Pest & Crop Newsletter

Bean Leaf Beetle Winter Survival and Early Season Damage– (John Obermeyer and Larry Bledsoe)

Bean leaf beetle may feed on soybean cotyledons, foliage, and pods. For an animation depicting the bean leaf beetle biology and damage, go to the web site http://www.entm.purdue.edu/entomology/ext/fieldcropsipm/flash/blb14.html. Occasionally, beetle numbers and damage warrant insecticide treatments to protect soybean stands, yield, and/or seed quality. Recently there’s been a renewed interest with this pest in the seed industry because of its propensity to vector the bean pod mottle virus (BPMV). Whereas BPMV is a major concern in the western Corn Belt, little of this disease has been seen in Indiana.

BPMV was first identified in the Midwest in the 1950s, and bean leaf beetle was quickly implicated as a major vector of this disease. It is believed that the virus over winters in wild and cultivated legumes (e.g., clovers). When the over wintering bean leaf beetle “awakens,” early in the spring, it begins to seek leguminous plants in which to feed. Should a plant be infected with BPMV, the bean leaf beetle will likely carry it to its next feeding site in its mouthparts. If soybean plants are emerging nearby, the bean leaf beetle may spread the virus to them when they colonize that field. Initial research studies from Nebraska and Iowa show that this early infection is most detrimental to soybean yield and seed quality.

Why BPMV incidence appears to be low and sporadic in Indiana is not understood. Logic tells us to look at the major vector…the bean leaf beetle. Bean leaf beetle numbers have been high in Iowa and Nebraska, and they expect large numbers again this season. Whereas the beetle numbers in Indiana have been relatively lower the last couple seasons, see Graph 1. Winter temperatures have an impact on the survival of several field crop insects (see Pest&Crop #1, February 21, 2003). Frankie Lam, Regional IPM Specialist in southwestern Indiana, while at Iowa State University developed a winter survival model for the bean leaf beetle. Graph 2 shows the expected mortality of bean leaf beetle, from temperatures gathered at West Lafayette, Indiana. Should the model be correct, bean leaf beetle numbers are expected to be low this spring.

Graph 1. Bean Leaf Beetle, Soybean Sweeps, Indiana Regions, 1997-2002 (number/100 sweeps)	Graph 2. Percent Expected Mortality of Over Wintering Bean Leaf Beetle.

Some have suggested that controlling the beetle while applying post-emergent herbicides will “kill two birds with one stone.” Given the current situation with the bean leaf beetle and BPMV, we certainly don’t recommend prophylactic insecticide applications. More importantly, the timing of bean leaf beetle feeding activity and herbicide application do NOT coincide. The tried and true approach is to use timely field scouting and economic thresholds based on research. For cotyledon- and unifoliolate-stage soybean, refer to the following threshold values:   

Another management option for the bean leaf beetle is planting date. Avoiding an early planting date for soybean grown for seed should avert over wintering beetles flocking to the field. Rather, they will infest the first emerging soybean fields in the area. This is not suggesting “late” planting, because later maturing fields in the fall are prone to second generation beetle damage to the pods. Pod scarring may lead to discolored and shriveled seed from disease organisms other than BPMV. The key seems to be timely planting of soybeans to lower the incidence of bean leaf beetle feeding and BPMV disease!

Close-up of the bean leaf beetle mouthparts.

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Time to Think Like a Black Cutworm Moth– (John Obermeyer and Larry Bledsoe)

Look at his week’s “Black Cutworm Adult Pheromone Trap Report.” You can see that several moths have been trapped by our dutiful cooperators; this is quite normal for this time of year. Sometime in the next week or two we will begin our heat unit accumulations to predict cutting, published in future Pest&Crop, “Weather Update.” We still await the moth arrivals of later April, which usually account for our crop damage in May and early June.

Weedy winter annuals in the spring.

