Knapweed Root Weevil and Seed Head Removal Update

Knapweed Root Weevil and Seed Head Removal Update

January 9, 2017

Jeff Clarke reports on the status of Knapweed seed head removal, and Knapweed Root Weevil relocation efforts.

Knapweed Root Weevil Relocation Project Summary Knapweed Seed Head Removal Project Summary

Knapweed blooms from June through September and may produce a few hundred flowers per plant. Each plant can live nine years and their seeds may remain viable for seven years. We think that if we remove knapweed flowers from an area for at least ten years, the seed bank will vanish, the existing plants will perish, and native plants on the landscape will have a better chance to flourish.

hoto shows the same hillside that we cut and did not cut one month after the treatment. The cut hillside has few flowers. The uncut area is choked out by knapweed biomass.

Female knapweed root weevils will deposit up to 100 eggs on the root collars of many plants. When the eggs hatch, the larvae burrow into the knapweed’s root and feed until they emerge as an adult the following summer. The larvae hollow out the root as they feed which makes it more difficult for the plant to produce biomass.

We found that some areas on the ranch had rich populations of weevils while other areas had few or no weevils at all. Due to their flightless nature, their populations don’t spread fast. For the last five years we’ve collected weevils from the prolific populations and transferred them to areas where few were detected.

This map shows where we relocated weevils in the boondocks. Yellow pins represent 2012 release sites, blue flags represent 2014 release sites.

In 2013 we added 5,000 weevils to the Clubhouse Floodplain to boost the population. In 2016 there appeared to be fewer knapweed flowers where we had released weevils.

In 2014 and 2015 we spread close to 22,000 weevils across the Summit of Baldy in an effort to establish a healthy population to attack the cut and uncut knapweed invasion. In 2015 we added another 9,000 weevils to a healthy knapweed invasion on 22 Bench.