amount of defoliation was visually determined 14 d after infestation. Field experiments were conducted at Athens, CA, by infesting plants in a cage with eggs or neonate larvae of CEW and at Midville, GA, with natural populations of BAW. Two visual estimates of amount of defoliation were made st Athens and three at Midville. There were no differences for levels of defoliation among the cultivars in MG V. There were significant (P < 0.05) differences among the cultivars in MG VI, VII, and VIII. 'Coker 686' and 'Deltapine 566' (MG VI), 'Braxton' (MG VII), and 'Coker 6738' (MG VIII) were the most resistant to defoliating insects in the greenhouse and field. These cultivars showed 50% of the resistance of insect-resistant checks. Partial correlation coefficients, after removing the effect of maturity, indicated the mean greenhouse ratings of the 56 genotypes were positively associated with the Athens field cage (r = 0.72) and Midville field (r = 0.72) ratings (significant at P = 0.01).