Northern corn rootworm (NCRW) and western corn rootworm (WCRW) are economically important pests of corn. They are responsible for nearly 1 billion dollars annually in crop losses and control costs.
LIFE CYCLE: Eggs overwinter in the soil and begin to hatch in late May or early June. Larvae feed on corn roots, which can decrease yield potential and increase the risk of root lodging. Larvae take about 3 weeks to pupate. Adult emergence begins in early July with peak emergence occurring around the beginning of August. Adult beetles feed on corn leaves, pollen, and soft kernels, but cause the most damage when feeding on silks. Female beetles return to the soil to lay eggs during the later part of August and into September.
IDENTIFICATION: CRW larvae are creamy white with a brown head. Adult WCRW are yellow to light green and generally have three black stripes on their wing covers, though this can vary. Adult NCRW are about ¼ inch long and pale to dark green without markings.
CRW larva (left), adult Western CRW (center) and adult Northern CRW (right).
SCOUTING: Scouting for both larval damage and adult beetles can help to make decisions for next year. Since root damage cannot be seen above ground, root digs are imperative to help assess your current corn rootworm management strategy. Scouting for CRW larval damage should take place in mid-July. To do so, dig a series of corn root balls (one foot in diameter), shake off the soil, and wash roots. Root damage from corn rootworm larval feeding consists of brown feeding scars often along the side of the roots, tunneling inside the larger roots, and root pruning with the roots eaten back toward the base of the stalk. Injury may be limited to a single root, or consist of multiple nodes of roots chewed back.
In areas with extremely high pressure, such as areas that plant multiyear corn on corn, it is beneficial to scout for adult CRW beetles. Adult CRW beetle counts help estimate the extent of CRW larval damage a grower might expect the next year. Scouting should take place during the key months of July and August when peak CRW beetle activity occurs. Thresholds vary by state and planting density. In general, if adult beetle populations exceed ¾ - 1 beetle per plant, the potential for significant yield loss the next season may exist if no control tactics are instituted.
MANAGEMENT: Product selection is key to managing CRW. The introduction of Genuity SmartStax corn, that contains the combination of Monsanto’s YieldGard VT Rootworm/RR2 technology and Dow AgroSciences’ HERCULEX RW Insect Protection technology, provides consistent protection against WCRW and NCRW larvae.