The Western corn rootworm, Diabrotica virgifera virgifera LeConte, is one of the most devastating corn rootworm species in North America, especially in the midwestern corn-growing areas such as Iowa. A related species, the Northern corn rootworm, D. barberi Smith and Lawrence, co-inhabits in much of the range, and is fairly similar in biology; in the discussion below, many comparisons are made between the two species.
Corn rootworm larvae can destroy significant percentages of corn if left untreated. In the United States, current estimates show that 30 million acres (120,000 km²) of corn (out of 80 million grown) are infested with corn rootworms and area is expected to grow over the next 20 years. The United States Department of Agriculture estimates that corn rootworms cause $1 billion in lost revenue each year, which includes $800 million in yield loss and $200 million in cost of treatment for corn growers (The Dow Chemical Company).
There are many similarities in the life cycles of the northern and western corn rootworm. Both overwinter in the egg stage in the soil. Eggs, which are deposited in the soil during the summer, are football-shaped, white, and less than 0.004 inches long. Larvae hatch in late May or early June and begin to feed on corn roots. Newly hatched larvae are small, less than 1/8 inch long, white worms. Corn rootworms go through three larval instars, pupate in the soil, and emerge as adults in July and August. There is one generation per year. Larvae have brown heads and a brown marking on the top of the last abdominal segment, giving them a double-headed appearance. Larvae have three pairs of legs, but these are not usually visible without magnification. After feeding for several weeks, the larvae dig a cell in the soil and molt into the pupal stage. The pupal stage is white and has the basic shape of the adult. Adult rootworms are about 1/4 inch long. Western corn rootworms are yellowish with a black stripe on each wing cover. Northern corn rootworm beetles are solid in color and vary from light tan to pale green (Wright).
Timing of egg hatch varies from year to year due to temperature differences and location. Corn rootworm males begin to emerge before females. Emergence often continues for a month or more. In years with hot, dry summers, numbers of western corn rootworm beetles may decline rapidly after mid-August, although in summers with less extreme conditions they may be found up until the first frost (Wright).
Females mate soon after emergence. Western corn rootworm females need to feed for about two weeks before they can lay eggs. Temperature and food quality influence the pre-oviposition period. Females typically lay eggs in the top 8 inches of soil, although they may be laid more than 12 inches deep, particularly if the soil surface is dry. Western corn rootworm females are more likely to lay some of their eggs below the 8-inch depth than northern corn rootworm females (Wright).
Rootworm larvae can complete development only on corn and a few other species of grasses. Western corn rootworm adults feed primarily on corn silk, pollen, and kernels on exposed ear tips, although they also will feed on leaves and pollen of other plants. If western corn rootworm adults begin emerging before corn reproductive tissues are present, adults may feed on leaf tissue, scraping away the green surface tissue and leaving a window-pane appearance. However, adults quickly shift to preferred green silks and pollen when they become available. Northern corn rootworm adults also feed on reproductive tissues of the corn plant, but rarely feed on corn leaves. Northern corn rootworm adults are more likely than western corn rootworm adults to abandon corn and seek pollen or flowers of other plants as corn matures (Wright).
Most of the damage in corn is caused by larval feeding. Newly hatched rootworms locate corn roots in the soil and initially begin feeding on the fine root hairs and burrow into root tips of the corn plant. As larvae grow larger, they feed on and tunnel into primary roots. When rootworms are abundant, larval feeding and deterioration of injured roots by root rot pathogens can result in roots being pruned to the base of the stalk. Severe root injury interferes with the roots' ability to transport water and nutrients into the plant, reduces plant growth, and results in reduced grain production. Severe root injury also may result in lodging of corn plants, making harvest more difficult. Silk feeding by adults can result in pruning of silks at the ear tip, commonly called silk clipping. In field corn, beetle populations are occasionally high enough to cause severe silk clipping during pollen shed, which may interfere with pollination (Wright).
There are many different management practices aimed at the control of corn rootworms. These practices include corn variety selection, early planting, insecticides, crop rotation and transgenic corn varieties.
There are no commercial non-transgenic rootworm-resistant corn varieties. Several hybrid corn traits may reduce damage from larval rootworm feeding by increasing stalk strength and root mass size. These characteristics allow a plant to better tolerate rootworm feeding, with reduced likelihood of lodging (Wright).
