What are Diffuse and Spotted Knapweed?
Originally introduced from the Eastern Mediterranean region and Europe, diffuse and spotted knapweed are invasive plants that form tumbleweeds and readily invade dry and disturbed sites throughout Colorado. The two knapweed species are closely related and may hybridize. Diffuse and spotted knapweed plants reproduce solely by profuse seed production and are typically biennial to short lived perennials. The seeds may remain viable in soil for up to 10 years, germinating during fall or spring depending on water availability which causes wild population fluctuations year-to-year. Diffuse and spotted knapweed have invaded around 1.8 to 3.5 million acres of land over 17 western states. Both species are found throughout Colorado and will continue to spread without active management.
How to Identify Diffuse and Spotted Knapweed
The initial growth of these knapweeds is a rosette formed by many deeply lobed, hairy, gray-green leaves, each of which can be around 8 inches long and 2 inches wide and fairly flat to the ground. When the knapweeds flower, they send up a many-branched flower stalk that reaches up to 3 feet tall and contains single urn-shaped flowers at the tip of each stem. The flowers have many white to purple petals and are wrapped in brown to tan vertically lined bracts (overlapping leaf-like structures at the base of the flower). Spotted knapweed's distinction is that the flower bracts will have a dark fringe at the tips, giving them a spotted appearance. Each plant will have a single large creamy white taproot. When the flowering stalk dries out at the end of the season it can break off at ground level and form tumbleweeds that distribute seed along their rolling path.
Effects of Diffuse and Spotted Knapweed
Both diffuse and spotted knapweed can greatly decrease forage for livestock and wildlife. The unpalatable knapweeds have spines on the tips of the flowerhead bracts which may damage the mouths and digestive tracts of animals. Decrease in forgeability has been measured to be over 80% in highly impacted sites. Colonies of these plants reduce native plant diversity and can increase soil surface water runoff and soil sedimentation. Both plants are known to have allelopathic root compounds that decrease grass germination and the competitiveness of native species. The sap and spines of knapweed can cause irritation and sometimes severe reactions in people.
The Palisade Insectary has released a number of biocontrol agents against diffuse and spotted knapweed including two root boring beetles, two seed head feeding flies, seed head feeding weevils, and seed head feeding moths. Several of these are well established in Colorado and are probably having an impact on the knapweeds.
The most successful agent for use against diffuse knapweed is the seed head feeding weevil, Larinus minutus. The beetle feeds on the seed heads but is also a foliage feeder as an adult. Large-scale control against diffuse knapweed has been seen at a number of locations along the Front Range. The Insectary currently releases this agent as the primary biocontrol for diffuse knapweed.
The root-boring weevil Cyphocleonus achates weakens plants by destroying the root system. This large weevil has been effective against both diffuse and spotted knapweed and collectible populations have recently been seen on the Front Range. The Insectary will distribute these beetles as numbers allow.
Life Stages of the Seedhead Weevil
Larinus minutus adults emerge from soil and leaf litter 2-4 weeks before diffuse and spotted knapweed begins budding, typically sometime in late May to June. The new adults feed on stems, leaves and preferentially on flowerheads, especially the females who require flower feeding to become reproductively active. The weevils begin mating and when ready, the females lay their eggs in the flowerhead. Once the eggs hatch, the larvae feed on the developing seeds of the knapweed, destroying up to 100% of seed production. Larvae pupate within the flowerhead and will chew characteristic holes in the side of the seedheads to emerge as adults. The new adults will then burrow into the soil and leaf litter to overwinter until the following year.
Damage and Field ID:
Adult Larinus minutus are a 4-5 mm round gray-brown weevil with a very bulbous snout. Newly emerged adults will often be covered in a light yellow fuzz. They may be found at the flower tips of diffuse and spotted knapweed from June until August. Signs of damage include defoliation of the flowering knapweed stalks and characteristic emergence holes in the seedheads from new adults chewing out of their pupation chambers.
Life Stages of the Root Boring Weevil
Adult Cyphocleonus achates, cyphos for short, begin emerging from the ground late July through September. They feed on the tender leaves of knapweed but are commonly found at the tips of the flowerstalk branches where they blend in remarkably well with the flowerheads. They mate and the females begin laying eggs just below the soil surface on the taproots of the knapweed plants. The eggs will hatch and the larva begin burrowing and eating their way into the taproot. Multiple larvae may be found in a single root. The feeding of the larvae will weaken and stunt the plant, sometimes killing the rosette outright through the introduction of soil pathogens. Larvae will develop inside the root and emerge the following year as adults.
Damage and Field ID:
Cyphocleonus achates are very large, 14-15mm, mottled gray-brown weevils. Their coloring strongly resembles the spotted appearance of spotted knapweed flowerheads and allows them to blend in well on the plants or on soil. They will drop off plants and play dead when disturbed which can make them extremely hard to spot. Adult feeding damage is typically not noticeable but the larval development in the taproots causes stunting of the rosettes and galling of the roots. When the roots are dissected large white c-shaped grubs will be found.
- What else will the weevils attack?
Neither Larinus minutus or Cyphocleonus achates are known to feed or reproduce on any other Centaurea spp. except diffuse and spotted knapweed.
- Will the weevils kill diffuse or spotted knapweed?
The seedhead weevils, Larinus minutus, do not kill diffuse or spotted knapweed plants. The adults will feed on and cause some damage to the plants but the main action is the larvae’s destruction of the seedheads. This will eliminate new seed from being formed and reduce knapweed over time. The root weevil, Cyphocleonus achates, can kill diffuse or spotted knapweed plants. The primary means of mortality is through the larval feeding on the roots which reduces root energy stores and decreases nutrient uptake which can prevent the knapweed from flowering. The feeding damage can also open the plant up to infections from soil pathogens.
- What if I spray the knapweed with herbicide or mow?
Mowing and herbicide use on knapweed is incompatible with either the seedhead or root weevils. Release sites should be free from disturbance for several years to allow the agents to establish and begin reducing the weed populations.
- When is the best time to release agents?
Larinus minutus emerge from the soil in May-June and the typical season to field collect, ship and release these agents is July through August. Cyphocleonus achates is typically collected August to September.
- How long will it take to control diffuse or spotted knapweed?
Control of diffuse and spotted knapweed will depend highly on the seedbank already present in the soil. While the seedhead weevils will destroy up to 99% of all new seed produced and the root weevils will weaken and kill developing rosettes, new plants will continue to sprout from seed for up to 10 years. Reduction of knapweed plants should be observable within 3 years of release.
- What is the best release site?
The ideal release site for diffuse or spotted knapweed weevils are colonies of knapweed with one to three feet of space between plants, coarse well-drained and semi-exposed soil, and south, west or east facing slopes. Vegetation that is too thick makes it difficult for the weevils to lay eggs on the root crowns or move through the plant colony.
- Where do I get these agents?
You can place an order for diffuse or spotted knapweed agents on the Request-A-Bug page.
- Is there a charge?
Yes, the seedhead weevils are $30.00 per release of 200 adults and the root weevils are $30 per release of 50 adults.