Issue No. 9, Article 4/May 22, 2009
Standing Water in Fields and Corn Rootworm Survival
Due to the very wet spring and the severe storms that plagued most of the state on May 15, many fields are saturated, and large portions of some have standing water. How will these flooded conditions affect corn rootworm survival? Many articles have been written on this topic in the Bulletin through the years. In general, survival of corn rootworms depends on the duration of the standing water, its temperature, and the point in the season when "ponded" conditions develop. If flooding occurs very soon after larval hatch, and before larvae can establish in root systems, a high percentage of larvae succumb to these environmental conditions. The timing of larval hatch varies considerably, but in most years it typically occurs near the end of May across central Illinois. In some years with very cool springs, hatch has occurred as late as mid-June in central Illinois (June 12, 1996; June 13, 1997). Certainly, a late hatch that extreme has been the exception. However, the very cool spring experienced thus far may result in a delayed hatch and also occur at a point when saturated soil conditions no longer exist, improving corn rootworm survival.
Flooded field, Champaign County, May 18, 2009.
A review of the accumulated degree-days (base 52°F) from January 1 indicates that for several locations in Illinois, a delayed hatch is a distinct possibility. When 684 to 767 degree-days (base 52°F) have accumulated from January 1, approximately 50% of corn rootworm larvae should have begun to hatch (Levine et al. 1992. Journal of Economic Entomology 85: 2425-2432). By May 18, only 296, 162, and 119 degree-days had accumulated for Belleville, Champaign, and Freeport, respectively. For Champaign, these degree-day accumulations are considerably below the 11-year average of 305 for this location. However, above-average temperatures for the next week to 10 days could result in a typical hatch by late May.
A few published papers on standing water and corn rootworm survival may shed some light on this topic:
- Riedell, W.E. and Gerald R. Sutter. 1995. Soil moisture and survival of western corn rootworm larvae in field plots. Journal of the Kansas Entomological Society 68(1): 80-84. In 1993, researchers conducted an experiment near Brookings, South Dakota, using artificial infestations of western corn rootworm eggs. Heavy precipitation (early to mid-June) resulted in saturated soil conditions in experimental plots. One of the replicates that was most affected by the rain had a 10-fold reduction in western corn rootworm emergence as compared with the other three blocks.
- Hoback, W.W., T.L. Clark, L.J. Meinke, L.G. Higley, and J.M. Scalzitti. 2002. Immersion survival differs among three Diabrotica species. Entomologia Experimentalis et Applicata 105: 29-34. When third instars (western corn rootworms) were placed into vials containing dechlorinated tap water with reduced oxygen levels in dark chambers, 50% of larvae were dead after 26 hours at 25°C. Lowering water temperature improved survival of third instars. At 10°C, approximately half of third-instar western corn rootworms survived 72 hours in the water-filled containers. Less mature larvae (western corn rootworms) survived immersion better--50% of second instars survived for 56 hours at 25°C.
The following are some points to keep in mind regarding corn rootworm survival and standing water. If saturated or flooded conditions exist while corn rootworms are in the egg stage, overall survival will not be greatly affected, particularly if the water remains cool. Likewise, if larvae have been able to establish within corn root tissue, they are better able to survive short durations of standing water. If flooded conditions in fields occur during larval hatch, anticipate significant reductions in corn rootworm numbers. Warmer conditions will decrease survival of larvae even further.
If extreme planting delays occur (into early to mid-June), starvation of corn rootworm larvae may occur following hatch. A 1989 article by Terry Branson (Journal of the Kansas Entomological Society, 62: 521-523) indicates that first instars must quickly locate a suitable host to begin feeding on. If first-instar western corn rootworms fail to locate a host within 24 hours following hatch, 45% of them will fail to establish successfully and reach the adult stage. According to Branson, after 3 days of starvation following hatch, only 4.7% of western corn rootworms reached the adult stage.
For now, producers should not assume that corn rootworm densities have been negatively affected by the wet weather and delays in corn planting this spring. As planting resumes in many areas of the state, standard corn rootworm management practices are still warranted, including the use of Bt corn rootworm hybrids or soil insecticides.--Mike Gray