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Issue No. 23, Article 3/October 3, 2008

Preliminary Node-Injury Ratings from University of Illinois Rootworm Product Efficacy Trials Near DeKalb, Monmouth, Perry, and Urbana

As we have done in several previous years, we are sharing the preliminary root rating results from our corn rootworm product efficacy trials conducted at University of Illinois Research and Education Centers located near DeKalb, Monmouth, Perry, and Urbana. Producers will soon begin making seed selection choices and pest management plans for the 2009 growing season. Hopefully, the results from our trials will help growers make informed decisions about optimizing corn rootworm management in 2009. Table 1 shows root rating results from "digs" conducted during mid- and late July 2008. Because we have observed later-season root pruning, primarily of brace roots on some Bt corn hybrids since 2004, we continue to provide root ratings from evaluations of selected plots in August (Tables 2 and 3). We offer our thanks to Joshua Heeren, Nicholas Tinsley, and the entire summer crew for their dedicated efforts in obtaining these data.

Information About the Experiments

A few explanations about our 2008 trials, both generalities and specifics, are in order.

Trap crops. The trials in 2008 were planted in areas that had been planted with a trap crop in 2007 (late-planted corn hybrids interplanted with pumpkins). Such trap crops usually attract large numbers of gravid (pregnant) female western corn rootworms late in the summer, when most other cornfields have brown silks and reduced nutritional quality. Large numbers of eggs deposited in these areas usually result in large numbers of rootworm larvae in the trial areas the following year. Our objective is to create a situation that significantly challenges corn rootworm control products. Many producers want to know how well rootworm control products perform when infestations of rootworm larvae are moderate to severe, levels of injury that often result in yield loss. In addition, because most fields are not scouted sufficiently for rootworm adults, producers generally can't accurately predict the level of a corn rootworm infestation the following season. Therefore, we believe that data from our trials provide growers a good assessment of what they can expect from various products even when infestations are heavy.

Precipitation in 2008. The growing season started out very wet and then turned dry at some sites, particularly at Urbana. The abundant rainfall led to saturated conditions and may have contributed to poor survival of rootworm larvae at some of our locations. Lending support to this speculation were the numerous observations from around the wet Midwest this year about very low numbers of corn rootworm adults in fields. Rainfall at each of our experimental locations was abundant, bordering on excessive, for the months of April through July (Table 4). More than 20 inches of rain was recorded for each of our trial locations from April through August.

Corn hybrids. In the past when insecticides were the only treatments in our trials, we could select and plant one corn hybrid for the entire trial, making all comparisons among treatments equivalent. However, the availability of an ever-growing number of corn hybrids with genes that express Bt proteins to control corn rootworms creates a situation for which planting one hybrid for the entire trial is not feasible. Consequently, we have to select a "base hybrid" for comparisons among untreated checks, soil-applied insecticides, and seed-applied insecticides, as well as the Bt corn hybrid that is a near-isoline of the base hybrid. If we include more than one Bt corn hybrid in the trials, we also must include the near-isoline non-Bt corn hybrids. In addition, different companies have different interests in comparisons among Bt corn hybrids and insecticides. And to cap all of this off, it is difficult to obtain corn seed that has not been treated with the 0.25 mg-per-seed rate of either Cruiser or Poncho. Consequently, the information for each treatment should be examined carefully to enable appropriate interpretations.

The "base hybrid" for all four corn rootworm product efficacy trials in 2008 was Pioneer 34P89, the non-Bt near-isoline of Pioneer 34P94, a Herculex XTRA hybrid. Pioneer 34P89 was not treated with a seed-applied insecticide. Four other pairs of rootworm Bt hybrids and their non-Bt near-isolines were also included in the trials, and the seeds of all of these corn hybrids were treated with the 0.25 mg-per-seed rate of either Cruiser or Poncho (refer to the footnotes for Tables 1, 2, and 3). In addition, due to logistics and requests, soil insecticides were applied to certain hybrids and not to others, and some rootworm Bt corn hybrids were treated with soil insecticides.

Because of the different types of treatments in our trials, we have categorized the data in Table 1 for easier reference--soil- and seed-applied insecticides, rootworm Bt corn hybrids, soil insecticides + rootworm Bt corn hybrids, and untreated checks. Node-injury ratings from some plots treated with experimental products were included in the analyses, but the data are not included in the tables.

Summer employee "art" associated with 2008 root digs--Boredom or anger? (University of Illinois)

Results and Discussion

Following is a series of questions that relate to our interpretation of these preliminary findings.

