When I think of weevils, I think of fields of cotton in the South, and I often start humming, “We’re looking for a home…” So you can imagine my surprise when a friend posted a picture of a rather large weevil on her deck. I was even more shocked when the same kind showed up at our house. Sure enough, there are weevils in Montana.

The tale of the boll weevil

Most of us heard about the boll weevils (Anthonomus grandis) decimating the cotton crops of the South in the early 1900s. Native to Central America, the weevil moved north following the seemingly never-ended food source of cotton fields. Each female lays up to 200 eggs five times per year, and the larvae are absolutely voracious. Plus, the period between egg to adult is a mere three weeks. You can imagine the numbers produced during a long, hot summer.  It’s no wonder they were once called “a wave of evil.”

Battling this pest continued well into the 1960s and ‘70s until researchers discovered the value of pheromone traps, as well as figuring out if you sprayed the insects in the autumn when they’re not maturing as quickly, it’s a more effective kill rate. By 2009, boll weevils were eradicated everywhere but in Texas. 

This weevil is a good guy

With a history like that, you can understand my initial concern with the sighting of the rather large and odd looking insect on our porch. Although it’s like so many horror movies begin, the knapweed root weevil (Cyphocleonus achates), a native of Eastern Europe and Asia, was released into the United States in 1987 to combat spotted and diffused knapweed. 

The ongoing knapweed battle

This weevil is host-specific, therefore it does not feed upon desirable plants and is a useful tool in the battle of this noxious weed. They’re rather large, typically over 1/2 inch long, and are gray with brown mottling. They blend in perfectly with the knapweed. Females lay three eggs a day and up to 100 during her brief life. The larvae feed upon the roots weakening, and hopefully, killing the knapweed plant. 

Sheep and Goats are Adorable Weed Eaters

These insects do overwinter and it’s legal to capture them if you have a knapweed problem and don’t want to use herbicides. The recommendation is to release 50 to 100 insects per square meter. This doesn’t mean that’s how many you need in each small area to be effective, but it’s how the researchers discovered they are the most effective. 

For anyone who has battled knapweed, you’ll take any tool available to keep ahead of it. With the ability to produce over 1000 seeds per plant — many with viability lasting at least a decade — each plant is a powerhouse of reproduction. The deep tap roots don’t give up easily when it comes to pulling them. And herbicide is expensive, must be timed just right, and is not good for the environment. Even though releasing any insect-related to one that caused such crop damage as the boll weevil sounds like a very bad idea, I’m now very happy to see the knapweed root weevil. Next time I’ll be sure to take it over to a knapweed plant to find them a new home.