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Influence of Irrigation on Susceptibility of Selected Soybean Genotypes to Soybean Looper

USDA-ARS Southern Insect Management Lab., P.O. Box 346, Stoneville, MS 38776
USDA-ARS, Soybean Research Unit, P.O. Box 343, Stoneville, MS 38776

^* Corresponding author (llambert{at}ag.gov).

Soybean [Glycine max (L.) Merr.] is usually grown under water deficit conditions in the southern USA and is often attacked by defoliating insects. Field studies were conducted during a 2-yr period with ‘Centennial’ (insect susceptible) and D75-10169 (insect resistant) bean genotypes. Plants were grown either with or without irrigation to determine the influence of water-stressed plants on soybean looper [Pseudoplusia includens [(Walker)] oviposition, development, survival, and damage to plants. There were no significant (P < 0.05) differences in egg deposition on the two plant genotypes, and effect of irrigation on egg deposition was not consistent. At 2 wk after insect release on plants, neither genotype nor irrigation treatment significantly affected number of larvae. Larvae developing on nonirrigated plants of both genotypes were significantly smaller than those developing on irrigated plants. All surviving larvae on Centennial pupated and all larvae on D75-10169 died by 18 d after the onset of oviposition. Defoliation by insects of irrigated plants of both genotypes was more than 50% greater than defoliation of nonirrigated plants. Without insects, seed yields of the two genotypes were similar and were greater from irrigated than from nonirrigated plants. Insect infestation significantly reduced yield of both genotypes, but the decrease in Centennial yield was greater. Yield reduction resulting from insect infestations was greater for irrigated than for nonirrigated plants. These findings show that soybean plants growing under water deficit conditions may allow a delay in initiation of soybean looper control measures, especially since yield potential and resulting profit potential are low.

Received for publication May 6, 1994.