Knapweed Biological Control

Overview

Diffuse (Centaurea diffusa), meadow (C. jacea nothosubsp. pratensis), spotted (C. stoebe) and squarrose (C. virgata subsp. squarrosa) knapweeds are susceptible, to varying degrees, to a suite of biological control agents. These plant species are closely related and some of them hybridize. Thirteen species of insects that attack these plants have been approved for release in the USA, but only 9 are approved for use in California (see table below). These insects may differ in their host plant preference and do not attack all of these plant species equally well. Knapweed agents have been most extensively released in Colorado, Montana and Oregon, where these knapweeds are most invasive. The status of agents in California are reported below. No damage to non-target species has been reported.

Diffuse knapweed. Diffuse knapweed occurs primarily in Trinity and Siskiyou counties. The two seed head weevils, Bangasternus fausti and Larinus minutus are well established in California and attack up to 85% of flower heads. The gall fly Urophora affinis appears to have gradually disappeared after establishment of the two seed head weevils. Knapweed densities decreased to about 1/3 over an 8-year period after introduction of seed head weevils. The root beetle, Sphenoptera jugoslavica, was released and occurs at low levels. The density of diffuse knapweed seed heads per square meter decreased gradually from 1998 to 2008 at a release site. The rust fungus Puccinia jaceae var. diffusa (not listed in table below) is an unpermitted natural enemy of diffuse knapweed that arrived on its own, but it infects up to 60% of plants in California.

Meadow knapweed. The seed head weevils, Larinus minutus and L. obtusus, were released in Siskiyou County in 2001, and two years later attacked 74 to 78% of flower heads. The flower heads of meadow knapweed are larger than those produced by spotted and diffuse knapweed, and not all seeds are destroyed by the seed head weevils. The seed head fly, Urophora quadrifasciata, dispersed to this plant by itself and attacked 4 to 38% of flower heads.

Spotted knapweed. Two seed head flies (Urophora quadrifasciata and U. affinis), the clearwing fly (Terellia virens), and the seed head weevils (Larinus minutus and Eustenopus villosus) are established on spotted knapweed in Shasta County. During a 3-year study, L. minutus attacked an average of 43% of flower heads, U. affinis40%, and U. quadrifasciata 34%. The hairy weevil (Eustenopus villosus) released on yellow starthistle was found to attack 9% of the flower heads. Terellia virens attacked less than 3% of flower heads. The density of spotted knapweed stems decreased by 78% over 5 years. Attack by E. villosus is likely a temporary 'spillover' effect due to insects coming from nearby yellow starthistle populations that declined and deprived the weevils of their preferred host plant. The root weevil (Cyphocleonus achates) persists at low levels.

Squarrose knapweed. Two seed head weevils, Bangasternus fausti and Larinus minutus, and the root beetle, Sphenoptera jugoslavica, have been introduced and successfully established over most of squarrose knapweed's range in CA. Urophora quadrifasciata arrived unaided and occurs throughout most of this weed's range. The two weevils generally infest more than 90% of seedheads, and B. fausti tends to displace L. minutus at most sites over about a 6-year period. Squarrose knapweed populations have now declined to very low levels.

Insect biology

Agapeta zoegana, the sulfur knapweed moth(Leidoptera: Cochylidae), has one generation per year (Fig. 1). The short-lived adults are active at night during summer and early fall. White eggs are laid on knapweeds and nearby vegetation. Larvae crawl down and mine into the roots (Fig. 2). The exposed sides of tunnels are covered with a silk web. Larvae persist through winter, and pupation occurs inside roots the following spring. This species does best at dry, well-drained, open sites that do not have dense vegetation.

Bangasternus fausti, thebroad-nosed seedhead weevil (Coleoptera: Curculionidae), feeds on leaves and lays eggs on leaflets or stems close to flower buds (Fig. 3). The eggs are covered by a black anal excretion. Larvae tunnel up into the flower bud and consume the developing seeds. Pupation occurs inside the flower head, and adults emerge in late summer and disappear to hibernate in sheltered sites. Adults have a shorter snout than the twoLarinus species.

Cyphocleonus achates, the knapweed root weevil (Coleoptera: Curculionidae), has one generation per year (Fig. 4). Adults feed on leaves and lay eggs in the root at the soil surface. Larvae feed inside the root and create a gall-like swelling (Fig. 5). Larvae persist through the winter, and pupation occurs inside the root in the spring. Adults emerge in early summer and lay eggs throughout the summer.

Larinus minutus, the lesser knapweed flower weevil (Coleoptera: Curculionidae), feeds on leaves and lays eggs inside the flower head (Fig. 6). Larvae consume most or all of the developing seed and pupate inside the flower head (Fig. 7). Adults emerge in the late summer and hibernate in sheltered sites. Larinus obtusus, the blunt knapweed flower weevil,is very similar toL. minutus, but is a little larger.

Sphenoptera jugoslavica, the bronze knapweed root borer (Coleoptera: Curculionidae), lays eggs at the base of rosette leaves (Fig. 8). Larvae tunnel into the root and persist through winter, completing development in the spring (Fig. 9). Pupation occurs inside the root, and adults emerge in mid-summer. Larvae have a swollen thorax and characteristic "J" shape.

