No. 12 Article 2/June 13, 2008

Standing Water, Corn Rootworm Survival, and Insecticide Performance

As mentioned last week in the Bulletin, corn rootworm larvae were detected in central Indiana on June 4, and Larry Bledsoe of Purdue University estimated that hatch most likely occurred on June 2. This year's hatch coincided with heavy precipitation across much of the Corn Belt and has resulted in many fields left with standing water for several days to over a week in some cases. Corn rootworm larval survival will be very poor in those ponded areas of fields.

So should producers be concerned about soil insecticide or Bt performance with respect to corn rootworms? A look back to 1993, a year when many areas of Illinois also received excessive rainfall, may help answer this question, at least with respect to soil insecticides.

Wet conditions persist in many fields (near DeKalb, IL, June 9, 2008).

In 1993 we planted our corn rootworm soil insecticide efficacy plots on May 13, 17, and 14 near Champaign, DeKalb, and Monmouth, respectively. Collective precipitation totals for May, June, and July were 15.72 inches (Champaign), 19.04 (DeKalb), and 21.82 (Monmouth). July was the wettest month for Champaign (8.42 inches) and Monmouth (10.81 inches). DeKalb received 9.87 inches of rain in June followed by 6.28 inches in July. Root injury ratings (1 to 6 scale) in the untreated control plots (checks) were 5.82 at Champaign, 3.76 at DeKalb, and 3.71 at Monmouth.

In recent years we have become accustomed to the 0 to 3 node root injury scale, so a brief refresher on what these ratings reflect is probably warranted. The 5.82 root rating in Champaign indicated that nearly 3 nodes of roots were destroyed in the check. In DeKalb and Monmouth, the root injury was less intense in the check plots, with untreated plants averaging almost 1 node of roots pruned. Despite these wet conditions and intense corn rootworm pressure at the Champaign location in 1993, the soil insecticides protected the root systems very well. Each of the following products kept root injury below the suggested economic injury index of 3.0 (several roots pruned within 1.5 inches of the plant, but less than the equivalent of 1 node): Aztec 2.1G (band), Counter 15G (furrow), Force 1.5G (band), Fortress 5G (furrow), and Lorsban 15G (band). At the DeKalb site, these treatments kept root injury below 2.0 (roots with feeding scars) in an experiment in which the check had nearly 1 node of roots (3.76) destroyed. At the Monmouth location, the soil insecticides also kept root injury below the economic injury index (Aztec 2.1G was not tested at this site). These data suggest that the granular soil insecticides continued to provide very good root protection under these very wet soil conditions in 1993. Obviously, we have no similar data set for the transgenic Bt corn rootworm hybrids.

For producers still contemplating replanting those areas of cornfields that have been flooded, it seems doubtful that corn rootworm populations will reach very high densities. In addition, any corn rootworm survivors from ponded areas of cornfields will undoubtedly face starvation by late June if corn roots are not available. This assessment takes into account the late hatch (June 2) that occurred across central Illinois and Indiana.

A note of caution is warranted. The following paragraph is from a Pest Management and Crop Development Bulletin article (No. 13, June 12, 1993) on this topic:

"The timing of precipitation and duration of saturated soil conditions can also significantly affect corn rootworm survival and root damage. Research conducted at the South Dakota laboratory (Northern Grain Insects Research Laboratory) revealed that if the soil was saturated during egg hatch and for roughly two weeks beyond the egg-hatch period, corn rootworm establishment was significantly reduced. Standing water in cornfields during late May and early June doesn't always spell the end for corn rootworms. If areas of a given field are ponded for only a few days, enough corn rootworms may survive and still inflict serious root damage. In 1991, a corn rootworm experiment near Urbana was severely flooded soon after planting because of heavy rains. Root damage ratings were taken in July for this experiment and revealed that the corn rootworms survived in sufficient numbers to inflict impressive damage."

Thanks for sharing your observations this season; let's hope we see lots of sunshine the rest of June.--Mike Gray

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