Western corn rootworm
|Western corn rootworm|
|Diabrotica virgifera virgifera
|Wikispecies has information related to: Western corn rootworm|
The Western corn rootworm, Diabrotica virgifera virgifera, 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 American football-shaped, white, and less than 0.004 inches (0.10 mm) 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.
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.
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 (200 mm) of soil, although they may be laid more than 12 inches (300 mm) 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 (200 mm) depth than northern corn rootworm females.
Rootworm larvae can complete development only on corn and a few other species of grasses. Studies have shown that rootworm larvae reared on other grasses (specifically, yellow foxtail) emerged as adults later and had smaller head capsule size as adults than larvae reared on corn. 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.
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.
History of invasions
The Western corn rootworm rapidly expanded its range in North America during the second part of the 20th century. It is now present from the southwestern region of the US Corn Belt to the east coast of North America. It was introduced at the end of the 20th century into Europe, where it was first observed near Belgrade, Serbia in 1992. The Serbian outbreak now extends from Greece to Poland and from eastern Italy to Ukraine. In addition to this large area of continuous spread in Central and southeastern Europe, a number of discontinuous outbreaks have been detected in Europe. The first was discovered near Venice, Italy, in 1998. New ones were then detected in northwestern Italy (Piedmont) and Switzerland (canton Ticino) in 2000, northeastern Italy in 2002 (near Pordenone) and 2003 (near Udine), northern Italy (Trentino), Eastern France (Alsace), Switzerland, Belgium, the United Kingdom and the Netherlands in 2003, and the Parisian region, France in 2002, 2004 and 2005. Outbreaks detected in north Switzerland, Belgium, the Netherlands and the Parisian region did not persist over time and are currently extinct. A recent population genetics study showed that the distribution of the European corn rootworm results from several introductions from North America to Europe. At least three successive introductions gave rise to outbreaks detected in Serbia in 1992, the Italian Piedmont in 2000, and Ile-de-France in 2002. A more recent study by the same team showed that the European outbreaks observed in Alsace in 2003 and Ile-de-France in 2005 came from two additional introductions from North America, bringing to five the number of transatlantic introductions of the Western corn rootworm. The exact North American origin of the European introductions has not yet been found, but the north of the United States appears as the most likely region of origin. Regarding the European populations, it has recently been shown that small remote outbreaks in southern Germany and north-eastern Italy most likely originated from long-distance dispersal events from Central and southeastern Europe. The large European outbreak is thus likely expanding by stratiﬁed dispersal, involving both continuous diffusion and discontinuous long-distance dispersal. This latter mode of dispersal may accelerate the expansion of the Western Corn Rootworm in Europe in the future.
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.
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. Aldrin resistance was probably introduced independently, at least twice, from North America into Europe. Organophosphates, such as methyl-parathion, may still provide effective control of both larval and adult Diabrotica virgifera virgifera in Central and southeastern Europe and in North West Italy.
Crop rotation is a consistent and economical means of controlling rootworms the following season in many corn-growing areas where rootworm beetles primarily lay eggs in corn. As a way to reduce rootworm densities, it is more effective than insecticides (Wright). Corn rootworm larvae 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 (510 mm) through the soil in search of food (Peairs). There are, however, two different biotypes of corn rootworms that have been found to adapt to the practice of crop rotation. The soybean variant of the western corn rootworm was first discovered in central Illinois in the late 1980s and has since spread throughout Illinois, Indiana, southern Wisconsin and into eastern Iowa (Rice). Instead of laying eggs into a corn field, the females of the soybean variant will mate and then fly into 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 1980s 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. This adaptation has been observed in areas of northern Iowa, Minnesota and South Dakota.
Companion or second crop planting can also increase rootworm populations dramatically. Corn with pumpkins or corn following pumpkins are examples of planting patterns that often produce particularly extreme rootworm feeding pressure.
|This section is outdated. (May 2012)|
|This article relies largely or entirely upon a single source. (July 2008)|
Transgenic corn, in combination with other pest management practices, is a method used for minimizing corn rootworm damage. Bt crops are 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 two most common brands of transgenic Bt corn are Genuity and Herculex. Genuity Smartstax combines the benefits of Monsanto's VT Triple Pro, Roundup Ready 2, and Acceleron Seed Treatment System technologies, as well as Dow's Herculex Xtra and Liberty Link technologies. Acceleron, Herculex Xtra, and VT Triple Pro are for protection from insect damage.
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.
Recently, however, strains of rootworms have been discovered in several Midwest US states that exhibit resistance to Bt proteins. According to Monsanto, the “YieldGard® VT Triple and Genuity® VT Triple PRO™ corn products” are affected. In 2009, four strains of western corn rootworm in Iowa were found to have field-evolved resistance to Bt corn.
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- The Dow Chemical Company
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