The variant western corn rootworm in Iowa

Corn producers in eastern Iowa should expect to see changes in corn rootworm behavior. The variant of the western corn rootworm has now been confirmed in eight Iowa counties. Their presence may influence corn producer management decisions, especially in corn rotated with soybeans.

What is the variant western corn rootworm?

This insect is the familiar western corn rootworm (Diabrotica virgifera virgifera). It is called several different common names including the "eastern variant of the western corn rootworm" or the "soybean variant of the western corn rootworm." It is the same species (or subspecies to be entomologically correct) of western corn rootworm that has occurred in Iowa since at least 1955.

Western and northern corn rootworm beetles
The two most common rootworm species in Iowa: �the western corn rootworm (left) and northern corn rootworm. (Marlin E. Rice)

What makes the variant western corn rootworm different?

Female western corn rootworms typically lay their eggs in corn. When eggs are laid in corn, then crop rotation is a successful management tool for breaking the life cycle of this insect. The variant western corn rootworm, however, no longer has this fidelity to corn. After emerging from the soil, adult females will leave a corn field and fly to soybeans to lay their eggs. Corn that is planted into this field the following year is now exposed to rootworms from the eggs that were laid in soybeans the previous summer.

Where did the variant western corn rootworm come from?

The "honor" of the variant origin has been given to Piper City in east-central Illinois. Since the early 1990s, damage to first-year corn has spread throughout Illinois and into Indiana and southern Wisconsin. The variant has now been documented in most, if not all, of the corn-producing counties in the northern two-thirds of Illinois although economic damage is not occurring in all fields in these counties.

Where does the variant western corn rootworm occur in Iowa?

Several years prior to 1999, a corn producer in northeastern Iowa near Decorah had been reporting lodging in corn following soybeans. Brian Lang, extension field crop specialist, decided to place cages in the first-year corn field and trap rootworm adults that emerged from the soil. What he found was surprising: in 12 cages the beetle count averaged 14 western corn rootworms and 1.5 northern corn rootworms. The variant western corn rootworm was now confirmed in Iowa for the first time. The trap counts also indicated that the variant western corn rootworm was mostly responsible for the lodging and not the extended diapause northern corn rootworms.

Reports of lodging in rotated corn also have increased for several years in eastern Iowa. In 2005, we placed emergence cages in 19 first-year corn fields in eight eastern Iowa counties. Adult western corn rootworms emerged from 14 of 19 rotated corn fields while northern corn rootworms emerged from 15 fields. These data indicate that rootworm survival in rotated corn was fairly common in those fields. The map indicates those counties where the variant in Iowa has been confirmed. The numbers of both rootworm species were low, however, with the exception of two fields in Jones County and one field in Cedar County (see chart). In these counties the northern corn rootworm emergence was greater than the westerns, indicating that extended diapause poses a more serious threat to rotated corn at the present time than the western laying eggs in soybeans.

Counties with variant western corn rootworm populations
Counties with confirmed variant western corn rootworm populations, 1999-2005.

Can a variant problem be determined before spring planting?

No. Only field scouting or trapping information from last summer can be used to predict economic damage for 2006. The risk of damage in first-year corn in eastern Iowa is very small, and in the absence of field information from 2005, an informed decision cannot be made for this spring's planting.

What is the significance of the variant western corn rootworm in Iowa?

Undoubtedly producer concern over economic damage from the variant will increase with time. This will increase insecticide use or transgenic corn rootworm hybrids and a corresponding increase in production costs in rotated corn fields. However, the presence of the variant is not a harbinger of economic damage in a field. Only field scouting can reliably predict a problem. If sampling was not conducted in 2005, the only information that is available for 2006 planting decisions is whether or not corn lodged in 2005. If a grower has rotated corn that lodged and it was diagnosed as corn rootworm larval feeding, they may be justified in protecting their rotated corn in 2006.

Additional information on the variant western corn rootworm may be found in the 2005 Integrated Crop Management Conference Proceedings, pages 85-91 or at online (online version requires Flash Player).

Western and northern corn rootworm emergence
Adult western and northern corn rootworms emerging from first-year corn fields in eight eastern Iowa counties, 2005.

This article originally appeared on pages 53-54 of the IC-496 (4) -- March 13, 2006 issue.

Updated 11/08/2006 - 5:43pm