SOYBEAN YIELD AND PEST-MANAGEMENT AS INFLUENCED BY NEMATODES, HERBICIDES, AND DEFOLIATING INSECTS

Pest management strategies based on crop losses from single stress fac tors may be inadequate where multiple stresses occur. Consequently, ex periments were conducted in 1989, 1990, and 1991 to quantify interacti ve effects of stresses from soybean cyst nematode (Heterodera glycines Ichinohe), acifluorfen chloro-4-(trifluoromethyl)phenoxy]-2-nitro-ben zoic acid) plus bentazon -methylethyl)-(1H)-2,1,3-benzothiadiazin-4(3H )-one 2,2-dioxide) herbicides, and green cloverworm [Plathypena scabra (F.)] on seed yield of soybean [Glycine max (L.) Merr]. Treatments we re combinations of at-planting soybean cyst nematode soil density, aci fluorfen plus bentazon rate, and green cloverworm defoliation level. H erbicides were applied at V6 soybean development, while green cloverwo rm feeding was simulated during R2 to R4 soybean development. Plot yie ld and yield components (pods per plant, seeds per pod, and weight per seed) were determined. Herbicide injury and defoliation independently reduced yield in 1990 and 1991. Yield reductions from defoliation wer e attributed to fewer pods per plant, while herbicide injury decreased pods per plant (1990) and weight per seed (1990 and 1991). Moreover, herbicide injury and defoliation interacted over 1990 and 1991 to decr ease yield. Although stress from soybean cyst nematode alone never aff ected yield, interactions with herbicides and with defoliation reduced yield in 1990. No treatment effects on yield were found in 1989, prob ably due to uncontrolled environmental factors. Because of the 2-yr he rbicides x defoliation effect, lower economic injury levels for green cloverworm were developed for soybean treated with acifluorfen plus be ntazon. To account for potential interactions involving soybean cyst n ematode, growers will be advised to consider adjusting weed and insect management strategies for soybean fields infested with soybean cyst n ematode.