Bean Leaf Beetle Early
Seedling Stage Threshold

Many, many calls are coming in from concerned growers and reps worried about the damage that bean leaf beetles (BLB) are doing in their soybeans. So I thought I’d do a recap on a previous article to make sure growers are not spending money on management when it is not needed.

Surprisingly it does take a lot of beetles before it is actually worth spraying, especially for soybeans grown for crushing. Unless the plants are being clipped off, they can grow ahead of much of the injury, putting out new leaves, as long as there are not too many beetles on each plant doing the damage in the first place. But what is the actual number? Like most management decisions, it depends on the price of the crop and the cost of control. Researchers from Iowa State University Entomology Department, Jeffrey Bradshaw, Marlin Rice and David Dorhout, have been working on BLB for many years and have determined the economic thresholds for BLB during the seedling stages of soybeans (Table 1). One thing is clear. It takes quite a few beetles per plant before a spray is necessary, especially when the price of soybeans is low.

This does not take into consideration bean pod mottle virus vectoring (carry and transmit), which is more of a concern for those growing soybeans for seed or export. Even in this situation, the overall viral load in overwintering beetles tends to be very low in Ontario with those in Essex and Kent at potentially a slightly higher risk. In recent surveys conducted by AAFC Harrow (Roberto Michelluti) and OMAFRA Ridgetown (Albert Tenuta and Tracey Baute) the amount of BPMV found in grower fields declined in the past two years (see Table 2).

Two hundred and twenty two commercial fields from Windsor to Ottawa, were sampled and tested in 2005, using DAS-ELISA (Agdia Inc.), for the presence of Alfalfa mosaic virus (AMV), Bean pod mottle virus (BPMV), Soybean mosaic virus, and Tobacco ringspot virus (TRSV). Twenty one fields were found positive for AMV, four for BPMV, sixty two for SMV, and twelve for TRSV. Refer to Table 2 for a comparison between other years.

Research data from Kentucky and Iowa suggest that the ability of overwintering beetles to infect young soybeans is extremely low (0 to 0.5%). The virus overwinters in the digestive system of the beetle and during this time it losses its ability to directly infect soybean plants when the beetle feeds soon after emerging. This means that the overwintering beetles need to feed on an infected soybean or other infected legume in the field, fencerow, etc to regain the ability to infect. Then the virus is able to spread through beetle feeding.

Knowing whether the field has had a history of bean pod mottle virus is critical in the decision process. So what should you be looking for when determining field history of BPMV? Start by asking yourself the following questions?

• In the previous year did I see a significant yield reduction?
• Was the seed harvested from the field mottled or discolored? Did a seed test verified the presence of BPMV?
• Did I see a delay in maturity (“Green Stem Syndrome”) on some plants in the field?
• In the fall, was there a large bean leaf beetle population in the field?

Fields close to overwintering sites (e.g. wooded areas) or early spring food sources (e.g. alfalfa) may be at higher risk. When possible, try to have seed production and food grade soybean fields emerge later than other neighboring soybean fields

Excerpt from Integrated Crop Management Newsletter, Iowa State University, May 15th, 2006 issue.

If you are growing seed or “Identity Preserved ” IP soybeans and have answered yes to all these questions then you may need to control the over-wintering bean leaf beetle population as soon as they see them in their fields. For the rest of the growers, spraying is only necessary if BLB populations have reached the thresholds shown above.

Once the beans are in the V3 stage, the crop has put out enough foliage that it is not nearly as susceptible to the feeding injury. In fact, once the crop is in the V3 stage, the defoliation charts are used instead to assess the damage and it takes 30% defoliation before spraying is necessary (Fig. 1). This 30% defoliation applies until the crop has reached bloom stage.

Figure 1. 30% defoliation of a soybean leaflet. (Marlin Rice, Iowa State University)

There is still much that we need to learn about the interaction between the virus and its insect vector, we do know the virus can survive in infected soybean seed and in overwintering beetles. An area that we now very little about and it may be the most important aspect in the entire disease cycle is on what other perennial legumes harbour the virus during the winter. There has been evidence from Iowa (John Hill) that Canada Tick trefoil (Desmodium canadense) and other Desmodium weed species could be one group that harbours the virus during the winter.

A group of Agriculture and Agri-Food Canada researchers are looking at other alternate weed hosts as part of the project ‘Agroforestry in Agricultural Production Systems (AAPS)’ which is part of the national shelterbelt/biodiversity initiative. In 2005, the group recovered BPMV from Mulberry (Morus alba) within these fencerows (shelterbelts). There are most likely many other alternate hosts that allow the virus to survive the winter and more researcher is needed.

What is the importance of these alternate hosts? Emerging beetle then feed on these infected alternate hosts thus acquiring the virus and then spread it to soybeans. The question around alfalfa keeps coming up and the research indicates that even though you can find bean leaf beetles feeding in alfalfa, alfalfa is not a host of BPMV (which is a good thing!).

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