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1996 Movement & Dispersal Research

Corn Rootworm Injury to First-Year Corn: New Research Findings

Joseph L. Spencer, Eli Levine, and Scott A. Isard

Dept. of Entomology, University of Illinois; Center for Economic Entomology, Illinois Natural History Survey; Dept. of Geography, University of Illinois

Western and northern corn rootworms are the most serious insect pests of corn grown after corn in the Midwest. Both species 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. This larval feeding may reduce the amount of water and nutrients supplied to the developing corn 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, the practice of growing corn in rotation with soybeans, wheat, oats, or alfalfa provided excellent control of corn rootworms because their eggs are laid almost exclusively in cornfields, and larvae must feed on corn roots the following season to complete their life cycle. Corn rootworm injury to first-year corn following soybeans in Illinois, Iowa, Minnesota, and South Dakota in the mid-1980's was attributed to a prolonged egg diapause in the northern corn rootworm; that is, eggs with this trait pass through two or more winters before hatching rather than the normal single winter pattern. Larvae from such eggs could go on to damage corn after a one-year rotation with another crop (Levine et al. 1992b). Lately, populations of northern corn rootworms have been rather low in Illinois, and injury to corn by this species has been of less concern than that of the western corn rootworm. Until recently, prolonged diapause has not been found in the western corn rootworm. Investigators have suggested that its absence was due in large part to the limited diapause capabilities of this species. However, studies in Illinois and Ontario, Canada showed that as many as 0.2% of western corn rootworm eggs hatched only after passing through two simulated winters in an environmental chamber or two winters in the field (Levine et al. 1992a). Whether the incidence of prolonged diapause in the western corn rootworm will increase beyond being a "mere curiosity" remains to be seen (prolonged diapause in the northern corn rootworm started out this way in the mid-1960s [Levine et al. 1992b]).

Western corn rootworm injury to first-year corn following soybeans has been increasing in severity for several years in east-central Illinois and north-western Indiana. 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 western corn rootworm 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).

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1994 Problems and Prolonged Diapause Follow-up

During the summer of 1994, a number of reports of rootworm larval injury to first-year commercial corn following soybeans were received, all in east-central Illinois. One field near Dewey, several fields near Crescent City, and a couple of fields near Sibley sustained severe larval rootworm injury. In the fields near Dewey and Crescent City, the predominant species was the western corn rootworm. The fields near Sibley also contained high densities of northern corn rootworm adults, so prolonged diapause in the northern corn rootworm could not be ruled out as the cause for rootworm injury there. Pyrethroid use in the vicinity of all these fields was minimal.

To check for the prolonged diapause trait, eggs were obtained from western corn rootworm beetles reared from larvae collected at Crescent City and Dewey in June 1994 as well as from beetles collected in August 1994 at Crescent City and Sibley. In the laboratory, these eggs were subjected to soil temperature conditions they would normally experience in the field. By September 1995, none of the eggs from beetles that were reared from larvae collected at Crescent City (n = 34 eggs) and only 9.8% of the eggs from beetles that were reared from larvae collected at Dewey (n = 61 eggs) remained unhatched, but appeared to be in good condition. In addition, by September 1995, only 5.6% of the eggs obtained from beetles collected at Crescent City (n = 360 eggs) and 3.1% of the eggs obtained from beetles collected at Sibley (n = 160 eggs) remained unhatched but appeared to be in good condition. All of these seemingly viable eggs were subjected to another overwintering cycle. All of these eggs died over the course of the "winter". Based on these observations, prolonged diapause in the western corn rootworm was unlikely to be the cause of the 1994 root injury problem in the Crescent City, Dewey, and Sibley fields.

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1995 Problems and Prolonged Diapause Follow-up

During the summer of 1995, many new reports of rootworm larval injury to first-year commercial corn following soybeans were received, again all in east-central Illinois. All larvae that were collected and reared to the adult stage (n = 85) were western corn rootworms (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. These potentially viable eggs are being subjected to another overwintering cycle. If most of the potentially viable eggs from Gifford, Strawn, and Saunemin hatch in 1997, this could be cause for concern and point the finger at prolonged diapause as a cause for some of the problems. As mentioned earlier, the percentage of any western corn rootworm eggs with the trait has been less than 0.2%. As it stands now, factors other than pyrethroid insecticide use and prolonged diapause seem to be involved in the problems producers have been experiencing with western corn rootworms in first-year corn following soybeans.

