Joseph Spencer, Illinois Natural History Survey
Scott Isard, Dept. of Geography, University of Illinois
Eli Levine, Illinois Natural History Survey
David Onstad, Dept. Natural Resources & Environmental Sciences, University of Illinois

Funded by C-FAR and the Illinois Soybean Program Operating Board.

Since the mid-1980s, a behavioral adaptation to annual crop rotation by the western corn rootworm beetle has rendered our most environmentally benign rootworm management tool useless over an expanding area of Illinois, Indiana, Ohio and Michigan. Rotation resistance involves frequent beetle flight between corn and crops rotated with corn to lay eggs and feed in both areas. Beetles that once laid all of their eggs in cornfields are now laying many of their eggs in soybeans and other crops. By hedging their egg-laying bets and depositing eggs in many places, female beetles assure that some of their eggs will hatch in cornfields the following spring where their larvae will find the corn roots they need to survive. Insect movement and migration are at the heart of the problem of WCR rotation resistance; understanding movement is key to developing management strategies for the WCR. Our research on WCR abundance, movement and dispersal was conducted at multiple spatial scales in 2000.

Statewide Sampling. Because of the mobility of the WCR, and the spreading nature of this problem, we have conducted extensive statewide surveys of WCR abundance in Illinois soybean fields since 1997. Our 2000 survey was the most extensive to date. Between July 26 and August 6, we traveled to 61 Illinois counties and collected100 sweep samples from over 340 soybean fields. All samples were frozen immediately (to preserve them for later dissection and analyses) and returned to our laboratory for analysis. In addition to counting WCR beetles, we also recorded the numbers of other economically important insect species (northern corn rootworms, grape colaspis, Japanese beetles, bean leaf beetles, ladybird beetles, and others) found in the samples. These data were used to generate insect abundance maps for each species by county in Illinois.

A number of notable pest abundance patterns were evident in the results. The WCR continues to be present at high abundance throughout east central Illinois. A trend toward higher numbers in northern Illinois over the past 2-3 years seems to be consistent with our findings. The lack of a significant westward expansion of the region where WCR are abundant in soybeans is consistent with simulation model predictions. WCR populations in 2000 were approximately 2-7 times higher than those we observed during 1999. The prevailing winds and movement of summertime convective storms are believed to facilitate the long distance transport of the rotation-resistant WCR. The current pattern of WCR distribution in soybean fields is consistent with a Ford, Co. Illinois nidus for the rotation-resistant WCR strain.

Seasonal Patterns of WCR Abundance in Champaign Co., Illinois Corn and Soybean Fields. Movement of WCR adults between corn and soybean fields begins shortly after beetles first emerge from cornfields. Contrary to popular perception, WCR move back-and-forth among corn and soybean fields throughout the growing season. Extensive temporal sampling has revealed that there is a daily periodicity of beetle movement that is gated by periods of atmosphere instability. When wind and temperature create buoyant (unstable) atmospheric conditions (typical of mid morning and early evening/dusk), biota may be transported upward into the air with greater ease than during times of stability (e.g., midday). The greatest WCR population movements occur in the morning and evenings during periods with unstable atmospheric conditions.

Though most of WCR flight activity is likely of short duration and primarily between fields within 1 meter of the local plant canopy, during unstable conditions some WCR are carried high into the atmosphere and moved long distances. Understanding the distinction between when, where, and under what conditions WCR move long or short distances may help us study migration of this important pest.

In 2000, we expanded our local sampling and observation capabilities to collect and monitor WCR abundance and movement under a variety of conditions. Standard abundance metrics were employed to monitor populations in corn and soybean fields. Nearly every day, multiple 100 sweep samples were collected in the soybean field canopy and multiple live collections were made in cornfields (live collections involve walking through a cornfield for a fixed time interval and knocking as many WCR adults as possible off of plant surfaces and into a slippery funnel that directs insects into a collection jar). Over the course of the season, such regular sampling generates a picture of changing insect abundance in the two crops.

Since we were interested in WCR flight, we complimented soybean sweep and cornfield live collections by establishing directional malaise traps (n=8) around the edges of soybean fields to capture WCR that were flying in either direction between the two crops at canopy level. The natural tendency of the WCR to move upward is exploited by our malaise trap design which employs a narrow necked insecticide chamber into which the insects are easily directed, but from which they cannot escape. WCR caught in the insecticide chambers of each trap were collected everyday. So that flight at higher elevation could also be monitored, two additional sets of malaise traps were mounted at 4 m and 8 m elevation in 10 m tall scaffolding towers erected near the ground level malaise traps (which caught insects flying just above soybean canopy).

