The Trapping Line: April 23

Black cutworm trap

Black cutworm pheromone sticky trap

UPDATE(4/25): Additional volunteers needed. Kelly Estes, Coordinator of the Illinois Cooperative Agricultural Pest Survey suggests anyone interested in monitoring traps contact her at: or 217-333-1005.

Currently, black cutworm moth pheromone traps are active in the University of Illinois insect monitoring program.  Traps for other species will be brought online as the season progresses. Extension Educators and volunteers monitor the traps and report the results through the North Central IPM PIPE system, hosted by Iowa State University. We will keep you updated about the trapping network here in the Bulletin.

Here is the current list of black cutworm moth captures:

County 11 12 15* 16 17 18 19 22* 23
Lee 1 0 0 1 0 0
Grundy 0 2 5 6
Champaign 1 1
Logan 1 2 4
Menard 0
Macon 3 1 0
Macoupin 1 0 0 1
Fayette 0 0 1 0 0 0 2
Madison 0 0 0 0 0 2
Edwards 0


The full website and current map can be found here.

The first date to expect damage is calculated based on growing degree day accumulations after the first intense flight, nine or more moths in a 2-day period.

Pyramided Bt Cotton and Factors Leading to Potential Compromised Longevity: Cautionary Findings for Bt Corn and Western Corn Rootworm?

In late March 2013, an article was published in the Proceedings of the National Academy of Sciences (PNAS) titled “Potential shortfall of pyramided transgenic cotton for insect resistance management.” The authors of the paper were as follows: Thierry Brévault, Shannon Heuberger, Min Zhang, Christa Ellers-Kirk, Xinzhi Ni, Luke Masson, Xianchiun Li, Bruce E. Tabashnik, and Yves Carrière. The scientists pointed out in their introduction that the first generation of Bt cotton plants, those expressing the Cry1Ac protein, were first used by US producers beginning in 1996. During that same year, corn producers began to grow Bt corn hybrids for European corn protection across many Corn Belt states. From 2003 to 2011, the authors of the PNAS paper indicated that the use of cotton plants expressing a single Cry protein were gradually phased out as producers began to use more pyramided products (either Cry1Ac + Cry2Ab or Cry1Ac + Cry1F). The scientists who wrote this PNAS article outlined several assumptions that explained the justification and increased use of pyramided cotton plants: 1) assumed – evolution of field resistance could be delayed by using more pyramided Bt cotton products, 2) assumed – insects that were resistant to one Cry protein would be killed by the other Cry protein expressed in plant tissue (redundant killing), 3) assumed – recessive resistance and only insects that had two resistant genes (homozygous for resistance) for each Cry protein (toxin) would survive on pyramided cotton plants, and 4) assumed – insects that are resistant to both Cry proteins are very rare within the overall population (assumed that target insect population has not been exposed to either Cry protein). Central to the effectiveness and long term durability of pyramided Bt plants is the concept of redundant killing. Redundant killing can be potentially compromised according to the authors of the PNAS article by the following factors: 1) some susceptible insects survive even though exposed to the Cry proteins, 2) as plants mature, Cry protein concentrations may decline, potentially increasing the number of survivors, 3) if an insect population has an inherently reduced susceptibility to a given Cry protein, exposure to reduced Cry protein levels as plants age may increase overall survivorship of the target insect pest, and 4) if cross resistance exists between Cry proteins being expressed in Bt plants.

The investigators reported that a Helicoverpa zea (cotton bollworm) strain that had been selected for resistance to the Cry1Ac protein had increased survivorship on pyramided cotton plants. They also concluded that cross resistance occurred between the Cry1A and Cry2A proteins. By using these data, they ran some simulation models and determined that the use of pyramided Bt plants – designed to substantially delay resistance development – could be compromised for Helicoverpa zea. They further pointed out that in order to prolong the usefulness of pyramided Bt plants to control Helicoverpa zea, large refuges may be needed as part of an IPM program that integrates several management strategies.