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Egg Sampling Studies, 1995

To determine if rootworms were laying their eggs in soybean fields, soil samples were collected in 1995. On July 24-25, two 10-cm deep by 10-cm diameter cores of soil were taken at each of 12 locations in soybean fields located adjacent to problem cornfields in Loda, Saunemin, and Urbana, Illinois. The two cores from each sampling site were screened through 0.64 cm hardware cloth. From that composite sample, a 0.5 liter of soil was saved for later processing. The three soybean fields were again sampled for eggs on October 5 (Urbana) and October 19 (Loda and Saunemin). Soil samples were processed using the methods of Shaw et al. (1976) to separate rootworm eggs from the soil. Eggs were identified to species based on chorionic sculpturing with a compound microscope (Atyeo et al. 1964).

Five western corn rootworm egg shells (eggs had hatched) and one viable northern corn rootworm egg were found in the July samples from Loda; two western corn rootworm egg shells, one viable northern corn rootworm egg, and two northern corn rootworm egg shells were found in the July samples from Saunemin; and four western corn rootworm egg shells were found in the July samples from Urbana. The fact that no viable western corn rootworm eggs were found in the July samples suggests that prolonged diapause in the western corn rootworm was not the cause for the injury to first-year corn. That is, eggs that were laid in 1994 hatched in June and early July of 1995.

Ten, 14, and 18 viable western corn rootworm eggs were found in the October soil samples (12 soil samples per field) from Loda, Saunemin, and Urbana, respectively. No viable northern corn rootworm eggs were found in any of the samples. These results are in marked contrast to a 1989 soybean planting date study conducted in Urbana. In that study, located less than 2 km away from the 1995 Urbana field, only two western corn rootworm eggs were found in 240 half liter samples of soil (Levine and Oloumi-Sadeghi 1996). These results confirm that the damage to first-year corn is being caused by the western corn rootworm, that egg-laying in soybeans appears to be the mechanism, and that this change in egg-laying behavior has occurred fairly recently (since 1989).

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Rootworm Beetles in Soybean Fields across Illinois, 1996

Rootworm beetles are very frequently found in crops such as soybeans and alfalfa during the growing season. That by itself does not necessarily mean that they are laying their eggs in these locations. However, over the years we have seen an increase in the number of western corn rootworm beetles reported in soybean fields. For example, historical data from Urbana shows that between the years 1979 and 1982, western corn rootworm 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, western corn rootworm beetle counts never exceeded 16 beetles per 100 sweeps in Champaign-Urbana soybean fields (n = 5 fields) in mid-August. In contrast, western corn rootworm 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, western corn rootworm 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.

Three collecting trips across the mid-section of Illinois were made in mid-August 1996 to sample population densities of western corn rootworm beetles in soybean fields. All trips originated in east-central Illinois where severe injury to first-year corn following soybeans was reported in 1995 and terminated in west-central Illinois. Sampling at intermediate locations was also made for a total of 12 locations. Periodic stops were made at adjacent corn and soybean fields; soybean fields were sampled only if western corn rootworms were present in the adjacent cornfield. If that was the case, four sets of 100 sweeps each 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. Table 1 presents the results of these trips. Locations in the central part of the "1995 problem area" (Urbana, Dewey, and Forrest) had the highest density of western corn rootworm beetles in soybeans (59-102 beetles per 100 sweeps). In these fields, western corn rootworm beetles even outnumbered bean leaf beetles, a soybean pest. Locations in the western part of the state (Laura, Rapatee, and Roseville) had the lowest density of western corn rootworm beetles in soybeans (0-1 beetle per 100 sweeps). The remaining locations in the central part of the state (Bellflower, Graymont, McLean, Congerville, and Roanoke) or the northern fringe of the "1995 problem area" (Dwight) had an intermediate density of western corn rootworm beetles in soybeans (2-16 beetles per 100 sweeps). If western corn rootworm beetles present in soybeans fields are laying eggs in these locations (the vast majority of these beetles are female, see Table 1), the results suggest that egg-laying in soybean fields west of the Illinois river is probably minimal.

