Biological Control

• Cinnabar moth
• Ragwort seed fly
• Ragwort flea beetle

Cinnabar moth

Biological control of tansy ragwort in the U.S. was initiated in 1959 with the California release of the cinnabar moth, Tyria jacobaeae L. (Lepidoptera: Arctiidae). This agent is now well-established in California, Oregon, Washington, and in northwestern Montana. The cinnabar moth flourishes in lower elevation (below 3,000 ft./900 meters) open canopy areas with warm, sunny summers and large, high-density infestations of tansy ragwort. Adult moths are 0.60-0.88 in (15-22 millimeters) long with a wingspan of 1.08-1.40 inches (27-35 millimeters) and have a distinctive appearance: the black forewing is marked by a few irregularly-shaped crimson spots and a crimson line along the outer edge of the wing while the hind wing is entirely crimson (see Figure 4).

Figure 4. Cinnabar moth. Image: Eric Coombs, Oregon Department of Agriculture. Available at Bugwood.org

Cinnabar moths typically emerge by late spring, mate and finish depositing eggs by mid-summer. Moths can be roused into flight during daylight hours when host vegetation is disturbed. Eggs are laid on the underside of rosette leaves in small groups. The eggs hatch after several weeks’ development and emerging larvae begin feeding on foliage closest to the hatch site. The damaging larval stage of this agent is easily recognized by its alternating bands of bright orange and black (see Figure 5).

Figure 5. Larval stage of cinnabar moth. Image: Eric Coombs, Oregon Department of Agriculture. Available at Bugwood.org

Pupation begins in late summer when the fully-grown fifth-instar larvae seek out suitable sites in debris or soil, or under bark; the cinnabar moth overwinters as a 0.80-1.00 inch (20-25 millimeters) long dark reddish-brown pupa.

Roving groups (10-30 individuals) of the gregarious larval stage of this agent aggressively feed on the leaves, flowers and apical meristems of bolting tansy ragwort, leaving plants stripped of all foliage. Larval feeding can lead to significant reductions in seed production and stand density, although impact varies considerably. The larval stage of this agent is optimal for redistribution onto uncolonized infestations of tansy ragwort. Caterpillars are easily collected by shaking plants over collecting pans (such as kitty litter trays); retain larvae under cool, dry and uncrowded conditions in ventilated (with pin holes) containers provisioned with fresh host plant material for as short a period as possible before releasing. Because endogenous toxic alkaloids sequestered from tansy ragwort are present in both the larval and adult stage of this agent, implications for livestock health and management should be carefully considered when choosing sites for releasing the cinnabar moth.

The efficacy of the cinnabar moth alone in controlling tansy ragwort is limited with successful control reported only when larval defoliation was accompanied by favorable environmental factors (frost). Although this agent is highly effective in combination with the ragwort flea beetle, its deployment has been surrounded by controversy. Non-target attack on the exotic weed common groundsel Senecio vulgaris, two native species Senecio triangularis and Packera pseudaurea (formerly Senecio pseudaureus) and the ornamental silver ragwort, Senecio bicolor, have been reported when moth population densities outstrip local tansy ragwort resources.

Ragwort seed fly

The ragwort seed fly Botanophila seneciella Meade (Diptera: Anthomyiidae) was first introduced to the U.S. in California in 1966 and was the first tansy ragwort agent to become well-established east of the Cascades. Botanophila seneciella was released to additively improve tansy ragwort biocontrol by bolstering the low levels of control realized with the cinnabar moth. Adult flies (see Figure 6), 0.20-0.28 inch (5-7 millimeters) in length, emerge in spring and deposit eggs on tansy ragwort flower buds through early summer .

Figure 6. Ragwort seed fly - adult. Image: Eric Coombs, Oregon Department of Agriculture. Available at Bugwood.org

The 0.16-0.24 inch (4-6 millimeters) long creamy-white larva tunnels into the flowerhead and throughout the receptacle, eventually moving back up to the seed head. Evidence of infestation by this agent is obvious: larval feeding generally destroys all seed within mined seed heads, and flowers under attack are typically marked by frothy spittle. Mature larvae exit host seed heads in late summer to pupate in the soil. The ragwort seed fly overwinters as a dark brown 0.2 inch (5 millimeters) long pupa. Because this agent shares the same habitat preferences as the cinnabar moth, their joint release is often marked by resource competition for favorably located host plants. The seed fly usually loses out because the moth’s phenology allows it to begin ovipositing earlier so that caterpillars have generally stripped all flowerheads from plants in open canopy areas by the time the seed fly is ready to begin oviposition. Reproductive success is significantly reduced when resource competition forces the fly to lay eggs on host plants located in moist, shaded areas. This agent has self-distributed comprehensively throughout the Pacific Northwest; redistribution is generally unnecessary but if desired is best accomplished by transplanting fly-infested plants in late spring.

Ragwort flea beetle

The light gold-colored ragwort flea beetle (see Figure 7), Longitarsus jacobaeae Waterhouse (Coleoptera: Chrysomelidae), first introduced to the U.S. in 1969, is now established in Montana, Washington, Oregon, and California.

Figure 7. Ragwort flea beetle - adult. Image: USDA ARS European Biological Control Laboratory. Available at Bugwood.org

Intensive foliar feeding by adult flea beetles, termed "shot holing," is a clear indicator of whether or not a tansy ragwort stand has been colonized by this agent. Ragwort flea beetles introduced to the U.S. before 2002 were collected in Italy; the so-called Italian strain of this agent is credited with exceptional control and continued suppression of tansy ragwort infestations west of the Cascades. A Swiss strain of this agent with a completely different phenology than the Italian strain and believed to be better adapted to higher elevations, colder winters and shorter growing seasons typical of tansy ragwort infested areas east of the Cascades was released in Montana in 2002.

Adult Italian-strain ragwort flea beetles emerge briefly in spring before returning to the soil to estivate (summer dormancy) until prompted by late summer/early fall rains to begin mating. Eggs laid from October through November produce slender white larvae 0.08-0.16 inch (2-4 millimeters) in length that feed throughout the winter on the host plant roots. Larvae leave the roots in the spring to pupate in the soil. Pupae are white and 0.08-0.16 inch (2-4 millimeters) long.

Swiss-strain adult flea beetles emerge from their pupation sites in the soil in late spring to early summer and begin feeding, mating and laying eggs immediately. Development in the eggs of Swiss-strain beetles is delayed through the rest of that summer, fall and winter and does not begin until the following spring. Larvae feed on and in tansy ragwort roots until pupating in the fall. The Swiss strain of the ragwort flea beetle is unique in that it overwinters both in the egg and pupal stage. Adults of both strains are 0.08-0.16 inch (2-4 millimeters) in length with the males approximately 0.04 inch (1 millimeter) shorter than the females.

Tansy ragwort rosettes appear to be the most susceptible stage of the weed to flea beetles as significant control is realized from both larval root mining and adult foliar feeding that specifically targets vegetative (pre-reproductive) plants. The adult stage of the flea beetle is best suited to successful collection for redistribution, either by vacuum collection or sweeping, October-November for the Italian strain and mid-summer for the Swiss strain.

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Last Modified: 06/18/2009