"Down, But Not Out":

1998 Status of the Illinois

Western Corn Rootworm Problem

in first-year corn

J.L. Spencer, S.A. Isard1, and E. Levine2

Department of Natural Resources and Environmental Sciences, University of Illinois and
Center for Economic Entomology, Illinois Natural History Survey 607 E. Peabody Drive, Champaign, IL 61820.
1Department of Geography, University of Illinois, 607 S. Mathews Ave, Urbana, IL 61801.
2Center for Economic Entomology, Illinois Natural History Survey, 607 E. Peabody Drive, Champaign, IL 61820

Introduction

The western corn rootworm (WCR, Diabrotica virgifera virgifera) is the most serious pest of corn grown in the Midwest (Levine and Oloumi-Sadeghi, 1991). Since the late 1980's, the previously effective practice of annually rotating corn with soybeans for WCR control has failed across an expanding area in Illinois and Indiana, and portions of Ohio and Michigan (Onstad et al. 1998). Previously, adult WCR displayed a strong fidelity toward cornfields for feeding and their egg-laying. Once mated, many WCR beetles dispersed from their natal field and settled in nearby cornfields where they continued feeding and laid their eggs. For a female's offspring to survive, her eggs must be laid and hatch where corn will be grown the following year. Successful development of WCR larvae requires that they feed on the roots of corn (or a limited number of other grasses) (Gray and Luckmann, 1994). Crop rotation exploits the WCR's allegiance to corn and their habit of laying eggs in cornfields, by annually alternating the production of corn with another crop (usually soybeans) on which WCR larvae cannot develop. Under a corn-soybean rotation, WCR eggs which were deposited in corn the previous year, and overwintered in the field, hatch in the midst of a soybean field. Emerging larvae are unable to develop on soybean roots and they die, breaking the WCR life cycle (Levine and Oloumi-Sadeghi, 1991).

As a management tool for WCR, crop rotation has been a great success; however, because of it's wide adoption and effectiveness, the practice itself has been a very strong selecting force for adaptations that would allow corn rootworms to reproduce in spite of rotation (Gray et al. 1996, 1998). The northern corn rootworm (NCR, Diabrotica barberi) adapted to crop rotation through selection for a prolonged egg diapause. Where rotation is prevalent, a high proportion of NCR eggs are able to lie dormant in rotated fields for two or more winters, instead of the normal one. By doing so, the NCR effectively waits until corn is once again available where the eggs were originally laid (Levine et al. 1992). A similar prolonged diapause was initially suspected when the recent problem with WCR in first-year corn was recognized. However, we now know that WCR populations in areas where rotation is failing ('problem areas') circumvent crop rotation by leaving cornfields to lay eggs in soybeans and other crops rotated with corn (Spencer et al. 1997, 1998). What the NCR accomplished by shifting it's developmental schedule, WCR have achieved through a change in their egg-laying behavior (Levine and Oloumi-Sadeghi 1996).

We have been engaged in efforts to further characterize the behavior, physiology, and distribution of the new WCR variant in Illinois for several years. This report details our latest progress in monitoring the spread of the problem and our search to uncover aspects of WCR biology and behavior that will improve and enlarge our management toolbox.

General Methods

Weather Monitoring: A micrometeorological measurement station (Campbell Scientific, Inc.) was established in one of three 4 acre soybean fields (Pioneer 9333 Roundup Ready) NE of Urbana on land owned by the University of Illinois Foundation (Lost 40) (Isard et al., 1998). Data from this station enabled us to study the influence of temperature, wind speed and direction, atmospheric pressure, precipitation, solar insolation (sunshine intensity) and other measures on WCR populations and their movement. This field was the focus of our most intensive WCR monitoring activity. The solar powered weather station, located at the center of the field, made continuous measurements during the study period recording mean or totals for all parameters at 10 minute intervals.