From the tops of the scaffolding towers, we used lightweight aerial insect nets to catch WCR which were actually in flight at 10 m above the soybean canopy. These insects experience a very different flight environment than those making trivial flights between corn and soybean at canopy level. Weather recording instruments record significantly higher wind speeds at the 8 m level. Observation of beetles moving by at 10 m suggested many were not in control of their flight direction. Individuals frequently were maintaining a flight posture aimed at moving in one direction, while they were being carried at several meters per second at 90-180 degrees away from their 'intended heading'. WCR were collected from tops of the towers only in the morning and evenings and only for short periods of time (30-90 minutes).

During the period when WCR were being collected as described above, a study of male mate finding was under way; a near-daily census of observed mating WCR pairs was also being undertaken. At the conclusion of our studies, when the various measures of WCR abundance and activity were collated, an interesting picture of WCR ecology, behavior, and movement was evident. The very first WCR adults were first detected in corn on June 26. At our field site, peak WCR mating activity coincided with the period of corn pollination (indicated by the shaded portion on each graph). During this time, male WCR were observed to arrive at cages of virgin females at an average rate of one per every 6.6 minutes. Independent counts of mating pairs on corn plants indicated mating activity was intense throughout the fields were male mate finding was being studied. Adult WCR captures in malaise traps began as soon as adult insects were becoming common in cornfields. In the highest elevation (8 m) malaise traps, the greatest daily captures of flying WCR occurred just after the time of peak mating activity in corn. Flying WCR captures in malaise traps positions at just above soybean canopy level (1 m) peaked several days later than the peak at 8 m and coincided with the greatest mean WCR collection rates in soybean.

WCR collections made with aerial nets from the tops (10 m) of the scaffolding towers revealed that significant numbers of insects were moving at high elevation. Some WCR were flying many meters above the towers and could not be collected. Capture rates during these collections were very sensitive to changing weather conditions and were more variable than results obtained from methods that integrated captures over a 24 hour period. Processing and dissection of high-flying WCR revealed that the ground level malaise traps caught WCR at a rate 10x of the traps at 4 or 8 m. Also, WCR captures high above the soybean canopy were >85% female while at canopy level the flying population was only 65% female. Eighty percent (79.8%) of high-flying females were newly mated (determined by presence of large or small spermatophores). Gut content analysis indicated that most females moving at high elevation had originated in cornfields shortly before taking flight; the vast majority of females caught at just above the soybean canopy had been feeding on soybean tissues. Observation of WCR captured at high elevation indicate that they differ behaviorally from beetles caught while flying at canopy level. From this ongoing study, we conclude that high-flying WCR are very probably engaged in migratory flight and that they are different from individuals making trivial flights between corn and soybean fields at canopy level.

	Japanese beetles (Popillia japonica Newman) have become an increasingly common annoyance to persons throughout east central Illinois. Known to feed on 100s of plant species, our sampling efforts indicate that Japanese beetles are rapidly becoming more abundant in soybean fields, where they feed en masse on soybean foliage.
Grape colaspis are occasionally serious pests of corn, their distribution, along with that of other secondary pests of corn may become very important once transgenic corn varieties that specifically target corn rootworms are commercialized. Growers who choose a transgenic solution over soil insecticide use may find that historically minor pests, like grape colaspis, become more significant because the general suppressive effects of some soil insecticides are absent.
	Bean leaf beetles(Cerotoma trifurcata, BLB) are important soybean pests. The BLB potential to transmit plant disease is significant and along with their defoliation capabilities they constitute an important yearly concern for soybean producers. BLB transmit Bean Pod Mottle Virus (BPMV), which is implicated in some occurrences of a soybean condition known as 'greenstem' that can impede harvesting and reduce soybean yields. A cooperative effort between USDA-ARS, University of Illinois, and Illinois Natural History Survey scientists aimed at using preserved BLB (from our 1999 state sampling trips) to survey statewide soybean disease and insect virus transmission potential was expanded in 1999 to include WCR after 80% of field-collected individuals tested positive for Bean Pod Mottle Virus (BPMV). Since that time, we have shown that WCR can transmit BPMV between soybean plants (WCR are extremely abundant in soybean fields in areas where rotation resistance is a problem for first year corn producers). Our state surveys now include collection of soybean foliage from all fields where WCR or BLB were collected.
The Asian multicolored ladybeetle (Harmonia axyridis) is an imported beneficial insect that is earning a bad reputation among homeowners and others. Harmonia have become very common inhabitants of both corn and soybean fields in the last several years, where they feed on small arthropods (they also will eat corn plant tissues!). Observation of dense Harmonia populations near local concentrations of the aphid suggest that there is potential for this ladybug 'pest' to help redeem itself by preying on Aphis glycines. Though identification of the soybean aphid in Illinois soybean fields occurred after our organized state sampling was completed, the county data indicate Harmonia are relatively abundant in areas where the soybean aphid is known to occur. Research and extension activities targeting Aphis glycines are poised to commence in 2001