Verbascum Thapsus - Agricultural Impacts and Control

Agricultural Impacts and Control

Because it cannot compete with established plants, Great Mullein is no longer considered a serious agricultural weed and is easily crowded out in cultivation, except in areas where vegetation is sparse to begin with, such as Californian semi-desertic areas of the Eastern Sierra Nevada. In such ecological contexts, it crowds out native herbs and grasses; its tendency to appear after forest fires also disturbs the normal ecological succession. Although not an agricultural threat, its presence can be very difficult to completely eradicate, and is especially problematic in overgrazed pastures. The species is legally listed as a noxious weed in the American state of Colorado (Class C) and Hawaii, and the Australian state of Victoria (regionally prohibited in the West Gippsland region, and regionally controlled in several others).

Despite not being an agricultural weed in itself, it hosts a number of insects and diseases, including both pests and beneficial insects. It is also a potential reservoir of the cucumber mosaic virus, Erysiphum cichoraceum (the cucurbit powdery mildew) and Texas root rot. A study found V. thapsus hosts insects from 29 different families. Most of the pests found were western flower thrips (Frankliniella occidentalis), Lygus species such as the tarnished plant bug (L. lineolaris), and various spider mites from the family Tetranychidae. These make the plant a potential reservoir for overwintering pests.

Other insects commonly found on Great Mullein feed exclusively on Verbascum species in general or V. thapsus in particular. They include mullein thrips (Haplothrips verbasci), Gymnaetron tetrum (whose larva consume the seeds) and the Mullein Moth (Cucullia verbasci). Useful insects are also hosted by Great Mullein, including predatory mites of the Galendromus, Typhlodromus and Amblyseius genera, the minute pirate bug Orius tristicolor and the mullein plant bug (Campylomma verbasci). The plant's ability to host both pests and beneficials makes it potentially useful to maintain stable populations of insects used for biological control in other cultures, like Campylomma verbasci and Dicyphus hesperus (Miridae), a predator of whiteflies. A number of pest Lepidoptera species, including the Stalk Borer (Papaipema nebris) and Gray Hairstreak (Strymon melinus), also use V. thapsus as a host plant.

Control of the plant, when desired, is best managed via mechanical means, such as hand pulling and hoeing, preferably followed by sowing of native plants. Animals rarely graze it because of its irritating hairs, and liquid herbicides require surfactants to be effective, as the hair causes water to roll off the plant, much like the lotus effect. Burning is ineffective, as it only creates new bare areas for seedlings to occupy. G. tetrum and Cucullia verbasci usually have little effect on V. thapsus populations as a whole. Goats and chickens have also been proposed to control Mullein. Effective (when used with a surfactant) contact herbicides include glyphosate, triclopyr and sulfurometuron-methyl. Ground herbicides, like tebuthiuron, are also effective, but recreate bare ground and require repeated application to prevent regrowth.