Vial traps: Vial traps were used to establish patterns of WCR abundance in a variety of settings. Traps were made from 60-ml amber-colored plastic vials with snap caps as described by Levine and Gray (1994). The bottoms of the vials were replaced with wire screen to prevent condensation within the traps. Ten-5 mm diameter holes were drilled in the sides of the vials to allow beetles to enter. Inserts for the vial traps were prepared by spraying both sides of 21.6 cm x 27.9 cm sheets of acetate transparency film with a 1:1 mixture (by volume) of carbaryl insecticide (Sevin XLR+; Rhone-Poulenc, Research Triangle Park, NC) and water. Powdered squash was sprinkled on the film and allowed to dry. The film was then cut into 2.5 x 7.6 cm strips and inserted into the vial traps, one strip per trap. Approximately 0.5 grams of squash was applied to each insert. The powdered squash came from a dried Cucurbita andreana x C. maxima cross grown in 1979. Only fruit with high levels of cucurbitacin was used. Cucurbitacins are a group of compounds found in bitter squash, cucumbers, and melons that make rootworm beetles feed compulsively. Beetles randomly enter the trap and feed on the powdered squash, and in the process ingest a lethal dose of the insecticide.

Vial trap transects: To monitor the distribution of WCR across crop interfaces vial trap transects were established between adjacent fields of corn and soybeans (or other crops, including alfalfa (Pioneer 5312) oats (Ogle)). Vial traps were tied to 150 cm tall PVC posts spaced 30 m apart across corn and soybean fields from the edge nearest the adjacent cornfield. Traps height in soybeans was determined by the level of the soybean canopy, and traps were adjusted upward as the plants grew. Vial traps in corn were kept at ear height. Five trap locations were used in each corn or soybean field, and were checked weekly from late-June until early-September. Because vial traps contain the cumulative collection of insects over a sampling interval, they reveal abundance patterns typical of the week prior to when they were serviced.

Sweep samples: In soybeans, oats, and alfalfa, WCR were sampled by making 100 sweeps across the soybean canopy of one soybean row using a 38 cm diameter sweep net while walking at a steady pace. Sweep samples provide an instantaneous measure of insect abundance in sampled locations. To immediately halt feeding and insure beetles were returned the laboratory in good condition for later dissection, WCR collected with sweep nets were immediately killed by transferring them into a cooler of dry ice. In the laboratory, the insects in each frozen sample were sorted; economically important insects were identified to species and sexed. All sweep sample WCR were transferred to small vials and stored at -23°C until dissection.

Live collections: Live collection methods allow retrieval of fresh WCR from cornfields, or other locations were WCR are easily visible. Containers for making live collections consist of a glass canning jar, with a screw top modified to accept a large plastic funnel mounted in the metal lid. While walking down cornfield rows for a fixed period of time, the operator knocks as many WCR adults as are encountered into the mouth of the funnel, whereupon they tumble into the collection jar through the narrow outlet of the funnel. If the insects are not destined for experimentation, dry ice is placed in the collecting jar. Intense cold and the carbon dioxide released from small pieces of dry ice in bottom of the jar instantly kill the WCR. Like WCR sampled with a sweep net, these were sorted, sexed, and kept frozen for later dissection.

Directional Malaise Traps: Monitoring the movement of WCR involved the use of directional paired "malaise" traps whose openings permitted insect entry from only one direction. Each trap was made of 3.8 cm outside diameter schedule 40 PVC pipe covered with approximately 13 m2 of Toule (bridal veil fabric) and had a 1 m2 opening. The sides of the trap converged at the back to give the trap a triangular cross-section. The roof of the trap sloped upward to the apex where a 10 cm circular opening was cut in piece of Plexiglas at the top. Mounted in the hole was a plastic collar into which a pair of nested 2-liter plastic soda bottles were inserted. Insects entering the trap passed up into the lower bottle and through its neck into the upper bottle. The interior surfaces of the upper bottle were coated with liquid Teflon to prevent insects from escaping before they were killed by an insecticide cube affixed in the cap of this bottle. Each morning, the trap tops were removed and the accumulated insects were sorted by species and sex. On July 8th, 16 traps were placed around the perimeter of two soybean fields in eight pairs. Two pairs of traps were also positioned a first-year cornfield adjacent to our field site. Two additional pairs of malaise traps, one in alfalfa and the other in oats, were placed opposite those in corn. At each site, one trap's opening faced away from the field and the other faced to the interior. Trap height was adjusted throughout the season so that the bottom of the 1 m2 opening was at the top of the plant canopy. Traps in corn were kept at the elevation of those in the adjacent soybean, oat, or alfalfa field.

The malaise traps described above were used to monitor WCR immigration and emigration within the first meter of air above the oat, alfalfa, and soybean plant canopies. Insects trapped in malaise traps were collected daily between July 8 and August 21. On July 10th, in the soybean field with the weather station, a 9 m tall scaffold was erected. Two additional malaise trap pairs were attached to this structure at 4 m and 8 m heights above the soybean canopy. These traps permitted measurement of WCR movement at relatively high elevation.

