Biological Control of Diffuse and Spotted Knapweed
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United States Department of Agriculture Biological Control of Spotted Animal and Plant Health Inspection and Diffuse Knapweeds Service Program Aid Number 1529 Cover: Adult root-boring weevil The U.S. Department of Agriculture (USDA) Cyphocleonus achates, originally from prohibits discrimination in its programs on Greece, rests on a spotted knapweed flower the basis of race, color, national origin, sex, head. Female adult weevils produce up to religion, age, disability, political beliefs, and 100 eggs. The larvae feed in the root crown marital or familial status. (Not all prohibited and below, causing a gall-like formation in bases apply to all programs.) Persons with the damaged area, reducing plant vigor, and disabilities who require alternative means for making the plant more susceptible to communication of program information disease. (braille, large print, audiotape, etc.) should contact the USDA Office of Communications Photo credits: Cover photograph and figures at (202) 720-5881 (voice) or (202) 720-7808 1, 6–11, 14, 16–18, 22, 24, and 27—R. D. (TDD). Richard, USDA-APHIS. Figures 5, 13, 15, 19–21, 23, 25, 26, and 29—V. J. Farquhar, To file a complaint, write the Secretary of USDA-APHIS. Figures 4 and 28—R. F. Agriculture, U.S. Department of Agriculture, Lang, USDA-APHIS. Maps and graphic Washington, DC 20250, or call (202) 720- adapted from Selected Weeds of the United 7327 (voice) or (202) 720-1127 (TDD). States (U.S. Department of Agriculture USDA is an equal employment opportunity 1970). employer. Issued December 1994 2 The Problem Spotted and diffuse knapweeds (Centau- rea maculosa Lam. and C. diffusa Lam.) have infested millions of acres of rangeland in the United States. These invasive, exotic perennial weeds have reduced the carrying capacity of native rangelands by up to 90 percent. The weeds have very low palatability to both livestock and wildlife and have severely diminished the quality of wildlife habitat and the returns from livestock produc- tion. In Montana alone, a 1984 report estimated the value of annual forage losses caused by knapweeds to be $4.5 million (Bucher 1984). Figure 1—Spotted knapweed may be attractive to the uninitiated but is a menace to livestock range and wildlife habitat. 3 Herbicide control of these weeds is Figure 2—Spotted fairly effective but can be cost prohibi- knapweed was first introduced from Eurasia in tive for several reasons. First, the contaminated alfalfa and weeds’ numerous seeds have a unique clover seed around 1893. ability to stay dormant in the soil for 8 to Although scattered 10 years. Therefore, chemicals must be populations are found in at least 34 States, spotted used in successive years to exhaust the knapweed infests in excess “seed bank” in the soil. Second, of 7.25 million acres in just because the weeds now occupy a vast 8 States and the Canadian expanse of dry mountainous rangeland Provinces of British Columbia and Alberta. of limited value, chemical control may not be economically feasible for most landowners. Knapweeds’ growth in rough terrain further complicates chemical control. To reduce these problems, the U.S. Department of Agriculture (USDA) is conducting a major biological control Figure 3—Diffuse knapweed, first reported in program that involves importing, Washington in 1907, propagating, and distributing the weeds’ currently covers more than natural enemies. Biological control 3.21 million acres in 10 specialists in USDA’s Animal and Plant States and the southern part of British Columbia. Health Inspection Service (APHIS) are concentrating on insects that have evolved to feed only on spotted and diffuse knapweeds. Collectively, the feeding insects inhibit the weeds’ growth and reproduction, reducing the plants’ ability to compete with desirable native range plants. 4 Figure 4—An old sewer line presents a perfect opportunity for spotted knapweed to Figure 7—The basal leaves of the knap- establish itself. Spotted and diffuse Figure 6—Stems of spotted knapweed weed rosette grow up to 6 inches long and knapweeds thrive in dry, sandy soils in old terminate in a solitary pink or purple flower. divide into leaflets along a common axis. fields or disturbed sites, such as roadsides, The well-developed taproot system enables gravel pits, or abandoned railway beds. the plant to survive in dry areas. Although selective overgrazing of fall, the stems become brittle and the desirable native vegetation by herbi- seedheads break off easily. Automobiles vores enhances the knapweeds’ ability driven through such weed patches can to invade rangeland, the weeds’ transport the noxious weed seeds for success is primarily the result of their many miles into previously uninfested prolific seed production and ability to areas. In diffuse knapweed’s case, the distribute the seeds. Each flower plants may break off at the base and produces 12–35 seeds, resulting in blow in the wind for a short distance, 400–25,000 seeds per plant, depending shaking out their seeds as they tumble upon the soil moisture available to the along. plant. When the seeds ripen in early Figure 5—These mature spotted knapweed heads have already distributed most of their seeds. 