Mention must be made that there has been a resurgence of the northern corn rootworm. As discussed earlier, a large percentage of the eggs of this species are capable of a prolonged egg diapause (14-51%) and can therefore damage corn following a rotational crop if egg densities are sufficiently high. Very large densities of this beetle were found in cornfields near Dewey, Forrest, Graymont, McLean, Congerville, Laura, and Roseville. Large densities were also found in soybean fields near Forrest, Graymont, and McLean (Table 1). Whether these beetles are laying eggs in soybean fields is not known. Until this year, 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 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.

Table 1. Number of western corn rootworm (WCR) and northern corn rootworm (NCR) beetles per 100 sweeps with a sweep net in Illinois soybean fields in mid-August 1996
Nearest town (county)	Date sampled	Number WCR	Number NCR	% female WCR
Urbana (Champaign)	Aug. 15	59	7	91%
Dewey (Champaign)	Aug. 09	83	4	76%
Forrest (Livingston)	Aug. 22	102	69	71%
Dwight (Livingston)	Aug. 13	12	1	87%
Bellflower (McLean)	Aug. 09	16	9	76%
Graymont (Livingston)	Aug. 13	14	30	82%
McLean (McLean)	Aug. 09	7	103	59%
Congerville (Woodford)	Aug. 22	2	0	78%
Roanoke (Woodford)	Aug. 13	4	16	88%
Laura (Peoria)	Aug. 13	1	8	--
Rapatee (Knox)	Aug. 22	0	3	--
Roseville (Warren)	Aug. 22	0	1	--

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Effect of Adult Western Corn Rootworm Feeding on Soybean Yield

In last year's proceedings of this conference (Levine and Gray 1996b) we noted that western corn rootworm beetles readily devoured soybean flowers. We have seen this feeding behavior in both the laboratory and in the field. Although soybean flowers pollinate themselves before the blossoms open, we nonetheless decided to take a closer look at the effect of this feeding on soybean yield.

Twelve soybean plants ('Williams' cultivar planted on June 7, 1996) were grown in separate 5 gallon plastic pots in a protected outdoor area in Urbana. All plants were covered with fine mesh fabric bags with drawstrings (the bags were supported by tomato cages) on August 12. Some pods had already set by this time. That evening, six of the cages were randomly selected to receive 25 western corn rootworm beetles each (mixed sex, but predominantly female) that had been collected earlier that day in a first-year cornfield in Urbana. The remaining six cages served as controls and were not infested with beetles. The cages were removed on 31 August when no more flowers were present. Pods were harvested by hand from each plant on 2 October and weighed.

The number of harvestable pods (pods with soybean seeds in them) from the uninfested plants averaged 95.5 pods per plant, whereas the number of harvestable pods from the infested plants averaged 89.5 pods per plant. This difference was not significant at the 5% level of probability (paired t-test). The weight of pods (unshelled weight) from the uninfested plants averaged 105.9 grams per plant, whereas the weight of pods from the infested plants averaged 99.4 grams per plant, a loss of 6.1%. Again, this difference was not significant at the 5% level (paired t-test).

We plan to repeat this study again next year with western corn rootworm beetles collected in soybean fields and with greater replication to make certain these insects are not having a negative impact on soybean yield.

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Injury to First-Year Corn following Crops other than Soybeans in 1996

Going into this study, we did not know if female western corn rootworm beetles were laying their eggs indiscriminately (in many different crops) or depositing their eggs only in certain corn and soybean fields. Several growers in east-central Illinois that experienced severe injury to their first-year corn following soybeans in 1995 identified for us some of their other fields that were planted to crops other than corn or soybeans in 1995 and that were to be planted into corn in 1996. These farmers agreed to leave untreated areas (no soil insecticide) in their cornfields so that we could obtain corn root injury ratings. On July 15 to 16, 1996 we evaluated corn rootworm larval injury to corn following oats, wheat, and soybeans at Rossville, Dewey, and Melvin, Illinois. Portions of one of the Rossville fields were planted to both oats and wheat in 1995. Portions of the Dewey field were planted to both wheat and soybeans in 1995. This allowed us to compare the effect of these crops more directly on injury to corn 1-year later. Twenty roots from each of the six fields were rated for injury using the Iowa State University 1-6 scale. Table 2 presents the results of this study. Injury was light, never exceeding a rating of 3. However, the fact that rootworm injury occurred demonstrates that rootworm egg-laying had taken place in all crops. The Rossville location is complicated by the fact that many northern corn rootworms beetles were observed in late July in these fields. Therefore, we can't determine whether egg-laying in 1995 by the northern corn rootworm, the western corn rootworm, or both species was responsible for the injury observed in corn in 1996. Significantly (P = 0.003, t-test) greater injury to corn following soybeans than corn after wheat at Dewey suggests that western corn rootworm beetles may find soybeans a more attractive place to lay their eggs than wheat, but further study is needed confirm these findings.

