Authors: Buyung Hadi, Robert Wright, Janet Knodel, Phillip Glogoza, Mark Boetel, R. Jeff Whitworth, Holly Davis, and J.P. Michaud
Corn rootworms are larvae of several species of leaf beetles. There are three species of corn rootworms that may be found on corn in Northern Plains, western corn rootworm (Diabrotica virgifera virgifera), southern corn rootworm (Diabrotica undecimpunctata howardi) and northern corn rootworm (Diabrotica barberi).
All three corn rootworm species have larvae that are similar in appearance. They are slender, white in color with brown heads. The fully-developed larvae are about 0.5 in long with a brown plate at the last segment of the body.
Adult northern corn rootworm are 0.25 in (6 mm) long with uniformly greenish to yellowish color and no markings on the wing covers. Adult southern corn rootworm are between 0.25-0.31 in (6-8 mm) long, with a yellow-green body, black head and antennae, and 12 black spots on their wing covers. Adult western corn rootworm are about 0.25 in. (6 mm) long with the females having yellow wing covers with three black stripes and the males having a solid black wing covers with yellow tips. Male western corn rootworm adults tend to be smaller than the females.
Life Cycle and Seasonal History
The northern and western corn rootworms have a similar life cycle. The eggs overwinter in soil and hatch in late May and early June. The resulting larvae feed on corn roots for 3-4 weeks. The most damage due to corn rootworms usually occurs in June and July. Mature larvae pupate in the soil within earthen cells. Adults emerge to feed and mate starting from late July through August. Adults feed on foliage, pollen and silk. Beginning in late July, mated females lay eggs in the soil near the base of corn plants.
Apart from corn, northern corn rootworms can utilize a number of other grass hosts including foxtails. Moreover, some populations of northern corn rootworms show an extended diapause, in which the eggs do not hatch in the following spring but overwinter for the second year.
The southern corn rootworms overwinter as adults in the southern states. Adults become active in spring when average temperatures exceed 70˚F (21˚C). The adult beetles move into corn fields soon after emergence and lay eggs. The eggs do not undergo diapause and hatch within two weeks. The resulting rootworms feed on corn roots, develop through three instar stages and pupate in the soil. The hatching adults emerge from the soil and feed on above-ground plant parts.
Plant Injury and Damage
Throughout their life cycle, corn rootworms injure different plant parts. The larvae injure corn roots while the adult beetles injure above-ground plant parts. The most serious injury is caused by larval feeding of the roots. Injuries on the root system due to rootworm feeding may cause significant damage in continuous corn fields by causing lodging, reducing vigor and yield. The impact of rootworm feeding on the roots may be exacerbated by environmental stresses such as drought or weed competition.
The adult beetles emerging in vegetative stage corn will feed on corn leaves. Later they feed on pollen and move to ear tips to feed on green silk. Silk feeding by adult beetles may interfere with pollination. However, feeding by adult beetles of corn rootworms rarely produce serious economic damage.
Normally corn rootworm cannot survive on the roots of soybean, sunflower or alfalfa, making these crops to be excellent candidates for rotation crops to manage corn rootworms. In some populations of northern corn rootworm extended egg diapause may occur. In populations with extended egg diapause, the eggs remain dormant in the soil for two or more winters before hatching. This may result in larval injury to corn after soybeans or other non-host crop. Populations with extended egg diapause can be managed by extending the rotation, incorporating two non-corn crops into the rotational mix.
Transgenic corn (Bt-corn) capable of producing various endotoxins against corn rootworms are available in the market. It is required by law to plant a non-Bt-corn refuge (20 percent of the total area) within or adjacent to each field of Bt-corn registered against rootworm on each farm. strategy. Refuge requirements are changing as new hybrids are registered. See additional resources below for more information. A refuge is expected to lessen the selection pressure on corn rootworm populations towards Bt-corn, thus delaying the evolution of Bt-corn resistant rootworms and increasing the durability of Bt-corn as a rootworm management
In summer 2011, western corn rootworm populations in northeastern Iowa were reported to have lower than expected mortality rate when feeding on Bt-corn hybrids expressing Cry3Bb1 Bt-toxin. These western corn rootworm populations were all collected from fields where corn hybrids expressing Cry3Bb1 had been used for at least three consecutive years. This finding signals the necessity to adopt a more integrated approach to corn rootworm management. Practices like rotating different Bt-toxins, rotating corn with non-host crops and incorporation of conventional insecticide in the corn rootworm management program should decelerate the evolution rate of corn rootworm populations toward a particular Bt-toxin.
Soil insecticide has been an option available for corn rootworm management. Historically, corn rootworm populations developed resistance towards broadcast applications of cyclodiene soil insecticides, starting in Nebraska. This trait later spread across the corn belt. Later soil insecticides applied in-furrow or in a band have not produced resistant populations. Insecticidal seed treatment is another option to manage corn rootworm. Insecticidal seed treatment with neonicotinoid products works well to protect corn roots in moderate corn rootworm population levels, but it may not be adequate in high rootworm population levels.
Other Online Resources
Radcliffe's IPM World Textbook, Maize Insect Pests in North America