Data analysis: All figures depict mean values ± standard error of the mean (SEM), unless otherwise indicated. Following significant ANOVA, multiple comparisons were performed using Fisher's Protected LSD procedure at a=0.05. Data were normalized before analysis when variation in WCR abundance was the result of seasonal or geographical variation (i.e., changing population sizes over the season, higher WCR populations in some areas than others due to more or less corn growing around a site).

Statewide Sampling

During late July and August 1998, 271 sweep samples were taken from soybean fields in 44 counties across Illinois, including all counties at risk for WCR egg-laying in soybeans (based on 1997 population densities of WCR in soybean fields and documented economic injury). In most counties, two different fields were sampled (100 sweeps per sample) at each of three separate locations in the county. Only soybean fields with nearby cornfields were selected for sampling. After collection, insects were transferred into coolers of dry ice and immediately frozen for later dissection. Figure 1 depicts the average number of WCR collected per 100 sweeps with a 38 cm sweep net in 1998. If a county was sampled in both 1997 and 1998, the 1997 average is presented in parentheses below the 1998 average. Abundance of WCR, northern corn rootworms (NCR), bean leaf beetles (BLB), and Japanese beetles are summarized for state regions and presented in the smaller maps.

Compared to 1997, the average abundance of western corn rootworms (WCR) in Illinois soybeans appears to have declined by almost 10-fold in 1998. WCR abundance in soybeans declined in all but one of the counties sampled in both 1997 and 1998 (Moultrie Co.; 1997= 0.0 WCR per 100 sweeps; 1998=2.0 WCR per 100 sweeps). WCR were significantly more abundant in the area previously identified as being at highest risk for egg-laying by WCR in soybeans, than in areas north, south, or west of the problem area (F=33.8, P<0.0001, df=270). WCR were not detected in 14 of the sampled counties. These data suggest there was no significant expansion of the 1997 WCR problem area in 1998. WCR continue to be present at low or undetectable levels in soybean fields west of the Illinois River and south of Interstate 70.

WCR collected during state sampling in 1997 were dissected. Females from soybean fields in problem area counties were found to carry significantly more eggs than females from soybeans in nonproblem counties or females in corn from any county (Figure 2). The proportion of females whose gut contents included both corn and soybeans was significantly higher for individuals in soybean fields (0.046) than for those in corn (0.007) regardless of the location's problem/nonproblem status (F=11.0, P=0.0009, df=716). Females collected in soybeans and dissected were always mated (n=518), whereas a small portion (15/857=1.8%) of those from live collections in corn were unmated. The difference in proportion mated between corn and soybean fields was significant (n=1375, alpha=9.17, P=0.0009; Fisher's exact test), all but two of the unmated females were collected in early July.

Remote Illinois Sampling Locations: In addition to sweep samples around the state, vial trap transects running between adjacent corn and soybean fields were maintained at four University of Illinois Field Research and Demonstration Centers remote from the problem area: Northern Illinois Agronomy Research Center near Shabbona (DeKalb Co.), Northwestern Illinois Agricultural Research and Demonstration Center near Monmouth (Warren Co.), Orr Agricultural Research and Demonstration Center near Perry (Pike Co.), and Dixon Springs Agricultural Center near Dixon Springs (Pope Co.). On-site cooperators deployed five traps each in a pair of adjacent corn and soybean fields. Sweep samples were taken weekly from the soybean field, and vial traps in corn and soybeans were emptied weekly at which time the insecticide-treated strips were also replaced. Identically configured vial trap transects were also deployed at two sites in the Champaign Co. area (see below). The results from vial traps at the four sites and the local problem area trap transects are presented in Figure 3.

WCR were scarce in soybean field vial traps at the four remote sites. WCR adults were found in corn at all sites, though they were always more abundant in corn at the Champaign Co. sites. Of the sites, WCR were more abundant in soybeans than in corn only at the Champaign Co. problem area sites. High WCR abundance in soybeans continues to distinguish problem from nonproblem areas.

Champaign Co. Sampling

During the 1998 growing season, vial traps, vial trap transects, sweep samples, live collections, and malaise trapping methods were used to monitor the emergence, development, and dispersal of WCR populations .

