Western Corn Rootworm on the Move:

Monitoring Beetles in Corn and Soybeans

Joseph L. Spencer (UI), Scott A. Isard (UI) , Eli Levine (INHS), David W. Onstad (UI), Mark Nasser (UI), Tom Mueller (UI), and Dawn Coppin (UI), Mark Belding (ISWS), Tony Armstrong (UI), Matt Coyner (UI), Ericka Bailey (UI).
Funded by C-FAR and the Illinois Soybean Program Operating Board. 

The western corn rootworm (WCR) (Diabrotica virgifera virgifera LeConte) is the most serious insect pest of corn grown after corn in the Midwest. WCR have a single generation per year. The adult beetles are present in cornfields from July through frost. They feed on corn pollen, silks, immature kernels, and foliage. From late July through September, egg-laying occurs primarily in cornfields; few eggs are normally laid in other crops. The eggs remain in the soil until the following spring. Egg hatch begins in late May and early June. The larvae can survive only on the roots of corn and on the roots of a limited number of grasses. Larval feeding on corn roots may reduce the amount of water and nutrients supplied to developing plants. Extensive root injury makes plants more susceptible to lodging (plants lean over or elbow). Larval feeding may also facilitate infection by root and stalk rot fungi, resulting in further damage. Yield losses also result from difficulty in harvesting lodged corn (Levine and Oloumi-Sadeghi 1991).

For many years, growers have controlled WCR with crop rotation, a method where fields are planted with corn and soybeans (or another non-corn crop) in alternate years. Crop rotation disrupts the WCR life cycle because soybeans (or another crop whose roots do not support the development of larvae) are grown where corn had been planted the previous year. Since WCR lay their eggs almost exclusively in cornfields, and the larvae must feed on corn roots to develop, planting corn away from where WCR eggs were laid kills the young larvae by separating them from their food supply. Using rotation, first-year corn (corn planted after another crop, usually soybeans) could be produced without the need for WCR-targeted soil insecticide treatments. Unfortunately, a behavioral change in some WCR populations is threatening the effectiveness of crop rotation, which has been one of the great "success stories" for controlling this pest.

Beginning in 1993, reports of serious WCR larval injury to first-year corn in east-central Illinois and northwest Indiana began to increase. Growers who had successfully used crop rotation for WCR control suffered serious crop losses. Use of pyrethroid insecticides, primarily permethrin for corn earworm control in seed corn production fields, was initially suspected in the early Illinois outbreaks of 1987-1992. It was thought that these insecticides repelled adult WCR beetles from treated seed cornfields to nearby soybean fields where they laid their eggs (Levine and Oloumi-Sadeghi 1996). In 1993, 1994, and especially in 1995, the problems became increasingly more frequent and severe and included many fields of commercial corn that were not near cornfields treated with pyrethroid insecticides the preceding year (Levine and Gray 1996a, 1996b).

Subsequent studies ruled out a prolonged egg diapause and repellancy by pyrethroid insecticides as likely explanations, leaving open the possibility that the corn-soybean rotation itself had selected for a WCR strain that circumvented crop rotation by laying eggs in soybean fields (where their larvae will emerge into corn the following spring and survive). Egg sampling and field-collection of larvae confirm that a behavioral change has occurred and WCR in east-central Illinois are now laying eggs in both corn and soybean fields.

Since the initial reports of WCR damage in first-year corn, the incidence of WCR injury to first-year corn following soybeans has been increasing in east-central Illinois and northwestern Indiana. Unlike 1995 when WCR problems were common in first-year corn, favorable weather conditions during the summer of 1996, enabled corn plants to tolerate root damage by WCR larvae and few reports of significant WCR damage were received. Monitoring of WCR abundance in east-central Illinois during the 1996 growing season revealed that beetles were moving out of corn and into soybean fields shortly after they were first detected in corn. Thereafter populations of WCR remained higher in soybeans than in corn for the remainder of the season (Spencer et al. 1997).

Before 1996, WCR were thought to move between corn and soybean fields for egg-laying only after corn silks, tassels, and foliage were either no longer available or attractive as food. However, early in the 1996 growing season, behavioral observations revealed that WCR were moving frequently between corn and soybean fields and feeding on soybean foliage in areas where WCR egg-laying in soybeans was known to occur. A startling aspect of this new behavior was that WCR were often feeding on soybean foliage in the vicinity of nutritionally suitable corn plant parts. The combination of movement between corn and soybean fields (and perhaps other crops) during the growing season and soybean foliage feeding suggested that the diet of WCR may also be broadening (Spencer et al. 1997). However, subsequent laboratory studies demonstrated that soybean foliage and other plant parts are not a suitable host for adult WCR females. In fact, insects which had fed on soybeans were judged to be significantly less vigorous than those feeding on corn plant parts. The vigor of soybean-feeding insects could be restored if they were fed corn plant parts after experience with soybeans. A necessity to compensate for the negative effects of feeding on soybean foliage may be at the heart of WCR movement between corn and soybean fields. In light of evidence for extensive movement by WCR between corn and soybean fields during the 1996 growing season, a concerted effort to explore factors and conditions that influence WCR movement was deemed important.

