MN Fruit & Vegetable IPM News - Vol. 1, No. 4

The bean leaf beetle (BLB), Cerotoma trifurcata (Foster), is a common pest of soybeans and snap (green) beans in Minnesota. In the upper Midwest, populations of BLB have increased during the last 4-5 years, possibly as a result of mild winters, good snow cover, and increasing soybean acreage. Adult BLB prefer to feed on the leaves of soybeans and snap beans although they have been found feeding on other crops such as peas, clover and dry edible beans.

Unlike many of us who prefer to fly south for the winter, BLB are able to overwinter as adults throughout the Midwest. They seek shelter under oak leaves or within curled ash leaves (and other leaf litter) in wooded areas close to bean fields, or within soybean stubble left in fields. In early spring, the beetles disperse from their overwintering sites and rapidly colonize newly emerging snap bean or soybean seedlings. Their feeding damage is characterized by round holes (3-4 mm diameter). Extensive feeding can result in complete defoliation, and possibly death of young seedlings (Bennett et al. 2003). Adult beetles feed and live for about 4-6 weeks. Female BLB lay their eggs in the soil, presumably close to the base of legumes; hatching occurs within one to three weeks depending on temperature. Larvae prefer to feed on root nodules, common in legumes. Later in the year, the F1 adults can also cause significant feeding damage to soybean and snap bean pods.

To develop better predictive models of BLB emergence and potential damage, we began collecting additional overwintering data on adult survival. As part of this effort, the purpose of this study was to assess the relationship between monthly winter survival and supercooling points (SCP; i.e., the temperature at which spontaneous freezing will occur) for BLB adults. In this article we summarize the results of our 2003-2004 study. We also contrast these results with another important vegetable pest, the striped cucumber beetle, Acalymma vittatum (Fabricius).

Methods

In late fall 2003, overwintering BLB were collected by sorting through the leaf litter in woods close to soybean field at the Rosemount Research and Outreach Center, Rosemount, MN. Groups of 20 BLB were placed into small (12”x 12” x 6”) wire mesh cages filled with dried leaves. The cages were secured to the ground in a grove of trees and spaced 12” apart. Dry leaves were placed between the cages until the cages were covered. Air and soil temperatures adjacent to the cages were recorded every 4 hours throughout the winter using HOBO data loggers. Soil temperatures summarized for this report were recorded at the soil surface. Once each month, 1-2 cages were removed from the study site, and all contents were allowed to thaw at room temperature for 24 hours. After 24 hours, alive and dead BLB were separated from the leaf litter inside the cage and placed in plastic containers until mortality was assessed. BLB were considered alive if they could walk.

To measure the SCPs, live beetles were attached to the end of a copper-constantan thermocouple with the help of a thin smear of high vacuum grease (Carrillo et al. 2004). The thermocouple and BLB arrangement was then placed at the center of a polystyrene cube designed to provide a constant cooling rate of 1°C min -1 when placed inside a -80°C freezer. The SCP was recorded as the lowest temperature reached before the release of latent heat of fusion (e.g., Koch et al. 2004).

Results—Bean Leaf Beetle

Overwintering BLB appeared to have a fairly high survival rate with 40-45% of the beetles still surviving between March and April 2004 at Rosemount Minnesota (Table 1). Our estimates of BLB survival are similar to observations by Len Dobbins (FMC Corp., Indiana), who recently reported estimates of 50-65% survival in the Midwest. It appears that surface soil temperatures on average did not surpass the mean SCP; therefore, many of the adult beetles were able to avoid freezing of their body contents, and thus able to survive the winter. However, in addition to temperature, it is known that winter mortality is also a function of the time of exposure to low temperatures. For these reasons, mortality observed this spring may have also occurred as a result of chill injury. It is also interesting to note that in April the SCP increased to -5.96 ºC. This finding suggests that the beetles were beginning to acclimate to warmer spring temperatures.

Finally, as noted by Dr. Dave Ragsdale (Dept. of Entomology, University of Minnesota), an additional key mortality factor for BLB appears to be the amount of spring rainfall that BLB is exposed to, particularly in April. BLB may survive winter temperatures very well, then experience high mortality in early spring due to high precipitation. Although there may be some direct affects of rainfall, the primary factor under these conditions is a fungal disease, Beauveria spp. This past spring, however, was relatively dry for much of southern Minnesota, and therefore may not result in significant BLB mortality.

