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Western Corn Rootworm in First-Year Corn: The Threat of Returning to the Insecticide Treadmill - Final Report

Disclaimer

This report was prepared by an EPA assistance agreement recipient and represents only the views of the author rather than EPA.

X 985324-01-0
Project Period-10/1/96-12/31/97

Project Personnel

Corey Gerber C. Richard Edwards
George Barna John Obermeyer
Mike Stanyard Larry Bledsoe
Ron Blackwell Philip Boeve
Department of Entomology, Purdue University

Introduction

Growing corn in rotation with soybean has been adopted widely by corn producers in the midwestern Corn Belt. One benefit of this short rotation is the management of corn rootworm larval populations without the use of insecticides, either soil applied for corn root protection, or foliar applied to prevent females from laying eggs. However, an apparent adaptation of the western corn rootworm beetle to corn grown in rotation with soybean has occurred in northwest Indiana and east-central Illinois, resulting in economically important numbers of rootworm eggs being laid in soybean. Therefore, growing corn in rotation with soybean is no longer a viable option as the sole management tactic against the corn rootworm for producers in this region.

Sampling plans and economic thresholds are available for corn rootworm eggs, larvae, and adults infesting continuous corn. Currently, no sampling plans or economic thresholds are available for use in soybean fields to determine if a soil insecticide is needed for corn planted the following year (first-year corn). As a result, an estimated 80% of corn producers in northwest Indiana and east-central Illinois may apply a soil insecticide to first-year corn to protect the root system from rootworm injury and/or other soil insect pests. This is up dramatically from 10-15% of the acres just a few years ago. There is indication that the problem area is expanding eastward, as well as northward, and the use of soil insecticides is expected to expand as well.

Study Objectives

  1. to determine the economic threshold for western corn rootworm in first-year corn (corn/soybean rotation) based on counts of beetles infesting soybean the year prior to corn planting.
  2. to devise a sampling program for western corn rootworm populations in the soybean using practical sampling tools and methods; and,
  3. to transfer the technology generated from this study to farmers to ensure that wise decisions are made regarding the use of insecticides for corn rootworm management in first-year corn.

Materials and Methods

Seventeen pairs of corn/soybean fields in 6 northwest Indiana counties, Clinton, White, Jasper, Newton, Benton, and Fountain, were used in this study. A field pair consisted of a soybean field adjacent to (or directly across the road from) a corn field. Field size varied from 30- 114 acres. Tillage, planting date, and all crop inputs were not controlled in this experiment, with the exception of the soil insecticide application to corn. The number of treated and untreated rows or strips (multiple rows) in corn depended on (1) the size of the planter used and (2) if the producer wanted to make a yield comparison between treated strips and untreated strips. In fields where check strips were established, producers would either shut off the entire planter or half of the planter while making a pass in the field. Similar passes were made at different points in the field. In fields where treated and untreated rows were established, row ‘3’ was untreated in fields planted with 6 and 8-row planters and rows ‘3’ and ‘9’ were untreated in fields planted with 12 and 16- row planters. This resulted in the corn fields having treated and untreated rows across the entire field. Treated rows received a full rate of soil insecticide at planting time. Seven producers used Counter 20CR, 3 producers used Lorsban 15G, 2 producers used Force 1.50, and 2 producers used Fortress 50. Untreated rows in the first 2 planter passes were identified by flags.

In 11 fields, 2 pairs of rows were identified as the sampling block. In 6 fields, only 1 pair of rows was identified as the sampling block. Each block consisted of an adjacent treated and untreated row. All subsequent sampling procedures were performed in these rows.

On 5 and 12 June 1997, 10 roots were sampled randomly from untreated rows in each field. Roots were examined for the presence of rootworm larvae in the laboratory using a dissecting scope.

The rootworm beetle population was characterized within 8 fields using emergence traps. Twelve traps were placed in untreated rows within each field on 7 July. Traps were placed 50-100m from the field edge and 50-lOOm apart. White, plastic deli-style cups (16 oz.) coated with insect trap coating (The Tanglefoot Company, Grand Rapids, Ml 49504) were changed weekly until beetle emergence ceased (25 August). The number of corn rootworm beetles in each cup was recorded. Sex ratio was determined for each of the 8 fields by examining 100 beetles from emergence cages each week.

