As we begin August, observations of severe corn rootworm larval injury continue across northern Illinois. During the past week, rains have softened soils just enough to allow plants with poor root systems to lodge. In many of these fields, brace roots have been pruned by corn rootworm larvae. However, don't automatically begin "pointing" to corn rootworms as the only potential explanation for lodging.|
Shallow root systems, in part caused by severe soil compaction, can predispose plants to lodge later in the season. This often occurs in fields with top-heavy plants following storms with heavy rains and wind. Fields that are severely lodged, especially in dry summers, often have very disappointing yields. Light interception by plants in these fields is not very efficient; at the same time, plants are devoting resources to regenerate root tissue when, instead, the ear should be the primary sink. I have observed fields with severe lodging caused by corn rootworm damage that ultimately produced "respectable" yields. In these cases, soil moisture was not a limiting factor during the pollination period. Slower and less efficient harvesting of lodged fields also needs to be factored into the final costs of corn rootworm damage.
Here's an observation from the field. Earlier this week, a research team led by Joe Spencer, an entomologist with the Illinois Natural History Survey, attempted to collect western corn rootworm females from soybean fields in northwestern Illinois. If your initial reaction to this statement is that there shouldn't be many western corn rootworms in soybean fields in this part of Illinois, you're correct! Joe's research team confirms that very few western corn rootworm adults can be found in soybean fields in the northwestern region of the state. This continues to be good news and supports our recommendation that crop rotation can still serve as an effective pest management tactic for producers in this area of Illinois.
After approximately 26,000 sweeps in soybean fields near Monmouth, they collected 250 western corn rootworm females. I believe this translates into 1 female captured for every 104 sweeps. In east-central and northeastern counties of Illinois, it's not uncommon to collect 5 or more western corn rootworm adults per sweep. As mentioned in the Bulletin last week, now is the time to make some scouting trips to your soybean field and determine the level of western corn rootworm activity. This investment in time will be of great assistance when determining the potential need for a soil insecticide next spring.--Mike Gray
Root system removed from field with severe soil compaction.
Lodged corn plants.