A survey of Bean pod mottle virus (BPMV) in Iowa counties was conducted and the virus was found throughout the state. A long-term monitoring study (1989 to 2002) of the main BPMV vector, the bean leaf beetle, Cerotoma trifurcata, indicated that, in 2002, populations reached the highest abundance recorded in 14 years. Three potential sources for an early season primary inoculum source were found: (i) soybean (Glycine max) seed, (ii) overwintered bean leaf beetles, and (iii) alternate BPMV host plants. Examination of 5,804 and 8,064 soybean seedlings of two cultivars yielded 0 and 3 seedlings, respectively, infected with BPMV. In a separate test, BPMV was detected in mottled and nonmottled soybean seed. Some mottled seed did not contain BPMV, indicating that soybean seed coat mottling is an unreliable indicator for presence of the virus in seed. Of 194 naturally overwintered bean leaf beetles, only 1 transmitted BPMV to soybean. BPMV was detected serologically only in 1 alternate host, Desmodium canadense, out of 23 naturally occurring plant species collected from the field. The three inoculum sources discovered in Iowa in this study could be important primary sources when vector populations are high and indicate starting points for future epidemiological investigations.
Bean pod mottle virus (BPMV) is a management concern for soybean, Glycine max (L.), producers in the North Central states because it can cause yield loss and reduce seed quality by induction of seed coat mottling. The main vector of BPMV is the bean leaf beetle, Cerotoma trifurcata (Forster). An experiment was conducted in 2000 and 2001 at two locations in northwestern and central Iowa to test three insecticide treatments for suppression of bean leaf beetles, and subsequently, BPMV. Treatments of insecticide applications with lambda-cyhalothrin were 1) a single early-season application (23 g [AI] /ha) (2.5 oz/acre) at the VE-VC soybean developmental stage; 2) two early-season applications, the first the same as treatment 1 and a second at the same rate 9-13 d later; 3) a single early-season application the same as treatment 1, followed by a mid-season application (28 g [AI] /ha (3.2 oz/acre) at approximately R2 (flowering, near 15 July); and 4) an unsprayed control. Application of lambda-cyhalothrin after soybean emergence and again as first-generation bean leaf beetles emerged in northwestern Iowa in 2000 (treatment 3) significantly reduced beetle densities through mid-season, BPMV field incidence by 31.5%, and seed coat mottling by 31.2%, compared with the unsprayed control. Similar effects were measured at the same location when insecticide was applied twice at early season (treatment 2). Yield was 453.7 kg/ha (6.74 bu/acre) greater in treatment 2 and 525.20 kg/ha (7.80 bu/acre) greater in treatment 3 than in the unsprayed control at the northwestern site in 2000. At both locations in 2001 fewer treatment effects were observed, which was likely related to lower beetle populations in that year. Early-season insecticide sprays targeted at overwintered beetles on VC-VE reduced the initial population of vector insects and may have contributed to a lower first-generation population because of reduced overwintered beetle oviposition. In 1 year at one location there was a benefit to an additional mid-season insecticide spray, although effectiveness of spraying at this time could vary based on the magnitude of the vector population.
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