Local Monitoring: Northeast of Urbana, a 41.5 km2 network of vial traps was deployed in corn, soybeans, woods, and a commercial tree farm at road intersections between July to September (the same sampling locations were also used during the 1997 growing season). Twenty-five growers were involved in this effort. One vial trap per field was placed 10 m into each selected field. These traps were emptied weekly.

Even over this relatively small geographic region, WCR abundance was quite variable. However, just as in 1997, WCR were collected at all locations where a trap was stationed. Averaged across the 41.5 km2 area, significantly more WCR were collected in corn (2.3-±0.3 WCR/day) and soybean field (2.0 ± 0.3 WCR/day) traps than in traps in woods (0.02 ± 0.01 WCR/day) or the tree farm (0.02 ±0.01 WCR/day; F=4.97, P=0.0002, df=449). In 1997, 8.6 ± 0.5, 17.1 ± 0.9, 0.6 ± 0.2, and 0.9 ± 0.4 WCR/day were caught in corn, soybeans, woods and the tree farm, respectively. The number of WCR collected from soybean field vial traps was overwhelmingly higher than that in cornfields (F=35.6, P<0.0001, df=1437) in 1997. The modest 1998 result represents an average population reduction of approximately 7.5 times compared to 1997.

Spatio-Temporal Variation and Periodicity of Daily WCR Abundance: Eight soybean and seven cornfields located at five road intersections across the local monitoring network were selected as sites to study the periodicity of WCR abundance. Each Thursday, sets of sweep samples (100 sweeps/field) and live collections (5 minute) were obtained between 8-9 am, 12-1 pm, 3-4 pm and 7-8 pm from the same area of each corn or soybean field, providing spatial and temporal resolution on WCR abundance. After collection, insects were immediately transferred into coolers of dry ice to preserve them for later dissection.

WCR abundance in both corn and soybeans varied significantly during the day (Figure 4). In soybeans, WCR were significantly more abundant in the morning and evening than in mid-day. In corn, the greatest WCR abundance occurred in the evening.

Champaign Co. Vial Trap Transects: Vial trap transects were deployed between adjacent first-year corn and soybean (as well as adjacent corn and oats/alfalfa plots) fields at three locations in Champaign Co. (two NE of Urbana, one NW of Champaign). A transect consisted of 10 vial traps (five in corn and five in the crop adjacent to corn). Traps were changed weekly at which time the insecticide treated strips were also replaced. At our primary research and observation site "Lost 40", located northeast of Urbana, traps were changed twice weekly. Vial traps were first deployed on June 26th and 30th. A sixth vial trap was deployed in the Champaign field on July 8th.

In addition, to the transects, another 66 vial traps were distributed in a grid like fashion across five of eight 4-acre research plots at the Lost 40 site. Twenty-four vial traps each were distributed in a 4 x 6 grid in a four acre soybean field and another field divided into two 1-acre alfalfa and two 1-acre oat plots. These plots were immediately east of a cooperating grower's first-year cornfield. Transects beginning in this first-year cornfield extended into soybeans, oats, and alfalfa. Six vial traps were placed in each of two other soybean fields and an alfalfa field. These vial traps were changed twice a week at the time the transects were serviced.

The first adult WCR males were observed on June 28th. By the end of week 1 (July 5-11), WCR adults were common in cornfield vial traps (Figure 5). Because these vial trap data represent insects captured during the week prior to their retrieval, the timeline of events they depict is shifted later by one week compared to instantaneous measures of abundance (e.g., beetle counts on plants and sweep samples). Between weeks 1 and 6, WCR abundance in soybeans increased dramatically while numbers in corn remained relatively steady. Outside of corn, the percentage of female WCR increased rapidly with time, by week 2 females comprised about 65% of all WCR captured; by week 7 females represented >90% of all WCR collected in vial traps outside of corn. Peak WCR abundance in soybean vial traps occurred on or around August 6th (week 5). Peak WCR abundance in oats and alfalfa occurred during week 6. Heavy rains were recorded during week 5, when 31 cm of rain fell at our field site. Conditions remained wet at the field site for several weeks. A high incidence of fungal infection among insects collected from the field at this time suggests disease may have contributed to the general population decline between weeks 6 and 7. Vial traps in soybean caught significantly more WCR than those in corn, oats or alfalfa (F=81.4, P<0.0001, df=1751) (Figure 6). Though vial traps in corn caught significantly fewer than those in soybeans, they performed significantly better than traps in either alfalfa or oats.

