Biological Control
Eric M. Coombs, Gary L. Piper, Mark Schwarzländer, Jennifer Andreas, and Joseph Milan
Revised October 7, 2008
Classical biological control involves the introduction and management of selected natural enemies of a weed. Many of our worst weeds originated in foreign countries. These newly introduced plants, free from the natural enemies found in their homelands, gained a competitive advantage over native plants. Once they were out of control, other methods of weed management were not always economical or physically possible. The need for a method of weed reduction that was economical, self-sustaining, and environmentally safe provided opportunities for biological control.
After noxious weed infestations are out of control, it is often assumed that biological control will solve the problem. But at this time, each new introduction of a biological weed control agent (BCA) in the United States has only about a one in three chance of success. There are several well-documented successes of biological control in our area: Mediterranean sage, St. Johnswort (Klamath weed), and tansy ragwort. Emerging successes on the following weeds (and the most effective agents) include Dalmatian toadflax (Mecinus janthinus), diffuse knapweed (Larinus minutus), leafy spurge (Aphthona spp.), purple loosestrife (Galerucella spp.), and yellow starthistle (Eustenopus villosus).
Biological control is a slow process, and its efficacy is highly variable. Many BCAs are so new that their ability to control their host weed has not yet been determined. Only after several years of monitoring weed populations will the impact of newly introduced agents become evident. 
Biological control agents impact weeds directly and indirectly. Direct impact destroys vital plant tissues and functions. Indirect impact increases stress on the weeds, which may reduce their ability to compete with desirable plants.
Biological control can be integrated with other management practices to reduce weed populations. For example, once weeds are weakened by BCAs, competitive plantings may be used to out-compete the weeds. 
Biological control is not a panacea; it will not eradicate noxious weeds. When using BCAs, you must expect residual levels of the weed populations; the agents’ survival depends on the density of their host weeds. After host weed populations decrease, populations of BCAs will decrease correspondingly. This is a natural cycle. Therefore, a resurgence of weed populations may occur due to seed reserves in the soil, missed plants, and lagging populations of BCAs. In areas where the BCAs do not provide sufficient control, other methods may need to be integrated or the search for additional BCAs must be pursued.
The BCAs released in our area have been tested to ensure they are host specific. This is an expensive and time-consuming task that must be done before the agents are allowed to be introduced into the United States. Some BCAs cleared for use may have only a minor impact on host plant density. Other BCAs that may be effective in reducing weed populations may not be cleared for introduction. One reason for this may be that they are not host specific; that is, the agents in the absence of their host weeds may switch to crops, native flora, or endangered plant species.
Biological control of certain weeds may not work in your area, even if it does in others. Climate variations such as cold winters and plant biotype differences may account for some failures in the past. To ensure maximum success, trained personnel should supervise biological control programs. Biological control agents require specific conditions to survive. 
Once release sites for BCAs have been selected, it is essential that those areas be protected. BCA releases are often made with small numbers. It can take 3 to 5 years to establish collectable populations. After BCA populations have sufficiently increased at nursery sites, they may then be introduced into new areas. The types and status of BCAs used in the Pacific Northwest are found in the tables in this section. In areas where control was achieved but the host plants make a resurgence—for example, after severe weather—reintroducing the agents may be required.
Biological control has many benefits and some disadvantages. Benefits include reducing herbicide residues in the environment, host specificity on target weeds, long-term self-perpetuating control, low cost per acre, searching ability to locate hosts, synchronizing agents to hosts’ life cycles, and the unlikelihood that hosts will develop resistance to agents. Disadvantages of biological control include the limited availability of agents from their native lands, the dependence of control on plant density, the slow rate at which control occurs, biotype matching, the difficulty and expense of conflicts over control of the weed, and host specificity when host populations are low.
Biological control programs require consistent funding, expertise, and interdisciplinary cooperation. Biological control of weeds is not a perfect solution for all noxious weed control programs. It can, however, be integrated as an additional tool with other weed management methods. Interdisciplinary vegetation management teams are working on noxious weed problems to minimize adverse environmental and socioeconomic impacts.