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Behavioral Observations

Development of an action plan to address problem egg laying behavior by western corn rootworms must incorporate a thorough understanding of mechanisms behind the phenomenon. Observation of western corn rootworm behavior in the lab and in the field is an important component of our ongoing investigations. Direct and indirect measures of behavior have been used to examine how movement, feeding and host choice may influence distribution of western corn rootworm between corn and soybeans.

Wind tunnel study. Throughout the summer of 1996, adult female western corn rootworms from different field sites in east-central Illinois were tested for their preferences for corn or soybeans in a very large (24 ft x 8 ft x 6 ft) compartment within a wind tunnel. Measuring any differences in preference for corn or soybeans between insects from different sites and plantings were the primary goals of this study. Preliminary experiments in early summer defined conditions suitable for flight testing; air temperature and relative humidity during trials were kept between 85-95íF and 40-60% RH. Wind speeds between the two potted corn and two potted soybean plants in each 4 ft x 4 ft compartment were approximately 0.1 meters/second; above the plant canopy wind speed was approximately 0.3-0.4 meters/second. On the afternoon before flight testing, insects from four field-collected populations were marked with different colored fluorescent powders. Marking individuals allowed insects from multiple populations to be tested simultaneously, and facilitated recovery of insects within each plant-filled compartment. Western corn rootworms were chilled briefly just before release from a 1 meter tall platform at the center of the wind tunnel and allowed to move freely up and downwind through the entire array of alternating corn and soybean plants filling the chamber. After 30 minutes, cloth partitions within the wind tunnel were lowered and the insects contained in the six isolated compartments (each containing two corn and two soybean plants) were censused. Location of insect capture (e.g., on corn, soybeans, ceiling, walls or floor), distance moved from the release platform and insect marking color were recorded. Modification of the plant configuration allowed additional hypotheses about movement to be tested within the wind tunnel.

Over 2000 female western corn rootworm beetles, from five sites and three different planting histories (corn after corn, corn after soybeans, and soybeans after corn) were tested during the course of the summer. Recapture of released insects was excellent; we recovered 91.8% of the insects released on a given day. On average, 50% of released insects were recovered on corn or soybeans; of these, there was no significant preference for either plant, though insects were recovered on corn approximately 60% of the time. Insects originally collected under different field conditions showed no significant preference to alight on either corn or beans (paired t-test: corn after corn, t = 1.4, P = 0.21; corn after beans, t = 1.3, P = 0.22; beans after corn, t = 0.7, P = 0.51). The proportion of insects alighting on corn was not significantly different for insects from problem areas (Champaign and Ford Counties, mean ± SEM = 0.63 ± 0.03) or a distant non problem area (Mead, Nebraska, mean ± SEM = 0.61 ± 0.12; F = 0.04, P = 0.84). Our earliest experiments had suggested a strong preference for western corn rootworm to alight on corn, however, when height differences between soybeans and corn were compensated for by elevating the potted soybeans, this preference all but disappeared, suggesting that flying western corn rootworms may simply land on whatever plants lie in their way. This was especially true for those insects recovered nearest the release site, where 55% were resting on corn. Notably, as distance from the release point increased, so did the probability that insects were found on corn, though not significantly so.

The majority of western corn rootworms did not move very far; 71% were collected on plants in compartments on either side of the release point. Concerned that some insects were just walking onto the plants around the release platform, we removed those plants immediately next to it; a majority (60%) of captures were still recorded in the closest compartment. Nonetheless, we observed many strong flights to both the up- and downwind ends of the wind tunnel.

Field observations of behavior. On several occasions, direct assessments of western corn rootworm behavior were made in the field. The observer, seated in the soybean field near the corn-soybean interface made continuous observations of randomly selected females for 15 minutes each. In corn, the observer stood in the corn and recorded behaviors of individual western corn rootworm on a hand held tape recorder for up to 30 minutes.

