Dispersal Rotation-Resistant Western Corn Rootworm to Illinois Soybean Fields

Illinois State Representatives: Scott Isard*, Mike Irwin**, and Gail Kampmeier***
*Dept. of Geography, University of Illinois at Urbana-Champaign
**,***Dept. of Natural Resources & Environmental Sciences, UIUC
and Center for Economic Entomology, Illinois Natural History Survey*,**,***

Dispersal of Rotation-Resistant Western Corn Rootworm to Illinois Soybean Fields [Joseph L. Spencer (INHS Center for Economic Entomology), Scott A. Isard (UI Dept. of Geography), and Eli Levine (INHS Center for Economic Entomology) with cooperations over the past few years from Randy Nelson and Mike Gray (UI Dept. of Soil Science) and University of Illinois students Anthony Armstrong, Erica Bailey, Dawn Coppin, Matt Coyner, Paul Dabisch, Robert Daniels, Emily Davis-Isgrig, Roger Diercks, Wendy Gierhart, Zihna Gordon, Allen Inman, Tim Mabry, Tom Mueller, Shalonda Reed, and Ryan VonHolten]. The research was funding by the Illinois Council on Food and Agricultural Research ($150,000 for 2-years) and the Illinois Soybean Program Operating Board ($160,000 for 3-years).

Project objectives are to document the spread of WCR into soybean fields and to quantify relationships among crop development, WCR population growth, weather, and WCR movement. Objective 1 is to drive throughout the state each August, sampling soybean fields for WCR to monitor the intensity and rate of spread of this problem insect. Objective 2 is to establish a sampling grid within producers' fields to collect WCR throughout the growing season from the problem area. The goal is to determine: a) differences in WCR (density and sex) among soybean and cornfields in the problem area, b) when WCR densities are increasing in soybean and cornfields, and c) relationships between crop phenology and changes in WCR densities. Objective 3 is to monitor the aerial movement of WCR between soybean fields and surrounding cornfields with directional insect traps and an automated weather station. The aim is to determine when during the day WCR are flying to soybean fields and the types of weather conditions that prohibit and encourage interfield movement. Objective 4 is to determine the effect of soybean feeding on WCR movement, survival, and reproduction.

Many results are presented on the Rotation-Resistant Western Corn Rootworm homepage [archived from 1999]

Monitoring WCR Spread in Illinois during 1997, 1998, 1999. We measured WCR abundance in soybean fields at 72 different sites in 29 counties in 1997 and at 271 sites in 44 counties in 1998, and 249 sites in 74 counties in 1999. Fig 1. depicts the mean number of WCR collected per 100 sweeps. Compared with 1997 data, WCR abundance declined by almost 10-fold in 1998 likely due to heavy June rains. WCR were not detected in 14 of the sampled counties. These data suggest that there was no significant expansion of the 1997 WCR problem area in 1998. Populations rebounded in 1999 to levels intermediate between the previous two years. WCR were present in moderate numbers in northern Illinois west of the Chicago suburbs (DeKalb and surrounding counties). This year, WCR continue to be present at low or undetectable levels in soybean fields west of the Illinois River and south of Interstate 70. In 2000, we will need to pay particular attention to sampling in northwestern Illinois because in 1999, WCR injury to first-year corn was reported in northeastern Iowa.

Local Monitoring. A 41.5 km2 network of vial traps was deployed in corn, soybeans, woods, and a commercial tree farm near road intersections in Champaign County from July through September. Compared with 1997 and 1998 results from the same network, the 1999 captures were intermediate; WCR abundance in 1997 was about twice that of 1999 and 7.5 fold that of 1998. As in 1997 and 1998, WCR abundance in 1999 was highly variable and WCR were collected at all locations. More WCR were captured in cropland than in woods and the tree farm. The number of WCR collected with vial traps in soybean fields was greater than in cornfields.

