United StatesDepartment ofAgriculture

Animal andPlant HealthInspectionService

Program AidNumber 1529

Biological Control of Spottedand Diffuse Knapweeds

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The U.S. Department of Agriculture (USDA)prohibits discrimination in its programs onthe basis of race, color, national origin, sex,religion, age, disability, political beliefs, andmarital or familial status. (Not all prohibitedbases apply to all programs.) Persons withdisabilities who require alternative means forcommunication of program information(braille, large print, audiotape, etc.) shouldcontact the USDA Office of Communicationsat (202) 720-5881 (voice) or (202) 720-7808(TDD).

To file a complaint, write the Secretary ofAgriculture, U.S. Department of Agriculture,Washington, DC 20250, or call (202) 720-7327 (voice) or (202) 720-1127 (TDD).USDA is an equal employment opportunityemployer.

Issued December 1994

Cover: Adult root-boring weevilCyphocleonus achates, originally fromGreece, rests on a spotted knapweed flowerhead. Female adult weevils produce up to100 eggs. The larvae feed in the root crownand below, causing a gall-like formation inthe damaged area, reducing plant vigor, andmaking the plant more susceptible todisease.

Photo credits: Cover photograph and figures1, 6–11, 14, 16–18, 22, 24, and 27—R. D.Richard, USDA-APHIS. Figures 5, 13, 15,19–21, 23, 25, 26, and 29—V. J. Farquhar,USDA-APHIS. Figures 4 and 28—R. F.Lang, USDA-APHIS. Maps and graphicadapted from Selected Weeds of the UnitedStates (U.S. Department of Agriculture1970).

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The Problem

Spotted and diffuse knapweeds (Centau-rea maculosa Lam. and C. diffusa Lam.)have infested millions of acres ofrangeland in the United States. Theseinvasive, exotic perennial weeds havereduced the carrying capacity of nativerangelands by up to 90 percent. Theweeds have very low palatability to bothlivestock and wildlife and have severelydiminished the quality of wildlife habitatand the returns from livestock produc-tion. In Montana alone, a 1984 reportestimated the value of annual foragelosses caused by knapweeds to be $4.5million (Bucher 1984).

Figure 1—Spotted knapweed may be attractive to the uninitiated but is amenace to livestock range and wildlife habitat.

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Herbicide control of these weeds isfairly effective but can be cost prohibi-tive for several reasons. First, theweeds’ numerous seeds have a uniqueability to stay dormant in the soil for 8 to10 years. Therefore, chemicals must beused in successive years to exhaust the“seed bank” in the soil. Second,because the weeds now occupy a vastexpanse of dry mountainous rangelandof limited value, chemical control maynot be economically feasible for mostlandowners. Knapweeds’ growth inrough terrain further complicateschemical control.

To reduce these problems, the U.S.Department of Agriculture (USDA) isconducting a major biological controlprogram that involves importing,propagating, and distributing the weeds’natural enemies. Biological controlspecialists in USDA’s Animal and PlantHealth Inspection Service (APHIS) areconcentrating on insects that haveevolved to feed only on spotted anddiffuse knapweeds. Collectively, thefeeding insects inhibit the weeds’growth and reproduction, reducing theplants’ ability to compete with desirablenative range plants.

Figure 2—Spottedknapweed was firstintroduced from Eurasia incontaminated alfalfa andclover seed around 1893.Although scatteredpopulations are found in atleast 34 States, spottedknapweed infests in excessof 7.25 million acres in just8 States and the CanadianProvinces of BritishColumbia and Alberta.

Figure 3—Diffuseknapweed, first reported inWashington in 1907,currently covers more than3.21 million acres in 10States and the southernpart of British Columbia.

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Figure 4—An old sewer line presents aperfect opportunity for spotted knapweed toestablish itself. Spotted and diffuseknapweeds thrive in dry, sandy soils in oldfields or disturbed sites, such as roadsides,gravel pits, or abandoned railway beds.

Figure 6—Stems of spotted knapweedterminate in a solitary pink or purple flower.

Figure 7—The basal leaves of the knap-weed rosette grow up to 6 inches long anddivide into leaflets along a common axis.The well-developed taproot system enablesthe plant to survive in dry areas.

Figure 5—These mature spotted knapweedheads have already distributed most of theirseeds.

Although selective overgrazing ofdesirable native vegetation by herbi-vores enhances the knapweeds’ abilityto invade rangeland, the weeds’success is primarily the result of theirprolific seed production and ability todistribute the seeds. Each flowerproduces 12–35 seeds, resulting in400–25,000 seeds per plant, dependingupon the soil moisture available to theplant. When the seeds ripen in early

fall, the stems become brittle and theseedheads break off easily. Automobilesdriven through such weed patches cantransport the noxious weed seeds formany miles into previously uninfestedareas. In diffuse knapweed’s case, theplants may break off at the base andblow in the wind for a short distance,shaking out their seeds as they tumblealong.

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Figure 8—Plants of spotted knapweed haveone or more stems that branch 1–3 feet tall.