Corn roots were dug between 22 July and 14 August 1997. Twenty-five roots were dug from each treated row and untreated row, for a total of 100 roots per field (in the fields that contained a sampling block of 2 pairs of rows), and a total of 50 roots per field (in the fields that contained a sampling block of 1 pair of rows). We assumed that the number of roots were comparable. Roots were washed to remove soil and rated using the Iowa 1-6 scale.

Corn yields were estimated by harvesting all corn ears in 5.31m of row at 6 locations in treated and untreated rows, for a total of 12 samples per field. Ear corn was weighed using a dial scale (Chatillon, New York, NY) and the percent moisture was determined with a grain moisture tester (Dickey-John Corp., Auburn, 11. 62615). Ear weights were converted to bushels/acre of No. 2 shelled corn at 15.5% moisture. Lodging scores (each plot was given a single score on a 1-5 scale with 1 = all plants erect, 2 = most plants leaning about 22° from vertical, 3 = most plants leaning 45° from vertical, 4 = most plants leaning about 68° from vertical, and 5 = most plants horizontal) and plant population estimates were also made on the 5.31m of row that was sampled for yield.

A. Multi-County Corn Rootworm Sampling and Threshold Development Study
Rootworm Beetle Sampling

Adult rootworm populations were estimated by using the Pherocon® AM unbaited sticky trap (Trece Corp.) in paired soybean/corn fields at 2 sites in White County, IN; at 2 sites in Jasper County, IN; at 3 sites in Newton County, IN; at 1 site in Benton County, IN; at 2 sites in Clinton County, IN; and at 1 site in Fountain County, IN. Corn rootworm beetle populations were sampled weekly from 28 July to 9 September 1997. Also each week, a sample of at least 30 western corn rootworm beetles were removed from the corn and soybean fields and placed in a small jar containing ethyl acetate for sex determination, as well as to evaluate ovarian development.

In each of the soybean fields, 1 trap line was established and consisted of 6 Pherocon® AM unbaited sticky traps that were placed equidistant down the middle of the fields. Pherocon® AM traps were attached to 1 .2m wood lath that had been driven into the ground. Traps were positioned at the top of the canopy. These traps were adjusted each week to maintain position relative to the soybean canopy. In several fields, the wood lath was not long enough to maintain the traps at the top of the soybean canopy throughout the trapping period.

In each of the corn fields, a similar line of Pherocon® AM traps was established; however, the traps were placed in the first row of corn next to the soybean field. The traps in the corn were tied with wire directly to the corn at ear level. At 5 of the 11 sites, the corn fields were not adjacent to the soybean fields, but were located across the road. In these fields, the Pherocon® AIvI traps were placed in the end rows closest to the paired soybean fields.

The Pherocon® AM traps and the jars containing 30 beetles were placed in plastic bags (by field), returned to the laboratory, and frozen. The number of western and northern rootworm beetles on the Pherocon® AM traps were recorded.

B. Corn Rootworm Sampling Method Study

Rootworm Beetle Sampling

Adult rootworm populations were estimated weekly, from 29 July to 10 September 1997, using 5 sampling methods at 6 paired soybean/corn fields in Benton County, IN. Sampling methods included 1) the Pherocon® AM unbaited sticky trap; 2) Olson sticky trap (Olson Products Inc.); 3) cucurbitacin vial trap; 4) an experimental corn rootworm trap (Trece Corp.); and 5) a 14.5cm diameter sweep net. These data will be correlated with root damage ratings obtained in 1998. Additionally, sample method variability will be evaluated.

