Weed control board offers info on biological control

Biological weed control is the deliberate use of living organisms to help reduce plant populations. Natural enemies may be insects, mites, fungi and other plant pathogens. Biological control of weeds is killing them directly, weakening or stressing weeds to the point where other plants can out-compete them, destroy seed formation or create lesions. The objective in biological control is not eradication but rather the reduction of a weed population.

The majority of the noxious weeds in the United States are introduced without their natural enemies. Potential bio-control agents are selected and tested to determine their host range. The agents that have a very limited host range are approved for release into the U.S.

One advantage of bio-control is once the control agent becomes established it will reproduce, increase in numbers and continue to attack the target plant. Another advantage is that the bio-control agents will move to other host plants and will not damage surrounding vegetation.

A disadvantage of bio-control is that it often takes many years for the populations of the introduced agents to increase to levels that permanently decrease the pest plant population. Other insect predators will attack the bio-control agent and environmental conditions can also affect the desired bio-control agent.

Perennial weeds are the most logical targets for a bio-control program because they live for more than two years. Before introducing a bio-control agent the landowner should first select a site that is at least 20 acres solid of infested weeds. Select the best bio-control agents for their habitat and protect the release area from grazing, sprays and other disturbances until the insects are well established, possibly up to five years.

Landowners can collect their own biological weed control or purchase them from different agencies such as Gary Piper from WSU.

Biological control agents have been released throughout Washington State since 1948. Several have become established in Eastern Washington and are collected by government agencies and landowners. The collected agent can then be transported to a new site.

Biological agents found in Adams County are affecting Canada thistle, rush skeletonweed, poison hemlock, St. Johnswort, Dalmatian toadflax, diffuse and spotted knapweeds.

Two varieties can be found on Canada thistle. Look for a gall formation on the stem of the thistle. Collect the gall in the fall or early spring by cutting them from the old plant. Inside the gall, the larva of Larinus planus (Canada thistle bud weevil) or Urophora cardui (Thistle stem gall fly) will develop into adults and emerge in the spring. This gall can be dropped into an area of Canada thistle, where it will dry and open for the adults to emerge.

Different small beetles are found on St. Johnswort, Dalmatian toadflax and the knapweeds. The adult beetles can be carefully collected by using nets or shaking the infected plant into a container. All insects need to be kept cool and transported to the new site within 24-48 hours.

The chrysolina beetle is found on St. Johnswort. The adult and larvea stage attack the leaves in the fall and spring.

A stem-boring weevil (Mecinus janthinus) is found on Dalmatian toadflax. It is collected May-June.

The knapweed flower weevil (Larinus minutus) attacks the flower head of the knapweed. It can be collected in the adult stage May-July.

Rush skeletonweed has three varieties of bio-agents that are common in this area and often all three can be found on the same plant. To find the skeletonweed gall mite, look for a gall formation at the flower heads of the mature skeletonweed. The gall mite is 0.006 to 0.01 inch in size. The gall midge is a tiny flying insect that lays its eggs on the leaves and stem of the skeletonweed. These infected areas form small galls that are raised and purple-reddish in color.

The third agent on skeletonweed is the Skeletonweed Rust. To collect infected plants, cut off the entire plant, bag and transport to the new location. Place the cut plants on top of the plants in the new area. As the cut plant dries, the bio-agents will move onto the new plant. Rush skeletonweed bio-agents can be collected from July to mid-October.

For more information, contact the Adams County Noxious Weed Control Board office at 201 West Broadway in Ritzville.