Figure 10—Plants of diffuse knapweed,more branched than those of spottedknapweed, grow 1–2 feet tall. Compared tospotted knapweed, the leaves of diffuseknapweed are small, and the numerousflower heads are narrow with spiny bracts.

Figure 11—The flower of the diffuseknapweed plant is generally white but canbe lavender or purple.

Figure 9—The seed,tipped with a tuft ofbristles, measures about1/8 inch long.

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Biological Control

In Europe and Asia, where spotted anddiffuse knapweeds originated, sufficientnatural enemies have developed overtime to keep the knapweeds in balancewith other plants native to those areas.Where insects and disease havestressed the knapweeds, the weeds arenothing more than wild flowers and exertno detrimental effects. However,because the weeds invaded NorthAmerica without their natural enemies,they have reproduced unchecked herefor more than 100 years.

USDA, APHIS—in cooperation withscientists in the USDA’s AgriculturalResearch Service (ARS), AgriculturalExperiment Stations (AES), and Interna-tional Institute of Biological Control (IIBC)—has imported, propagated, andredistributed hundreds of thousands ofbiological control agents. Scientistsemployed by these agencies scour theEuropean landscape for new species ofbeneficial insects. This extensive programrelies upon cooperation from manyFederal and State agencies, along withprivate landowners.

Currently, there are 12 beneficial insectspecies from Europe cleared by USDAfor release in the United States. Most ofthese biological control agents attack

burrow into the root, they allow naturallyoccurring soil components such as fungiand bacteria to enter the plant’s system,further weakening the plant.

Eventually, scientists hope to propa-gate and redistribute sufficient insectpopulations to hold spotted and diffuseknapweeds to tolerable levels similar totheir population in Europe.

both species of knapweed, but a few arespecific to just diffuse knapweed. Theinsects work together by feeding on theseeds or in the roots of the weeds. Eachattack reduces the knapweeds’ defensesby inhibiting seed production, either fromdirect destruction of seeds or by stuntingoverall growth and strength of the plant.In some cases, when the insect larvae

Figure 12—Location of biocontrol agent-induced damage on spotted anddiffuse knapweed plants.

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Line Art

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Root-Boring InsectsLarvae of these insects mine the

root of the plant, reducing storagecapacity and increasing susceptibility toinfection from fungi or bacteria. Plantsattacked before they reach the criticalsize for flowering experience delayedbolting for 1 or more years, reducingflowering and production. Continuedfeeding by root-mining insects onweakened plants kills the weeds.Larvae of Cyphocleonus achates (coverphoto) feed in the root crown andbelow, causing a gall-like formation inthe damaged area.

Pterolonche inspersa (fig. 15), abrown moth from the northeasternMediterranean region, andSphenoptera jugoslavica (fig. 16), aEuropean buprestid beetle, are bothspecific to diffuse knapweed. Theirlarvae mine the root of diffuse knap-weed, reducing plant vitality andproviding a port of entry for fungalinfection.

Figure 13—Agapeta zoegana, a brightyellow moth, originates from theMediterranean region of Europe.

Figure 14—Agapeta zoegana larvae possessthe ability to move from one root to anotherupon depleting the food supply. Severallarvae can occupy large roots.

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Figure 15—Pterolonche inspersa adult moth.

Figure 16—Sphenoptera jugoslavica adultbeetle.

Figure 17—Sphenoptera jugoslavica larvamining the taproot of diffuse knapweed.

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Seedhead InsectsLarvae 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 alsocause gall-like structures in theseedhead. These galls are formed bythe plant in response to the larvalfeeding and further deplete the plant’sresources.

Figure 20—Terellia virens, originating fromthe Mediterranean region, joins Chaetorelliaacrolophi as APHIS’ two newest weaponsagainst knapweeds. Originally introduced in1993, T. virens’ larvae also attack the youngseeds, burrowing inside and consuming thegerm. In addition, the adult flies feed on theknapweed flowers.

Figure 19—Two U. affinis galls flank anexposed larva. The larvae of U. affinis andU. quadrifasciata inhibit seed production. Inthe process, they form a gall inside theseedhead, which depletes the plant’s energyresources.

Figure 18—Urophora affinis adult fly.U. affinis and U. quadrifasciata are the mostwidely distributed of all the knapweedbiocontrol agents.

Figure 21—Chaetorellia acrolophi, fromwestern Switzerland, can lay up to 12 eggsper day. This seedhead fly’s larvae burrowinto the center of the bud, eat their waythrough a floret down into the seed, andcontinue to another floret, usually destroyingthe entire contents of the seedhead.

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Figure 23—Larinus obtusus adult weevil. Figure 25—The Larinus minutusdevelopment chamber occupies most of theseedhead. Note the nonviable seed,remnants of larval feeding, “glued” to thesides.

Figure 22—Bangasternus fausti adult weevil.

Bangasternus fausti, Larinus obtusus,and L. minutus (figs. 22–24) are threespecies of seedhead weevils fromcentral Europe and the Mediterranean.The adults of these weevils feed on theleaves, stems, and florets but preferflower heads when available.