At what location was corn rootworm injury most severe in the 2008 trials? Over the past few years, rootworm injury has been the most severe at our Urbana research site. However, this was not the case in 2008; the level of root pruning was most severe at our DeKalb site. Each of our five untreated checks at DeKalb had essentially 3 nodes of roots completely destroyed. The overall average node-injury rating for the five checks in DeKalb was 2.95. Overall averages for the five checks at Monmouth, Perry, and Urbana were 1.46 (1-1/2 nodes of roots destroyed), 0.88 (nearly 1 node of roots destroyed), and 1.86 (almost 2 nodes of roots destroyed), respectively. The trial in Urbana received more than 6 inches of rain in both May and June (Table 3), which very likely reduced the level of rootworm pressure at the site. As we have discussed from time to time in articles in the Bulletin, corn rootworm larval survival is diminished when larvae hatch from eggs in flooded or saturated soils. Although the DeKalb site had almost 5 inches of rain in May, only about 3 inches of rain fell on that site in June.

There was more rootworm injury in the untreated check plots at the Perry site in 2008 than we had observed the first two years we established a trial there. The average node-injury ratings in the untreated checks ranged from 0.5 to 1.31, depending on the hybrid.

Severe rootworm larval injury to corn roots from untreated check plots, DeKalb 2008. (University of Illinois)

Given the wet spring we experienced, how well did soil insecticides protect roots from rootworm injury? Were they as effective as the rootworm Bt corn hybrids? There was variation in the performance of soil insecticides at all locations. For instance, nearly 1-1/2 nodes of roots were destroyed in the plots treated with Counter 15G at DeKalb. In the Monmouth and Urbana trials, where rootworm injury in the untreated checks was less severe, node-injury ratings for Counter 15G were considerably lower--0.05 and 0.36, respectively. In general, the soil insecticides provided better protection against corn rootworms at Monmouth, Perry, and Urbana than at DeKalb. This also was the case in 2007, particularly with the organophosphate insecticides Counter, Fortress, and Lorsban, each averaging nearly 1 node of roots destroyed at our DeKalb site last year.

Aztec provided relatively consistent control of rootworms, with node-injury ratings lower than or equivalent to the injury ratings for most of the rootworm Bt corn hybrids at all locations.

Roots from a plot treated with Aztec 2.1G at the Urbana site in 2008. (University of Illinois)

The node-injury ratings for the granular and liquid formulations of Force were typically less than 1/2 node of roots pruned, although Force 3G was not included at the DeKalb site. In DeKalb, the mean node-injury rating for Force CS was 1.06, not significantly different from the ratings for four of the five rootworm Bt corn hybrids. Force 3G provided slightly better root protection than Force CS at the Monmouth site.

Neither Cobalt (a mixture of chlorpyrifos [active ingredient of Lorsban] and gamma-cyhalothrin [active ingredient of Proaxis]) nor Fortress 5G provided adequate control of corn rootworms at most of the locations where these products were evaluated. Node-injury ratings for Lorsban 15G ranged from 0.66 at Urbana to 1.55 at DeKalb.

Did Poncho at the "corn rootworm rate" provide adequate root protection in 2008? No. Node-injury ratings for Poncho 1250 were 2.85, 1.21, 0.74, and 2.07 at DeKalb, Monmouth, Perry, and Urbana, respectively. These means were not significantly different from the means for most of the untreated check plots at all locations. Relying on this treatment for satisfactory and consistent root protection in a non-Bt corn refuge is risky.

How did the rootworm Bt corn hybrids perform? Generally, all of the hybrids had very little rootworm injury at Monmouth and Perry and relatively more injury at DeKalb and Urbana (Table 1). The node-injury ratings for the HxXTRA Bt hybrids (Mycogen 2T789, Pioneer 32T85, Pioneer 34P94) ranged from 0.65 to 1.01 at DeKalb and from 0.39 to 0.84 at Urbana. Most HxXTRA roots had characteristic bites, scarring, and tunneling that resulted in a proliferation of secondary roots, a characteristic we have mentioned previously in the Bulletin. The node-injury ratings for the YieldGard VT hybrids DKC 61-69 and 63-42 were 0.42 and 0.25, respectively, at Urbana and 1.2 and 1.17, respectively, at DeKalb.

Significant rootworm injury to the roots of an HxXTRA hybrid from the DeKalb site in 2008. (University of Illinois)

Caption: Significant rootworm injury to the roots of a YieldGard VT hybrid from the DeKalb site in 2008. (University of Illinois)

There were few differences in mean node-injury ratings between the first and second evaluations of rootworm injury to rootworm Bt corn hybrids at all locations (Tables 2 and 3). Interestingly, the most noticeable increase in injury between the first and second digs occurred for the YieldGard hybrids at DeKalb, the site where the time interval between first and second digs was shortest (2 weeks). The mean ratings for YieldGard VT hybrids DKC 61-69 and DKC 61-72 represented approximately 1-1/2 nodes of roots pruned. Other noticeable increases in rootworm injury between the first and second digs occurred for some HxXTRA hybrids at Monmouth and Urbana, with mean ratings from the second evaluations that ranged from 0.66 (2/3 of a node of roots pruned) to 1.36 (1-1/3 nodes of roots pruned) at Urbana.