The threat to crop rotation constituted by WCR egg-laying in soybeans is one in which alteration of movement patterns play an important role. In addition to continued measurement of seasonal change in WCR population densities and observation of behavior during the 1997 growing season, we also measured the effect of large and small scale weather conditions on WCR movement abundance at several different spatial scales. Identification of circumstances, in particular those related to daily weather conditions, that favor or inhibit WCR movement, will allow us to better interpret results of sampling programs and foster development of models to predict the future course of this developing problem.

Prolonged Diapause Studies on 1995 Eggs

During the summer of 1995, many new reports of rootworm larval injury to first-year commercial corn following soybeans were received, all in east-central Illinois. All larvae that were collected and reared to the adult stage (n = 85) were WCR (collected in fields near Loda, Gifford, Strawn, and Saunemin). To check for the prolonged diapause trait, eggs were obtained from beetles reared from larvae collected at Gifford, Strawn, and Saunemin. These eggs were then subjected in the laboratory to soil temperature conditions they would normally experience in the field. By September 1996, 11.3% of the eggs obtained from beetles reared from larvae collected at Gifford (n = 160 eggs), 5.0% of the eggs obtained from beetles reared from larvae collected at Strawn (n = 160 eggs), and 41.5% of the eggs obtained from beetles reared from larvae collected at Saunemin (n = 41 eggs) remained unhatched, but appeared to be in good condition. All of these seemingly viable eggs were subjected to another overwintering cycle. These eggs all died over the course of the "winter" or immediately after they were incubated in the spring of 1997. Based on these and earlier observations of egg hatch, prolonged diapause is not likely to be the cause of the root injury problems producers have been experiencing with WCR in first-year corn following soybeans.

Survival of WCR Larvae on Soybean Roots

Repeated attempts to rear WCR larvae on soybean roots have been unsuccessful; WCR larvae do not develop on soybean roots. We are confident that the WCR beetles we find in soybean fields must have flown there from neighboring cornfields. The fact that when using vial traps we capture WCR beetles about a week sooner in corn than in soybeans is a further indication that beetles fly into soybean fields from nearby cornfields. The skewed mid-season sex ratio that we find in soybean fields (about 70% female, rather than the 50:50 ratio one finds in fields where the adults emerged) also points toward immigration of beetles (more females than males colonize new fields).

Effect of Adult WCR Feeding on Soybean Yield

We have repeatedly observed in the laboratory and the field that WCR beetles readily devour soybean flowers. Although soybean flowers self-pollinate before the blossoms open, we nonetheless decided to take a closer look at the effect of this feeding on soybean seed production. A preliminary study in 1996 indicated a small negative (a statistically insignificant 6.1% yield loss) impact on yield (Spencer et al. 1997). The study was repeated again this past spring and summer with greater replication (34 soybean plants). Again, no significant effect of WCR blossom feeding on soybean yield was detected. If feeding by WCR on soybean blossoms is having an impact on yield, it is probably very subtle.

Soybean Cultivar Testing

In an effort to screen for plant characteristics which may affect movement of WCR beetles into soybean fields, studies were begun in 1996 to evaluate WCR population densities in different soybean cultivars. In cooperation with Randall Nelson at the National Soybean Research Laboratory in Urbana, WCR vial traps were placed in small plots (3 meter x 3 meter) of 37 different soybean cultivars (one trap per plot) at the soybean germplasm plots in Urbana on August 28, 1996 and left in the field for 1 week. By combining trap catch data from cultivars sharing specific plant phenotypic traits and growth patterns we were able to compare the effect of several plant characteristics with respect to numbers of WCR captured in control cultivar plots ('Proto' and 'A3127'). Several interesting trends were found. Differences between chlorophyll deficient (yellow) and green-leafed plants were marginally significant (P = 0.11), as were comparisons of plants of varying height (P = 0.18). There were significant effects of pubescence color in a comparison of exotic germplasm lines (P = 0.03), but little effect for the same comparison among modern germplasm lines (P = 0.22).

