Evaluation of Organic Pest Management Treatments for Bean Leaf Beetle--Neely-Kinyon Trial, 2002

Abstract

Introduction Annual organic soybean [Glycine max (L.) Merr.] production in the U.S. has risen to more than 150,000 acres (USDA-ERS, 2005). Critical challenges associated with organic soybean production include weed control and bean leaf beetles (Cerotoma trifurcata Forster) – primarily as vectors for the seed-staining bean pod mottle virus (BPMV) and for providing sites for other seed-staining fungi such as purple stain [Cercospora kikuchii (Mastsumoto & Tomoyasu) M.W. Gardener] and Fusarium spp. Bean leaf beetles generally have two generations a year in Iowa, with overwintering adults from the previous year’s second generation primarily feeding on vegetative soybean stages. First-generation adults, which require an average of 1,212 degree days with a developmental base threshold of 46 °F, usually peak during the early reproductive soybean stage (Lam et al., 2001). Second-generation adults, whose numbers are dependent on the first-generation population size, peak during the pod-filling stage. Feeding by first-generation beetles on soybean leaves seldom results in economic yield losses, but when the second-generation adults emerge from the soil to feed on seed pods, crop damage in late summer can be very significant. The second-generation adults overwinter in the soil and leaf litter where they remain until spring of the following year. The severity of the over-wintering period is a key factor in determining insect survival, with snow cover (Lam and Pedigo, 2000a) and woodland areas (Lam and Pedigo, 2000b) aiding survival. Both generations of bean leaf beetles can transmit the BPMV, although disease incidence is generally greater during pod setting and filling because higher amounts of rainfall often create ideal conditions for spreading the disease. BPMV has been reported to cause yield losses >50% and in 1999, it was estimated that soybean yield losses reached 155,778 metric tons in Iowa due to soybean viruses. The soybean aphid (Aphis glycines Matsumura) is native to China and Japan, and was a new pest in Iowa in 2000. Soybean aphid can reduce yields by direct feeding, and interfering with photosynthesis and growth. Natural enemies, including beneficial fungi, such as Pandora neoaphidis, can infect aphids and give them a red color. Spraying fungicides can decrease the activity of this beneficial fungus.