For a few years now, we have observed variability in levels of rootworm injury among rootworm Bt corn hybrids, which was evident again in 2008. The level of injury to rootworm Bt corn hybrids at DeKalb and Urbana ranged from approximately 1/2 to more than 1-1/2 nodes of roots pruned, an amount of rootworm injury that many growers won't expect. It seems that conditions at the trial sites may play a role in this variability, and relative abundance of variant western corn rootworms may also play a role. Variant western corn rootworms (females will lay eggs in soybean fields) are firmly established in the areas around DeKalb and Urbana, but they are less well established near Monmouth and Perry. Regardless of the causes for such variability in protection against corn rootworms, it is evident that rootworm Bt corn hybrids are not silver bullets against corn rootworms.

What did the root protection look like when a soil insecticide was used in combination with a rootworm Bt corn hybrid? In 2007, we examined the effects of treating a YieldGard rootworm Bt corn hybrid with a soil insecticide (Counter 15G) applied at 3/4 rate (6 oz per 1,000 ft of row). Our results and a few results from other universities generated some interest in this combination, so we included multiple treatments of soil insecticides + rootworm Bt corn hybrids in our trials in 2008. The soil insecticides used were Aztec 4.67G (at Urbana), Counter 15G (at DeKalb, Monmouth, and Urbana), and Force CS (at all four locations). A quick review of the results (Table 1) reveals very low node-injury ratings for all combinations at all sites, except for Counter 15G + YieldGard VT DKC 63-42 (mean node-injury rating of 0.94, not statistically different from the mean node-injury rating of Counter 15G alone [1.46] or YieldGard VT DKC 63-42 alone [1.17]). There were no significant differences in mean ratings among the rootworm Bt corn hybrids with or without a soil insecticide at either Monmouth or Perry. However, at DeKalb and Urbana, where the rootworm pressure was greater, the combination of Force CS + most rootworm Bt corn hybrids resulted in significantly lower node-injury ratings. The combination of Fortress 5G + HxXTRA Pioneer 34P94 at DeKalb also resulted in a node-injury rating (0.13) that was significantly lower than the node-injury rating for either Fortress 5G alone (1.88) or HxXTRA Pioneer 34P94 alone (1.01).

An HxXTRA root system + Force CS (left) compared with a root system from an untreated check (right) from the DeKalb site in 2008. (University of Illinois)

A YieldGard VT root system + Force CS from the DeKalb site in 2008. (University of Illinois)

Does it always make sense to use a soil insecticide in conjunction with a Bt hybrid? No. If a field has a history of severe secondary insect infestations, such as wireworms and/or white grubs, and does not have a history of rootworm problems, a soil insecticide applied in-furrow is likely the best management choice. In fields where both rootworms and secondary insects have caused problems, applying a soil insecticide to a rootworm Bt corn hybrid also makes some sense. However, we are not certain that reduction in the rate of application of the insecticide is wise. The data presented here apply only to rootworm control, not to control of secondary insects, for which data are limited. In fields where growers have experienced unexpected levels of rootworm injury to rootworm Bt corn hybrids, the application of a soil insecticide to a rootworm Bt corn hybrid can be considered. Our data suggest that protection against rootworms in such situations improves when a soil insecticide is used with a rootworm Bt corn hybrid. However, this is not a recommendation for all corn growers in Illinois. Under most circumstances, either a rootworm Bt corn hybrid or a soil insecticide is sufficient for adequate protection against corn rootworms. Keep in mind that by applying a soil insecticide with a rootworm Bt corn hybrid, the rootworms are exposed to a neonicotinoid seed treatment (either clothianidin or thiamethoxam), an organophosphate or pyrethroid soil insecticide, and a Bt protein--all targeted at corn rootworms. In our opinion, although this approach may provide short-term solutions, the selection pressure on rootworm populations will be high. Western corn rootworms have had a nasty habit of adapting to many of our management tools when selection pressure has been high.

Will yields from our rootworm product efficacy trials be published? Yes. After harvest is completed, we will share the yield results in a future article of the Bulletin, in our annual summary of insect management trials (on Target), and at forthcoming Extension meetings, such as the Corn & Soybean Classics that will be held in January 2008.--Mike Gray, Kevin Steffey, and Ron Estes


Oleson, J.D., Y. Park, T.M. Nowatzki, and J.J. Tollefson. 2005. Node-injury scale to evaluate root injury by corn rootworms (Coleoptera: Chrysomelidae). Journal of Economic Entomology 98: 1-8. Also, information about this node-injury scale is available at www.ent.iastate.edu/pest/rootworm/nodeinjury/nodeinjury.html.

Mike Gray
Kevin Steffey
Ron Estes

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