Insect Description

Eggs are laid in clusters under tansy ragwort leaves.  Initially yellow the eggs turn black with maturity.  Larvae are initially orange turning orange and black striped as they develop through three to six instars.  Larvae in the final instar are up to 25 mm long.  Adult moths have black forewings wings with two red dots and red on wing borders.  Hind or underwings are solid red.  Moth wingspan can be up to 4 cm.

 Field Identification

Larvae can be found in groups (10-30 larvae) on bolting or flowering tansy ragwort plants.  Plants may be completely defoliated.

Life Cycle

Pupae overwinter in shallow soil or leaf litter.  Adults emerge, mate and lay eggs in late spring or early summer.  Larvae begin to feed on undersides of leaves, eventually feeding on stems and buds as they mature.  Larvae in the final instar retreat to the soil to pupate in late summer.


Larvae can completely defoliate plants causing stress and reduced storage in the root system. Flowering and seed production may be greatly reduced.  This agent appears to be more effective in habitats with a drier, and shorter growing season.


Interstate shipment of the moth is prohibited due to observed non-target feeding on native species.  It may be possible to distribute insects that are already present in Montana to new locations.  For more information on this, contact the Montana biocontrol coordinator.


This biocontrol works well in conjunction with Longitarsus jacobaeae.  It has been shown to be more effective in areas shorter and drier growing seasons.

 Using the Agent

If restrictions against redistribution of this insect are not in place, insects can be collected in the larval stage and moved to intended areas.   Larvae can be collected by shaking plants to force larvae to drop into a collection pan or bucket. From 50-100 larvae should be released on sites where infestations are heavy and immediate eradication is not the goal.  Establishment can be monitored by looking for larvae the following summer and looking for defoliation of plants.

Where the biocontrol is well-established other physical, chemical or mechanical control methods should be considered carefully so as not to interfere with insect populations.

For more information on control methods and herbicide recommendations contact your local weed district or Extension Service.



Winston, R., C.B. Randall, R. D. Clerck-Floate, A. McClay, J. Andreas, M. Schwarzlander.  May 2014.  Field Guide for the Biological Control of Weeds in the Northwest.

Washington State University Integrated Weed Control Project.  Biological Control Agents. Accessed February 9, 2017.