Biological Control

Larinus minutus. Reductions in diffuse knapweed populations in Montana, Oregon, and Washington have been observed following the release of the lesser knapweed flower weevil, Larinus minutus. In Colorado, diffuse knapweed cover declined from 8.3 percent to 1.9 percent over one year after the insects were released, and in one area, diffuse knapweed disappeared entirely. Diffuse knapweed cover increased from 14.5 percent to 17 percent in a nearby area where the insects were not present. Reductions of 95 percent or greater in diffuse knapweed stand density have been reported from L. minutus release sites in Washington state. Adult weevils emerge from overwintering sites approximately four weeks before the diffuse knapweed flower buds begin to form. Due to their sensitivity to high temperatures, adult weevils are primarily active (and most apparent) during the relatively cooler morning or evening hours, from May through mid-September. Feeding damage to diffuse knapweed foliage, shoots and immature flower buds in spring or early summer when newly-emerged adult weevils are aggregated and feeding in high densities can result in significant shoot stunting, flowerhead abnormalities, or even plant mortality. Over-wintered female weevils must feed on the host plant before ovariole maturity can occur; oviposition then begins on newly-opened flowers. Eggs are deposited among individual pappus hairs on plants typically free of significant adult feeding damage. Larvae complete three instar stages over a four week period; at this time they actively feed on florets, developing seeds and pappus hairs and are capable of consuming all seeds produced in a head. Mature larvae (one per flowerhead) construct pupal cells from seed debris and pappus hairs. The pupal cell is a domed, straw-colored structure filling the capitulum (bottom) of an otherwise empty seedhead. Pupal color changes from white to brown just before adult emergence. Empty pupal cells have a large hole in the center marking the adult weevil’s exit point (see Figure 2).

Figure 2. The exit hole of an adult Larinus minutus weevil from the pupal cell in a diffuse knapweed flowerhead (photo by Bob Richards, USDA-APHIS).

The reproductively immature adults over-winter protected in the soil and litter near the base of plants. Adult weevils can be collected by aspirator (see Figure 3) when they congregate in large numbers around the root crown in spring, or they can be swept from the diffuse knapweed foliage before 50 percent bloom is attained. After 50 percent bloom the females are more likely to have laid their full compliment of eggs and will not effectively spread the insect population.

Figure 3. A simple aspirator used to collect adult Larinus minutus in the spring from diffuse knapweed root crowns (photo by Sharlene Sing).

In laboratory cultures of the weevil, the adult lifespan ranged from five to 14 weeks. Larinus minutus prefers hot, dry areas and does poorly at high elevations and in areas with prolonged rainfall. Livestock grazing while plants are bolting is detrimental to L. minutus population buildup because the females will disperse to find flowering plants. Diffuse knapweed is currently host to 12 other insects and one nematode released in North America for the control of knapweed species (see Table 1). In addition, Sclerotinia sclerotiorum is a common native soil fungus that infects the crowns of diffuse knapweed, and Puccinia jaceae var. diffusea attacks the leaves. These fungi are not cleared as bio-control agents and pathogenetic control of diffuse knapweed using fungi, bacteria, or viruses has had limited consistent success in field applications.

The Urophora seed head flies were released over 25 years ago in Montana and are now well established throughout most knapweed-infested areas in the western United States. These species have been observed to reduce seed production by 50 percent or more. Competition inhibits overall bio-control efficacy when multiple seed-feeding agent species are present in the same release sites. Other flower-head feeding insects are not as well distributed, but may be as effective as the Urophora flies. The other flower-head weevil, Larinus obtusus, and the Metzneria seed-head moth are established in Montana and believed to be effective in reducing seed production. Larinus weevils prefer hot, dry sites while Metzneria does best on sites with winter snow cover. The Bangasternus seed-head weevil is not widely established in Montana. However, it is reported to consume up to 100 percent of the seeds per flowerhead where it has established. Poor establishment of the Chaetorellia and Terellia flies is believed to be the result of competition with other flower-head insect species.

Of the root-feeding insects, the Agapeta root moth, Cyphocleonus root weevil, and Sphenoptera root borer are well established in parts of Montana. These species prefer hot, dry, open sites. The Pelochrista root moth first released in Montana in 1984 has been slow to establish for unknown reasons. The Pterolonche root moth was released and established in Oregon in 1986 but has not been recovered since 2000, presumably due to the locally dramatic control of diffuse knapweed by Larinus weevils.

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Last Modified: 08/21/2008