5 Figure 9—The seed, tipped with a tuft of bristles, measures about 1/8 inch long. Figure 11—The flower of the diffuse knapweed plant is generally white but can be lavender or purple. Figure 8—Plants of spotted knapweed have Figure 10—Plants of diffuse knapweed, one or more stems that branch 1–3 feet tall. more branched than those of spotted knapweed, grow 1–2 feet tall. Compared to spotted knapweed, the leaves of diffuse knapweed are small, and the numerous flower heads are narrow with spiny bracts. 6 Biological Control In Europe and Asia, where spotted and diffuse knapweeds originated, sufficient natural enemies have developed over time to keep the knapweeds in balance with other plants native to those areas. Where insects and disease have stressed the knapweeds, the weeds are 13-A nothing more than wild flowers and exert Line Art no detrimental effects. However, because the weeds invaded North America without their natural enemies, they have reproduced unchecked here for more than 100 years. USDA, APHIS—in cooperation with scientists in the USDA’s Agricultural Research Service (ARS), Agricultural Experiment Stations (AES), and Interna- tional Institute of Biological Control (IIBC) —has imported, propagated, and redistributed hundreds of thousands of Figure 12—Location of biocontrol agent-induced damage on spotted and biological control agents. Scientists diffuse knapweed plants. employed by these agencies scour the European landscape for new species of both species of knapweed, but a few are burrow into the root, they allow naturally beneficial insects. This extensive program specific to just diffuse knapweed. The occurring soil components such as fungi relies upon cooperation from many insects work together by feeding on the and bacteria to enter the plant’s system, Federal and State agencies, along with seeds or in the roots of the weeds. Each further weakening the plant. private landowners. attack reduces the knapweeds’ defenses Eventually, scientists hope to propa- Currently, there are 12 beneficial insect by inhibiting seed production, either from gate and redistribute sufficient insect species from Europe cleared by USDA direct destruction of seeds or by stunting populations to hold spotted and diffuse for release in the United States. Most of overall growth and strength of the plant. knapweeds to tolerable levels similar to these biological control agents attack In some cases, when the insect larvae their population in Europe. 7 Root-Boring Insects Larvae of these insects mine the root of the plant, reducing storage capacity and increasing susceptibility to infection from fungi or bacteria. Plants attacked before they reach the critical size for flowering experience delayed bolting for 1 or more years, reducing flowering and production. Continued feeding by root-mining insects on weakened plants kills the weeds. Larvae of Cyphocleonus achates (cover photo) feed in the root crown and below, causing a gall-like formation in the damaged area. Pterolonche inspersa (fig. 15), a brown moth from the northeastern Mediterranean region, and Sphenoptera jugoslavica (fig. 16), a European buprestid beetle, are both specific to diffuse knapweed. Their Figure 13—Agapeta zoegana, a bright yellow moth, originates from the larvae mine the root of diffuse knap- Mediterranean region of Europe. weed, reducing plant vitality and providing a port of entry for fungal infection. Figure 14—Agapeta zoegana larvae possess the ability to move from one root to another upon depleting the food supply. Several larvae can occupy large roots. 8 Figure 16—Sphenoptera jugoslavica adult beetle. Figure 17—Sphenoptera jugoslavica larva mining the taproot of diffuse knapweed. Figure 15—Pterolonche inspersa adult moth. 9 Seedhead Insects Larvae of these species can consume the entire contents of immature knap- weed seedheads, including the seeds, florets, and portions of the receptacle. Most of the larvae of these species also cause gall-like structures in the seedhead. These galls are formed by the plant in response to the larval feeding and further deplete the plant’s resources. Figure 19—Two U. affinis galls flank an exposed larva. The larvae of U. affinis and U. quadrifasciata inhibit seed production. In the process, they form a gall inside the seedhead, which depletes the plant’s energy resources. Figure 20—Terellia virens, originating from the Mediterranean region, joins Chaetorellia acrolophi as APHIS’ two newest weapons against knapweeds. Originally introduced in 1993, T. virens’ larvae also attack the young seeds, burrowing inside and consuming the germ. In addition, the adult flies feed on the knapweed flowers. Figure 18—Urophora affinis adult fly. Figure 21—Chaetorellia acrolophi, from U. affinis and U. quadrifasciata are the most western Switzerland, can lay up to 12 eggs widely distributed of all the knapweed per day.