WCR Flights between Corn and Soybean Fields. Using malaise traps and sweep net sampling in soybeans at our intensive monitoring fields we were able to complete our studies of WCR flight periodicity and the influence of weather on flight.

In summary, 68% of WCR collected in the soybean fields were female. A diel periodicity in WCR immigration and emigration was observed on days conducive to beetle flight in soybean fields. Flight activity was low during the early morning (500-700 solar time) when air temperatures were cool. Aerial movement peaked between 700 and 1100. During the first half of this interval (700 to 900), immigration of WCR to the soybean field exceeded emigration, thereafter emigration exceeded immigration. Before sunset (1700-1900), flight activity increased. WCR were not caught in the malaise traps between sunset and sunrise. Micrometeorological measurements from days when WCR captures were large indicate that WCR movement may be gated to late morning, when wind speed and air temperature gradients above the soybean crop frequently create unstable atmospheric conditions that facilitate flight. Because of the diel periodicity and high day-to-day variability of WCR movement to soybean fields, scouting tools, such as sticky traps and vial traps that integrate captures over an entire day or multiple days are likely to provide more reliable WCR population estimates in soybean fields than sampling techniques that measure WCR abundance at single points in time (e.g., plant counts and sweep net samples).

Feeding and Reproductive Status Studies. WCR feed on foliage while in soybean fields, however, soybean feeding significantly reduces adult WCR vigor. However, flying back to corn to feed after eating soybeans allows WCR to survive soybean feeding. The proportion of females with both corn and soybeans in their digestive tract closely mirrored daily patterns of WCR abundance established for the 1997 and 1998 populations. The majority of the beetles caught in 1999 have yet to be dissected. From our 1997 and 1998 data, we know that the greatest proportion with corn and soybeans in their digestive tract is found at times when WCR populations are on the move between corn and soybeans. We found that the reproductive status (a measure of egg maturity) of WCR females in soybeans was significantly greater than that of females in corn between weeks 3 and 8, thereafter, females from corn and soybeans had a similar reproductive status. A higher proportion of the females caught in problem area soybean fields were carrying mature eggs than WCR caught in non-problem area soybean fields.

We are beginning to believe that "rotation resistance" in WCR involves a complex pattern of flight and feeding behaviors. WCR emerge, mate, and initially feed in cornfields. Many female WCR then begin to fly back and forth between corn and soybean fields while their eggs are maturing. Females that eventually deposit eggs in soybean fields feed on nearby soybean plants afterward. Consumption of soybean foliage alone inhibits further egg development and will kill adult beetles. Adults WCR can survive soybean feeding if they return to cornfields to feed. The mechanism behind the behavior that stimulates WCR to fly back to cornfields after laying eggs in soybean fields is unknown. However, studies in our recently constructed laboratory wind tunnel have revealed an important relationship between WCR flight activity and adult soybean feeding (Fig 2). Beetles that feed on soybean foliage are more likely to initiate flight than WCR that feed on corn plant parts. We hypothesize that the enhanced likelihood of flight following soybean herbivory is the proximate mechanism that stimulates WCR movement back to cornfields. Corn feeding over subsequent days allows female WCR to mature more eggs, leading to flight and a return to soybean fields for egg laying. In addition, we believe that the presence of mature eggs in the ovaries of females in cornfields stimulates the behavior that is at the very root of rotation resistance in WCR, the flight of gravid female WCR into soybean fields.

Soybean Cultivar Studies. Field and lab studies indicated that WCR prefer some soybean varieties over others. Problem area WCR consumed foliage of Mexican bean beetle (MBB)-resistant soybean cultivars at significantly reduced rates than that of the MBB susceptible Williams 82 soybean line.

Research Directions for YR2000 and Beyond. We have submitted research proposals to the Illinois Council on Food and Agricultural Research and the Illinois Soybean Program Operating Board to continue our WCR research program. We want to continue to make sweep net samples in soybean fields (100 net sweeps/field) throughout Illinois to monitor the intensity and rate of spread of rotation-resistant WCR. This program, coupled with weekly sampling from UI experimental farms in Illinois, has provided growers and scientists with useful maps over the past 3 years and also provides our lab with beetles from around the state for biological studies.