Early planted fields that have completed pollen shed are not very attractive to rootworm beetles, and therefore have less egg laying activity. Early fields also will have relatively larger root systems when rootworm feeding starts. This makes them somewhat more tolerant to rootworm damage. Practices that promote strong root systems and a generally vigorous crop will make corn more tolerant to rootworm feeding and damage (Peairs).
Soil-applied insecticides have been shown to effectively control corn rootworms. The use of an insecticide may be warranted in areas which have a history of moderate to high rootworm damage. The number of adults present during the previous growing season is the best guide for selecting the fields to be treated (Peairs). However, in areas of high insecticide use in central Nebraska, populations of corn rootworm beetles have become resistant to certain insecticides.
Crop rotation is the most consistent and economical means of controlling rootworms. As a way to reduce rootworm densities, it is more effective than insecticides (Wright). Corn rootworms must feed on corn roots in order to develop and mature properly. If they hatch in a field rotated out of corn, they will starve to death because they cannot move more than 10 to 20 inches through the soil in search of food (Peairs). Also, rootworm beetles rarely lay eggs anywhere but in corn. There are however, two different biotypes of corn rootworms that have recently been found to avoid the practice of crop rotation. The soybean variant of the western corn rootworm was first discovered in central Illinois in the late 1980’s and has since spread throughout Illinois, Indiana, southern Wisconsin and in to eastern Iowa (Rice). Instead of laying eggs in to a corn field the females of the soybean variant will mate and then fly in to a soybean field to lay her eggs, thus allowing the larvae to hatch in a field that is likely to be planted with corn the following year. In the 1980’s northern corn rootworm began to be a problem by beating the corn rotation practice with extended diapause eggs (Willson). The extended diapause eggs remained for two years or more in the soil before hatching, thereby avoiding the year where soybeans are planted in a corn-soybean rotation.
Companion or second crop planting can also increase rootworm populations dramatically. Corn with pumpkins or corn following pumpkins are examples of planting patterns that produce particularly extreme rootworm feeding pressure.
Above all other options, transgenic corn varieties are the best method for minimizing corn rootworm damage. They are the most effective at reducing root damage from feeding and are safer and often cheaper than insecticide use. The transgenic genes, isolated from the common soil bacterium Bacillus thuringiensis strain (often referred to as Bt), produces the insect control protein. Bt was first discovered in 1901 by the Japanese biologist S. Ishiwatari as the source of disease that was killing large populations of silkworms. Bt was first used as an insecticide in 1920, and spray formulations containing either Bt bacteria or Bt proteins have been used for more than 40 years for crop protection, including organic farming operations. EPA-approved Bt insecticides saw expanded use and development in the 1980s as an alternative to synthetic chemical insecticides. Beginning in the 1980s, the genes responsible for making Bt proteins were isolated and transferred into corn plants. Bt was commercially approved in transgenic corn seed in the mid-1990s. Compared to conventional Bt spray formulations, transgenic plants with the Bt protein provide much more effective insect protection throughout the growing season. Other Bt proteins have been used to genetically modify potatoes, cotton, and other types of commercial corn (The Dow Chemical Company).
Bt must be ingested to kill the insect. A susceptible larva eats the protein, which then binds to specific receptors in the larval gut. Binding initiates a cascade of effects in the larva that ultimately leads to death. Bt proteins are highly selective on certain categories and species of insects, eliminating insecticide use and its harmful effects to non-target organisms (The Dow Chemical Company).
The Dow Chemical Company. Product Safety Assessment (PSA): Herculex RW Rootworm Protection. September 26, 2006. URL: http://www.dow.com/productsafety/finder/herculex.htm. Viewed February 3, 2007.
Peairs, F. B. and S. D. Pilcher. Western Corn Rootworm. Colorado State University. March 24, 2006. URL: http://www.ext.colostate.edu/PUBS/insect/05570.html. Viewed February 3, 2007.
Rice, M. E. and J. J. Tollefson. The variant western corn rootworm in Iowa. Iowa State University. March 13, 2006. URL: http://www.ipm.iastate.edu/ipm/icm/2006/3-13/variant.html. Viewed February 3, 2007.
Willson H. R. and J. B. Eisley. Monitoring Western Corn Rootworm Activity in Soybeans to Predict Rootworm Injury in First-Year Corn. August 2001. URL: http://ohioline.osu.edu/ent-fact/0017.html. Viewed February 3, 2007.
Wright, R., L. Meinke and K. Jarvi. Corn Rootworm Management. University of Nebraska. July 1999. URL: http://ianrpubs.unl.edu/insects/ec1563.htm. Viewed February 3, 2007.