Insect pests are a major factor in the loss of agricultural crops. 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. 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 to western corn rootworm. A third corn rootworm species, the Southern corn rootworm, D. undecimpunctata howardi, causes significant economic damage in other regions.
Corn rootworm larvae can destroy significant percentages of corn if left untreated. In the United States, it is presently estimated that 30 million acres (120,000 km2) of corn (out of 80 million grown) are infested with corn rootworms, and that the 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.
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 reducing plant growth and resulting 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.
Corn rootworms of the genus Diabrotica (Coleoptera: Chrysomelidae) are among the most important insect pest of agricultural crops in the United States. For example, the Southern corn rootworm (SCRW), Diabrotica undecimpunctata howardi Barber is an economically important pest of corn, cucurbits and peanuts. SCRW Diabrotica undecimpunctata howardi Barber, or the spotted cucumber beetle, is widely distributed in North America, occurring in most areas east of the Rocky Mountains, in southern Canada, and in Mexico. It is most abundant and destructive in the southern United States. This insect is multivoltine and overwinters as adults in the southern parts of its range (Branson & Krysan (1981) Environmental Entomology 10:826-831). Southern corn rootworms infest the roots of many grass crops and weeds, as well as those of peanuts, alfalfa, and occasionally cucurbits. Annually, 20 to 25 million acres of corn are treated with soil insecticides to protect the crop from corn rootworm larval feeding damage (Fuller et al. (1997) J Econ Entomol 90:1332-1340). Soil insecticides applied for the corn rootworm represent one of the major uses of insecticide in the United States. Costs associated with insecticides applied to control larval damage to corn roots and adult damage to corn silks, along with crop losses can approach $1 billion annually (Metcalf (1986) In M. Kogan [ed.], Ecological Theory and Integrated Pest Management Practice. John Wiley & Sons, New York).
Biological control of insect pests of agricultural significance using a microbial agent, such as fungi, bacteria, or another species of insect affords an environmentally friendly and commercially attractive alternative to synthetic chemical pesticides. Generally speaking, the use of biopesticides presents a lower risk of pollution and environmental hazards. Biopesticides also provide greater target specificity than is characteristic of traditional broad-spectrum chemical insecticides. Biopesticides are often less expensive to produce and, thus, improve economic yield for a wide variety of crops.
Microbial insecticides, particularly those obtained from Bacillus strains, have played an important role in agriculture as alternatives to chemical pest control. Certain species of microorganisms of the genus Bacillus are known to possess pesticidal activity against a broad range of insect pests including Lepidoptera, Diptera, Coleoptera, Hemiptera, and others. Bacillus thuringiensis and Bacillus papilliae are among the most successful biocontrol agents discovered to date. Insect pathogenicity has also been attributed to strains of B. larvae, B. lentimorbus, B. sphaericus (Harwook, ed., ((1989) Bacillus (Plenum Press), 306) and B. cereus (International Application Publication No. WO 96/10083). Pesticidal activity appears to be concentrated in parasporal crystalline protein inclusions, although pesticidal proteins have also been isolated from the vegetative growth stage of Bacillus. Several genes encoding these pesticidal proteins have been isolated and characterized (see, for example, U.S. Pat. Nos. 5,366,892 and 5,840,868).
Recently, agricultural scientists have developed crop plants with enhanced insect resistance by genetically engineering crop plants to produce pesticidal proteins from Bacillus. For example, corn and cotton plants have been genetically engineered to produce pesticidal proteins isolated from strains of B. thuringiensis. These proteins are known as δ-endotoxins or Cry toxins (see, e.g., Aronson (2002) Cell Mol. Life Sci. 59(3):417-425; Schnepf et al. (1998) Microbiol Mol Biol Rev. 62(3):775-806). These genetically engineered crops are now widely used in American agriculture and have provided the farmer with an environmentally friendly alternative to traditional insect-control methods. In addition, potatoes genetically engineered to contain pesticidal Cry toxins have been sold to the American farmer. While they have proven to be very successful commercially, these genetically engineered, insect-resistant crop plants provide resistance to only a narrow range of the economically important insect pests.
Accordingly, there remains a need for new Bt toxins with a broader range of insecticidal activity against insect pests including, for example, toxins which are active against a greater variety of insects from the order Coleoptera. In addition, there remains a need for biopesticides having activity against a variety of insect pests and for biopesticides which have improved insecticidal activity.