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Controlling WCR Beetles in Soybean Fields Where First-Year Corn Rootworm Problems Exist- (John Obermeyer, Rich Edwards, and Larry Bledsoe)
Where problems with western corn rootworm (WCR) in first-year corn (corn/soybean rotation) have shown up, some have asked about controlling the rootworm adults in this year’s soybean fields. The idea behind this strategy is to control the adults in soybean before they have laid sufficient eggs to create a rootworm larval problem in next year’s corn. Although this strategy sounds good, and in theory is possible, it requires a knowledge of beetle biology and a very high level of management. You don’t just spray fields and forget them when it comes to adult control! Fields need to be scouted before to determine beetle presence and after treatment to determine if a second treatment is needed. Even 100% attention to detail will not guarantee that failures will not occur. Of course, one can argue that this is also the case with soil insecticides. However, soil insecticides are more consistent in their control based on timing and the level of management that is required. The idea of controlling adults to prevent egg laying and subsequent larval damage is not new, the management of rootworm adults in continuous corn using this strategy has been around since the 1970’s. Many midwestern states researched this strategy with varying degrees of success. The primary problem was the timing of the application and the amount of field monitoring that was required to ensure that the rootworm beetles were adequately controlled, and, if an economic reinfestation occurred, that these new beetles were quickly controlled. Also, follow-up was needed the next year to make sure that rootworm larvae were not present in high enough numbers to result in economic root damage. If a critical larval population was detected the next year, a cultivation application to rescue the field was required. At this time, it is not possible to utilize this strategy at the same level in soybean since we do not have beetle threshold information. We have research underway that will hopefully provide these numbers. However, since these numbers are not presently available, we are reluctant to recommend this as a control strategy. It could be that without these thresholds many fields will be sprayed and no economic return will be realized. Another possible concern is that preliminary results from a new study indicate that more WCR eggs are laid in soybeans earlier than first believed. Should this be the case, adult control to prevent egg laying may be next to impossible. How about the economics of adult suppression in soybean versus larval control in corn? As previously mentioned, it will likely take two foliar insecticide applications to prevent enough egg laying in soybean to cause economic damage to next year’s corn. Therefore, when comparing foliar insecticide and application inputs, it is about the same cost if not more, as granular insecticides at planting. Knowing that precise scouting and insecticide timing are critical over the rootworm beetle’s egg laying period (mid-July through August), it seems that money is best put toward larval control in the spring. Refer to last week’ Pest&Crop article “Monitoring and Decision Rules for Western Corn Rootworm Beetles in Soybean,” for monitoring adults to determine the need for a rootworm insecticide in next year’s corn.
Soybean Aphid Update– (John Obermeyer) So far, so good! Soybean aphid numbers have been very low. This is not only true for Indiana but surrounding states are reporting much the same. A population explosion in August is certainly possible. Watch for future updates. Soybean at a Critical Stage for Defoliation– (John Obermeyer, Larry Bledsoe, Rich Edwards) Soybean plants can withstand high amounts of defoliation (leaf removal) before economic losses occur. During the vegetative stage of growth, defoliation above 40% will not significantly affect yield. During pod set and fill, defoliation of greater than 15% can result in economic losses. Determine the average defoliation level throughout the plant, not just in the tops of plants where most of the damage is noted. Remember, defoliation always looks worse than it really is. Refer to the table below for treatment thresholds for insect defoliated soybeans. Refer to Extension Publication E-77-W, Soybean Insect Control Recommendations-2002 (Revised 1/2002) for abbreviated treatment thresholds and control materials. This publication can be downloaded at: <http://www.entm.purdue.edu/entomology/ext/targets/e-series/fieldcro.htm>. Click for Table. |
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Some Droughty Fields Experiencing Silk Emergence Delay– (Bob Nielsen) Not unexpectedly, silk emergence has been delayed in some drought-stressed corn fields throughout the state. Lengthy silk emergence delays relative to pollen shed disrupts the pollination process, resulting in ears with varying degrees of barrenness. What caught my eye this week in several fields was the appearance of ears with unusually long fresh (unpollinated) silks. One of the consequences of severe drought stress near pollination is a delay in silk emergence. Sometimes the length of the delay is great enough that little to no pollen remains in the tassels by the time the silks finally appear. Unusually lengthy silks that are still “fresh” during the pollination period are therefore a symptom that pollination has not been successful. Drought-stressed fields exhibiting visibly long fresh silks should be inspected for possible pollination failure. I recently described the “ear shake” technique for evaluating silk detachment to verify the extent of pollination success soon after pollen shed is complete (P&C Newsletter, 26 July). It doesn’t take long to check a number of ear shoots for silk detachment. Also look at the tassels for evidence of anthers and pollen yet to be released. If the “ear shake” technique indicates that little successful pollination has yet occurred AND if there is little visual evidence that pollen remains in the tassels, then the prospect for complete kernel set on the ears is unfortunately very dismal. If, on the other hand, there appears to be pollen yet remaining in the tassels, then the opportunity still exists for some degree of kernel set. Don’t forget, this and other timely information about corn can be viewed at the Chat ‘n Chew Café on the World Wide Web at <http://www.kingcorn.org/cafe>. For other information about corn, take a look at the Corn Growers’ Guidebook on the World Wide Web at <http://www.kingcorn.org/>. |
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MSU Research Examines Soybean Aphid Predation– (Tyler Fox and Doug Landis, Entomology and Center for Integrated Plant Systems) To determine the effects of natural enemies in Michigan soybean fields we have been conducting experiments on soybean aphid predation for the past two years. Our studies have attempted to answer the following questions: 1) What predators are present in Michigan soybean fields? 2) Do they have a significant impact on soybean aphid establishment? 3) Can predation alone prevent soybean aphids from building to damaging levels? Who are the Predators? Predators Impact Soybean Aphid Establishment Predators Can Prevent Soybean Aphid Build Up Conclusions This article is being reprinted with permission from Michigan State University’s Field Crop Advisory Team Alert, Volume 8, Number 14, July 25, 2002. |
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