Because WCR soybean foliage feeding was so common in our 1996 field plots, we evaluated a soybean cultivar ('MBB-80-133') known to be resistant to leaf feeding by the Mexican bean beetle and the bean leaf beetle for possible resistance to WCR feeding. In these laboratory tests, WCR beetles consumed significantly less (P < 0.0001) 'MBB-80-133' foliage than foliage of a susceptible cultivar ('Williams 82'). The high incidence of WCR soybean foliage feeding in areas where egg-laying in soybeans is a problem suggest that these two behaviors may be linked.

In 1997, we continued our efforts to screen for plant characteristics which could affect movement of WCR beetles into soybeans. Again, in cooperation with Randall Nelson, vial traps were placed in replicated 3 meter x 3 meter plots (one trap per plot) of four different soybean cultivars at the Urbana soybean germplasm plots on August 14, 1997 and left in the field for 1 week. Cultivars included: 'Savoy' (early to middle Group II, tawny pubescence), 'Chapman' (early to middle Group II, gray pubescence), 'Iroquois' (early to middle group III, gray pubescence), and 'A3127' (early to middle group III, tawny pubescence). The number of beetles captured was not significantly different for the four cultivars (P > 0.05; an average of 276, 308, 289, and 267 WCR beetles were captured in traps placed in plots of 'Savoy', 'Chapman', 'Iroquois', and 'A3127', respectively), suggesting that the WCR beetles were similarly attracted to the four cultivars.

In cooperation with Charlie Helm and Robert Wiedenmann of the Illinois Natural History Survey, we also placed vial traps in small plots of soybean cultivars judged to be resistant ('MBB-80-133' and 'PI 567.327') or susceptible ('Pioneer 9352' and 'PI 567.765C') to the Mexican bean beetle and/or bean leaf beetle (based on laboratory and field preference and antibiosis tests). Vial traps were installed in the Champaign plots on August 28, 1997 and were removed 8 days later. The number of beetles captured was not significantly different for the four cultivars (P > 0.05; an average of 123, 109, 161, and 111 WCR beetles were captured in traps placed in plots of 'MBB-80-133', 'PI 567.327', 'Pioneer 9352', and 'PI 567.765C', respectively), suggesting that WCR beetles were similarly attracted to the four cultivars, even though earlier laboratory tests showed less feeding by WCR beetles on 'MBB-80-133' than a susceptible cultivar. Most interestingly, these plots of soybeans were located at least 0.5 mile away from the nearest cornfield. How this many WCR beetles could locate this small field of soybeans is quite remarkable.

Vial Trap Transects

The extent to which WCR were moving from corn into soybeans over the growing season was monitored by establishing vial trap transects across corn and into adjacent soybean fields at three east-central Illinois sites in the Urbana area; (Results from a fourth transect, deployed at an intensively monitored site, are discussed separately). In addition, sweep samples and live collections were made in the same fields.

The vial 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 traps were tied to posts placed at 30 meter intervals across the soybean field from the edge nearest the adjacent cornfield. Traps in soybeans were positioned in the soybean canopy and adjusted upward as the plants grew. Vial traps in corn were tied at ear height to stalks also spaced 30 meters apart beginning at the edge of the corn nearest the soybean field. Five trap locations were used in each corn or soybean field, and were checked weekly from mid-July to mid-September. Live collections in corn where made by capturing all adult WCR encountered during a fixed sampling interval while walking through the cornfield. In soybeans, four 50-sweep samples were taken in the vicinity of the vial trap transect. Insects recovered via all three sampling methods were sorted by species and sex (numbers of other pest Diabrotica were also noted) and frozen at -80·C.

At each of our vial trap transects, WCR were captured in soybeans one to two weeks after their first capture in corn (Fig. 1). Last season (1996), we were amazed to find that foliage feeding by WCR on soybeans was quite common. Dissection of the very first WCR collected in soybeans during the 1996 growing season revealed that (59%) of those first immigrant females collected in soybeans (10 of 17 female WCR collected on July 25, 1996) had consumed soybean foliage. This year (1997) we again saw frequent soybean foliage feeding, however an upsurge in the population of Japanese beetles, whose feeding traces in soybean leaves closely resemble those of WCR, made a planned field assessment of the rootworm contribution impossible. Dissection of beetles from the 1997 season is underway.

Overall, populations of WCR in corn and soybeans reached a peak in mid- to late August. Unlike last year (1996) when WCR populations were much higher in soybeans than in corn for almost every sampling date (Spencer et al. 1997; O'Neal et al. 1997), WCR population densities in corn and soybeans were very similar (Fig. 1). This may be interpreted as an indication that WCR populations were higher this year than in 1996. Though the sweep and live collection methods are not directly comparable, peak beetle abundance during the week of August 7-14, 1997, measured using two different collection methods (Fig. 2) corroborated the time of the WCR population peak as determined from vial trap samples.

