Issue No. 13, Article 2/June 18, 2004
Reports of Corn Rootworm Larval Injury Continue
As first noted in last week's edition of the Bulletin ("Corn Rootworm Larval Injury Early and Impressive in Some Locations"), significant levels of root pruning and numerous corn rootworm larvae have been observed in Illinois cornfields. This week's article offers much of the same.
Shawn Jones, Pioneer Hi-Bred International, has continued his vigilant search for corn rootworm larval injury. He reports some low levels of rootworm larvae in first-year corn in Christian County and heavy feeding and high larvae counts in a field in Shelby County.
In notes from Dr. Joe Spencer, Center for Economic Entomology at the Illinois Natural History Survey, and Kevin Black, Growmark, finding second- and third-instar corn rootworm larvae is not uncommon. Joe examined corn roots in a research plot northeast of Urbana on June 13 and found that 5% to 10% of the larvae in unprotected first-year corn roots were in the third instar! He also noted that many of the roots were significantly pruned and he suspected that, if not for the more-than-adequate soil moisture, many of the plants would be wilting, especially with hot days we've been experiencing. In those same plots, sticky traps monitored during the summer of 2003 were all above threshold (7.8 to 10.4 western corn rootworms per trap per day). Kevin reported that in trials put out by Jerry Harbour, Lincolnland FS, second- and third-instar larvae were found, along with corn rootworm pupae.
Undoubtedly, corn rootworm pressure has been high this spring. Mike Hellmer, Pioneer Hi-Bred International, reported severe larvae feeding in Champaign County. Without sifting through the soil, he found 5 to 6 rootworms per plant in an insecticide-treated field. With the extent of larval injury seen this spring, concerns of lodging, spread of the variant, and insecticide efficacy continue.--Kelly Cook