The important point now is .... will pregnant black cutworm moths arriving in the Hoosier state, find your fields attractive to lay eggs in? Moths are particularly attracted to winter annuals, such as chickweed and mustards. Fields that are showing plenty of green are at highest risk for cutworm damage. Remember, corn is one of the black cutworms least favorite foods, it just so happens it is the only plant remaining by the time larvae have emerged and weeds have been killed. Research has shown that cutworm larvae starve if weeds are treated with tillage or herbicide 2-3 weeks before corn emergence. Something to be said for controlling weeds in order to manage this insect pest.  More later!

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Click for Table.
Black Cutworm Adult Pheromone Trap Report.

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Catch the Drift?- Hopefully Not!- (Glenn Nice, Thomas Bauman, and Bill Johnson)

As we enter the busy spray season, we felt it was important to review a few issues concerning spray drift. Spray drift is becoming an important issue in many communities in both the rural and urban settings. There are a number of extension publications on spray drift and recent ones by the University of Wisconsin titled Managing Pesticide Drift in Wisconsin: Field Sprayers (X1000) and University of Missouri Extension titled Controlling Drift of Crop Protection Materials (guidesheet G1886) address in more detail many of the specific issues responsible for minimizing pesticide drift. Remember, drift increases as spray droplet size decreases because smaller droplets fall more slowly and are more easily moved by wind; when there is no air movement a 50 mm droplet will fall at a theoretical rate of 15 ft./min.  A 200 mm droplet will fall 150 ft./min.

Many of the conditions that effect drift effect droplet size and the percent of small droplets produced.  Below is a list of variables that can affect droplet size (Table 1).  No matter what the situation, there will always be a certain amount of small particles.  The key is to reduce the percentage of these small particles. Drift increases as wind speed increases because droplets are carried farther before they can be deposited; as one would expect wind is one of the larger considerations.  Wind direction can be both a tool and a problem.  As a simple tool, try to spray when the direction may lead drift away from sensitive plants and residences.  As a problem: spraying up wind of sensitive plants and residences.  However, we can always wait around for the “winds to change.” Drift increases as nozzle height from target surface increases because droplets take longer to reach the target surface; drift increases as temperature rises and relative humidity falls because water in spray droplets evaporates, making droplets smaller.

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Early Season Weed Control in Corn – (Bill Johnson, Glenn Nice, and Tom Bauman)

Corn planting is underway and will continue in many portions of the state during the next week if the weather allows. Residual herbicide activity is very important in corn production due to the prevalence of weed species that emerge later in the growing season. If lack of or excess rainfall raises concern over the performance of soil-applied herbicides, you will need to scout corn fields within 1 to 2 weeks after planting to determine if weeds are escaping soil-applied treatments. Atrazine and atrazine premixes (Bicep II Magnum, Cinch ATZ, Harness Extra, Degree Extra, Fultime, Keystone, Guardsman, Guardsman Max, Leadoff, and Lariat) are used on a widespread basis. Most herbicide labels for these herbicides state that 0.5 to 1 inch of rain is needed within 1 to 2 weeks after emergence for proper activation and efficacy on target weeds. If the herbicide is not activated (moved into soil solution), it cannot be taken up by weeds.
If weeds emerge with the crop and do not die shortly thereafter, additional methods of weed control will be needed. Rotary hoeing will break up the soil crust, dislodge small weeds and reactivate soil-applied herbicides if an activating rainfall was lacking. This inexpensive, mechanical technique is probably underutilized in corn production in most areas.

In addition to mechanical techniques, there are several herbicides that can be used as early post emergence treatments in corn to control small, emerged weeds, provide some residual activity, and extend the window of residual control in the crop (see Table 1). This information should also be useful if untimely rain prevented spraying after planting, and the crop has emerged. Adjuvants will be needed with most of these products to maximize activity on emerged weeds. Consult WS-16 Weed Control Guidelines for Indiana http://www.btny.purdue.edu/Pubs/WS/WS-16/ and the manufacturer’s label for appropriate rates and adjuvants to use with these products.

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