Bean leaf beetle: Management decisions

For food-grade soybeans, bean pod mottle virus can be an important management problem. Soybean grain is subjected to stringent standards of seed-coat (or testa) quality when sold for food. However, for soybean grain where seed-coat quality is not a concern, this virus is of lesser concern because BPMV does not affect the color of a soybean seed beneath the seed coat. However, for some soybean varienties yield may also be affected by BPVM and bean leaf beetles. In particular, soybean fields with a recent history of a large number of bean leaf beetles may be at risk for yield and quality reductions from both bean pod mottle virus (BPMV) and bean leaf beetles.

Predicting outbreaks for management decisions
As on previous pages, bean leaf beetle populations essentially have boom and bust cycles. Cold winters are thought to keep beetle abundance in check. For a few years Integrated Crop Management, Iowa State University, has published the predicted winter survival (or conversely mortality) of bean leaf beetles from the nine Iowa Crop Reporting Districts. Often these predictions are made in April at the end of the beetle's overwinter period. However, it might be helpful to predict beetle mortality throughout the overwintering period and to compare our predictions to yearly trends. Predicting outbreak years earlier in the winter could help growers make better-informed management decisions (e.g., planting date and seed treatments) further ahead of planting. The "average date of predicted mortality" table helps with this concept. The table shows the average date for 50 and 70% predicted mortality of overwintering bean leaf beetles as derived from a 18-year average using the predictive model of Lam and Pedigo (2000) for the nine crop reporting districts in Iowa. In other words, assume that these dates represent times when beetle populations normally reach 50 or 70%. Maintain a running total of cumulative bean leaf beetle mortality to the current date and compare it to the expected day for a given level of mortality (e.g., 50% on 13-Jan for the Northwest CRD). If the current year's 50 or 70% mortality level occurs earlier than normal that it is likely a high mortality year for the bean leaf beetle.

Average bean leaf beetle mortality while overwintering: Average (based on 18 winters, 1989-2007) predicted mortality of overwintering bean leaf beetles by crop reporting district. Based on the bean leaf beetle predictive mortality model of Lam and Pedigo (2000).Average bean leaf beetle mortality while overwintering: Average (based on 18 winters, 1989-2007) predicted mortality of overwintering bean leaf beetles by crop reporting district. Based on the bean leaf beetle predictive mortality model of Lam and Pedigo (2000).

Chemical Control
Currently no thresholds are available for managing bean pod mottle virus; however, insecticidal control has shown potential for managing this virus by reducing vector abundance(Krell et al. 2004). Pyrethroid sprays at two times during the season -- at the arrival of overwintered beetles in late May or early June seedling soybean and at the presence of teneral, first-generation beetles in July. The idea behind this management tactic is to reduce the initial inoculation of the field (targeting the overwintered beetles) and the mid-season inoculation and spread of the virus (targeting the teneral first-generation beetles).

Recently a study was completed at Iowa State University concerning the effect of insecticidal seed treatments on bean leaf beetles, bean pod mottle virus, and how they might fit within the current management program (Bradshaw et al. 2008). Under high vector abundance seed treatments alone reduced BPMV incidence. However, yield was only improved with the application of a mid-season insecticide application.

Cultural Control
Planting date, although it can protect pods from damage (Pedigo and Zeiss 1996), may (Giesler et al. 2002) or may not (Krell et al. 2004) have a consistent affect on bean pod mottle virus incidence. However, it seems likely that having the first soybean emergence date in a region may increase a crop’s risk of an economically important level of infection.

Managing soybeans for pod damage can be economically important for any soybean grade. Even if a later planting date is employed (Pedigo and Zeiss 1996), monitoring for bean leaf beetles is still very important. For this monitoring a degree day mottle has be developed (Zeiss et al. 1996). The degree days for the first-generation adults are estimated to be 1212 degree days with a developmental base threshold at 46°F. Overwintered female beetles usually begin to lay their eggs after colonizing the bean fields. The degree-day estimation for the first-generation adults is calculated by accumulating the temperature at the week of soybean emergence. For adult beetles use the following:

• Determine what week your soybean plants emerged from the soil.

• Sample your soybean fields one week after the predicted peak emergence. If the number of beetles reaches or exceeds the threshold (Table 2), stop sampling. If the sample is below the threshold, sample the following week. If the sample remains below the threshold, sample a third and final week. If the threshold is not reached, an economic infestation of bean leaf beetles should not occur in your pod-stage soybeans.

• If the first-generation population is above the threshold, do not spray now, but scout the fields again in late August to monitor for the first-emerging beetles of the second generation. When the first beetles appear, spray the field with an insecticide (45-day preharvest interval or less). Based upon the population size of the first generation, it is expected that the second generation will exceed the economic threshold. Fields can be sampled for first-generation beetles by using either a drop cloth or a sweep net.

Bean leaf beetle and bean pod mottle virus decision guideBean leaf beetle and bean pod mottle virus decision guide