Pest & Crop Newsletter

Soybean Aphid Numbers Increasing in Northen Indiana– (John Obermeyer and Larry Bledsoe)

Damaged leaves from soybean aphid.

Field yellowing from soybean aphid

Soybean plants may be looking yellow and puny, but the soybean aphid doesn’t seem to mind. Research fields in northern Indiana, along U.S 30, have had a dramatic increase in aphid numbers over the past week. In these fields, 100% of the plants sampled had on average 14-30 aphids/plant. Winged aphids were present, indicating that they are moving to other locations. This population surge is not being seen in monitored fields further south, near Lafayette. The University of Illinois is reporting threatening populations in the state’s northern counties.

Presently the lack of beneficial predators, parasites, and pathogens in these infested fields has been disappointing. The most common predator that Purdue researchers are reporting is the minute pirate bug (Orius tristicolor). This has been the most consistent natural enemy found in soybean fields, even before soybean aphid was known to occur in the Midwest. The minute pirate bug as the name implies, is very small. It is one of the first predators to appear in early growing soybean plants and is thought to keep most early invading aphids in check. The much larger and obvious Asian lady beetle has been very low in numbers this season. Reports from heavily infested fields in northern Illinois are noting an obvious decline in this beetle’s population from previous seasons.

Nobody knows for sure when is the best time to treat for soybean aphid, a.k.a., economic threshold. There has been much discussion about aphids/plant and aphids/trifoliolate leaves, etc., but no hard and fast rules. At this time, our recommendation is that commercial soybean fields should not be treated until symptoms become evident. When high stress areas of a field are beginning to yellow (e.g., low potassium levels, sandy soils, soybean cyst nematode) and aphids are quite evident (live aphids and sticky plants from honeydew) as you walk through the field, then treatment may be justified. It is extremely important to assess aphid-infested fields for beneficial organisms before management decisions are made.

Treating soybean with an insecticide for the remainder of the season may tip the balance in the favor of soybean aphid. In other words, natural enemies recover slowly from broad-spectrum insecticides compared to aphids. In general, toxic levels of insecticide are absorbed by ingestion (eating treated leaves) and/or contact (walking over treated areas). Aphids are sucking insects and ingest only internal plant fluids. As well, except for mature females, they are relatively stationary on the bottom sides of leaves; obviously a difficult location to get thorough coverage. A very important note is that surviving aphids can repopulate fields at break-neck speed, certainly outpacing natural enemies.

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Now is a Critical Time for Soybean Defoliation– (John Obermeyer and Larry Bledsoe)

We have begun to sample soybean fields with sweep nets for the presence of western corn rootworm beetles. While doing so, we capture and record other insects. So far, overall insect numbers seem to lower than “normal.” That’s good news. However, an occasional field with no known reason will have a surprisingly high number of insects and damage. This only emphasizes that each field should be scouted separately.

Bean leaf beetle, Japanese beetle, grasshoppers, and green cloverworm all feed-on soybean leaves. And even though soybeans have the amazing ability to withstand damage from defoliation, yield losses can occur. The impact of defoliation is greatest during pod fill because of the importance of leaf area to photosynthesis and, ultimately, yields.

The best management guidelines for soybean defoliators involve identifying the insect pests and then characterizing the level of defoliation and growth stage of the beans. Then, management decisions will depend on anticipated market price of the soybeans, cost of treatment, the level of damage, the growth stage of the soybean, and potential yield. At mid pod fill, consider treatment when defoliation exceeds approximately 15 to 20% and the defoliator(s) is still present and actively feeding. Refer to the following table for treatment thresholds for insect defoliated soybeans.

Western corn rootworm beetles they will feed on soybean foliage, but this is not of concern to soybean yield. However, their presence in soybean could signal the need to control larval population in next year’s corn. Refer to last week’s
Pest&Crop on sampling in this year’s soybean fields with yellow sticky traps. A Flash animation of sampling rootworm beetles in soybean can be viewed at <http://www.entm.purdue.edu/entomology/ext/fieldcropsipm/animation.htm>.

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Click for Table
Black Light Trap Catch Report

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Dandelion Control with Late Sprng Applied Treatment in No-till Corn-  (Earl Creech and Bill Johnson)

The adoption of no-till cropping systems in Indiana has led to an increase in troublesome perennial weeds in crops.  Dandelion is one such weed that has become a cause of concern for many people throughout the state.  To help us better address the many questions that have been directed toward us this spring, we established a couple of field experiments to examine a number of herbicide programs directed toward dandelion control.

In the first experiment, our goal was to evaluate dandelion control with late spring applied treatments.  A primary objective was to obtain a relatively quick burn-down of top growth.  On May 8, herbicide treatments were applied to no-till plots that were subsequently planted into corn.  Initial ratings were collected 14 days after treatment (DAT) and revealed that the best treatments were generally those which contained Gramoxone Max (Table 1).  Lumax alone controlled dandelion 71% while the addition of Gramoxone Max increased control to 94% or greater.  Similarly, Bicep II Magnum + 2,4-D and Bicep II Magnum + 2,4-D + Gramoxone Max provided 38% and 89% control of dandelion, respectively.  At 41 DAT, dandelion control in the Lumax + Gramoxone Max treatments had decreased to ~80%, a level similar to Lumax + 2,4-D (84%).  A much more significant reduction in control was observed with Bicep II Magnum + Gramoxone Max which had decreased to ~50%.  In contrast to the initial evaluations, the treatment combinations containing Roundup Weathermax received the highest ratings (92%) at 41 DAT.

Based on our observations concerning Lumax + Gramoxone Max at 14 DAT in the previous study, we established another trial to examine the interaction between these two products more closely.  Callisto (one component of the Lumax premix) at 3 and 6 oz/A controlled dandelion 68% and 80%, respectively (Table 2).  The addition of atrazine to Callisto has been reported to improve Callisto activity on a number of weeds but had no effect on dandelion control in this study.  Although initial (11 DAT) activity was ~4-fold higher than Callisto alone, dandelion control in the Callisto + Gramoxone Max treatments was 8-15% less than a similar rate of Callisto alone at 35 DAT.  This decrease in activity may be the result of the Gramoxone Max destroying the above-ground plant tissue so rapidly that the Callisto did not have adequate opportunity to absorb through the foliage and translocate into the roots.

These studies demonstrate the burn-down potential of a number of herbicides on established dandelion.  Gramoxone Max is a contact product that rapidly destroys the above ground dandelion parts.  Translocated herbicides such as Lumax, Callisto, and Roundup Weathermax were essential to control dandelion 5-6 weeks down the road. 

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