In addition to the three Urbana sites, we also ran a vial trap transect in an adjacent corn and soybean field in Oak Hill (Peoria County, Illinois), a "non-problem" area. A single vial trap was placed in the cornfield one row in from the common border with the soybean field, and five traps were placed 30 meters apart in the adjacent soybean field, beginning one row in from the common border. Traps were installed on August 22 and retrieved on August 31, 1997. During this 9 day period, 54 WCR beetles were captured in the vial trap in corn, whereas 25, 2, 3, 1, and 7 WCR beetles were found in respective vial traps leading into the soybean field interior. In the trap closest to the cornfield, 20 of the 25 beetles captured were female. These captures in soybeans were a far cry from our captures along our Urbana transects. During this same period, vial traps in soybeans at the Urbana sites caught consistently more WCR beetles than the first vial trap in corn at the Oak Hill site. The contrast between transect results from problem and non-problem areas is remarkable. Conditions created by the soybean-corn rotation at both locations reward individuals that lay eggs in soybeans, yet the population of WCR in soybeans remains low at Oak Hill. Without knowledge of what sparked the behavioral change that created the problem, we are left to wonder just how important the number of females captured in soybeans might become.

Vial Trap Height and WCR Capture

The effect of vial trap height on WCR capture in soybeans was tested specifically at one transect location (Stipps field southeast of Urbana) during the month of August. "Top" traps were positioned in the top of the soybean canopy with the lid of the vial trap level with the highest leaves, "bottom" traps were placed about 2 cm above the ground, and "middle" traps were placed equidistant between the "top" and "bottom" traps.

More WCR beetles were captured in either the "top" or "middle" traps than the "bottom" traps for all 4 weeks of August and this difference was significant (P < 0.05) for the last 3 weeks of this test. In 1996 studies, traps placed 30 cm above the soybean canopy caught significantly fewer beetles than traps placed in the top of the soybean canopy (Spencer et al. 1997).

Sweep Sample Collection Patterns

During the growing season, rootworm beetles are frequently found in crops outside of corn, like soybeans and alfalfa. This observation by itself does not necessarily mean that WCR are laying their eggs in these locations. However, over the years we've seen an increase in the number of WCR beetles reported in soybean fields in areas where egg-laying in soybeans has become a problem in first-year corn. For example, historical data from Urbana shows that between the years 1979 and 1982, WCR beetle counts in soybean fields in mid-August (generally the peak period for rootworm egg-laying) ranged from 6 per 100 sweeps (with a 15 inch sweep net) in 1980 to 16 per 100 sweeps in 1979 (corn-soybean rotation; Helm, unpublished data). Even as late as 1994, WCR beetle counts never exceeded 16 beetles per 100 sweeps in Champaign-Urbana soybean fields (n = 5 fields) in mid-August.

In contrast, WCR beetle counts in soybean fields near problem cornfields in east-central Illinois (n = 5 fields) ranged between 23 and 100 beetles per 100 sweeps in mid-August 1994. In 1995, WCR beetle counts in Urbana soybean fields (n = 3 fields) increased moderately to as many as 32 beetles per 100 sweeps in mid-August. The trend continued in 1996 with as many as 59 beetles per 100 sweeps in Urbana soybean fields (n = 2 fields) in mid-August, and 273 beetles per 100 sweeps in mid-August in Urbana fields (n = 3) in 1997. The Urbana location has the distinction of having the highest numbers of WCR to be collected in Illinois soybean sweep samples to date (273 WCR beetles per 100 sweeps in mid-August 1997). Not to be outdone, however, entomologists in Indiana reported a staggering 1081 WCR beetles per 100 sweeps in soybeans in August 1997 (Blackwell 1997)! What the upper limit is, we don't know!

Changing Patterns of WCR Abundance Within an Agricultural Landscape: Movement of WCR in Illinois Soybean Fields

WCR egg-laying in soybean fields is a problem of insect movement. Because beetles disperse some distance from their place of development and move frequently between and among different fields in the agricultural landscape, the damage potential in any given field can be influenced by conditions that prevail some distance away. We know mass WCR movement is occurring; populations of WCR in soybeans have a female biased sex ratio very similar to that seen in the past when WCR migration into first-year corn from continuous cornfields was the rule. To begin to understand problem egg-laying by WCR and formulate adjustments to current IPM strategies for WCR control, we felt it essential to focus on beetle movement.

In 1997, we initiated a program to monitor this pest's population densities, movements, host crops, and the atmospheric motion systems (weather) that may aid WCR dispersal. Our work toward interrelating the effects of weather and WCR biology was conducted at three different spatial scales: statewide via sweep samples and live collections in soybeans and corn across Illinois, over a large section of a county (16 mi2 NE of Urbana, Ill.) with sticky traps (unbaited Pherocon AM traps) and vial traps, and within one intensively-monitored soybean field adjacent to a first-year cornfield.

