Summary

 

·   Decades of an intense corn-soybean rotation in east-central Illinois and west-central Indiana have selected for a strain of western corn rootworm (CRW) that deposits eggs in soybeans rather than corn fields.

·   The new strain of western corn rootworm has since spread to parts of western Ohio, southwestern Michigan, southern Wisconsin and southeastern Minnesota in addition to continued expansion in Illinois and Indiana.

·   As a result, crop rotation is no longer an effective corn rootworm management strategy in some areas of the central and eastern Corn Belt.

·   Producers in areas with high infestations of the variant CRW biotype have found it necessary to treat their corn following soybeans with a soil insecticide.

·   University researchers have developed a system of monitoring CRW adult populations in soybean fields. This should help growers to determine the need for treating corn following soybeans.

·   Pioneer hybrids with Poncho 1250 seed-applied insecticide provide another treatment option for growers in areas with moderate infestations of the CRW variant.

·   This Crop Insights discusses the development of the western corn rootworm variant biotype, the approximate areas of infestation, and management practices to deal with this increasing problem. 

 

Introduction

 

Corn rootworms are the most destructive pest of corn in the major corn-growing regions of the U.S. and Canada. Producers have managed corn rootworms primarily by crop rotation, or if planting corn after corn, by use of soil-applied insecticides. However, changes in western corn rootworm egg-laying behavior are rendering crop rotation ineffective in some areas.

 

Historically, western corn rootworm adults have deposited virtually all of their eggs in corn fields near the base of corn plants. Eggs overwinter and hatch in late spring of the following year. Corn rootworm larvae, which feed only on corn and a few grassy weed species, quickly starve to death unless corn is planted the year they hatch. Thus, crop rotation was very effective in breaking the life cycle of the existing rootworm population in a field, allowing corn to be safely planted every second year.

Figure 1. Western corn rootworm adult on silks of corn plant. (Photo courtesy of Jim Kalisch, University of Neb.)

 

Development of Variant Rootworm Biotype 

In an area centered in east-central Illinois and west-central Indiana, a new biotype of western corn rootworm has developed that severely damages corn in rotation. In this area, a strict corn-soybean rotation had been practiced for decades. Among the original corn rootworm population were a few adults that preferred to lay eggs in soybean fields. These eggs hatched during the corn year, so a high percentage of larvae survived. In the normal population, on the other hand, few larvae survived when hatching in a soybean field.

Over many years, the variant type laying eggs in soybeans became predominant in the population. As this new population has spread, more and more corn fields following soybeans have experienced severe corn rootworm larval feeding, lodging and significant yield loss. Rotation is no longer an effective management strategy against this new rootworm strain.

Extent of the Western CRW Variant Problem

 

The western CRW variant biotype is now considered a severe problem in at least 40 counties in the eastern Illinois/western Indiana area (Figure 2).

Figure 2. Western corn rootworm variant populations in Illinois and Indiana, 2002 (Ratcliffe, et al., 2003 and Purdue News, 2003).

Figure 3 shows the results of a root damage survey conducted by University of Illinois entomologists in 2002 and 2003. The ten-county area outlined in red in represents the epicenter of the variant population development in Illinois.

Figure 3. Average root ratings by county in Illinois in 2002 and 2003 (Schroeder and Ratcliffe, 2003). Higher numbers indicate more root damage (see next page for Iowa 1-6 scale).

First-year corn in these counties has been damaged by corn rootworm feeding for several years. Growers are now treating first-year corn to keep root feeding damage below the economic threshold. However, in counties to the west and northwest of this red zone, most growers have not yet responded to variant corn rootworm population movement, and damage levels are higher. Many fields in these counties are experiencing economic losses as a result of corn rootworm feeding in first-year corn fields.

In addition to Illinois and Indiana, the western CRW variant problem is also appearing in areas of northwest and west central Ohio, southwest Michigan, southern Wisconsin and southeast Minnesota. Infestations have mostly remained low or moderate in these states, but some fields in Ohio have experienced economic damage. Growers in affected areas are encouraged to address this problem by beginning to monitor western CRW beetle populations in soybean fields and treating the following corn crop if necessary.

