Since 1971, Oregon has used a flea beetle against tansy ragwort; 7 years later, the state said the biocontrol had cut infestations by 90%

Oregon's real weapon against a toxic weed has been a beetle smaller than a grain of rice. In 1971, the Oregon Department of Agriculture introduced a tiny flea beetle, Longitarsus jacobaeae, to control tansy ragwort, a poisonous plant that was costing ranchers around five million dollars a year in livestock losses, according to a 2022 factsheet. Seven years later, officials said the beetle had reduced ragwort levels by 90 percent. More than five decades on, the Oregon Department of Agriculture still lists the flea beetle as an active, ongoing part of its ragwort management program, not a one-time fix.

What is tansy ragwort, and why did it scare farmers

Tansy ragwort, a weed from Europe and Asia, was discovered in Oregon in 1922 and spread rapidly along the Pacific Coast. It grows two to six feet tall with seeds that can last for as long as fifteen years in the soil; hence, even after clearing the land, there is still a possibility of finding the weed back in the area within fifteen years or more. However, the greater risk is its harm to animals. The weed contains alkaloids that damage the livers of cows and horses. Even dead tansy ragwort can contaminate milk and honey, causing serious economic losses for farmers.

Meet the beetle that was hired for the job

Instead of relying only on herbicides, Oregon turned to biological control, or "biocontrol": introducing an insect to target the weed while minimizing harm to other species. Three insects were brought in over time: the cinnabar moth in 1960, a seed-eating fly in 1968, and finally the flea beetle in 1971, which emerged as the champion. Adult beetles chew small holes in the leaves, which can damage young plants, but the larvae do the most serious damage underground by feeding on the root crown, which helps the plant survive the winter. Destroying it kills that individual plant outright, though the seed bank can keep reintroducing new plants for years afterward, which is why beetle populations have to stay established season after season rather than working as a one-time treatment. A 2024 study published in The Canadian Entomologist found that the beetle was being studied as a possible tansy ragwort agent as early as the 1960s, with the study tracking how the beetle was released and redistributed across sites in Canada over the following decades.

So did the 90 percent figure actually hold up

While a figure like '90 percent' can be misleading without context, independent field research, both in Oregon and in similar coastal ecosystems where the same insects were released, supports Oregon's report. A 1990 study in the journal Entomophaga, 'Biological control of Senecio jacobaea in northern California, an enduring success,' found that ragwort populations near Fort Bragg, California, dropped sharply and stayed suppressed for years after flea beetles and cinnabar moths were introduced, evidence that the effect wasn't a short-lived blip. Similarly, field research conducted in the Cascade Head Scenic Research Area in Oregon found that when the flea beetles and cinnabar moths were introduced into the area from 1978 to 1980, the ragwort density dropped to only 3 percent of its original density within a few years.

Where the beetle hasn't worked

Biocontrol is not foolproof. The Oregon Department of Agriculture says the goal was never to wipe out tansy ragwort entirely, only to reduce it to a level that no longer damages farms and ranches. In eastern Oregon, for example, the seed-eating fly does most of the work because monitoring revealed that the flea beetle never really got a foothold. There’s a genetic wrinkle as well. The same 2024 Canadian Entomologist study found that a closely related but different beetle species, Longitarsus flavicornis, had been accidentally released along with the target species in British Columbia in the 1970s, a reminder that even a bug release involves careful scientific work.

The takeaway

It's a rare case of a chemical-free fix holding up over the long run; the beetles reproduce on their own, so ranchers aren't paying for repeat treatments the way they would with annual herbicide spraying. But the program only worked because Oregon kept tracking infestation rates for years afterward, catching problems like the eastern Oregon gap and the accidental beetle mix-up in British Columbia instead of assuming the job was done in 1978. A weed that once cost the state millions a year is now a minor, manageable problem, maintained by an insect most people will never see or hear about.