Learn About Whitehall Bio Control: 6/12/2024

A weed is any plant that grows where you don't want it to grow. A noxious weed is a plant that causes environmental and/or economic damage.

The Whitehall Project has now made six releases of the whitetop (hoary cress) flower galling mite (Aceria drabae) in the Whitehall Area. We are excited about the future potential of this bioagent. Time will tell...

Our next insect available in small numbers is the yellow toadflax stem boring weevil (Mecinus janthinus). Yellow toadflax (butter and eggs) is an herbaceous perennial with vertical and horizontal creeping roots. The whole patch of this Montana state-designated noxious weed will often be connected via the root system. Stems are erect, branched, and six inches to three feet tall. Leaves are pale green, narrow, hairless, pointed at both ends and about two inches long. Yellow toadflax flowers are clustered, bright yellow, tinged orange, and are snapdragon-like in appearance. The weed occurs in scattered patches throughout Jefferson County and the surrounding area.

A weevil is a beetle with a long snout. These small, skinny, dark brown or black-yellow toadflax stem boring weevils emerge from toadflax stems in early spring. The adult weevils feed on yellow toadflax leaves. They lay eggs within the stems of the plant. The larvae soon hatch and feed on the inside of the stem, causing tunnels and other damage. The weevils pupate within the stem in late summer. They soon develop into adults but stay within the stem all fall and winter until they emerge again in early spring.

The larval stem mining (boring) deforms the stems, suppresses flowering, and may often kill the plant. Adult feeding can greatly decrease seed production and further stress the plant when in high numbers.

We collect these weevils by tapping the stems over a collection bucket or pan (a method called pan-taping). The weevils drop into the container and "play dead" for a while. We sort them into releases of 100-200+ weevils and transport them in paper containers similar to food take-out containers. We add some yellow toadflax leaves to the containers for them to cling to and feed on. We can store them for up to a week in a refrigerator or a cooler with ice packs.

To release the insects, dump them out of the container onto a patch of yellow toadflax. They are strong flyers and will naturally disperse over a large area within a few years. If you release on a hillside, it is better to release on the downhill part of the weed patch, as the weevils tend to fly uphill while dispersing. They have the best winter survival rates, where the snow is deep enough to cover and insulate the stems. However, the weevils seem to survive in great enough numbers to severely damage yellow toadflax at most Montana locations and elevations.

Although most of our yellow toadflax stem boring weevil collection sites have crashed (meaning the weeds are mostly gone and thus there are few weevils to collect) we still can collect a few releases in the spring. If you have a patch of yellow toadflax on your land and would like to try using the insects to control the weed, feel free to give us a call during normal business hours to request a release. Please call the project coordinator, Todd, at 406-498-5236 or AmeriCorps volunteer Ava, at 310-237-3353.

We are also attempting to rear the yellow toadflax stem galling weevil (Rhinusa pilosa) in our insectary located in Whitehall. These hairy brown weevils cause a gall (swelling) in the stem that usually stops stem growth and flower formation. We have made a few releases in the area but are not having much success in rearing collectable numbers, so we do not currently have any available for release. Hopefully, in the future...

To help keep weeds from spreading, always wash the undercarriage of your car or ATV after you have been in areas with noxious weeds. Weed seeds, flowers, or roots can be caught on the vehicle and fall off in other areas, thus spreading the problem.