What kind of Bean Leaf Beetle Activity are we expecting as Soybean Seedlings Emerge?
Eileen Cullen, Extension Entomologist,
UW Entomology Department
Bean leaf beetle, Cerotoma trifurcata, belongs to the insect family Chrysomelidae. These are the leaf feeding beetles. Bean leaf beetles are related to another well known Chrysomelid, corn rootworm (Diabrotica spp.), which we’ll talk about soon enough this season. Bean leaf beetle overwinters in the adult stage beneath leaf litter. In early spring, before soybeans are out of the ground or even planted, adult bean leaf beetles become active and start to feed on cultivated (alfalfa) and wild (e.g. showy tick trefoil) legume plants. Although overwintered bean leaf beetle adults feed on legumes, they wait until soybeans emerge to lay eggs. As soybean cotyledons, unifoliate and first trifoliates emerge, overwintered bean leaf beetle adults move into soybean fields to continue feeding and lay eggs. (Bean leaf beetle larval development on soybean roots is not documented to be of economic concern).
Depending on temperature, peak activity for overwintered bean leaf beetle will continue throughout the month of May. A window of vulnerability exists for early planted (late April to early May) soybeans. Field scouting is critical for the rest of May as seedling soybeans are present at the same time overwintered bean leaf beetles are feeding and laying eggs. By early June, the overwintered bean leaf beetle population will taper, usually markedly so by mid-June, after having done their job of laying eggs in soybean to secure a first-generation. Later planted soybean fields, generally May 15th and beyond, emerge later in May, escaping peak overwintered bean leaf beetle adult feeding and egg-laying activity.
At this time of year, I occasionally hear people refer to bean leaf beetle adults as “first generation”. They mean overwintered population. The adults you’ll see in seedling soybean fields now are actually 2nd generation from the 2004 season, successfully overwintered and laying eggs that give rise to first-generation bean leaf beetle which we’ll pick up in July during later vegetative growth stage soybeans.
Photo: UW Madison, Entomology Kathryn Thomas
Photo: UW Madison, Entomology Kathryn Thomas
In addition to the yellow/tan and crimson color forms featured here, with wing cover spots and a black line around the margin, remember that both color forms can also occur without any spots. You will, however, always have the black triangle behind the bean leaf beetle head to confirm you are dealing with Ceratoma trifurcata.
For the past two growing seasons, Wisconsin has not experienced threshold level bean leaf beetle defoliation. Bean leaf beetle populations (overwintered, first generation and second generation) have been
low, noneconomic events. On the BPMV side, during 2004, both UW Field Crops Entomology and WI DATCP field monitoring and laboratory assays found nearly zero BPMV in soybean leaves and bean leaf beetles collected throughout the state. Please refer back to Wisconsin Crop Manager Article “Bean Pod Mottle Virus – A No-Show in Wisconsin Soybeans in 2004” Vol. 12, No. 1, Jan. 17, 2005 http://ipcm.wisc.edu/wcm/pdfs/2005/05-1insect1.html for details on the recent lack of bean leaf beetle defoliation damage or BPMV transmission in Wisconsin.
With a warmer 2004-2005 winter season, compared to past years, we are expecting relatively greater overwintered bean leaf beetle survival. Still, as a general rule, we are not expecting big overwintered bean leaf beetle problems this year, because last season’s 2nd generation that went into overwintering was not high. Greater survival of a moderate to low overwintering population does not lead us to predict extremely high overwintered adult population activity this May. With regard to BPMV potential, the situation is similar to that which I discussed for corn flea beetle and Stewart’s bacterial wilt of corn in Wisconsin Crop Manager Vol. 12, No. 7, April 21 2005 http://ipcm.wisc.edu/wcm/pdfs/2005/CullenApril19.pdf. Again, warmer 2004-05 winter temperatures predict greater survival of overwintered corn flea beetles. In both cases however, the respective associated plant pathogens (Corn flea beetle : Stewart’s wilt bacterium Erwinia stewartiiAND Bean leaf beetle : BPMV) were extremely low to non-existent in Wisconsin during 2004. This bodes well for the 2005 season since these insects would need to pick up and/or overwinter with disease inoculum in order to transmit this spring 2005 in seedling corn and soybeans, respectively. With such low, or zero, BPMV (and Stewart’s wilt) incidence in 2004, inoculum should be very minimal even if insect populations build. In both cases, however, scouting should be maintained this May into June as crops emerge and progress through seedling stages.
Scouting soybeans is recommended now and throughout May for overwintered bean leaf beetle feeding activity to make sure the soybean defoliation thresholds are not reached. Defoliation from overwintered bean leaf beetles on emerging soybeans at V1 (unifoliate) to V2 (1st trifoliate) is noticeable and can look dramatic, especially given how small the plants are at this time. Keep in mind however, that in terms of economic damage due to defoliation, the plant is very resilient.
In a very good website, the North Central Soybean Research Program’s Plant Health Initiative at http://www.planthealth.info/blb_basics.htm hosts a comprehensive, yet concise and user-friendly page on bean leaf beetle in soybean. Fortunately, feeding by first-generation beetles on soybean leaves seldom results in economic yield losses as seedlings can often recover. The University of Illinois recommendations suggest a density of 16 adults per row foot in the early seedling stage before economic injury from physical feeding will occur. By the V2 stage, 39 beetles per row foot are required before economic injury will occur. (see web site above, click on “Agronomic impact” link at left). Also check stand to make sure cotyledons (seed leaves) and particularly regrowth are OK. (Plants can withstand some cotyledon scarring).
In the event that overwintered bean leaf beetle adults reach economic thresholds, several foliar insecticides are registered for adult control. These are listed on page 116 the UWEX Publication A3646 Pest Management in Wisconsin Field Crops 2005 with product choices, rates and label remarks and precautions. Consult A3646 for insecticide recommendations if economic thresholds are reached http://cecommerce.uwex.edu/pdfs/A3646.PDF. Soybeans that were planted with a seed applied insecticide application of Cruiser (a.i., thiamethoxam) should sustain noticeably less overwintered bean leaf beetle feeding damage during soybean emergence. Finally, the cultural control of late planting will permit fields to escape peak overwintered bean leaf beetle feeding.
For more information on Bean Leaf Beetle –
BPMV complex in Wisconsin Soybeans
Link to the following sites:
UW SOYBEAN PLANT HEALTH (Home: http://www.plantpath.wisc.edu/soyhealth/index.htm)
Bean Leaf Beetle in Wisconsin
http://www.plantpath.wisc.edu/soyhealth/viruscomplex/ blb.htm
Bean Pod Mottle Virus
http://www.plantpath.wisc.edu/soyhealth/viruscomplex/ bpmv.htm
Soybean Insect-Virus-Complex in Wisconsin
http://www.plantpath.wisc.edu/soyhealth/virus.htm
Symptoms of Infection by Soybean Viruses
http://www.plantpath.wisc.edu/soyhealth/virusymp.htm
NORTH CENTRAL SOYBEAN RESEARCH PROGRAM PLANT HEALTH INITIATIVE (Home: http://www.planthealth.info/)
Bean Leaf Beetle Page with links to Basics, Life Cycle, Scouting, Distribution, Agronomic Impact, Management and Links to State Resources.
http://www.planthealth.info/blb_basics.htm