A Consultant's Observations from the Field

April 28, 2000
During the past week, Dave Mowers, Mowers Soil Testing Plus, Inc., observed a variety of insects already at work this spring in several fields. In a Stark County cornfield that had been planted in late March, Dave found numerous corn flea beetles with more than five adults per plant in some areas of the field. In issue no. 2 of the Bulletin, we suggested that corn flea beetles were likely to be a problem this spring, even in northern counties. Dave's observations seem to suggest that vigilant scouting for this pest makes a lot of sense, especially in those early-planted cornfields devoted to seed production or sweet corn varieties. If susceptible inbreds (to Stewart's disease) are infested before the 5-leaf stage and two to three adults per plant are found, and 10 percent of the plants are silver or white because of flea beetle injury, a rescue treatment may be warranted. As Dave's observations seem to verify, early-planted fields are most at risk, particularly those fields planted with inbreds or varieties susceptible to Stewart's disease.

In addition to aboveground insects, Dave reported that white grubs and wireworms are both commonly showing up in bait stations within producers' fields. Because a rescue treatment for these soil insects is not an option, producers must make decisions prior to planting whether or not a soil insecticide may represent a good investment. Please refer to Bulletin issues no. 1 and no. 3 for more detailed management information for wireworms and white grubs, respectively. Many of the white grub observations this spring have been confirmed to be annual white grubs. As we've suggested previously, early-planted cornfields (early to mid-April) are at greatest economic risk from these annual species. Corn that is planted in early May is less prone to root injury from annual grubs because most root feeding by these species begins to diminish by mid- to late May. If true white grubs are found in bait stations or churned up in spring tillage passes, producers should consider the use of a soil insecticide at planting. Recall that true white grubs have a 3-year life cycle, and during the second year they feed all summer long on the root systems of corn plants.


Seedling soybean injury caused by bean leaf beetle adults.


Bean leaf beetle adults.

Although most soybean fields have yet to be planted, Dave observed numerous bean leaf beetles that are eagerly awaiting the emergence of soybean plants. Bean leaf beetles overwinter beneath leaf litter outside of soybean fields and undoubtedly came through the winter in great shape. Following their spring awakening, bean leaf beetle adults travel initially to stands of alfalfa. As soon as soybeans begin to emerge, the adults don't waste much time in locating tender seedlings to begin feeding upon. In an upcoming issue of the Bulletin, we will provide more detailed information on bean leaf beetle management strategies. For now, keep in mind that the earliest-planted soybean fields are at most risk to economic injury caused by bean leaf beetle feeding. If you find bean leaf beetles in these early-planted soybean fields, don't panic. Densities of 16 adults per foot of row in the early seedling stage of soybean growth are typically necessary before economic losses begin to occur. By the V2 stage, 39 bean leaf beetle adults per foot of row are required to deliver an economic punch! We thank Dave Mowers for his early-season observations from north-central Illinois.--Mike Gray

Author: Mike Gray