Terellia virens, the green clearwing knapweed fly (Diptera: Tephritidae), lays eggs among the florets in open flower heads (Fig. 10). Larvae feed on developing seeds and overwinter inside the flowerheads. This insect has one generation per year.

Two tephritid flies form galls in the flower heads: thebanded gall fly, Urophora affinis (Diptera: Tephritidae), has three faint parallel dark bands on its wings (Fig. 11) and the UV knapweed seedhead fly, U. quadrifasciata, has a dark "UV" pattern on each wing (Fig. 12). Adults lay eggs in flower buds, and larvae convert an immature flower ovary into a gall (Fig. 13). Multiple insects can develop inside one flower head (Fig. 14. Two or more generations can occur during the summer, and larvae of the late summer generation remain in the flower head to hibernate until spring. Urophora quadrifasciata appears to be a better disperser, but U. affinis often displaces it after it arrives at a site. Both flies can be displaced by seed head weevils.

Biological Control Agents

Species	Common name	Host Attacked	Plant part attacked	Distribution	Impact	Notes
Agapeta zoegana	sulfur knapweed moth	spotted knapweed	roots	limited	low	first released in CA in 1993
Bangasternus fausti	broad-nosed knapweed seedhead weevil	spotted, diffuse, squarrose	seeds	widespread	high	first released in CA in 1994
Cyphocleonus achates	knapweed root weevil	spotted knapweed	roots	limited	low	first released in CA in 2001
Larinus minutus	lesser knapweed flower weevil	spotted, diffuse, squarrose	seeds	wide	high	first released in CA in 1995
Larinus obtusus	blunt knapweed flower weevil	meadow	seeds	limited	unknown	first released in CA in 1999
Sphenoptera jugoslavica	bronze knap­weed root borer	diffuse, squarrose	roots	wide	moderate	first released in CA in 1980
Terellia virens	green clearwing knapweed fly	spotted knapweed	seeds	limited	low	first released in CA in 1995
Urophora affinis	banded knapweed gall fly	spotted, diffuse, squarrose	flower gall	wide	moderate	first released in CA in 1976
Urophora quadrifasciata	UV knapweed seedhead fly	spotted, squarrose, meadow	flower gall	wide	low	initially spread into CA on its own, first recovered in 1990

How the Technique Is Employed

Collect adult insects (flies and beetles) in the field by sweep net or aspirator; however, the root weevil will drop off the plant as soon as it sees you, and their camouflage coloring makes them extremely hard to find on the ground unless they move. Adult weevils can also be collected by holding an open container under them and allowing them to fall into it when they try to escape.

Adult sulfur knapweed moths can be collected in the evening by attracting them to a vertically-hung sheet that is illuminated by bright light or black light.

The presence of insects can also be detected by opening mature flower heads or roots and looking for larvae and/or signs of damage.

For additional information see Winston et al. (2012) listed below.

Special Tips

It is not known where the seed head weevils overwinter, but it is likely in sheltered places, such as in tree bark, under rocks or in leaf litter. The seed head flies overwinter inside flower heads as larvae, and may be susceptible to predation by rodents, birds or goats eating flower heads.

Caveats

Knowing what insects are present may help you to integrate other management strategies. The flower head weevil species emerge in the late summer and overwinter as adults away from the plant. The seed head fly species overwinter inside flower heads as larvae. Thus, for example, fall grazing of flower heads by goats would kill most of the flies, but not affect the weevils.

Larinus minutus and L. obtusus have similar biologies and are very difficult to distinguish.

Where Can I Get These?

Some insects may be available from your county Agricultural Commissioner.

Some insects are commercially available through Integrated Weed Control (www.integratedweedcontrol.com) and Weed Busters Biocontrol (www.weedbustersbiocontrol.com).

References

APHIS. Biological Control of Spotted and Diffuse Knapweeds. United States Department of Agriculture, Animal and Plant Health Inspection Service Program Aid Number 1529
https://www.invasive.org/publications/aphis/knapwpub.pdf

Coombs, E.M., J.K. Clark, G.L. Piper and A.F. Cofrancesco, Jr. (eds.). 2004. Biological Control of Invasive Plants in the United States. Oregon State University Press, Corvallis. 467 p.

Pitcairn, M.J. 2018. Weed biological control in California, USA: review of the past and prospects for the future. BioControl 63: 349-359.
doi.org/10.1007/s10526-018-9884-6

Wilson, L.M. and C.B. Randall. 2003. Biology and Biological Control of Knapweed (Getting to Know Knapweeds). USDA Forest Service FHTET-2001-07. 2nd edition.
http://www.invasive.org/weeds/knapweed/chaptr1.html

Winston, R., M. Schwarzlander, C.B. Randall and R. Reardon. 2012. Biology and Biological Control of Knapweeds. U.S. Forest Service Forest Health Technology Enterprise Team. FHTET-2011-05. 149 p.

Woods, D. and V. Popescu. 2009. Diffuse knapweed biological control in California: 1976-2008. In D.M. Woods, (ed.), Biological Control Program 2008 Annual Summary. California Department of Food and Agriculture, Plant Health and Pest Prevention Services, Sacramento, California. pp. 43-46.

Contributing Authors

Dr. Michael J. Pitcairn, Program Manager, California Department of Food and Agriculture

Dr. Lincoln Smith, Research Entomologist, USDA.