As observed along other transects in Champaign Co. (i.e., Lost 40)., WCR were most abundant in soybean field vial traps (Figure 7). WCR were collected in oats and alfalfa but at a lower rate. Results from the grid of vial traps arrayed around Lost 40 indicate that WCR abundance was not significantly different in separate plantings of the same crop (soybeans and alfalfa), nor did abundance vary significantly within a plot.

Sweep Samples and Live Collections: Sweep nets and live collection methods were used extensively at the Lost 40 location to monitor WCR population fluctuations during the 1998 growing season. Extensive sweep sampling in soybeans, oats and alfalfa and live collections in corn were made throughout the season at this site multiple times per day and at multiple locations around the site.

Sweep sample and live collection data corroborated abundance patterns established using vial traps (Figure 8). Sweep samples collected in soybeans indicated the WCR population peaked during weeks 4 and 5, and fell by week 6 (the same pattern was observed for sweep samples in oats and alfalfa (data not shown)). The proportion of females in soybean sweeps samples rose dramatically from 55% to >84% between weeks 1 and 2, and remained at 85-100% female for all subsequent weeks. As observed in vial traps, significantly more WCR were collected in soybeans than in either oats or alfalfa (Figure 6).

Patterns of WCR abundance were observed to fluctuate significantly during the day (Figure 9). In general, significantly more WCR are caught during morning or evening than are caught during midday in soybeans and alfalfa. There were no significant differences in abundance in oats. The proportion of females in plantings outside of corn also declines significantly in the evening (0.75) compared to the proportions in morning (0.88) and midday (0.90) samples.

Similar patterns of WCR periodicity were observed during the 1997 growing season (Spencer et al. 1998). In 1997, over 20,000 adult WCR were collected during sweep samples and live collections in the soybean and cornfields at Lost 40; 2,300 of these were dissected and scored for reproductive development, mating status, and contents of their digestive tract (gut contents). We found that the reproductive status (a measure of the state of egg development and an indication of the potential for egg-laying) of WCR females in soybeans was significantly greater than that of females in corn between weeks 3 through 8 (Figure 10), thereafter females from corn and soybeans had a similar reproductive status.

Based on patterns of WCR capture in directional malaise traps during 1997, we hypothesized that WCR are moving frequently of between corn and soybeans. Dissection of WCR collected in 1997 from the fields where movement was monitored, revealed that a high proportion of WCR females had both corn and soybean plant parts in their digestive systems at the time of capture (Figure 11). The presence of corn plant parts in the guts of females collected in soybeans indicates that these individuals had been feeding in both locations within the last hour (it takes approximately 1 hour for a piece of WCR food to completely pass through and exit the insects digestive system). Two peaks of corn and soybean feeding were observed, coinciding with pollen shed and corn pollination in the local cornfields and that of a late-planted cornfield north of our sampling site.

Daily patterns were also observed in the gut contents of WCR collected in soybeans, the proportions of females with both corn and soybeans in their digestive systems closely mirrored daily patterns of WCR abundance established for the 1997 population (Figure 12). The greatest proportion of corn and soybeans in the gut is found at times when WCR populations have been observed to be on the move between corn and soybeans. The proportion of males with both corn and soybeans in the digestive system does not differ significantly over the time intervals. The daily patterns of WCR abundance in soybeans were remarkably similar between 1997 and 1998 (Figure 13).

Malaise Trap Monitoring of WCR Movement: Malaise traps capture insects flying in or out of fields. In 1998, using 28 malaise traps (20 were in soybeans), we caught only 1/10th as many flying WCR as we had with one third fewer traps (eight malaise traps were used) in 1997, for an overall estimated decline of approximately 25 times. The 1998 results for the soybean fields reveal that peak population occurred 21 days earlier in 1998 than it did in 1997 (Figure 14). The week of peak abundance in malaise traps coincided with peak WCR abundance in soybean sweep samples and cornfield live collections. As in 1997, daily WCR abundance was highly variable. Traps elevated to 4 and 8 m above the soybean canopy caught significantly fewer WCR than those collecting insects within 1 meter of the soybean canopy (F=44.3, P=<0.0001, df=833) (Figure 15). Females comprised a significantly greater proportion of the insects trapped in the elevated malaise traps than those at ground level (F=4.5, P=0.03, df=806) perhaps indicating that females are more likely to engage in flight behavior conducive to long distance dispersal.

