Western Corn Rootworm (Diabrotica virgifera virgifera) is one of four related chrysomelid beetles that make up the ‘rootworm complex’. Common names can be confusing, and we do not have all of these species in the PNW, but I will list them here for posterity’s sake:
Coleoptera : Chrysomelidae
Species – Common name(s)
D. virgifera virgifera – Western corn rootworm
D. baberi – Northern corn rootworm
D. undecimpunctata howardi – Southern corn rootworm
D. undecimpunctata undecimpunctata – Western variant of Southern corn rootworm; also known as Western spotted cucumber beetle or 12-spot beetle
Clear as mud? Good.
In general, adult rootworms are about 1/4″ long, black and yellow leaf beetles.
Patterns of the wing covers, as well as leg and antennal colors vary between species. Western corn rootworm is dimorphic (which means that patterning differs between males and females).
Larvae are difficult to distinguish between rootworm species. They are 3-12mm long and have brown sclerotized plates on the head and at the rear.
Adult beetles: Yellow sticky traps work well because this pest’s activity can be highly variable depending on time of day. Place them on a stake or post, level with the height of the crop to be monitored. Ideally, place a transect of 4-6 traps about 50m apart. Count, record, and replace the cards weekly. For active field scouting, note that adults are most active in the morning and late afternoon. If corn is at or near silking, walk briskly through the field and note activity in the ear zone. If adults are found, proceed with more intensive evaluation of silk clipping. If beetles are not flying around the ear zone, return in 2-3 days to recheck.
Larvae (rootworms): Randomly select 10 plants, (1-2 each from throughout a few different locations in the field) and dig vertically around the base of each plant, being careful not to cut the roots with the spade or shovel. Collect about a 7″ cube, lift the plant and soil out of the ground, and place them on a small piece of dark canvas or plastic. Slowly break the soil away from the roots and carefully examine the soil and roots for larvae. The dark background will enhance visibility of the small, white rootworms. Additionally, you can wash the soil from the root sample into a bucket as a means of extracting larvae. Adding salt to the water will help ensure that larvae float to the top, where they can be collected or counted. Repeat the process and get an average count of larvae per sample. For rootwashes, detection of 2 larvae per plant (or 8 larvae per plant if using a saline solution) may justify treatment. 1
BIOLOGY of western corn rootworm may or may not be similar to other rootworm species that we are more familiar with. In the Midwest, egg laying begins in July and it is the egg stage that overwinters. As temperatures warm in the spring, eggs develop into larvae and seek out corn roots to feed on. They pupate and become adults mid-summer. It is presumed that there is one generation per year.
*** We are in the process of quantifying activity data of D. virgifera in the PNW, both in field corn and sweet corn. Current data can be found here: http://bit.ly/CRWinOR ***
Cultural – Crop rotation is a main strategy, and economic damage usually only occurs when corn follows corn. There is some evidence that rootworm survival may be lower in very sandy soils due to physical abrasion of the cuticle. Muck soils tend to not have as much feeding damage as other soil types.
In areas with much larger annual acreage of corn, D. virgifera is managed through a combination of crop rotation and planting transgenic Bt-corn hybrids. Concerns about resistance to both of these tactics have been expressed.
Chemical – Consult the PNW Insect Management Handbook, and ensure products are labeled for your crop and site use.
Hodgson, E. Western Corn Rootworm. Utah State University Extension fact sheet, 2008. 4pp.
Murphy, A. et al. Western Corn Rootworm in Eastern Oregon, Idaho, and Eastern Washington. PNW Extension publication 662. 2014. 7pp.
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1 IMPORTANT: Thresholds are based on Purdue Field Crop Ext. Service recommendations, but have not yet been established for fresh or processed corn in Oregon.