By Sharon Eggenberger, Forrest W. Nutter, Jr., Andrew Gougherty, Alison Robertson, and Erin Hodgson
Bean pod mottle virus (BPMV) and bean leaf beetle (the primary insect vector of BPMV) have been important concerns for Iowa soybean producers during the past two decades. A statewide survey was conducted in 2010 and 2011 to assess current BPMV incidence and to collect information about insect vector abundance and site factors that might influence BPMV epidemics.
ISU field agronomists sampled three soybean fields within each county. Soybean fields were visited at growth stages V2-V3 (two to three trifoliate leaves) and R2-R3 (full bloom to beginning pod set). Insect sampling was conducted at five locations within each field. At growth stage V2-V3, insects were counted on plants and on the ground within 1-meter (39 inches) sections of row. At growth stage R2-R3, 50 sweeps were made at each stop in the field. Population densities of bean leaf beetle, the most efficient vector of BPMV, and minor vectors, including western corn rootworm, southern corn rootworm, Japanese beetle, and colaspis beetles, were counted and mapped.
Bean leaf beetle
Population densities of bean leaf beetle were lower in 2010 and 2011 than they had been in the previous decade. Early-season sampling (soybean growth stage V2-V3) detected only low population densities (0 to 2 beetles per m) in roughly 10 percent of the fields visited in 2010 and 2011 (data not shown). These densities are much lower than those of the previous decade, when researchers found fields with 300+ and 400+ beetles per 50 sweeps in May (in 2006 and 2002, respectively) (Bean leaf beetles return -- with a vengeance!). In 2011, bean leaf beetles were found in 55 percent of the soybean fields sampled at growth stage R2-R3. The highest mean density (133 bean leaf beetles per 50-sweep sample) occurred in a soybean field in Webster County (Fig. 1; data for 2010 is shown for comparison).
Figure 1. Mean number of bean leaf beetles per 50-sweep sample in Iowa counties in 2010 and 2011. Samples were taken at growth stage R2-R3, at 5 locations within three soybean fields in each county.
In 2011, Japanese beetle was found in 18 percent of the soybean fields sampled at growth stage R2-R3 (Fig. 2). Infrequent transmission of BPMV by Japanese beetles was reported in 2007 (Wickizer and Gergerich, 2007). Current economic damage thresholds for Japanese beetles are based on defoliation levels, not on insect densities (see ISU's Soybean Insects Guide).
Figure 2. Mean Japanese beetle population densities per 50-sweep sample in Iowa soybean fields in 2010 and 2011. Sampling was conducted at growth stage R2-R3, in three fields per county.
Southern corn rootworm adults were found in 68 percent of the soybean fields sampled at growth stage R2-R3 in 2011, but populations were substantially lower than they had been in 2010 (Fig. 3). Western corn rootworm adults were detected in only 5 percent of the surveyed fields (data not shown), and at low numbers (<5 beetles per 50 sweeps).
Figure 3. Mean southern corn rootworm beetle population densities in 50-sweep samples in Iowa soybean fields in 2010 and 2011. Sweep sampling was conducted at growth stage R2-R3, in three fields per county.
Two midwestern species of colaspis beetles are BPMV vectors (Giesler et al., 2002). Colaspis beetles were found in 48 percent of the soybean fields sampled in 2011, but in most fields the number of colaspis beetles was low (Fig. 4; data for Colaspis species is combined). Colaspis beetles are not considered to be economically-damaging pests of soybean, but they can cause early-season damage in corn.
Figure 4. Mean colaspis beetle population densities in 50-sweep samples in Iowa soybean fields in 2011. Sweep samples were conducted at growth stage R2-R3, in three fields per county.
Three successive cold winters (2007-2008, 2008-2009, and 2009-2010) have greatly reduced overwintering bean leaf beetle population densities. However, winter temperature is not the only factor influencing bean leaf beetle population densities. Maps of bean leaf beetle population densities for 2010 and 2011 (Fig. 1) show higher populations in the western half of the state, which were not indicated by model predictions (Lam and Pedigo, 2000). The use of insecticide seed treatments also impacts bean leaf beetle population densities (see Seed treatments in soybean: Managing bean leaf beetles).
This spring, record low winter bean leaf beetle mortality was predicted for Iowa's nine agricultural districts (see Mild winter for bean leaf beetle), and bean leaf beetle feeding has been reported in central Iowa (see Bean leaf beetles are moving to soybean). Bean leaf beetle populations can rebound quickly from one year to the next (Nutter, unpublished data). Low bean leaf beetle mortality during the 2011-2012 winter, considered with the bean leaf beetle population density map from 2011, suggests that soybean fields, especially those located in the western half of the state, should be scouted for bean leaf beetle this year. Growers can refer to the economic injury calculator for bean leaf beetle that was published in 2011 and find additional management recommendations in ISU's Soybean Insects Guide.
Giesler, L. J., Ghabrial, S. A., Hunt, T. E., and Hill, J. H. 2002. Bean pod mottle virus: A threat to U.S. soybean production. Plant Dis. 86:1280-1289.
Lam, W.-K. F., and Pedigo, L. P. 2000. A predictive model for the survival of overwintering bean leaf beetles (Coleoptera: Chrysomelidae). Environ. Entomol. 29(4):800-806.
Wickizer, S. L., and Gergerich, R. C. 2007. First report of Japanese beetle (Popillia japonica) as a vector of Southern bean mosaic virus and Bean pod mottle virus. Plant Dis. 91(5):637.
Sharon Eggenberger (Research Associate) and Andrew Gougherty (former Research Associate) work in the lab of Forrest W. Nutter, Jr., (Professor, Department of Plant Pathology and Microbiology). Erin Hodgson (Assistant Professor, Entomology) and Alison Robertson (Associate Professor, Plant Pathology and Microbiology), are ISU researchers with extension responsibilities.