Trécé attractant traps: In addition to vial traps, four pairs of Trécé attractant traps (Trécé, Inc., Salinas, California) were stationed in corn and soybean fields around the 41.5 km2 vial trap network. The traps consisted of a clear plastic cup bowl with removable rain shield baited with a proprietary floral lure and a block of Sevin-treated insect diet, and were mounted on 150 cm tall PVC poles. Trécé traps were located 30 m from the vial trap in selected fields. A fifth pair of Trécé traps were deployed at the Lost 40 site along a corn-soybean transect. The attractant traps were monitored when vial traps were changed.

Throughout the period when they were deployed, the Trécé traps collected tremendous numbers of WCR (Figure 16), so many that during peak WCR abundance, captured WCR were weighed rather than counted. The Trécé traps accumulated WCR at a rate that was over 40 times that of vial traps in soybeans and 15 times that of vial traps in corn. The trap lures were not changed during the entire summer.

Effect of Insecticide-free "Slam-less" SLAM application on WCR abundance in soybeans: SLAM is an insecticide developed to specifically target Diabroticite beetle pests and is manufactured by MicroFlo, Inc. (Lakeland, Florida). It contains cucurbitacins (behavioral arrestants and feeding stimulants upon which WCR and other corn rootworms will feed compulsively) formulated with the insecticide carbaryl. This combination of components allows much less active ingredient to be used to affect control. Slam has been applied for adult control in corn and soybeans as part of the Area Wide WCR Management Program for several years (Buhler et al., 1998).

On August 20, an experiment to evaluate the effect of applying SLAM formulated without insecticide ("slamless-SLAM") (supplied by MicroFlo of Lakeland Florida) on WCR populations in soybeans was executed. Because soybeans are an inadequate food for WCR egg production and long-term survival, it may be possible to significantly impact the reproductive potential of female WCR by enticing them to remain in soybeans for extended periods. The cucurbitacins in SLAM are powerful arrestants and feeding stimulants. We were interested to know if an application of just the formulated cucurbitacins in SLAM could increase WCR populations in soybeans (where WCR feeding damage has no effect on crop yield (Spencer et al., 1997, 1998). Perhaps female WCR arrested in soybean fields will fail to balance their diet and, as a consequence of reduced health, cease to mature eggs.

The "slamless-SLAM" was applied at the recommended rate of 0.375 lbs/acre in 16- 8 row x 50' soybean plots arrayed around all sides of the soybean field at Lost 40. In preparation, >2000 adult WCR were collected from area cornfields and transferred into cardboard containers in groups of 100 beetles. Half of the small plots were hand sprayed with the formulation on the evening of August 20 using a backpack reservoir and a hand held 2-row -spanning sprayer equipped with TeeJet 8002VS nozzles operating at 20 psi. After spraying, two containers of 100 WCR were placed in half of the sprayed plots, and half of the plots that had not been sprayed. The remaining sprayed and unsprayed plots received no WCR augmentation. The following morning before sunrise, the WCR were released from the containers. That evening each plot was sampled with a sweep net, and the number of beetles was counted.

Significantly more WCR were recovered from the plots where SLAM had been applied (F=8.7, P=0.01, df=1) (Figure 17); there was no significant effect of adding 200 WCR to some plots (F=0.14, P=0.71, df=1) nor any SLAM x WCR addition interaction (F=0.02, P=0.89, df=1). In addition, abundance of NCR and SCR were also significantly higher in plots with SLAM (data not shown). The abundance of other non-diabroticite beetles (e.g. bean leaf beetles and Japanese beetles) in the treated plots was not significantly altered by the treatments. This was to be expected if the effects of treatment were due to the cucurbitacin content of the spray and not some other factor or disturbance associated with the experiment.

Soybean Cultivar Testing: Consumption of soybean foliage by adult WCR has been observed in areas where WCR are a problem in first-year corn. It has been hypothesized that soybean varieties less attractive to WCR for feeding may be useful for reducing egg-laying in soybeans by reducing the time these insects spend in a field. In a laboratory feeding study, three Mexican bean beetle resistant soybean varieties (HC 83-193, PI 229358, and MBB 80-133) along with a MBB susceptible control (Elf), and the WCR susceptible 'Williams 82' (a variety shown to be readily consumed by WCR in previous studies) were presented to single soybean field-collected female WCR in no choice tests. Each female was given a single leaf disk (1.5 cm diameter) cut from one soybean variety while held for 2-3 days in a 20 x 60 mm Petri dish with a moist cotton dental wick. At the end of the experiment, leaf disks were removed from each container and the area consumed was determined. Control leaf disks, prepared just as the test disks and held under identical conditions during the period of insect feeding, were used to make weight per unit area determinations so that leaf area consumed could be standardized across the varieties. All of the MBB resistant soybean varieties, and the MBB susceptible control were consumed at a significantly lower rate than that of the susceptible Williams 82 (F=10.8, P<0.0001, n=100) (Figure 18).