The larvae of all three insects eat thepappus hairs and the entire contents ofindividual seeds and then prepare forpupation by forming a cell of pappushairs, seeds, and secretions in theseedhead.

Figure 24—Larinus minutus adult weevil.

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Figure 26—Metzneria paucipunctella adult moth.

The larvae of Metzneriapaucipunctella (fig. 27), a small brownmoth originating in Europe, feed ondifferent areas of the spotted knapweedseedhead as they mature. Larvae firstconsume the florets, move on to theyoung seed and the receptacle, andfinally consume the mature seed.

Figure 27—Metzneria paucipunctella larvainside a spotted knapweed seedhead.

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Establishing Exotic Insects

Scientists evaluated the 12 potentialbiological control insects in laboratoriesand experimental plots in Europe andexamined them in quarantine facilitiesin the United States. The entomologistsmade certain that insects intended forrelease were healthy, ensuring a suc-cessful establishment in North America,free of diseases and parasites.

Furthermore, they made sure thatthe new insects would be safe forrelease in the United States. Stringentimport regulations prevent the introduc-tion of foreign organisms that candamage desirable plants.

The scientists checked whether theinsects would attack crop and pastureplants, including alfalfa and safflower.They also studied the insects’ behaviortoward North American wild plantsclosely related to spotted and diffuseknapweeds, such as common yarrowand common sunflower. The 12 selectedbiological control agents have allpassed rigorous testing, and all areextremely particular about their diet.

After each agent was tested andapproved for release in the UnitedStates, APHIS faced the challenge ofsecuring a sufficient number of each

Figure 28—Nursery knapweed plants in thegarden insectary are trimmed several timeseach summer to enhance root-boring insectproduction.

Figure 29—Scientists use a variety ofmethods to collect the insects forredistribution, ranging from small hand-heldvacuums that suck the insects off the plantsto old-fashioned hands-and-knees searching.

species to release on the range. Toencourage rapid population develop-ment, specialists developed methods tomass-produce some species usinginsectaries where specially designedcages are placed over plots of knap-weed. The cages protect and contain theinsects, enabling them to grow andmultiply with minimal stress. Scientistsuse an intensive garden setting to raiseother insects and carefully nurture the

plants to foster maximum agent develop-ment. After sufficient populations of theinsects develop in the garden setting,specialists collect and redistribute someof them to new insectary sites managedby county, State, or Federal cooperators.In this way, populations of insects arespread across the areas of the UnitedStates already infested with spotted anddiffuse knapweed.

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Ranchers need to be patient in waitingfor the benefits from introducedenemies of spotted and diffuse knap-weed. Because biological control is agradual process, and because of theknapweed seed’s ability to lie dormantin the soil for many years, it will taketime to see results. Ranchers can helpthe process by restricting their use ofherbicides and insecticides at or nearinsect release sites, thus enablingbeneficial insects to multiply withoutharm from agricultural chemicals. Still,

it can take many years to build upadequate numbers of a weed’s naturalenemies at any specific location. Whenthe insects increase in number andknapweed becomes less abundant, thenatural enemies search for other standsof the weed to attack. This naturalprocess will continue for as long asknapweed is present.

Ranchers and outdoor enthusiastscan help by being conscientious andcareful not to spread knapweed intonew areas. Repeatedly, people un-aware they were doing any harm havespread spotted and diffuse knapweed.

The seed travels along unobtrusively onvehicles driven off maintained roads.Bumpers, doors, and undercarriagesshould always be checked beforemoving a vehicle to make certain theweeds are cleared away. Do not driveoff-road vehicles, including pickups,four-wheelers, motorcycles, bicycles,and snowmobiles, through knapweedinfestations.

Through conscientious monitoring ofland-use habits, landowners andrecreationists can help control thespread of these noxious weeds.

What You Can Do

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Selected References

Roché, Ben F. 1991. Seeds—from startto finish. In: Cooperative extensionknapweed, vol. 5, no. 3. Pullman, WA:Washington Interagency KnapweedCommittee: 1–2.

U.S. Department of Agriculture. 1970.Selected weeds of the United States.Washington, DC: U.S. Department ofAgriculture.

Whitson, Tom D., ed. 1987. Weedsand poisonous plants of Wyoming andUtah. Res. Rep. 116-USU. Laramie,WY: Cooperative Extension Service,College of Agriculture, University ofWyoming: 162–165.

Bucher, Robert F. 1984. Potentialspread and cost of spotted knapweedon range. Montguide MT 8423.Bozeman, MT: Montana CooperativeExtension Service. 3 p.

Lacey, C. A. 1989. Knapweed manage-ment: a decade of change. In: Proceed-ings of the knapweed symposium;4–5 April 1989; Bozeman, MT.Bozeman, MT: Plant and Soils Depart-ment and Extension Service, MontanaState University: 1–6.

Lacey, C. A.; Fay, P. K. 1984.Montana’s spotted knapweed aware-ness program. In: Proceedings of theknapweed symposium; 3–4 April 1984;Great Falls, MT. Bozeman, MT: Plantand Soils Department and ExtensionService, Montana State University:83–88.