Biological Pest Control on Spotted Knapweed

By: Gisa Seeholzer

Flower Weevil Enjoying Spotted Knapweed (Photo Courtesy of Elisa Christensen)

Biological pest control offers several benefits over chemical methods. It's more environmentally friendly, as it doesn't involve the use of toxic chemicals that can harm the ecosystem. It's also a sustainable approach, as it relies on natural predators and parasites to regulate pest populations. Additionally, biological pest control is often more targeted, affecting only the intended pest species and minimizing harm to beneficial organisms. This method also has the potential to be cost-effective in the long term, as it can lead to reduced reliance on expensive chemical treatments.

The utilization of L. obtusus, flower weevils, for biological control of spotted knapweed, Centaurea stoebe, in Los Padres National Forest involves introducing the beetle to the area. L. obtusus has been found to effectively control the population of spotted knapweed by feeding on the plant and inhibiting its growth and spread. The beetle's feeding activity reduces the vigor and reproductive capacity of the knapweed, contributing to its control without causing harm to other native plant species. This biological control method has been studied and implemented as an environmentally friendly approach to managing the invasive species in the forest ecosystem.

I had an opportunity to visit the spotted knapweed hot spots with PMC Resident Elisa Christensen. Christensen spoke to our weevil supplier in Montana (where knapweed is most invasive), and they had great news. They said that since spotted flower weevils were placed in certain hotspots last fall, and as far as we know, we have yet to release them in 2023, there is a good chance that they were released on the mountain in years past. This would have given them a few years to propagate and spread, which they do at a pretty good rate, given that flower weevils have wings. Christensen asked Joe Ladin, PMCPOA EC Officer, about this, and he said he remembered some discussion of weevils years back but couldn't recall the details. 

Flower Weevil (Photo Courtesy of Elisa Christensen)

On Monday, July 15th, Christensen and I checked several knapweed hotspots for flower weevils. If they are abundantly present, Christensen believes it unnecessary to order any. (FYI, the flower weevils are $90 per release, and the root weevils are $160.)

When we discovered the flower weevil present at several hotspots, we were very excited to see that the population did not die out after their release into the area over four years ago. 

"The big key to all this is keeping the knapweed from being ground up in the green waste site and sent back out as mulch into our properties. I understand the cost of hauling the green waste off the mountain to Lebec would be considerable. Still, perhaps we could have a brainstorming session with the transfer site employees and EC on how we can greatly reduce the likelihood of the knapweed returning to the community," says Christensen. 

What to look for in the coming weeks...

Starting this week and in the future, you will begin to see the pink flowers. Once this starts, and the flowers begin to turn brown and go to seed, it is critically important that we reduce the spread of the plant as aggressively as possible. If you see weevils on your knapweed, move them onto another plant or relocate them before throwing out the knapweed you collect throughout. 

As a reminder, please pick up the yellow bags from the PMCPOA EC Office for collecting knapweed and dispose of them properly at the transfer site.