Discussion

Compared to 1997, populations of WCR in Illinois soybeans were approximately 10 fold lower in 1998. A potentially devastating WCR problem anticipated for 1998 was likely averted by significant rainfall during the period of peak WCR egg hatch and larval establishment on corn roots. WCR populations were significantly reduced throughout most of Illinois (NCR populations were also much reduced across the northern half of Illinois). No new pockets of WCR infestation in soybeans were identified outside of the counties previously identified as at high or moderate risk for WCR damage. Statewide, WCR populations in soybean fields remained highest within the nine county 'problem area' identified in 1995. Monitoring at sites remote from the problem area reveals a stark a contrast between the situation in east-central Illinois and that where the WCR has yet to arrive.

WCR collected with vial traps in several Champaign Co. corn and soybean fields indicate that the population decline occurred in both corn and soybean fields. For comparison, during the 1997 growing season, vial traps in corn and soybeans averaged 70-80 WCR collected/day; however, in 1998 the season average for corn and soybeans was only 3.1 and 6.1 WCR/day, respectively.

Multiple sampling methods indicate that the WCR population emerged and later peaked about two to three weeks earlier than it had in 1997. Patterns of WCR abundance as measured in vial traps reveal a consistent pattern: WCR are initially detected in corn and remain most abundant there for approximately 3-7 days. Thereafter, WCR populations build in soybeans and rapidly exceed those in corn. In spite of generally low populations throughout Illinois counties most at risk for WCR damage in first-year corn, WCR continued to be abundant in soybean fields.

Dissection of insects from the 1997 growing season and identification of individuals with both corn and soybeans in their gut demonstrates that movement between corn and soybeans (and likely other crops) is extensive during the first month of the WCR season. This period coincides with a period when reproductive development of females in soybeans is at a peak. We now know that females in soybean fields in the problem area are carrying more mature eggs than those in corn or in corn or soybeans outside the region. An abundance of females in soybeans with high reproductive development early in the season may indicate that significant egg-laying in soybeans is happening early in the growing season.

WCR movement between corn and soybeans is occurring according to a consistent schedule. Adults are most numerous in soybeans during the morning and evening, populations are significantly reduced during mid-day. Based on 1997 malaise trap results, we had hypothesized that the daily periodicity of WCR abundance in soybeans could be explained by movement of between corn and soybeans. Dissection of insects collected during 1997 provides evidence that the periodicity is indeed due to WCR immigration and emigration between corn and soybean fields.

The origins of the migrating insects are revealed by the contents of their guts. The proportion of female WCR whose guts contained both corn and soybeans changes in step with that of WCR periodicity in soybeans. At times during the day when immigration to soybeans is suspected to account for high WCR populations there, a significantly greater proportion of WCR guts contained both corn and soybeans--a condition that is not possible unless individuals had been in both fields within the previous hour. At mid-day when movement out of soybeans is hypothesized, the proportion of females with both corn and soybeans in the gut is lowest. Male WCR display no periodicity in corn and soybean presence in their gut contents. With respect to feeding on corn plant parts and soybean foliage, male and female WCR behavior is very different.

Although WCR will feed on soybean foliage and a variety of other plants when outside of corn, an exclusive diet of soybean tissue will not support egg development or sustain a WCR for long. The high frequency of WCR adults that contained both corn and soybean plant tissues at some times of the day and season are evidence of the WCR's potential to achieve diet mixing that may compensate for the poor quality of a strictly soybean diet. Given the disadvantages of feeding on soybeans, it has been proposed that the soybean plant could be exploited to reduce the likelihood or ability of WCR to lay eggs in soybean fields. Our WCR feeding trials with soybean varieties resistant to the leaf-feeding Mexican bean beetle, indicate that characteristics of these varieties may confer resistance to WCR feeding. If WCR are deterred from feeding on soybean foliage, perhaps they will spend less time in soybeans and be less likely to deposit eggs outside of corn. Conversely, soybean varieties that stimulate excessive feeding in spite of the physiological costs to the insect of doing so, may disadvantage females enough that they suffer reduced reproductive potential as a consequence.

