Montana has several home-invading weevils that come into our homes in April through November. See pictures below. They are harmless to humans and structures.

They are root weevils, and the biology for most species is poorly understood. Several root weevil species are pests on ornamental plants. The most common ones we are seeing right now are small root weevils, Cathormiocerus spinosus (Fig. 1).

Cathormiocerus spinosus

Figure 1. Cathormiocerus spinosus (Size=2.5mm). Photo by 2016 Stephen P.L. Luk


many deciduous shrubs, inside buildings/homes


The characteristic damage from adults includes notching along the leaf margins from chewing (Fig. 4). The larvae feed on plant roots. The damage is primarily cosmetic (no treatment necessary).

Black vine weevil

Figure 2. black vine weevil (Size=6-8mm). Photo by C. Moorehead,

Strawberry root weevil

Figure 3. Strawberry root weevil (Size=4-7mm). Photo by W. Cranshaw, CSU,



Most often, no management is necessary. These weevils are difficult to control and can come in by the dozens for a couple weeks at a time in the late summer. Any small opening can allow these beetles to “squeeze” into the building. You will likely want to do the following:

  • Make sure door sweeps are installed at the base of the exterior doors and that they fit tightly.
  • Seal all cracks in siding.
  • One point of entry is the meeting of foundation and siding of a building. Caulk or fill this juncture.
  • Vacuum up all the adults that you can in the meantime.
Root weevil notching damage on lilac

Figure 4. Root weevil notching damage on lilac. Photoa by W. Cranshaw,


Chemical controls have not been very effective for these beetles because it is difficult to target their overwintering sites and their point of entry. However, you can do barrier sprays in the early fall to kill the insect before they enter the building. Here are some tips-

  • The chemical treatments need to be in place before the invasion is in full swing (before late fall).
  • The base of all doors and window should be treated.•Apply materials to a 2-6 foot wide band along the soil around the foundation and 2-3 feet up the foundation wall.
  • Sprays work well on surfaces.
  • Granular insecticides or sprays can be used for ground treatments.                                                                                                                                                                                                                                                                                                                                                         MSU Extension pest management logo

By Laurie Kerzicnik ([email protected])

Disclaimer: These recommendations are provided only as a guide. It is always the pesticide applicator’s     responsibility, by law, to read and follow all current label directions for the specific pesticide being     used. Due to constantly changing labels and product registration, some of the recommendations given in this writing may no longer be legal by the time you read them. If any information in these recommendations       disagrees with the label, the recommendation must be disregarded. No endorsement is intended for products   mentioned. The authors and Montana State University assume no liability resulting from the use of these     recommendations.

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