If you are considering importing biocontrol agents from another state, be aware that federal permits are required. To obtain USDA-APHIS-PPQ Form 526, write: USDA-APHIS-PPQ, Biological Assessment and Taxonomic Support, 4700 River Road, Unit 113, Riverdale, MD 20737; online at http://www.aphis.usda.gov/plant_health/permits/index.shtml
We encourage all who implement classical biological weed control to follow the International Code of Best Practices, presented at the 1999 International Symposium on Biological Control of Weeds. Briefly, this includes:
•	Releasing only approved agents
•	Using the most effective agents
•	Documenting releases
•	Monitoring for impacts on the target weed, nontarget species, and the environment
For more information about monitoring biocontrol agents, contact your state department of agriculture or university.We do not recommend using unapproved natural enemies (that is, accidentally introduced species such as Larinus planus, Chaetorellia succinea, and Cassida rubiginosa) as biological control agents because of their negative nontarget impacts. Also, the seed head weevil, Rhinocyllus conicus, is no longer approved for interstate shipment because of its negative impacts on native thistle species.
Current Status of Biological Weed Control Agents
in Oregon, Washington, and Idaho
		Distribution1		Attack Rate2		Control3		Availability4	
Weed	Agent	OR	WA	ID		OR	WA	ID		OR	WA	ID		OR	WA	ID	
Bindweed, field	Aceria malherbae	L	L	L		H	S	L		G	U	F		L	U	L	
Broom, Scotch 	Bruchidius villosus	L	W	U		M	M	U		G	G	U		M	M	U	
	Exapion fuscirostre	W	W	U		H	H	U		G	G	U		M	M	U	
	Leucoptera spartifoliella	W	L	—		M	M	—		F	P	—		M	L	—	
Cordgrass, smooth	Prokelisia marginata	—	L	—		—	M	—		—	L	—		—	L	—	
Gorse	Exapion ulicis	W	W	—		H	H	—		G	G	—		M	M	—	
	Tetranychus lintearius	W	W	—		L	L	—		P	P	—		L	L	—	
Knapweed, diffuse	Bangasternus fausti 	W	L	L		H	M	U		G	G	U		M	L	L	
	Cyphocleonus achates	L	L	L		L	M	M		F	E	M		L	L	L	
	Larinus minutus	W	W	W		H	H	H		E	E	E		M	M	M	
	Pterolonche inspersa	L	F	F		L	—	—		L	—	—		L	—	—	
	Sphenoptera jugoslavica	W	W	W		H	H	H		G	G	G		M	M	M	
	Urophora affinis	W	W	W		H	H	H		G	G	G		M	M	M	
	Urophora quadrifasciata	W	W	W		H	H	H		G	G	G		M	M	M	
Knapweed, meadow	Larinus minutus	L	L	—		M	M	—		F	F	—		L	M	—	
	Larinus obtusus	W	W	— 		H	H	—		G	G	—		M	M	—	
	Urophora quadrifasciata	L	L	—		L	M	—		F	G	—		L	M	—	
Knapweed, Russian	Subanguina picridis	L	F	U		S	—	U		F	—	U		L	—	U	
Knapweed, spotted	Agapeta zoegana	L	L	W		H	M	L		G	U	U		M	L	L	
	Bangasternus fausti	L	L	L		L	L	U		G	U	U		L	L	L	
	Chaetorellia acrolophi	L	L	  L		L	L	L		F	U	U		L	L	L	
	Cyphocleonus achates	L	L	L		H	M	M		F	F	F		M	L	L	
	Larinus minutus	W	W	W		H	H	H		E	E	E		M	M	M	
	Larinus obtusus	W	L	L		H	M	M		E	G	F		M	M	L	
	Metzneria paucipunctella	W	W	W		H	L	H		G	G	G		M	L	M	
	Sphenoptera jugoslavica	W	L	L		H	H	L		G	G	P		M	M	L	
	Terellia virens	L	L	L		M	U	U		G	U	U		L	U	U	
	Urophora affinis	W	W	W		M	H	H		G	G	G		M	M	M	
	Urophora quadrifasciata	W	W	W		H	H	H		G	G	G		M	M	M	
Loosestrife, purple	Galerucella calmariensis	W	W	W		H	H	H		E	E	E		M	M	M	
	Galerucella pusilla	W	W	W		H	H	H		E	E	E		M	M	M	
	Hylobius transversovittatus	W	L	L		H	M	L		G	F	F		L	L	L	
	Nanophyes marmoratus	W	L	W		M	M	L		F	F	F		M	M	M	
Ragwort, tansy	Botanophila seneciella	W	W	L		H	H	L		F	G	F		M	M	U	
	Longitarsus jacobaeae	W	W	U		H	H	U		E	E	U		M	M	U	
	Tyria jacobaeae	W	W	—		H	H	—		E	E	—		M	M	—	
Sage, Mediterranean	Phrydiuchus tau	W	L	W		H	L	H		G	F	G		M	L	M	
St. Johnswort	Agrilus hyperici	L	L	W		H	H	H		E	E	E		M	L	M	
	Aplocera plagiata	W	W	W		M	M	M		F	F	F		M	M	M	
	Chrysolina hyperici	W	W	W		H	H	H		E	E	E		M	M	M	
	Chrysolina quadrigemina	W	W	W		H	H	H		E	E	E		M	M	M	
Saltcedar	Diorhabda elongata	L	L	L		H	L	S		F	F	U		L	L	U	
Skeletonweed, rush	Bradyrrhoa gilveolella	U	—	U		U	—	U		U	—	U		U	—	U	
	Cystiphora schmidti	W	W	W		H	H	H		G	E	E		M	M	M	
	Eriophyes chondrillae	W	W	W		H	H	H		E	E	E		M	M	M	
	Puccinia chondrillina	W	W	W		H	H	H		G	E	F		M	M	M	
Spurge, leafy	Aphthona cyparissiae	L	L	L		H	M	L		E	F	G		M	L	M	
	Aphthona czwalinae	L	L	L		H	L	L		E	U	G		M	U	M	
	Aphthona flava	L	L	L		H	M	L		F	F	G		L	L	M	
	Aphthona lacertosa	W	L	W		H	M	H		E	U	E		M	L	M	
	Aphthona nigriscutis	W	L	W		H	M	H		E	F	E		M	U	M	
	Hyles euphorbiae	—	—	L		—	—	M		—	—	G		—	—	M	
	Oberea erythrocephala	W	U	W		H	U	M		G	U	G		M	U	M	
	Spurgia esulae	F	F	L		—	—	U		—	—	U		—	—	U	
Starthistle, yellow	Bangasternus orientalis	W	W	W		L	L	L		P	P	P		L	L	M	
	Chaetorellia australis	W	W	W		H	M	H		E	G	E		M	M	M	
	Eustenopus villosus	W	W	W		H	H	H		E	E	E		M	M	M	
	Larinus curtus	W	W	W		H	M	M		E	F	E		M	L	M	
	Puccinia jacea var. solstitialis	L	—	—		S	—	—		U	—	—		U	—	—	
	Urophora sirunaseva	W	L	L		L	L	L		F	F	U		L	L	L	
Thistle, bull	Urophora stylata	W	L	—		H	M	—		G	F	—		M	L	—	
Thistle, Canada 	Ceutorhynchus litura	L	L	L		H	M	L		G	F	U		M	L	L	
	Rhinocyllus conicus	W	W	W		H	H	F		F	H	F		M	M	M	
	Urophora cardui	W	W	L		H	M	M		F	F	F		M	M	M	
Thistle, Italian 	Rhinocyllus conicus	W	—	L		H	—	H		G	—	G		M	—	—	
	Trichosirocalus horridus	W	—	—		H	—	—		U	—	—		M	—	—	
Thistle, milk	Rhinocyllus conicus	W	L	—		H	H	—		G	G	—		M	M	—	
Thistle, musk	Rhinocyllus conicus	W	W	W		H	H	H		G	E	G		M	M	M	
	Trichosirocalus horridus	L	—	L		M	—	M		G	—	G		M	—	L	
Thistle, plumeless	Rhinocyllus conicus	L	W	W		H	H	H		G	E	G		L	M	M	
	Trichosirocalus horridus	—	L	—		—	L	—		—	U	—		—	U	—	
Thistle, slenderflower	Cheilosia corydon	W	—	—		M	—	—		F	—	—		M	—	—	
	Rhinocyllus conicus	W	—	—		H	—	—		G	—	—		M	—	—	
	Trichosirocalus horridus	W	—	—		H	—	—		U	—	—		M	—	—	
Toadflax, Dalmatian 	Brachypterolus pulicarius	W	W	W		M	M	H		P	P	E		M	M	M	
	Calophasia lunula	L	W	W		L	H	L		F	F	U		L	M	L	
	Mecinus janthinus	W	W	W		H	H	H		E	E	E		M	M	M	
Toadflax, yellow	Brachypterolus pulicarius	L	L	L		M	M	M		F	F	P		L	L	L	
	Calophasia lunula	—	L	L		—	M	M		—	F	F		—	L	L	
	Gymnetron antirrhini	W	L	L		M	H	M		U	G	U		L	L	L	
1 Distribution within host range: W = widespread; L = limited sites; F = failed to establish; U = unknown status; — = not released2 Attack rate host: H = heavy (> 70%); M = medium (> 30%); L = light (> 10%); S = slight (< 1%); U = unknown status3 Control ability on seeds and/or plant density: E = excellent; G = good; F = fair; P = poor; U = undetermined4 Availability for redistribution: M = mass collections; L = limited; U = unavailable. (Limited availability indicates agent populations are slow in building or are recently introduced. Work on these species should be coordinated through biological control specialists at the state department of agriculture or state university.)

Biological Agents and Their Roles
Classical biological agents introduced into the Pacific Northwest for the biological control of weeds and the general role of each agent.
Species	Role	
Aceria malherbae	bud/leaf gall mite 	
Agapeta zoegana	root boring moth 	
Agrilus hyperici	root boring beetle 	
Aplocera plagiata	defoliating moth 	
Aphthona cyparissiae	root/defoliating flea beetle 	
Aphthona czwalinae	root/defoliating flea beetle 	
Aphthona flava	root/defoliating flea beetle 	
Aphthona lacertosa	root/defoliating flea beetle 	
Aphthona nigriscutis	root/defoliating flea beetle 	
Bangasternus fausti	seed head weevil 	
Bangasternus orientalis	seed head weevil 	
Botanophila seneciella	seed head fly	
Brachypterolus pulicarius	flower beetle 	
Bradyrrhoa gilveolella	root boring moth	
Bruchidius villosus	seed beetle 	
Calophasia lunula	defoliating moth 	
Ceutorhynchus litura	crown/root weevil 	
Chaetorellia acrolophi	seed head fly 	
Chaetorellia australis	seed head fly 	
Chrysolina hyperici	defoliating beetle 	
Chrysolina quadrigemina	defoliating beetle 	
Cyphocleonus achates	root boring/gall weevil 	
Cystiphora schmidti	stem/leaf gall midge 	
Diorhabda elongata	leaf beetle	
Eriophyes chondrillae	bud gall mite 	
Eustenopus villosus	seed head weevil 	
Exapion fuscirostre	seed weevil 	
Exapion ulicis	seed weevil 	
Galerucella calmariensis	leaf beetle 	
Galerucella pusilla	leaf beetle 	
Gymnetron antirrhini	seed head weevil 	
Hyles euphorbiae	defoliating moth 	
Hylobius transversovittatus	root boring weevil 	
Larinus curtus	seed head weevil 	
Larinus minutus	seed head weevil 	
Larinus obtusus	seed head weevil 	
Leucoptera spartifoliella	twig mining moth 	
Longitarsus jacobaeae	root/defoliating flea beetle 	
Mecinus janthinus	stem boring weevil 	
Metzneria paucipunctella	seed head moth 	
Nanophyes marmoratus	flower bud weevil 	
Oberea erythrocephala	stem boring beetle	
Phrydiuchus tau	crown/root weevil 	
Prokelisia marginata	sap sucking plant hopper 	
Pterolonche inspersa	root boring moth	
Puccinia chondrillina	rust fungus 	
Puccinia jacea var. solstitialis	rust fungus	
Rhinocyllus conicus*	seed head weevil 	
Sphenoptera jugoslavica	root boring/gall beetle 	
Spurgia esulae	shoot tip gall midge 	
Subanguina picridis	stem/leaf gall nematode 	
Terellia virens	seed head fly 	
Tetranychus lintearius	spider mite 	
Trichosirocalus horridus	root/crown weevil 	
Tyria jacobaeae**	defoliating moth	
Urophora affinis	seed head gall fly	
Urophora cardui	stem gall fly	
Urophora quadrifasciata	seed head gall fly 	
Urophora sirunaseva	seed head gall fly 	
Urophora stylata	seed head gall fly 	
*Rhinocyllus conicus not recommended. It attacks native thistles.**Tyria jacobaeae not recommended east of Cascades. It attacks several native plants.