Last year we captured WCR engaged in long-distance movement at 4 and 8 m heights (using malaise traps mounted on a tower) above a soybean field at our UI field site. We have propose to collect WCR flying at the same altitudes from 4 towers and compare their sex ratio, flight behavior, mating status, egg load, and gut contents with that of WCR engaged in trivial flight between corn and soybean fields at canopy level. High- and low-flying WCR will be tested in our free-flight wind tunnel ( 2 x 1 x 1 m: length, width, height) to determine whether migratory WCR respond differently to color, shape, and odor cues than WCR making trivial flights.

In addition, we plan to extend the study of how diet and reproductive maturity influence flight using our wind tunnel to determine WCR responses to stimuli. Information that a particular combination of shape, color, and/or odor cues stimulates the flight of WCR carrying eggs could be used to make more effective traps for monitoring WCR in soybean fields. Including comparisons between rotation-resistant WCR and beetles from outside the affected area may provide a behavioral profile of rotation-resistant individuals that can be used to screen WCR from suspect populations. In addition, a behavioral screen would facilitate characterization of genetic markers diagnostic of the rotation-resistant WCR strain.

We also propose to continue monitoring rotation-resistant WCR beetle activity on different soybean cultivars. For each cultivar, we (a) quantify seasonal patterns of WCR abundance and dissect a subsample of the female beetles to measure corn-soybean diet-mixing and reproductive development, (b) measure defoliation by WCR, and (c ) determine the incidence of WCR egg-laying and subsequent larval injury to the roots of corn planted in the plots the following year. In laboratory feeding studies, we are determining longevity and fecundity of WCR on the foliage of different soybean lines.

Related publications over the past year:

Spencer, J.L., S.A. Isard, and E. Levine, 1998. Western corn rootworm flight activity in soybeans. Illinois Natural History Survey Reports 350: 2-3.

Isard, S.A., J.L. Spencer, E. Levine, D.W. Onstad, M.A. Nasser, T.R. Mueller, and D.M. Coppin, 1998. The influence of weather on interfield and regional movement of western corn rootworm, pp. 344-347. In Proceedings of the Thirteenth Conference on Biometeorology and Aerobiology, AMS, Boston.

Nasser, M.A. 1998. The influence of weather on western corn rootworm flight activity in a soybean field. M.A. Thesis, University of Illinois. 98 pp.

Isard, S.A., M.A. Nasser, J.L. Spencer, and E. Levine. 1999. The influence of weather on western corn rootworm flight activity at the borders of a soybean field in east-central Illinois. Aerobiologia 15: 95-104.

Spencer, J.L., S.A. Isard, and E. Levine, 1999. Western Corn Rootworm (Diabrotica virgifera virgifera LeConte) free flight to potted corn (Zea mays L.) and soybean (Glycine max L.) plants in a walk-in wind tunnel. Journal of Economic Entomology 92: 146-155.

Onstad, D.W., M.G. Joselyn, S.A. Isard, E. Levine, J.L. Spencer, L.W. Bledsoe, C.R. Edwards, C.D. DiFonzo, and H. Wilson, 1999. Modeling the spread of western corn rootworm (Coleoptera: Chrysomelidae) populations adapting to soybean-corn rotation. Environmental Entomology 28: 188-194.

Isard, S.A., J.L. Spencer, M.A. Nasser, and E. Levine, 1999. Aerial movement of western corn rootworm, Diabrotica virgifera virgifera (Coleoptera: Chrysomelidae): Diel periodicity of flight activity in soybean fields. Environmental Entomology (in press).

Spencer, J.L., S.A. Isard, and E. Levine, 1999. Western corn rootworm injury in first-year corn: What's new?, pp. 13-27. In 1999 Illinois Crop Protection Technology Conference, Cooperative Extension Service, UI.

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