 

Management of the Western CRW Variant

 

In response to the increasing severity of damage in many areas affected by the variant CRW biotype, producers have found it necessary to use insecticides in corn following soybean production. As a result, use of planting-time insecticides has increased significantly. Side-by-side tests conducted in 2002 by Pioneer agronomists in Indiana and Ohio confirm that this practice has paid off for most growers. The average yield advantage for insecticide-treated plots in 13 side-by-side tests was 28 bu/acre with a range of -4 to 72 bu/acre (Figure 4).

Figure 4. Comparison of CRW insecticide vs. no insecti-cide in 13 field-sized strip plots in east central Indiana and west central Ohio, 2002.

 

Monitoring Soybean Fields for Damaging Levels of Western CRW Beetles

In areas known to have high infestations of the variant biotype, the value of treating is evident. But in less-affected areas, growers need a reliable method of determining whether to use an insecticide in corn following soybeans.

University of Illinois extension entomologists have devel-oped a system of monitoring western corn rootworm adult populations in soybean fields in the affected area. This monitoring is necessary to determine if an insecticide will be needed the following year if corn is planted into the field. The monitoring system uses unbaited Pherocon AM yellow sticky traps (Figure 5.)

The researchers recommend that monitoring take place from the last week of July through the third week in August. A total of 12 traps per field, regardless of field size, should be distributed at equal spacing throughout the field during the last week of July. Traps should be positioned on steel posts so that they are just above the level of the soybean canopy. All adult western corn rootworms should be counted each week, and traps replaced to ensure accurate counts.

Based on this scouting program, University of Illinois entomologists have developed the table below to help predict the expected injury level in each field scouted based on the average number of beetles captured (Ratcliffe et al., 2003).

B/T/D	RR	Description
0-4	1	No damage or only a few minor feeding scars.
	2	Some roots with feeding scars, but no roots eaten off to within 1.5” of the plant.
5	3	Several roots eaten off to within 1.5” of plant, but never the equivalent of an entire node.
10	4	1 node of roots destroyed (or equivalent).
11+	5	2 nodes of destroyed (or equivalent).
	6	3 or more nodes of roots completely destroyed.

B/T/D=Beetles/trap/day. RR = Root Rating.

The relationship between root injury ratings and yield loss varies considerably from year to year and field to field. Entomologists generally agree that economic loss will occur with root injury ratings between 3.0 and 4.0. However, under extreme drought conditions, economic loss may occur with root injury ratings as low as 2.0-2.9.

A similar monitoring program to detect the presence of western corn rootworm adults in soybean fields outside or on the fringe of affected areas can be employed using only four Pherocon AM yellow sticky traps per field. If adults are detected, densities should be determined using 12 traps per field. Decisions regarding insecticide use should not be made based on monitoring with four traps per field. Additionally, trap captures should not be used to trigger insecticide applications to soybean fields to prevent egg laying by adult western corn rootworms.

Producers in and near affected areas should continue to use sound integrated pest management sampling methods such as those developed by the University of Illinois to assess the need for soil or seed-applied insecticide use against this new corn rootworm variant. These sampling programs must be used in each field each year to make informed decisions.

 

Poncho 1250 Seed-Applied Insecticide

 

Pioneer is introducing a new management option for CRW control in 2004 - Poncho¹ 1250. Growers in areas with low to moderate infestations of the western corn rootworm variant should consider the benefits of this new management tool. Poncho 1250 users will find advantages in convenience, handling, safety, grain marketing and hybrid availability over other corn rootworm management options.

Poncho 1250 comes on-the-seed and in-the-bag, eliminating the cost and time required to install soil-applied insecticide equipment, calibrate equipment and load products during planting, and handle and return insecticide containers. Poncho 1250 also allows growers to “stack” corn rootworm protection with the newest hybrid genetics and corn technologies, including the latest Pioneer® brand hybrids with Herculex² I or YieldGard Corn Borer³ insect protection, or the Roundup Ready³ trait.