Learn About Whitehall Bio Control: 6/19/2024

The Whitehall Biological Weed Control Project (based in Whitehall) of Jefferson County is in full swing for the summer season. The high school-aged students have now started working, and we are busy taking care of the insectary at the Whitehall football field next to the I-90 Whitehall exit, collecting and releasing biocontrol insects, and monitoring for landowners to see how the insects are impacting their noxious weeds. The whitetop mite and the yellow toadflax stem boring weevil release season is over for this year. We hope to have these bioagents available next spring in small numbers for you.

At our insectary garden, we rear insects and mites for several weed species:

1) two species that make galls (swellings) on Russian knapweed, a midge, and a small wasp,

2) two species that attack yellow toadflax, the stem boring weevil mentioned above and a stem galling weevil,

3) on spotted knapweed, we have a seed head weevil, a seed head fly, a root-boring moth, and a root-boring weevil, and

4) we just this spring began rearing the white top (hoary cress) mite mentioned above. It is quite fun to grow the noxious weeds, weed the weeds, fertilize the weeds, to grow the insect/mite bioagents, to kill the noxious weeds finally... If that all makes sense (LOL)

We are now monitoring, collecting, and releasing the Dalmatian toadflax stem-boring weevil (Mecinus janthiniformis). Dalmatian toadflax (Linaria genistifolia) is an herbaceous perennial plant with numerous upright stems 1-4 feet tall. It has a deep taproot with many lateral roots that often connect within the weed patch (this makes it hard to control just by pulling). The leaves are alternate on the stem, thick, waxy, heart-shaped, and blue-green. The flowers are bright yellow, snapdragon-like, and occur in spiked clusters. Each flower forms a round capsule seedhead holding 60-300 small, somewhat triangular seeds. D. toadflax reproduces both via seeds and by spreading roots. Flowering occurs all summer, and the seeds can stay viable in the soil for up to 10 years.

The Dalmatian toadflax stem boring weevil is a small, slim, bluish-black weevil (beetle with a long snout) up to 5mm (1/4 inch) long. The overwintering adults emerge from last year's stems in the spring. Their feeding produces a shot-hole pattern in the upper leaves. They lay their eggs singly (up to 45) in toadflax stems. The white grub-like larvae hatch and feed in tunnels chewed (called boring or mining) into the stems. In late summer, they pupate within the stems. The weevils develop into adults and stay within the stems all fall and winter until spring. The adults emerge from the stems in May to do it all again.

Feeding damage from the adults may stunt the shoots and limit flowering but does not usually significantly damage the plant. Larval feeding, however, damages the water/nutrient-conducting stem tissues, causing the stems to dry out and die or be deformed. This slowly starves the roots. High larval infestations will kill the plants over time. We see a major impact on Dalmatian toadflax stands throughout Montana, usually about a decade after the weevil release. The I-90 corridor from Drummond, Missoula, to Superior and the I-15 corridor from Boulder Hill to the Helena Valley are prime examples of the density of Dalmatian toadflax that has greatly decreased over time because of these fine little weevils.

You have several options to monitor your land for these weevils. First, you can carefully cut last year's stems lengthwise and look for old feeding damage. You may sometimes find dead adults who did not survive the winter entombed within the old stems. You can find adult weevils in May-June by shaking the green stem tips into your hand. The small adult weevils will fall out and "play dead" for a while. After a bit, they will try and crawl or fly away. They are strong fliers. You may also observe the shot-hole feeding damage they make in the leaves of the upper stem. In July and August, you can cut new stems lengthwise and observe larval feeding damage, the larvae, or the pupa within the stems. In the fall and winter, you can cut the stems and see adults overwintering within the stems. The weevils are quite small as they live within the stems for most of the year.

It must not be a very exciting insect life hanging out in the stems all summer, fall, and winter, trying not to freeze to death. We are glad they can survive Montana winters fairly well! If you need a release of these weevils on your land, please call us during normal business hours: Todd 406-498-5236 or Ava 310-237-3353. We are careful to monitor before we release these cool little weevils as most of the folks who call us already have them, and the weevils are slowly starting to decrease their toadflax. The weevil damage is often subtle and hard to see at first. We recommend taking yearly photos at the same time/location to monitor toadflax long-term (put it into your phone calendar!). We will be glad to come to monitor your land and educate you about all aspects of weed control if we can fit you into our busy schedule. The sooner you call, the better. The time to start managing your weeds is TODAY!!