Another approach to managing the WCR would be to arrest it in soybean fields (where it's feeding damage doesn't reduce crop yield) long enough so the costs of not balancing soybean consumption with corn feeding would reduce their ability to lay eggs subsequently. The potential for manipulation of movement was demonstrated with using a commercial formulation of the cucurbitacin-based insecticide SLAM which was specially prepared without any insecticide.

Conclusion

Despite unprecedented high levels of egg-laying by WCR during the 1997 growing season and a mild winter, populations of WCR beetles across Illinois counties in 1998 were down. Furthermore, the "problem area" in the state was not enlarged. Apparently, the heavy rains across Illinois during June 1998, killed large numbers of larvae in many cornfields. However, it should be noted that scattered pockets of high WCR abundance were reported, probably where the rainfall was less than elsewhere and/or on well-drained soils. Low WCR populations are good news for Illinois growers; on average we expect WCR egg-laying in Illinois corn and soybean fields to have been much reduced in 1998 than from the previous year.

WCR, like many other insects, have a high reproductive potential. Each female beetle can lay between 200 and 1,000 eggs (Levine and Oloumi-Sadeghi 1991, Gray and Luckmann, 1994). With reduced competition among larvae for corn roots next year, a large proportion of what eggs were deposited in soybean fields during 1998 could emerge as adults in 1999, unless weather conditions are again adverse. Next year, it is likely that some pockets of high abundance will appear in regions with only moderately high WCR count averages this year. Thus, WCR management decisions for 1999 should be based on local observations and the use of scouting procedures developed for WCR in soybean fields (O'Neal et al. 1997, 1998). WCR counts were clearly down in numbers, but they should not be left out of management decisions.

Hopefully multiple years will pass before WCR populations reach the unprecedented high levels like those recorded in 1995 and 1997 (Levine and Gray 1996, Spencer et al. 1998). This report indicates just how much we have learned about the behavior and physiology of the new WCR strain that developed in east-central Illinois. We believe that enough progress has been made to allow for development and testing of new tools for managing WCR in first-year cornfields. We are very concerned about the dramatic increase in chemical applications to first-year cornfields for WCR control in east-central Illinois (Pike and Gray 1992, Gray et al. 1998). Fortunately several biotechnology companies are perfecting genetically-engineered rootworm-resistant corn that should be available to Illinois producers in a few years (Kilman 1998). Ultimately, the success and long-term utility of rootworm-resistant transgenic corn will depend on communications between scientists and growers. Sound methods for deploying refugia areas planted with non-transgenic corn will be crucial to the success of this new phase of WCR control. Learning how WCR behave in refugia and transgenic corn plots will be critical to designing appropriate refugia. Consequently, not only do we need to develop more environmentally benign tools for controlling both WCR larvae and adults in first-year cornfields right now, but we must also explore strategies and tactics for managing refugia populations of WCR while preserving their susceptibility to transgenics for the not-so-distant future.

Acknowledgments

We thank Anthony Armstrong, Erica Bailey, Dawn Coppin, Matt Coyner, Paul Dabisch, Wendy Gierhardt, Zihna Gordon, Tom Mueller, Mark Nasser, Shalondra Reed, Rob Daniels and Ryan Von Holten for technical assistance on this project, and Randall Nelson for assistance with the various soybean cultivars. Mexican bean beetle resistant soybean lines were supplied by Richard Cooper,(USDA-ARS, OARDC, Wooster, Ohio) and Charles Helm (Illinois Natural History Survey). Slam formulated without insecticide was kindly provided by Scott Johns (MicroFlo Inc., Lakeland, Florida). Appreciation is also extended to the many producers in whose fields we worked, to Robert Dunker, Mike Plotner and the University of Illinois Crop Sciences Department for their assistance in planting, cultivation and field maintenance at our research site, and to John Shaw for the use of his backpack spray equipment, and assistance in its calibration. We very much appreciate the season-long help of Eric Adee, Ron Hines, Lyle Paul, and Glenn Raines for establishing and maintaining the vial trap transects at the four University of Illinois Field Research and Demonstration Centers. We also gratefully acknowledge funding from the Illinois Council on Food and Agricultural Research (C-FAR) and the Illinois Soybean Program Operating Board (ISPOB) that supported this research.

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