Statewide Sampling

Multiple collecting trips across Illinois were made between July 28 and August 28, 1997 to sample population densities of WCR beetles in soybean fields. All trips originated in east-central Illinois and covered areas north, south and west of Champaign-Urbana. Periodic stops were made at adjacent corn and soybean fields; soybean fields were sampled for WCR only if they were present in the adjacent cornfield. If that was the case, 100 sweeps were made in each soybean field using a sweep net. Beetles were also hand-collected in the adjacent cornfields. Beetles were frozen, sexed, and saved for future genetic study. A total of 72 different sites were sampled in 29 counties.

Table 1 presents the results of these trips. The seven counties we visited from within the nine county "problem area" (Will, Kankakee, Grundy, Livingston, Ford, Iroquois, McLean, Champaign, and Vermilion counties [we did not sample Ford and Iroquois Counties for WCR as part of this project in 1997]) had significantly higher densities of WCR in corn (10.7 ± 1.6/min (mean ± SEM)) and soybeans (89.6 ±15.3/100 sweeps) than counties sampled to the north (corn = 3.9 ± 0.7 WCR/min; soybeans = 16.0 ± 3.7/100 sweeps), south (corn = 4.1 ± 0.5 WCR/min; soybeans = 10.2 ± 3.1/100 sweeps), or west (corn = 3.3 ± 0.9 WCR/min; soybeans = 9.2 ± 3.3/100 sweeps), (corn; P<0.0001, n = 71; soybeans; P < 0.0001, n = 71;) with the greatest WCR counts in corn or soybeans consistently found in Champaign Co.

Locations in the western and northern parts of the state had the lowest density of WCR beetles in soybeans (0-1 beetle per 100 sweeps). The remaining locations in the central part of the state around the western fringe of the "1995 problem area" had intermediate densities of WCR beetles in soybeans. If WCR beetles present in soybeans fields are an indication of egg-laying in these locations (the vast majority of these beetles are female), these results suggest that WCR problems in first-year cornfields west of the Illinois river are still probably minimal.

In addition to WCR, northern corn rootworm and Japanese beetle abundance were also noted (Table 1). Japanese beetles were significantly more abundant in the southern counties than in any other area we sampled (P = 0.0003, n = 71). In some southern counties we noted Japanese beetles clipping silks and feeding in great numbers on soybean foliage. If the winter of 1997-98 is a mild one, we might expect to see more of this generalist pest in the 1998 growing season.

Significantly more northern corn rootworm beetles (NCR) are found in areas to the west and north of the WCR problem area than to the south (P = 0.0002, n = 52). A large percentage (14-51%) of the eggs of this species are capable of a prolonged egg diapause and can therefore damage corn following a rotational crop if egg densities are sufficiently high. Very high densities of these beetles were found in the soybean fields of LaSalle, Putnam, Marshall, and Grundy counties (Table 1). Whether these beetles are laying eggs in soybean fields is not known. Until recently, northern corn rootworm populations have been fairly low; laboratory studies have shown that the northern corn rootworm can better tolerate low soil temperature conditions than can the western species (Gustin, 1983) and the severe winter of 1995-1996 may have played a part in the northern corn rootworm's comeback. The "reappearance" of the northern corn rootworm certainly complicates the issue of which rootworm species is injuring first-year corn.

Local Monitoring

So that we might have an opportunity to learn how the population of WCR over a region grows and is redistributed during the growing season, we established a grid of 63 WCR monitoring stations in grower fields throughout a 16 square mile region NE of Urbana, Ill.

Shortly after the start of the 1997 growing season, a detailed map of the plantings within the region, bounded by High Cross and County Road 2000 E on the west and east, respectively and by Royal Rd. and County Road 1700 N on the north and south, respectively, was prepared. Within this area ("the grid") pairs of agricultural fields at road intersections were identified as potential sites for establishing WCR monitoring stations. The cooperation of 29 growers was obtained and plans were made to deploy three types of WCR monitoring devices (vial traps baited with cucurbitacin and carbaryl; unbaited Pherocon AM yellow sticky traps; and Lingren attractant traps baited with a volatile WCR attractant and an insecticide pellet) in a variety of plantings at different sites within the grid. We selected pairs of fields with a variety of compass orientations and plantings. Most of the paired fields at each of the road intersections ("corners") included a corn and/or a soybean field, however, a number of corners were surrounded with only corn or only soybeans, in which case two (or in several cases, three) fields planted with the same crop were monitored. We also elected to deploy our traps in several forested areas as well as in a commercial nursery at two locations.