Soybean fields were divided into fifths lengthwise, creating 4 transects running the length of the field. Thus, 4 trap lines were established and each line consisted of 1 of the 4 trap types. Eight traps were placed equidistant in each line. The cucurbitacin vial traps, Olson traps, and Pherocon® AM traps were attached to 1 .2m wood lath that had been driven into the ground. Traps were positioned at the top of the canopy. These traps were adjusted each week to maintain position relative to the soybean canopy. In several fields, the wood lath was not long enough to maintain the traps at the soybean canopy throughout the trapping period. The corn rootworm traps were positioned on 1.8m x 1.27cm PVC pipes. Eight sweep samples were taken within a field. The samples were obtained in the Olson trap line in each of the soybean fields and consisted of 30 sweeps. Sweeps were made in a continuous pendulum motion in the upper 1/4 to 1/3 of the soybean canopy. Sample contents were transferred from the net to pint-size plastic bags.

In each of the corn fields, a similar line of Pherocon® AM traps was established; however, the traps were placed in the first row of corn next to the soybean field. The traps in the corn were tied with wire directly to the corn at ear level.

The cucurbitacin vial traps, Olson traps, Pherocon® AM traps, experimental rootworm traps, and sweep net samples, for each field, were placed in a plastic bag, returned to the laboratory, and frozen. The number of western and northern corn rootworm beetles was determined for each sampling method.

C. (Farmer/Educator Cooperator) Corn Rootworm Sampling and Threshold Development Study
Rootworm Beetle Sampling

Additionally, another study was conducted at 10 sites (in Benton, Jasper, Newton, and White counties) in northwest Indiana. The methods for this study were similar to those mentioned in the multi-county corn rootworm sampling and threshold development study, except, farmers and county Extension Educators placed, collected, and processed the traps. Adding the data of this study to the regression analysis increased the sample size which provided a better fit of the regression line.

D. Analyses

Root ratings, lodging scores, yield estimates, and plant population size were analyzed with analysis of variance procedures, and mean separation using Fisher’s Protected Least Significant Difference test. Minimum sample size required for 15% and 25% level of precision was determined using Southwood’s (1978) formula ii = (s/Em)2 where s = standard deviation, E = desired level of precision, and m = mean. Finally, a regression was conducted between mean number of beetles collected per trap, during the sampling dates of 12 August through 3 September, and the mean root rating numbers. Due to the low number of beetle counts during the first 2 sampling dates, we felt that the final 4 sampling dates provided adequate numbers for a better fit of the regression line. For the regression, we pooled data from Parts A, B, and C.

Results and Discussion

Note: Tables and figures are available upon request.

A. Multi-County Corn Rootworm Sampling and Threshold Development Study

Since the sampling method (Pherocon® AM trap) component (Part B) was similar to the multicounty corn rootworm sampling and threshold development study (Part A), we were able to pool the data to increase the sample size to 17 fields. Only 5 of 17 (29%) corn fields had root ratings 3.5. We consider root ratings at or above 3.5 economically important. Overall, corn treated with an insecticide had an average root rating significantly less than those not treated with an insecticide (Table 1).

Table 1. Mean root ratings from 17 northwest Indiana corn fields in 1997

No differences were found among treated and untreated rows in plant populations (Table 2). However, significant differences were noted in lodging scores (Table 3) and yield estimates (Table 4) among treated and untreated rows. These differences were quite different than those noted in a similar experiment in 1996 where no significant differences were observed. Significant differences between treatments in lodging scores and yield estimates in 1997 were probably related to the environmental conditions as well as the moderate amount of rootworm pressure present in the experiment.

Table 2. Mean plant populations from 17 northwest Indiana corn fields in 1997
Table 3. Mean lodging scores from 17 northwest Indiana corn fields in 1997
Table 4. Mean yields from 17 northwest Indiana corn fields in 1997

There was a slight delay in the emergence of adults from what might be expected due to cool soil conditions in the spring and early summer. Adult populations peaked in late July (Fig. 1) in 1997, during the corn silking stage. An anticipated corn rootworm population decline in corn and a corresponding increase in soybean was not detected (Fig. 2). This may have been due to the delayed emergence, or perhaps a portion of the beetle population is exiting the corn shortly after emergence and entering soybean, thus somewhat balancing the population. A portion of the population seems to be preferring soybean over corn as a host.

Western corn rootworm beetle populations seem to be at a high level in their multi-year cycle. For example, the peak populations on the Pherocon® AM traps in corn were 6 times the economic threshold for continuous corn. Corn rootworm population size in this area of Indiana may explain some of the increased damage to first-year corn noted in recent years.