Our results for 2003-2004 are also similar to a previous two-year study in Minnesota during the 1980s (D. Ragsdale, unpublished data). We are in the process of comparing our results with this earlier study, and also plan to repeat the study in 2004-2005. Additional data are needed to better understand the relationship between overwintering survival and SCP measurements. We also plan to conduct further experiments to develop a relationship between the recorded soil surface temperatures, mortality rates, and SCPs to be able to predict the percentage survival of this pest following multiple weather scenarios.

Table 1 . Mean percentage survival, soil temperature and supercooling point (SCP; ±SEM) of bean leaf beetles throughout the 2003-2004 winter, Rosemount, MN.

Month	n	Survival (%)	Soil Temp. (ºC)	n	SCP (ºC)
January	41	66	-6.70	--	---
February	40	58	-3.40	19	-8.89 ± 0.15
March	40	40	0.70	14	-8.83 ± 0.30
April	20	45	8.10	8	-5.96 ± 0.21

Contrasting Bean Leaf Beetle and Striped Cucumber Beetle Survival: In addition to BLB, we also conducted a striped cucumber beetle survival study this past winter at Rosemount. To date, little information is known about the northern limits of cucumber beetle survival. Although the cucumber beetle is not closely related to BLB (i.e., different genus), the adults are similar in size and shape to BLB. In contrast to BLB, however, we found high mortality of cucumber beetles by December of 2003 with 100% mortality by February 2004. These results with cucumber beetle are also similar to those from the 2002-2003 winter. Although migration of beetles from southern states may continue to play a role in developing summer infestations in Minnesota, our winter mortality results suggest that locally overwintering beetles may only survive, and contribute to summer infestations, under very mild winter conditions.

References

Bennett, K., W.D. Hutchison, E.C. Burkness, R.L. Koch, & B. Potter. 2003. Bean Leaf Beetle—Snap Beans. VegEdge Fact Sheet, University of Minnesota Extension Service. University of Minnesota, St. Paul, MN. http://www.vegedge.umn.edu/vegpest/BLB.htm

Carrillo, M.A., N. Kaliyan, C.A. Cannon, R.V. Morey, and W.F. Wilcke. 2004. A simple method to adjust cooling rates for supercooling point determination. CryoLetters (In press).

Koch, R.L., M.A. Carrillo, R.C. Venette, C.A. Cannon, and W.D. Hutchison. 2004. Cold hardiness of the multicolored Asian lady beetle (Coleoptera: Coccinellidae). Environmental Entomology (In press).

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ACKNOWLEDGEMENTS

Co-Editors: Bill Hutchison, Department of Entomology, University of Minnesota, hutch002@umn.edu
Jeanne Ciborowski, Minnesota Department of Agriculture, Ag. Resources Division,jeanne.ciborowski@state.mn.us
Suzanne Wold-Burkness, Department of Entomology, University of Minnesota, woldx018@umn.edu

The Newsletter is published weekly from May through August, cooperatively, by the Minnesota Department of Agriculture (MDA) and the University of Minnesota (U of MN). Reports are posted on the U of MN and MDA web sites on Fridays. If you have suggestions and/or comments, please send your contributions by 4 p.m., Tuesday to Jeanne Ciborowski, 651-297-3217, jeanne.ciborowski@state.mn.us , MDA, 90 W. Plato Blvd., St. Paul, MN 55107-2094. You can access the Newsletter at the U of MN web site in htm. format at: www.vegedge.umn.edu/MNFruit&VegNews/mnindex.htm and at the MDA web site in pdf. format at: www.mda.state.mn.us/biocon/fruitreports/default.htm

Partial funding for this publication is provided through partnership agreements with the Minnesota Fruit and Vegetable Growers Association and the United States Department of Agriculture � Risk Management Agency (RMA) and the RMA Community Outreach and Assistance Partnership Program. These institutions are equal opportunity providers.

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Last Revised May 27, 2004.
The University, including the Minnesota Extension Service, is an equal opportunity educator and employer. ©1999-2004 Minnesota Extension Service, University of Minnesota. All rights reserved. Contact copyright@extension.umn.edu for information on reproduction or use of this material.