Traps were deployed during the week of June 29, 1997. In soybeans, traps were mounted on 1.5 meter tall schedule 20 PVC posts. Vial traps were placed at the level of the plant canopy and adjusted upward as the season progressed. Yellow sticky traps and attractant traps were mounted near or at the top of their poles in both corn and soybeans. In corn, the vial traps were attached to the plant directly at the level where the first ear would appear. The sets of traps were located 14 meters in from the corner of each plot; the row containing them was marked with a flag at the roadside. Insect trap data were collected twice a week (Monday and Thursday) beginning on July 7 and continuing until September 25th (traps were changed once per week starting on September 4th. (Author's note: final counting and analysis of these data were incomplete at the time of manuscript submission.)

Seasonal Patterns of WCR Movement and Changing Abundance in a Field of Soybeans

Some of the most important questions to ask about WCR egg-laying in soybean fields deal with the timing of WCR presence and ability/likelihood of those present to lay eggs. However, to make this information relevant to the development of improved IPM measures, seasonal patterns of WCR abundance need context. The most accessible and relevant framework in which to place WCR movement between corn and soybeans is that of local weather conditions.

Weather Monitoring

A micrometeorological measurement station (Campbell Scientific, Inc.) was established in a 4 acre soybean field NE of Urbana (Pioneer 9281; 30" rows; planted May 14, 1997) on land owned by the University of Illinois Foundation (Lost 40). Data from this station enabled us to relate the contributions of temperature, windspeed and direction, atmospheric pressure, precipitation, solar insolation (sunshine intensity) and other measures to WCR migration and abundance. This field was the focus of our most intensive WCR monitoring activity. The weather station, located at the center of the field and powered by solar cells with battery backup, made continuous measurements 24 hours per day, reporting means for all parameters at 10 minute intervals. A season long summary of some weather measurements is depicted in Figure 3a-e, along with a plot of some daily insect collection data (Fig 3 see 'Directional Malaise Traps' below).

Soybean Field Vial Trap Monitoring Grid

A grid of evenly spaced pairs of vial traps and yellow sticky traps was also established in this field. Five rows of five trap pairs were deployed at 50 meter intervals beginning at the 4th row from the south end of the field. With this grid we could develop a picture the flux of WCR over time as a surface, and relate the patterns relative to the surrounding fields (corn on the north, east and south, alfalfa on the west) and the prevailing winds during the period of sampling. (Author's note: sorting, counting and analysis of these data were incomplete at the time of manuscript submission.)

Vial Trap Transect

As was done at other local sites (see above), WCR abundance and movement patterns were measured by multiple methods in this intensively monitored soybean field and the surrounding fields. A vial trap transect beginning in the first-year cornfield immediately south of the soybean field and continuing across the soybeans and into a late-planted cornfield immediately to the north. Nineteen vial traps (five each in the south and north cornfields and nine across the soybean field) spaced at 30 meter intervals in a linear array constituted the transect. Each trap, like all of the other trapping and monitoring methods to be described, was serviced twice weekly from July 7th until September 3rd and every week thereafter.

The first WCR noted at our field site was collected in soybeans on July 7th. Along the corn-soybeans-corn transect the first WCR were detected in corn between July 11-14, in soybeans between July 17-21, and in the late-planted trap corn between July 21-24 (Fig. 4). We noted that WCR abundance dramatically increased in both corn and soybeans between the weeks of August 3 and August 10, with peak abundance (WCR/trap/day) occurring during the period of August 11-14 in all three plantings. The same general pattern was observed at the other three vial trap transects (see above) with peak abundance during the week of August 10-16. Unlike transects in 1996 (Spencer et al. 1997, located within 1.5 miles of this site) WCR abundance in corn and soybeans here and at our other sites (Fig. 1) was similar for most of the season.

Directional Malaise Traps

A third method of 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 1.5" diameter schedule 40 PVC pipe covered with approximately 15 square yards of 0.001 mesh Toule (bridal veil fabric) and had a 1 m2 opening. The sides of the trap converged at the back to form a triangular cross-section. The roof of the trap sloped upward to the apex where a 4" circular opening 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 and moving to the opening passed up the inside 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. At intervals, the trap tops were removed and the accumulated insects were sorted by species and sex. Eight traps were placed around the perimeter of the soybean field in four pairs. 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 1 m2 opening was at the top of the plant canopy.

Malaise traps were sampled at approximately daily intervals throughout the growing season to measure the WCR flux. On select days, the malaise traps were sampled at 30 minute intervals continuously throughout the daylight period. Results from these day-long "walkabouts" were used to establish daily patterns of immigration and emigration for male and female WCR. Comparisons between data collected on different days were important in studying the contribution of varying weather conditions to the pattern of WCR abundance.