As we move forward into the 2013 growing season, it is increasingly clear that more corn producers will rely upon pyramided Bt hybrids for corn rootworm control. Have some of the assumptions regarding redundant killing already been violated with respect to the western corn rootworm and the use of pyramided Bt hybrids? Unlike the use of high dose events for many lepidopteran pests, Bt hybrids for corn rootworms are generally recognized as low dose – there are survivors and occasionally severe pruning in some high pressure fields. The following passage in the PNAS paper is worthy of reflection: “Previous experimental evidence on the pyramid strategy comes primarily from a model system with diamondback moth and noncommercial Bt broccoli plants producing Cry1Ac and Cry1C. Although most of the optimal conditions for pyramids apply to this model system, they may not apply for some other pest-Bt crop combinations, particularly when pests have inherently low susceptibility to one or more of the toxins in the pyramid.” We also know that root protection generally declines with some corn rootworm Bt hybrids as the season progresses. Consequently, a late-hatch and prolonged feed period can result in more severe root damage. Thus far, there has been no confirmation of western corn rootworm cross resistance between Cry3Bb1 and Cry34/35Ab1. That’s good news. However, as new rootworm Bt proteins (e.g., eCry3.1Ab) enter the market place and are expressed in pyramid combinations, potential cross resistance will need to be continuously evaluated.

Soil Insecticide Use on Bt Corn Expected to Increase this Spring Across Much of Illinois

What a difference a year can make. Many of us will recall the record-breaking warm temperatures of last March across the Corn Belt of the United States. Those temperatures fueled a rush towards planting in April and the earliest emergence of western corn rootworm adults that I have witnessed. On June 7, 2012, I reported that severe rootworm injury had already occurred in a cornfield located in Cass County along with plentiful adults that were causing considerable injury to the corn leaves. I don’t anticipate a similar early emergence this season with snow still commonly found in many fields across the northern one-half of Illinois.

As I have done for many years, I used hand-held clickers (Turning Technologies) to poll producers at the 2013 Corn and Soybean Classics held in several Illinois locations in January.  Growers were asked if they intended to plant a Bt hybrid for corn rootworm protection in 2013? On average, for the five locations, nearly 92% (n = 568 responses) of the producers indicated that Bt hybrids would be targeted at corn rootworms for the upcoming growing season (Figure 1). The range in “yes” responses was 87.6% (Moline, n = 97 responses) to 95% (Champaign, n = 121 responses and Malta, n = 88 responses). Not surprisingly, and similar to previous growing seasons, the use of Bt hybrids will remain a key tactic used by producers in 2013 for corn rootworm protection.

As more pyramided Bt hybrids enter the market place, the type of refuge used will continue to change in a very significant manner. In 2012, the traditional 20% structured refuge was still the dominant refuge approach used by producers in the Corn Belt. Based upon responses received at the 2013 Corn and Soybean Classics, this refuge strategy will no longer remain dominant for Illinois producers. Overall, the most common refuge (Figure 2) that will be used by producers who took part in these regional meetings will be the 5% seed-blend (refuge-in-a-bag) with 43% (n = 572 responses) indicating they will be moving in this new direction. The second most common refuge tactic will be the 10% seed blend (refuge-in-a-bag) with 31% relying upon this approach. Together, these data indicate that nearly 3 out of 4 producers who responded to this question will use a seed-blend as their refuge management strategy for corn rootworms. The advantages of a refuge-in-a-bag (RIB) are straightforward – convenience, ensured compliance, and favorable from a resistant management vantage point for western corn rootworms (based upon emergence patterns and inter-field dispersal dynamics of adults).In 2013, I anticipate a sharp increase in the use of planting-time soil insecticides with corn rootworm Bt hybrids. On average, nearly 47% (n = 571 responses) of producers indicated they intend to use both a soil applied (at-planting) insecticide and a corn rootworm Bt hybrid this spring (Figure 3). For the Moline meeting, nearly 56% (n = 93 responses) of the producers are likely to follow this strategy for 2013. From my perspective, the escalation of soil insecticide use along with corn rootworm Bt hybrids has been fueled primarily over concerns of Bt resistance and high commodity prices. Producers who intend to use a soil insecticide with their corn rootworm Bt hybrid this spring offered several reasons for this approach (Figure 4). On average, concerns over secondary insect infestations and Bt resistance were the top issues cited for this practice. However, nearly 27% of the producers acknowledged that they view the use of a soil insecticide with a corn rootworm Bt hybrid as “cheap insurance.” Regarding the potential insect resistance management (IRM) benefits of a soil insecticide used in combination with a Bt hybrid, more research is warranted on this topic. In addition, I think it is worth mentioning that one of the key benefits touted concerning the use of Bt hybrids for corn rootworm management was the reduction of soil insecticide use. It is a bit surprising that 10 years after the first Bt hybrids entered the market place for corn rootworms (2003), that a heightened interest in the use of soil insecticides has surfaced in such a significant fashion.

Mike Gray