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Angie Peltier

Angie Peltier
Former Extension Educator, Commercial Agriculture

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Hill and Furrow

Current topics about crop production in Western Illinois, including field crops research at the NWIARDC in Monmouth.
Figure. Western corn rootworm beetles feed on corn silks.
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Abundant western corn rootworm beetles: Implications for 2014?

Posted by Angie Peltier - Insects

Recent observations. I recently spent the day in the field collecting foliar disease severity data in a corn fungicide trial at the Northwestern Illinois Agricultural Research and Demonstration Center (NWIARDC).  Very little disease was present in this particular hybrid.  Common rust was the most commonly observed disease, while gray leaf spot was just starting to develop along plot edges.

While disease was the purpose of this field visit, what was most striking was the sheer number of western corn rootworm (and some northern corn rootworm) beetles feeding on silks – between 3 and 8 per ear.  Corn rootworms are most notorious for the damage that they can cause, as worm-like larvae, to corn roots.  This damage and corresponding yield losses is what has made the western corn rootworm Midwest's most costly corn pest.

Bt technology. Bt stands for Bacillus thuringiensis, a naturally occurring soil bacterium. This bacterium produces a suite of naturally-occurring proteins that are toxic when ingested by specific corn insect pests.  Historically, this bacterium has been formulated into agricultural pesticides, some even approved for certified organic production.  More recently, plant scientists at several seed corn companies developed transgenic corn hybrids that express one or more of the insecticidal toxin genes derived from Bacillus thuringiensis, known as Cry proteins.  Ten years ago, the first corn rootworm Bt hybrids were released.  Although rootworm Bt technology has been quite successful in reducing rootworm-associated yield loss, it is not a 'silver bullet'.  Hybrids with rootworm Bt traits still sustain some level of larval feeding damage.  Additionally, there have been sporadic reports since 2011 of the development of Cry3Bb1-resistant western corn rootworm populations.

The pictures that you see displayed here are of western corn rootworms feeding on silks of a hybrid that expresses the Cry34Ab1 and Cry35Ab1 Bt rootworm proteins (Figure).

How can this be?  While young rootworm larvae feeding on corn roots are susceptible to rootworm-specific Cry proteins, corn rootworm beetles, which tend to feed on leaves, silks, and pollen are not. Widespread feeding damage on leaves was also evident on this particular hybrid (Figure).

Scouting for rootworm beetles. Entomologists typically recommend scouting for adult rootworm beetles weekly starting in mid-July through August.  Several different methods of corn rootworm scouting are detailed in an article written by Purdue University IPM personnel.  Silk feeding during pollination can result in yield loss if silks are clipped shorter than one half inch.  Entomologists typically don't worry about yield losses occurring from leaf feeding damage because damage typically occurs at a low level and before corn pollination and grain fill.

Although not typically resulting in in-season yield losses, western corn rootworm leaf and silk feeding damage and high beetle populations can be important observations.  Large beetle populations in a field suggest that significant mating and egg-laying are likely to occur in that field.  A proportion of these eggs are likely to survive the winter and hatch as larvae.  If corn is planted into the same field in 2014, larvae can feed on corn roots.

Dr. Mike Gray, University of Illinois' Extension Entomology Specialist, recently summarized the results of a state-wide corn and soybean insect survey in a Bulletin article.  In this article he also spoke about beetle thresholds.  In integrated pest management, research leads to the development of economic thresholds.  Thresholds are levels of feeding damage, insect population number or disease severity that when reached or surpassed trigger management recommendations.  For western corn rootworm beetles, thresholds are an average of 0.75 beetles per plant for continuous corn and 0.5 beetles per plant in first year corn.  Once these beetle thresholds are reached or surpassed, entomologists recommend that producers choose between: 1) rotating away from corn in 2014, 2) considering the use of a Bt rootworm hybrid or 3) applying a soil insecticide at planting.

To ensure the long-term efficacy of these important management tools, entomologist recommend rotating Bt traits by planting hybrids with different individual or stacked Bt traits from year to year.



Illinois IPM Fact Sheet: Western corn rootworm – Kelly Estes, Mike Gray, and Kevin Steffey

Illinois Agronomy Handbook: Ch. 13, Managing Insect Pests – Kevin Steffey and Mike Gray

Purdue University: Field Crops IPM: Corn Rootworms, 2009

Utah Pests Fact Sheet: Western corn rootworm - Erin W. Hodgson

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