Insects were collected using the malaise traps on a nearly daily basis from July 15 to August 22, and at intervals of several days thereafter. When traps could not monitored daily, the total for multiple days was divided by the days included in the sample interval and the average presented for each of the days in the sample. Result from a season of malaise trap monitoring are presented in Figure 3f. For presentation purposes, results from days with multiple samples/day have been combined to produce a daily total. No data were collected between August 16-20, as hail and strong winds destroyed the traps. The malaise trap data depict a discontinuous pattern of WCR movement around our field site. Notably, the pattern of WCR movement includes several peaks of very high WCR activity and other days of little or no apparent movement. Days with low WCR movement were found to be associated with higher average wind speeds, those with the highest WCR movement were frequently days of moderate to low wind and warm temperatures. Overall there was significantly more movement into soybeans than out.

The results from days with continuous daytime sampling reveal that the flux of WCR migration changes during the day (Fig 5). WCR movement peaks in early to mid-morning and just before sunset. A high rate of female movement into soybeans predominated in the morning and continued at a lower rate throughout the day. There was no net directional movement by males in the morning, however they tended to leave the soybeans during the middle of the day, but return strongly in the evening at a rate greater than that of females.

Intensive Sweep Sample Collections

The most intensive sampling methods involved taking multiple insect samples using a sweep net at several defined locations within the soybean field at different times during the day. Establishment of daily abundance patterns for WCR within the plant canopy were a major goal of this sampling. Unlike malaise trap samples or insects recovered from vial traps, both of which are collected over an extended period, sweep samples in soybeans (or live collections made in corn) represent point samples appropriate for correlating with instantaneous measures of weather conditions. These samples and others made at similar times also permit a method to compare the reliability of different sampling methods and corroborate daily and seasonal patterns of changing WCR abundance.

Using sweep sample and live collection methods, WCR were consistently collected in corn and soybeans beginning on July 14 and July 15, respectively (the first individuals in corn and soybeans were detected on July 7 and July 9, respectively). Rates of WCR collection in soybeans were greatest for locations nearest to the cornfield south of our soybean plot, but dropped significantly with increasing distance from the corn.

WCR abundance changed in a periodic fashion during the day (Fig. 6). The greatest WCR collection rates occurred in early to mid-morning samples and those made just prior to sunset. Significantly fewer WCR were in present in the soybean field during the late morning and afternoon. Inspection of foliage beneath the soybean canopy and on soil surfaces in the field during mid-day confirmed that the "missing" WCR were not resting out of the range of our sweep nets. The microclimatic conditions above and below the soybean canopy change drastically during the season (Fig 7). We suspect that the moderating effect a closed soybean canopy would have on the environment beneath make it a likely refuge from extreme conditions. Since it is also the area where females go to lay eggs, variation in these conditions may define when egg-laying occurs.

Concurrent with changing overall WCR abundance, was a daily pattern of increasing female representation in the population in soybeans until evening when a significant drop in the proportion female was observed (Fig. 8). These changes in WCR abundance may be explained by the insect immigration and emigration pattern from malaise trap data (Fig. 5). The malaise trap collections reveal a pattern of WCR flux that supports both the observed daily change in WCR abundance and the changing proportion of females during the day as measured with sweep sampling.

What's Happening When WCR Move Between Corn and Soybean Fields?

In 1996, we observed adult WCR moving between corn and soybean fields frequently and feeding upon soybean foliage beginning early in the season. Though soybean foliage feeding was commonly observed, tests at the time indicated that eating soybeans had negative effects on WCR adults. Field-collected WCR who had recently fed on soybean foliage were not as vigorous as those who had been feeding on corn, but that vigor could be restored if they were given corn to eat. In addition, other studies revealed that adult WCR, feeding on only soybeans (for two weeks), did not produce eggs, weighed less, and died before their siblings fed an artificial laboratory diet or one of corn plant parts (including silks, tassels, foliage and immature ears).

It was also noted during these studies, that beetles restricted to a soybean diet are much more "busy' in their cages relative to WCR on a corn or artificial diet. Such increases in activity or locomotory rate are often indicative of starvation (Bell 1991; pp. 233). Increasing propensity to move under conditions of starvation or host deprivation is a strategy which improves the chances of finding food by simply stimulating an animal to visit potential food sources at a greater rate. If a short period of soybean feeding increases movement propensity or activity levels (only 2 days of exposure to soybeans significantly affected WCR vigor in previous studies), perhaps the return to corn after feeding on soybeans is a consequence of feeling a malaise after eating an inadequate soybean foliage diet.

