There are 5 species of permitted and 3 accidentally introduced insects that attack yellow toadflax (Linaria vulgaris) and/or Dalmatian toadflax (L. dalmatica) in the USA, but only one of these is permitted in CA.

The stem weevil was initially believed to be one species of weevil ("Mecinus janthinus") that attacks the stems of both yellow and Dalmatian toadflax. However, molecular genetic analysis has clearly shown that there are two species of weevil: Mecinus janthiniformis (Coleoptera: Curculionidae) prefers Dalmatian toadflax, and M. janthinus prefers yellow toadflax. Both species were released in Canada and east of the Rocky Mountains (first in Montana in 1996). On its own, M. janthiniformis has spread into northeastern California (Modoc, Siskiyou, Lassen, and Shasta counties). CDFA issued a permit to intentionally release M. janthiniformis into areas south of 34.82° N latitude (the northern border of Kern County) because of concern that this weevil can attack a native snapdragon, Antirrhinum virga, which occurs in the Coastal Mountains in Lake, Napa, and Colusa counties.

Adult M. janthiniformis are good fliers and have been observed to disperse 2 miles in 4 years. There is one generation per year. Adults overwinter inside the plant stems, emerge in spring, feed on leaves and lay eggs in the stems. Larvae tunnel inside the stems and pupate inside the stems. This agent has been extremely effective in reducing Dalmatian toadflax populations by over 90% at a large infestation at Hungry Valley State Vehicular Recreation Area near Gorman, Los Angeles County, in about 4 years after release. In contrast, most populations of Dalmatian toadflax in northern California are relatively small (less than 5 acres) so the impact of the stem weevil at these locations has not been as dramatic.

The seed weevil, Rhinusa neta(Coleoptera: Curculionidae), is an accidental introduction on yellow and Dalmatian toadflax. It is a strong flier and spread on its own into California. It was first recovered in 2017 and occurs on almost all known infestations of Dalmatian toadflax in northern California. The adult female deposits eggs on the seed capsule, and the larvae consume the developing seeds. It has one generation per year. Field observations show that R. neta destroyed over 70% of seed in one toadflax population in Trinity County.

Biological Control Agents


Common name




Mecinus janthiniformis

stem weevil



First release in southern CA in 2008; not intentionally released in northern CA where it was first recovered in 2009 - Not a permitted agent north of Kern County

Rhinusa neta

seed weevil



Not intentionally released, first recovered in CA in 2017 - Not a permitted agent

How the Technique Is Employed

Check to see if the stem weevil is present at your site. Look for signs of insect damage to the stems (holes for laying eggs, tunneling in the pith) or adult feeding holes on the leaves. Adult exit holes should be apparent on old, prior year stems. If the weevil is present, then it is not worth releasing more.

Overwintering adults can be collected by cutting stems that were infested the previous summer (presence of egg holes). These can be held in a refrigerator until ready to release. Release adults when the plants begin to grow in the spring. Alternatively, active adults can be collected in the spring, when plants are bolting, using a sweep net or by knocking them into a container. Provide leaves for the adults to eat and hide in. Keep them cool and release them at the new site as soon as possible.

Detailed information and photographs can be found in (Sing et al. 2016).

Special Tips

This biological control agent will multiply if it has a suitable habitat, and the old stems are undisturbed to allow adults to survive until spring emergence.


There are no biological control agents permitted for control of Dalmatian toadflax north of Kern County. Both weevils are widespread and provide heavy feeding damage to their host plant. The seed weevil, R. neta, has been found in seed capsules of a native snapdragon, Antirrhinum virga, in Colusa County.

Killing plants in the middle of the growing season is likely to kill the next generation of weevils, so it is important to leave some areas untreated.

Mowing or burning would have the same effect of decimating the weevil population; however, cutting flower panicles to prevent seed set would not affect the stem weevil.

Herbicides that kill toadflax before it bolts will deprive the insects of a host plant, but will give them time to search for the remaining plants that have not been killed. Thus, biological control can be complementary to herbicide or other control treatments, especially if there are areas that are not treated (e.g., too difficult to access or too environmentally sensitive to treat).

Where Can I Get These?

There are no biological control agents currently permitted for use on Dalmatian toadflax north of Kern County. Contact the CDFA Biological Control Program about release information for toadflax populations from Kern County south.


De Clerck-Floate, R.A. and S.C. Turner. 2013. Chapter 52. Linaria dalmatica (L.) Miller, Dalmatian Toadflax (Plantaginaceae). In: Mason, P.G. and Gillespie, D.R. eds., Biological Control Programmes in Canada 2001-2012. CABI, pp. 342-353.

Sing, S.E., R. De Clerck-Floate, R.W. Hansen, H. Pearce, C.B. Randall, I. Tosevski and S.M. Ward. 2016. Biology and biological control of Dalmatian and yellow toadflax. FHTET-2016-01. US Department of Agriculture, Forest Service, Forest Health Technology Enterprise Team, Morgantown, WV. 141 p.

Contributing Authors

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

Dr. Lincoln Smith, Research Entomologist, USDA