Western corn rootworm feeding on soybean foliage was observed at multiple field sites. At Loda, Illinois, individuals or groups of western corn rootworms feeding on soybean foliage were common within inches of highly suitable corn silks and tassels. Feeding on and within green soybean pods (perhaps exploitation of existing bean leaf beetle and grasshopper feeding damage) was also noted. Individual females were observed to feed at multiple sites on different soybean plants, exploiting both existing damage and initiating new foliage feeding holes within the 15 minute observation periods. During observation of western corn rootworms in corn and soybeans (between September 11 and 24, 1996), females fed for 24 and 37% of the available time, respectively. Females were also observed to fly from corn into soybeans and vice versa. Sweep samples from soybeans and adjacent weeds at the time of these observations (September 24, 1996) indicated that western corn rootworms were plentiful outside of the corn; in the soybean field 39 western corn rootworms per 100 sweeps were netted at 10 meters from the corn and 11 per 100 sweeps 200 meters from the corn, and 16 westerns were collected in 100 sweeps from the weeds several meters to the east of the soybeans in the field margin. At a Champaign County site near Dewey (September 1, 1996), the western corn rootworm collection rate was higher in an adjacent alfalfa field (35 beetles per 100 sweeps) than in the nearby soybean field (13 beetles per 100 sweeps) where foliage feeding was occurring.

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Vial Trap Transects and Live Collections

The extent to which insects 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 sites in east-central Illinois (O'Neal et al., 1997). In addition, sweep samples and simultaneous live collections were made in the same fields. The effect of vial trap height on western corn rootworm capture in soybeans was tested specifically at two of these locations, both just northeast of Urbana, Illinois.

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 holes, each 5 mm in diameter, 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, ingesting a lethal dose of the insecticide.

Vial traps were tied to posts placed at approximately 30 meter intervals across the soybean field from the edge nearest the adjacent cornfield. Traps in soybeans were positioned at ground level, in the soybean canopy and 1 foot above the soybean canopy (the latter two were adjusted throughout the growing season). Vial traps in corn were tied at ear height to stalks also spaced approximately 30 meters apart beginning at the edge of the corn nearest the soybean field. Five trap locations were used in each field, and were checked weekly from mid-July to mid-September. Live collections in corn where made by capturing all adult western corn rootworms encountered during a fixed sampling interval while walking through the cornfield. In soybeans, four 25 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 both sites where vial trap height was varied, significantly more western corn rootworms were recovered from traps in the soybean canopy than at ground level or above the canopy (paired t-test; t = 8.0, P < 0.0001 and t = 7.5, P < 0.0001, respectively). Rootworm counts were at least 7.5 times greater within the soybean canopy than at 1 foot above the canopy (mean western corn rootworm/trap ± SEM = 93.7 ± 11.7 and 12.5 ± 1.3, respectively). We often found several carabid beetles (predaceous/scavenging beetles) along with the few western corn rootworms caught in the bottom traps; often there were only carabids in these traps leading us to suspect that many or most of the rootworms trapped in the bottom vial traps were eaten by ground beetles before they could be counted (mean western corn rootworm/trap ± SEM = 2.0 ± 0.4).

At these two and the three other sites where vial traps were deployed in adjacent corn and soybean fields (at sites in Champaign, Ford, and Livingston Counties, vial traps were only placed within the soybean canopy), western corn rootworms were captured in soybeans one to two weeks after their first capture in corn (Fig. 1).

Overall, populations of western corn rootworms in corn and soybeans reached a peak around mid- to late August (populations at a late planted site in Ford County peaked in early to middle September). Western corn rootworm populations were much higher in soybeans than in corn at almost every sampling interval sites (Fig. 1, and see O'Neal et al. 1997, Fig. 2). Live collections and sweep samples corroborate the pattern evident from the vial traps from the Urbana, Illinois sites (Fig. 2). Though the collection methods are not directly comparable, beetle counts using both live collections in corn and sweep samples in soybeans peaked on August 15.

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Soybean Cultivar Testing

In an effort to screen for plant characteristics which may affect movement of western corn rootworm beetles into soybeans, several studies were begun to evaluate western corn rootworm population densities in different soybean cultivars. In cooperation with Dr. Randy Nelson at the National Soybean Research Laboratory (USDA-ARS, Urbana), vial traps, identical to those used to sample western corn rootworm at our remote field sites, were placed in 10 ft x 10 ft plots of 37 different soybean cultivars (one trap per cultivar) at the Urbana soybean germplasm plots on August 28 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 western corn rootworms captured in control cultivar plots ('Proto' and 'A3127').

