Linaria vulgaris (common toadflax)

From Zouhar (2003), quoting Lajeunesse (1999):

Equipment should be checked for seeds and root pieces, and cleaned before moving it from infested to uninfested areas.

Livestock moving from infested areas to uninfested areas should be held in corrals or small pastures, for six days for cattle and 11 days for sheep, to allow any viable seeds to pass through the digestive tract.

Livestock holdings and areas where fill dirt has been imported should be monitored for L. vulgaris establishment.

Avoid buying seed or feed that could be contaminated with L. vulgairs seeds.

L. vulgaris is classified as a noxious weed or weed seed in many USA states, including Alaska, Colorado, Idaho, Montana, North Dakota, New Mexico, Nevada, Oregon, South Dakota, Utah, Washington and Wyoming (USDA-ARS, 2015), as well as several Canadian provinces (Zouhar, 2003).

Vigorous, healthy plant communities can often out-compete L. vulgaris seedlings and thus prevent their establishment. In areas where the weed is already established, any initial control needs to be followed by establishment of well-adapted, desirable plant species that will provide competition throughout the season at all levels of the soil-root profile. This can provide longer-term suppression. Proper grazing management to maintain the competitive ability of these plant communities is important for long-term control of L. vulgaris (Zouhar, 2003).

Seedlings can be readily destroyed by cultivation, but where plants already have an established root system, repeated cultivations will be needed over a period of 5-6 years to eradicate L. vulgaris. Saner et al. (1995) referred to successful control by means of intensive shallow cultivation, requiring at least 2 years, with 8-10 cultivations in the first year and 4-5 in the second year. Further control methods will be needed to prevent regeneration from seeds in the soil, perhaps for a further 10 or more years (Zouhar, 2003).

Mowing is unlikely to be effective, especially where competing vegetation is also removed in the process.

Zouhar (2003) reviewed the subject as follows: ‘Several insect species that feed on L. vulgaris have been purposely or accidentally released in the USA and Canada. The flower feeding beetle Brachypterolus pulicarius and seed-feeding Gymnetron antirrhini (= Rhinusa antirrhini) appear to be the most important insects for reducing seed production. B. pulicarius larvae develop inside floral ovaries, and adults feed on buds and young stems. B. pulicarius can reduce seed production by 80 to 90%. G. antirrhini can reduce seed production by 85-90%. McClay (1992) recorded a 74% reduction in seed viability due to infestation by Brachypterolus pulicarius; however, Mason and Gillespie (2013) stated that ‘B. pulicarius and G. antirrhini are already widespread in Canada but the impact is unknown. Egan and Irwin (2008) also obtained very variable results. There were reductions of seed and shoot production of L. vulgaris, but these were modest and minimal at high densities of the weed. Harris (1981), however, commented on a decline of the weed (in Canada) due to Brachypterolus pulicarius, Gymnetron antirrhinii and Calophasia lunula, and Quartes (2007) stated that seed weevils (such as G. antirrhini, have already led to the decline of L. vulgaris throughout Canada. The moth Calophasia lunula, a long-term accidental introduction to Canada, and two other insects, the moth Eteobalea serratella (approved for release in Canada in 1991) and the root-galling weevil Gymnetron linariae [Rhinusa linariae](approved for release in Canada in 1995) are referred to by McClay and De Clerck-Floate (2001), but were apparently not effective enough to deserve comment in later publications. Also, the seed capsule weevil R. neta appears to occur only in scattered populations in the eastern and western USA and Canada (Wilson et al., 2005).

Among other agents reviewed by Mason and Gillespie (2013), Mecinus janthinus has been released at numerous sites in Canada since 1996, but with mixed results, possibly because the closely related M. janthiniformis has sometimes been involved, which is more specific to L. dalmatica. Toševski et al. (2013) have now described a fast and accurate way of distinguishing the two Mecinus species haplotypes using PCR-RFLP diagnostic assay of the mitochondrial cytochrome oxidase subunit II (COII) gene. At a site in Alberta where M. janthinus was released in 1996, observations in 2012 found that the population of L. vulgaris had declined to a very low density, with the few remaining stems heavily infested with M. janthinus (Mason and Gillespie, 2013). More recently studies have started on another stem-boring weevil, M. heydenii (Tosevski et al 2016).

More recently, the stem-galling weevil weevil Rhinusa pilosa has been approved for release in USA (Gassmann et al., 2014). Although not quite specific to L. vulgaris, it is considered to be a suitable biocontrol species due to its high host specificity, minimal risk to related species in North America, robustness during rearing, impact on host growth and reproduction through galling and expected population release from the effects of a European gall intruder, the inquiline weevil Rhinusa eversmanni (Mason and Gillespie, 2013). 

The considerable genetic diversity in L. vulgaris (Ward et al., 2008) may help to explain its variable response to biocontrol organisms.

Due to the variable regulations around (de-)registration of pesticides, we are for the moment not including any specific chemical control recommendations. For further information, we recommend you visit the following resources: