Northern and Western Corn Rootworms
The northern and western corn rootworms are strictly pests of corn. The eggs, which are the overwintering stage, are laid in the soil in cornfields in late summer. The eggs hatch late the next spring (late May-early June), and the larvae feed on the roots of corn plants if corn has been planted in that field again. Because the larvae can feed only on corn and cannot move to another field, rotation to another crop will effectively reduce the rootworm population. A quick check for corn rootworm larvae can be made by examining the roots of a few corn plants selected at random throughout a field in late June and early July. Look for roots that are dead and broken on the tips. The larvae are small, elongate, and white, with a brown head and well-developed mandibles. They are found in the soil at the root zone as well as tunneling in the roots. Corn rootworm larvae pupate in the soil and the adults emerge around corn flowering. Corn pollen is an important food source for newly emerged beetles.
Root damage by corn rootworm (CRW) larvae weakens plants and may cause lodging or “goose-necking.” Severe lodging greatly complicates harvest and reduces yields by stunting the plant and reducing the ear size. During rainy late summers and falls, corn can lodge without rootworms. Adult CRW beetles, which are about 1/4 inch long, emerge from the soil about the time of corn pollination. CRW beetles are very active, feed on pollen and corn silks, and may cause some stripping of leaves resulting in a windowpane appearance in the affected area. Northern corn rootworm (NCRW) beetles are tan to pale green. Newly emerged beetles are light in color, gradually becoming greener with age. Females have slightly longer abdomens than males. Western corn rootworm (WCRW) beetles are yellow with a black stripe along the sides of their wing covers. Male WCRW beetles usually have a broad, dark band across their wing covers compared to female beetles of this species, which tend to have black stripes. Females have slightly longer abdomens than males. Beetles of both species survive until the first hard frost, laying eggs for the next year’s population. There is a single generation per year.
Corn rootworm may be a problem in the second or later crop year where extensive acreages of corn are grown. In some areas of the Midwest corn belt, rootworm has also become a problem in first year corn because the insect has adapted to the practice of annual crop rotation between corn and soybeans by laying their eggs in the soybean fields. However, this variant has not been reported in NY so crop rotation is still a viable rootworm management strategy.
Economic populations of CRW can be managed in continuous corn fields with the use of soil applied insecticide, seed applied insecticide or plant incorporated insecticide (GMO-BT-corn). Routine “insurance” applications of soil insecticides, seed applied insecticide or plant incorporated insecticides at planting time are strongly discouraged. Risk for economic CRW populations can be easily estimated with established field scouting guidelines by counting adult rootworm populations and estimating resulting larval populations the following spring.
The number of adult beetles in a cornfield this year provides an indication of the potential larval population in that field next year. Check fields for CRW beetles on the leaves, tassels, and silks of corn. This check should be made beyond the margins of the field. If 55 or more CRW beetles are found on 55 plants, rotate the field to another crop the next year if at all possible. Note: WCRW beetles pose a higher risk to yields than NCRW. Threshold for NCRW is 110 or more per 55 plants. In fields with mixed populations of CRW, divide number of NCRW observed per 55 plants by 2, add this number to number of WCRW observed per 55 plants to calculate total number of CRW observed. Treat with an insecticide (Table 3.6.1) or use one of the available BT-corn rootworm corn varieties if corn must be planted in that field again. Remember that an insecticide or a CRW resistant variety may cost $15 or more per acre. If the producer decides to plant a BT-CRW resistant corn variety, close attention must be paid to the label requirements regarding the required planting of a BT-CRW free refuge.
Planting the Refuge: Producers will be exposed to two different types of refuges depending on the corn variety they purchase. The first refuge type is the structured refuge where CRW susceptible corn is planted in a block within the field or immediately adjacent to the field. The size of the refuge can vary from 5% to 20% depending on the seed company and GMO trait. The second refuge type is the seed blend where the refuge seed is mixed in the bag before sale and the producer automatically plants the refuge when planting the field. This strategy is known as “Refuge in a Bag”. Some GMO-BT-corn varieties are permitted by EPA to be sold using the RIB refuge deployment. Make sure you are familiar with the refuge requirements for the seed you purchase before planting.
Why plant a Refuge? First of all, it is required by EPA as a condition of sale for GMO-BT corn. In addition, it is just good economic sense. A planted refuge allows the development of non-insecticide exposed CRW which mate with the surviving insects exposed to the insecticide and therefore delaying the development of insecticide resistance to the BT toxin within the corn plant. Prevention of resistance development allows the BT-plant incorporated technology to survive longer in the corn production areas and technology become less expensive over time. Producers in the SE Minnesota, and NW Iowa corn growing regions ignored the required refuge requirements for many yearsand they now have developed BT-resistant CRW populations. Producers in these areas are now required to adopt more expensive management strategies to prevent economic losses from this insect.
In New York, the feeding of the adult beetles on silks can occasionally be a problem. Clipping of the silks can prevent pollination, resulting in poorly filled ears. If 10 or more adults are found per plant at silking, less than 50 percent of corn silks are brown, and silks <0.5 inch long, treatments to control adults may be warranted, and pollination has not yet occurred, apply an insecticide (Table 3.6.1). Pollination generally occurs within three to four days after silk emergence. The silks quickly dry up after pollination.
Planting a corn variety that has a high standability rating may help reduce lodging from rootworm larval feeding in untreated fields. Planting early may help to avoid silk feeding by the adults. Planting fields last that are to be rotated the following year will attract CRW beetles to this last source of fresh corn pollen on the farm and may reduce their populations from other cornfields.
A sequential sampling method is available and greatly reduces sampling time when CRW populations are very low or are above economic numbers. The sequential sampling plan is available through the NYS IPM Program website at www.nysipm.cornell.edu/publications/crw_bro/default.asp.
- IPM for Insect Pests of Field Corn - a free Moodle course from the NYS IPM Program
- Don't Let Corn Rootworm Steal Yield (includes table with all Bt traits in corn)
- A Beetle Is Destroying U.S. Corn, so Scientists Are Punching at the Insect's Genes - Scientific American - March 2017 (links to article preview, requires subscription or purchase for full article)
- Sampling and Management of Corn Rootworm in New York Field Corn Fact Sheet from the NYS IPM Program (PDF)
- Cucumber Beetles, Corn Rootworms, and Bacterial Wilt from the NYS IPM Program (PDF)