Weevils are beetles with an iconic Jimmy Durante profile. They use their extended schnoz (which contain antennae and mouthparts) to chew holes into plants, flowers, seeds and nuts. The over 60,000 species of weevils generally develop very specific relationships with their host plants – so specific, in fact, that fossilized weevils can be used to reconstruct past climates and ecosystems.
Weevils in an agricultural setting have been lethal pests to a host of plants and seeds, especially the boll weevil, which decimated cotton crops. Many farmers cringe when they hear weevils are on the rise. Their damage is legendary and cannot be overlooked.
However, there are other friendly weevils now being used for biological control of invasive plants. Like Randy Newman sings, “You got a friend in me.”
Farmers need friends when it comes to controlling the allelopathic, invasive, purple-flowering knapweed plant that is taking over the Northeastern landscape, thriving on fallow pasture and hayfields. You can spray it, mow it, plow it under and mob graze it, but it produces so much seed, it’s pretty resilient and opportunistic – especially during dry periods when other plants wither.
Wisconsin-based environmental educator Kate Redmond (aka “the BugLady”) says quarter-inch-long knapweed seedhead-feeding weevils, blunt knapweed flower weevils or knapweed flower-feeding weevils are being released into fields around the country, potentially saving farmers money on control costs.
“There are five species in the genus Larinus in North America, and like the knapweed plant species, our knapweed-feeding weevils come from eastern Europe and the Middle East, all introduced for pest control,” Redmond explained. “The knapweed seedhead-feeding weevil was first released in the U.S. in 1991 in northern Colorado. Its first ‘applications’ were in the Western states, but now it is the bug of choice for spotted knapweed from Oregon to Arizona to Arkansas to Rhode Island to Michigan to Minnesota, and across the southern tier of Canada.”
But there is a local twist: The friendly weevils are here in Central New York without one dollar being spent!
Small but mighty, the knapweed weevils are perfectly evolved biocontrol agents for a particular invasive species. Photo by Troy Bishopp
When Dr. Jonathan Lundgren, founder and director of South Dakota-based Ecdysis Foundation, visited 15 New York farms last year as part of the 1,000 farms Regenerative Farming Project, he said, “Nature will find the control measure if we are patient and build a welcoming organic house where beneficial insects and organisms can thrive.”
The prophecy he spoke of has come with a snout. Recently, this farmer has witnessed thousands of “white” flowerless knapweed seedheads gorged on by these tiny little workhorses in Oneida, Madison and Chenango counties. They have come, perhaps, because the knapweed crop is so abundant and the conditions are right for their survival. Has the house been built?
As the BugLady said, “The weevil’s life is centered around the flower head of a knapweed plant. Courtship happens there as the plants start to flower; the eggs are laid there; the larvae hatch, dig in and feed on the developing seeds. They pupate there (in a case made of plant bits plus ‘secretions’); and the adults feed there, first on the foliage and then on the flowers. In fall, adults shelter in the dead vegetation below the plant, spending the winter there and restarting in spring.”
She added, “Knapweed is very good at what it does, but so are the weevils.”
According to the Cornell University College of Agriculture & Life Sciences, “L. obtusus larvae destroy nearly 100% of the seeds. Any seeds that are not eaten will become a part of the ‘cocoon.’ More than one larva may develop in a seedhead.”
The weevils greatly reduce the number of seeds that lurk on the ground in the seed bank, waiting to grow, and eventually another European – a Buprestid beetle called the bronze knapweed root borer – will pick off the plants.
As more damage is inflicted on the invasive knapweed plant by these tiny weevils, it’s important for farmers to look down and see if they have these biocontrol agents working in their fields for free – and share the news and pictures with other farmers and agronomy staff on control.
Monitoring specific field attributes, history and effectiveness, especially over a few seasons, will help everyone learn control strategies. Let observation be our guide to understanding the power of natural systems.
Now get out there and go scouting.
by Troy Bishopp
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