Beetle captures were highly variable across the array of soybean varieties (0 to 100 beetles per trap, mean ± SEM = 15.8 ± 3.7) in this preliminary screen. Differences between chlorophyll deficient (yellow) and green-leafed plants marginally approached significance (P = 0.11), as did comparisons of plants of varying height (P = 0.18). There were significant effects of pubescence color in a comparison of exotic germ lines (P = 0.03), but no effect for the same comparison among modern germlines (P = 0.22). Lacking true replication, these data must be interpreted cautiously. Results of this preliminary screen will serve to guide our choices for more focused cultivar tests.

Additional cultivars, kindly provided by Dr. Cecil Nickell (Dept. of Crop Sciences, University of Illinois), were grown for use in feeding choice tests. Individual western corn rootworms collected in corn or soybean fields were presented with leaflets from two different soybean varieties and allowed to feed for several days. Recently completed, data on leaf area eaten have not yet been complied. Preliminary assessment of feeding (damaged vs. undamaged) suggests that when they eat soybean leaves, western corn rootworms show little tendency to feed preferentially on one or another variety. It is notable though, that beetles in these tests appear to fall into two categories; those that eat soybeans and those that will not feed on any soybeans at all. Given widespread soybean foliar feeding in the field, this topic will be a major focus of our coming research.

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Do Problem Western Corn Rootworms Prefer to Lay Their Eggs in Soybeans or Have They Just Lost Their Fondness for Corn?

Expansion of western corn rootworm egg laying in soybeans is a serious threat to the continued efficacy of a corn-soybean crop rotation for rootworm management. Successfully adapting our management procedures to counter and regain control of western corn rootworm populations will depend on a thorough understanding of mechanisms which are at the root of this problem. Only when we answer some of the "how" and "why" questions can we hope to mount anything more than a delaying action against so formidable a foe as the western corn rootworm.

Without a doubt, crop rotation imposes heavy selection pressures on western corn rootworm populations and "rewards" females who deposit all or some of their eggs outside of cornfields. A consequence of crop rotation may be selection for individuals who lay their eggs away from cornfields, that is selection for females who lack strong selectivity. 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.

Indeed, our behavioral data suggest that western corn rootworms have no particular affinity for alighting on soybeans when they are presented along side corn. This was true without regard to where insects were collected (from corn or soybeans) and whether they came from problem or non problem areas. Movement of western corn rootworm beetles in large numbers into soybean fields is occurring early in the season, when silks and tassels are still plentiful. Collection of significant numbers of western corn rootworms in crops other than soybeans, growing adjacent to corn, and rootworm damage in first-year corn following wheat and oats (Table 2), suggests that a more general movement phenomenon "away from corn" rather than specific attraction to soybeans may be at the root of this problem, or contribute significantly to it.

Table 2. Rootworm injury to corn following wheat, oats, or soybeans. East-central Illinois, 1996
Crop sequence	Location	Root rating*
Corn after wheat	Rossville- Field 1	1.7
Corn after oats	Rossville - Field 2	2.9
Corn after wheat	Rossville - Field 2	2.2
Corn after wheat	Dewey	1.9
Corn after soybeans	Dewey	2.7
Corn after oats	Melvin	1.8

*The root rating system used was the Iowa State University root-rating scale: 1 - no visible damage or only a few minor feeding scars; 2 - Some roots with feeding scars but none eaten off to within 1.5 inches of the plant; 3 - Several roots eaten off to within 1.5 inches of the plant but never the equivalent of an entire node of roots destroyed; 4 - One node of roots destroyed or the equivalent; 5 - Two nodes of roots destroyed or the equivalent; and 6 - Three or more nodes of roots destroyed.

Western corn rootworm feeding away from corn, on soybeans or weeds, was more extensive this year than had been noted previously. Large numbers of western corn rootworms in soybeans (greater even than those simultaneously collected in adjacent corn) long before corn became a poor host, suggest that movement has not been forced due to hunger or crowding. Aggregated feeding on soybeans and foliage of velvet leaf was repeatedly observed within inches of acceptable corn foliage, silks, and tassels. Feeding on alfalfa by adult western corn rootworms was observed in large cage studies. During short observation periods, we have seen insects feeding in corn fly into soybeans, and vice versa. There is every indication that adult feeding preferences may be broader than what has historically been reported and that individuals are moving back and forth between corn and soybeans (or other plants) with some frequency.

