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Diffuse Knapweed

A noxious dilemma

Public domain photo via Wikimedia Commons
Public domain photo via Wikimedia Commons

Diffuse knapweed is a very common noxious weed that will take over any area that has the right conditions for it. The seeds are dispersed through agriculture, by wind and along waterways, and will attach to wildlife and pets. Diffuse knapweed can also be known as Centaurea diffusa. It prefers semi-arid and arid environments, light, dry, porous soil, and is intolerant of shade. Here are some facts on diffuse knapweed and its biological control agent.

Diffuse knapweed is a biennial, or short-lived perennial, reproducing entirely by seed and producing up to 18,000 seeds per plant. Flowers occur singly or in clusters; bloom July to September; and may be white, pink or lavender. The bracts of the flower heads are tipped with a long, slender spine fringed with smaller spines. Seeds are blackish-brown with vertical brown and grey stripes and about .1 inch long. Stems are usually 6 inches to 2 feet tall with deeply divided leaves. This plant is common along roadsides, in disturbed areas, and in abandoned sites. To date, 13 biological control agents have been approved for release for the knapweed complex, which includes diffuse knapweed.

Our solution to the knapweed problem is seed-head weevils. Larinus minutus, also known as the lesser knapweed flower weevil, or LA, is an abundant biocontrol agent that can utilize spotted, diffuse and squarrose knapweeds. LA weevils overwinter as adults that emerge in the spring when they begin to feed on the foliage. Females produce between 28 and 130 eggs that they lay in clusters in open flower heads. Eggs hatch into larvae, which feed on seeds and receptacle tissue for about a month. Larvae construct cocoons within the seed heads using pappus hairs and pupate. Emerging adults chew a characteristic round hole in the top of the cocoon that is visible when viewed from above.

If you have seen diffuse knapweed in an area that has no grazing, mowing, cultivation or spraying, be sure to give us a call at (209) 316-0355. Thank you for helping us control Idaho’s noxious weeds!

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