Analysis of the gut (stomach) contents of female WCR collected in corn and soybean fields during 1996 combined with data on the time required for food to pass through a WCR's digestive system told an interesting story of movement. Of WCR females collected in cornfields (n=229), 15% had a mixture of corn and soybean foliage in their guts. From feeding experiments, we know it takes approximately 1 hour for a WCR to completely process a meal. Combining the feeding rate data and the gut content analysis tells us that 15% of insects collected in corn had been in soybeans within the previous hour; confirmation of the suggested frequent movement between these two crops. Curiously, the percentage of WCR from soybeans with corn in their guts was much lower (0.5%). Based on 1997 observations of movement patterns and behavior, it may be that female WCR who move from corn into soybeans stop feeding more than 1 hour before flying. Alternatively, they may spend more than 1 hour engaged in other behaviors before they feed on soybean foliage; that other behavior may be egg-laying!

If females eat soybeans after laying eggs, we would expect insects with soybeans tissues in their guts to have fewer mature eggs than those without soybeans in theirs; this is exactly what we see. Among females collected in soybean fields, the mean number of eggs per female is only 1.7 ± 1.0 eggs (mean ± SEM, n = 186) for those with soybeans in their gut and 21.9 ± 7.1 eggs (n = 40) for females without soybeans in theirs. Also we would expect corn-collected females with soybeans in their guts to have few mature eggs; no females (0 of 27) collected in corn had any mature eggs. In light of these calculations and the behavioral data, we hypothesize that WCR in the problem area move from corn into soybeans shortly before they lay eggs. In the soybean field they deposit eggs and subsequently feed upon soybean foliage before moving back into corn where they again feed and may mature additional eggs. Given the great advantages of laying eggs outside of corn, the cost to a female of feeding on an inadequate food source following egg-laying may be rather small.

Dissection of frozen sweep samples and live collected WCR from 1997 will be used to test elements of this movement hypothesis. Since samples were collected at many different times during the day, we may also find daily patterns in soybean feeding that may reveal when egg-laden insects move in or out of our fields.

Summary

Egg-laying by WCR beetles in soybean fields continues to be a serious threat to the continued efficacy of a corn-soybean crop rotation for rootworm management. This past season, by focusing on WCR movement, we believe we have begun to assemble some of the key puzzle-pieces that eventually will be joined with others to provide a more complete picture of WCR biology in corn and soybeans. Identifying factors that influence the transit of WCR between corn and soybean fields is an important step down the road toward adapting our management procedures to an insect which is rapidly evolving in our midst.

Because crop rotation rewards female WCR who deposit all or some of their eggs outside of cornfields, evolution of the new behavior by the WCR can hardly be said to be unexpected. The precise nature of this behavioral change is still not clear. It would be instructive to know whether the insects who express the "egg-laying outside of corn" phenotype are less attracted to corn, or are attracted to soybeans, or perhaps have an increased propensity to move in general. Because the corn-soybean rotation dominates our agricultural landscape and few crops other than these are grown widely, insects leaving corn are more likely than not to encounter soybeans or another cornfield. It is important that in the future we also pay attention to the factors and influences which may stimulate insects to leave corn.

Our behavioral data reveal a population of insects that may be found in a number of different crops and weeds at times during the growing season when corn is nutritionally at its best. Aggregated feeding on soybeans (foliage and flowers), velvet leaf (foliage), giant ragweed (foliage and flowers), and Jerusalem artichoke (flowers) was repeatedly observed adjacent to corn plants on which other WCR were feeding. Large numbers of adult WCR beetles were also collected in alfalfa fields and a red clover field in the Urbana area. WCR feeding in other crops or on weeds suggest adult feeding preferences may be broader than what has historically been reported.

Perhaps close proximity to corn and a capacity to recover from poor food choices by eating corn is behind the WCR willingness to consume a plant (like soybeans) whose tissues are demonstrated not to support adult maintenance or reproductive development. The relationship between egg-laying and broadened feeding preferences should be established.

Last year we noted that "identifying problem insects with certainty stands as a key to efficiently dissecting this problem, but is one of the most challenging hurdles we have yet to clear". We still do not yet know if all females in the local populations laying at least some of their eggs away from corn, or if only a portion of the population express this proclivity. What is needed is a method for increasing the probability that the insects we chose for our studies are actually part of the problem. This is critical to efficient progress in genetic and behavioral studies that may put new monitoring and management tools into the hands of producers.

Acknowledgment

We thank Anthony Armstrong, Erica Bailey, Dawn Coppin, Matt Coyner, Zihna Gordon, Tom Mueller, Mark Nasser, and Ryan Von Holten for technical assistance on this project, and Randall Nelson, Charlie Helm, and Robert Wiedenmann for assistance with the various soybean cultivars. Appreciation is also extended to the many producers in whose fields we worked. 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.

References

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[ 1997 Research Index ]