In many cases, western corn rootworms in soybeans out numbered bean leaf beetles, even in soybean fields far from corn. Even if our lab studies had demonstrated that adult western corn rootworm feeding could significantly reduce soybean yields, it would be too soon to suggest that this beetle is becoming a soybean pest. However, it is not too soon to begin to ask how other behaviors, particularly feeding behavior in soybeans, other crops and weeds may contribute to the problem.

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 do not yet know if all insects in the local populations are equally likely to lay 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.

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Acknowledgments

We thank Anthony Armstrong and Erica Bailey for technical assistance on this project, Dr. Lance Meinke of the University of Nebraska for providing us with the Nebraska population of western corn rootworms for use in our wind tunnel experiments, and Drs. Randall Nelson and Cecil Nickell for assistance with the various soybean cultivars. Appreciation is also extended to the many producers in whose fields we worked. They include Dean Carmien, Alvin Christians, Larry Knox, Virgil Nieman, Greg Pool, Phil Shields, and Jean Stewart. We also gratefully acknowledge funding from the Illinois Council on Food and Agricultural Research (C-FAR) that supported this research.

References

Atyeo, W. T., G. T. Weekman, and D. E. Lawson. 1964. The identification of Diabrotica species by chorion sculpturing. Journal of the Kansas Entomological Society 37:9-11.

Levine, E., and M. E. Gray. 1994. Use of cucurbitacin vial traps to predict corn rootworm (Coleoptera: Chrysomelidae) larval injury in a subsequent crop of corn. Journal of Entomological Science 29:590-600.

Levine, E., and M. Gray. 1996a. Rootworm problems in first-year corn, an update. Illinois Natural History Survey Reports 339:2, 5.

Levine, E., and M. Gray. 1996b. First-year corn rootworm injury: east-central Illinois research progress to date and recommendations for 1996, pp. 3-13. In 1996 Illinois Agricultural Pesticides Conference, Cooperative Extension Service, University of Illinois at Urbana-Champaign. 160 pp.

Levine, E., and H. Oloumi-Sadeghi. 1991. Management of diabroticite rootworms in corn. Annual Review of Entomology 36:229-255.

Levine, E., and H. Oloumi-Sadeghi. 1996. Western corn rootworm (Coleoptera: Chrysomelidae) larval injury to corn grown for seed production following soybeans grown for seed production. Journal of Economic Entomology 89:1010-1016.

Levine, E., H. Oloumi-Sadeghi, and C. R. Ellis. 1992a. Thermal requirements, hatching patterns, and prolonged diapause in western corn rootworm (Coleoptera: Chrysomelidae) eggs. Journal of Economic Entomology 85:2425-2432.

Levine, E., H. Oloumi-Sadeghi, and J. R. Fisher. 1992b. Discovery of multiyear diapause in Illinois and South Dakota northern corn rootworm (Coleoptera: Chrysomelidae) eggs and incidence of the prolonged diapause trait in Illinois. Journal of Economic Entomology 85:262-267.

O'Neal, M., M. Gray, J. Spencer, K. Steffey, and E. Levine. 1997. Western corn rootworm in corn after soybeans: efforts towards an economic threshold. In 1997 Illinois Agricultural Pesticides Conference, Cooperative Extension Service, University of Illinois at Urbana-Champaign.

Shaw, J. T., R. O. Ellis, and W. H. Luckmann. 1976. Apparatus and procedure for extracting corn rootworm eggs from soil. Illinois Natural History Survey Biological Notes 96. 4 pp.

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Recent Publications on Black Cutworm Research

Armon Keaster (state representative)

University of Missouri, Columbia, MO

Keaster, A. J., J. A. Grundler, M. A. Jackson, M. D. McCorcle, W. B. Showers, M. O. Way, R. D. Parker, J. B. Giezentanner, K. Schwindt, and J. R. Raulston. 1995. Occurrence and winter activity of black cutworm moths along the Texas Gulf Coast, 1987­1991. Southwestern Entomol., Suppl. No. 18: 135­154.

Keaster, A. J., J. A. Grundler, W. S. Craig, and M. A. Jackson. 1996. Noctuid moths and other insects captured in wing-style traps baited with black cutworm (Lepidoptera: Noctuidae) pheromone on offshore oil platforms in the Gulf of Mexico, 1988-1991. J. Kansas Entomol. Soc. 69: 17-25.

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