Fig. 1. Mean (SEM) western corn rootworm beetle emergence by week in 8 NW Indiana first-year corn fields in 1997.

Fig. 2. Mean (SEM) western corn rootworm beetle population estimates from 11 NW Indiana corn/soybean fields (Multi-County Study) in 1997 using the Pherocon® AM unbaited sticky traps.

The majority of the beetles present in soybean and corn during the last week of August and the first 2 weeks of September were female (Fig. 3). During this period, approximately 85% of the beetles in soybean were female. This is advantageous for developing a sampling plan because the producer or crop consultant needs only to be concerned about total numbers of beetles present, and does not have to sex the beetles.

Fig. 3. Mean (SEM) percentage of female western corn rootworm beetles in 11 corn and soybean fields in NW Indiana (Multi-County Study) in 1997.

B. Corn Rootworm Sampling Method Study

The beetle population estimated by cucurbitacin vial traps peaked 2 weeks earlier than the estimated peak population from the Olson trap, the Pherocon® AM unbaited sticky trap in the corn, and the experimental corn rootworm trap. Additionally, this was 1 week earlier than the sweep net samples and the Pherocon® AM unbaited sticky trap samples in the soybean. Although the cucurbitacin vial traps estimated the peak population during the week of 12 August, it appeared that a resurgence took place during the week of 26 August (Fig. 4).

Fig. 4. Mean western corn rootworm population estimates from 6 NW Indiana soybean fields in 1997 using 5 sampling methods.

Pest management decision making requires an accurate estimate of the population and use of economic thresholds. Sample sizes for the Pherocon® AM traps, Olson traps, experimental corn rootworm traps, cucurbitacin vial traps, and sweep nets were determined for soybean (Table 5). Minimum sample size required for the 5 rootworm beetle sampling methods varied depending on sampling date. We estimated that the least number of traps to determine the adult rootworm beetle population would occur with the Pherocon® AM unbaited sticky trap and the experimental corn rootworm trap.

Table 5. Minimum sample size required for 5 corn rootworm beetle sampling methods in soybean at 2 levels

(Results and Discussion for the Regression Analysis)

After conducting a regression analysis for each of the trapping methods (in 1996) versus root ratings for each of the fields (in 1997), it was determined that the best fit of the regression line occurred with the Pherocon® AM unbaited sticky traps.

The regression analysis for mean number of beetles collected per Pherocon® AM unbaited sticky trap, during the sampling dates of 12 August through 3 September, and the mean root ratings produced an R2 value of 0.057 (Fig. 5).

Fig. 5. Regression analysis of mean western corn rootworm population estimates/trap/week from 27 NW Indiana soybean fields from 8/12-9/3, 1996 versus root ratings taken in these same fields during 1997.

Conclusions

Environmental conditions may have played a significant role in the level of rootworm injury sustained by corn following soybeans in northwest Indiana. Economic levels of rootworm injury to corn following soybean is expected to occur on an infrequent basis. However, the unpredictability of rootworm injury tied to the amount of damage that can possibly occur will likely result in widespread use of soil insecticides in northern Indiana for the foreseeable future.

When selecting a sampling method for corn rootworms in soybean, it is critical to obtain a balance between the number of traps actually needed per field, acceptance by the producers, commercial availability and how well the traps actually predict root injury the following season. The results suggest that the most efficient trap types to adequately estimate risk from rootworm beetles to a following corn crop are the Pherocon® AM unbaited sticky traps and the experimental corn rootworm traps. The Pherocon® AM unbaited sticky trap provided the best correlation of beetles collected per week (1996) versus root ratings (1997). Although these data are considered preliminary, they suggest that a mean of 22 beetles per trap per day is the economic threshold for western corn rootworm beetles in soybean grown in rotation with corn in northwest Indiana. Other studies, notably those conducted by University of Illinois researchers suggest that the threshold may be lower. These investigations are continuing at least two more cropping seasons to further refine the treatment threshold.

The full public transfer of these data must wait until greater confidence in the results is obtained.

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