Insect Description
Larvae are curved with a white body and head capsule. Adults are near black and somewhat of a hump-shape appearance. Adults approximately 2 mm long. Males, if present, are smaller than females which also have a more prominent abdomen. Their legs are light brown with darker areas nearer to the body. They have transparent wings and long antennae.
Field Identification
Presence of the gall wasp is best indicated by the presence of galls on Russian knapweed plants. Eggs laid in meristematic tissue are not visible under field conditions and adults may be difficult to see due to their size.
Life Cycle
Adult wasps emerge from the previous year’s galls in the spring – typically from May to June. . Females begin laying fully developed eggs immediately. Larvae hatch from eggs and begin feeding on plant tissue forming galls. Larvae develop through three instars with the third instar going dormant around mid-summer through the winter before pupating in early spring. Pupation occurs inside galls. Some larvae may hibernate for a second winter before pupating. A higher proportion of wasps are female than male.
Impacts
In the lab, galls stress plants and reduce flower and seed production but under field conditions impacts are still being determined.
Availability
Currently, insects are available in Montana through a limited number of sources. To obtain the contact information for these sources, reach out to the MT Biocontrol Coordinator.
Notes
In its native range it is well-adapted across a range of moisture regimes and environments though it seems to thrive best in areas with minimal disturbance from grazing and cultivation. Initial releases of this wasp were made in Montana in 2009 and therefore it is not yet widely established and the full impact on Russian knapweed is still undetermined.
Using the Agent
Wasps are best collected directly from galls as sweeping is difficult and can damage the delicate adults. Galls can be placed directly in the field in early-spring before adults emerge. Ideal release sites should have actively growing plants that are ~6 inches tall Success should be monitored by looking for galls on Russian knapweed plants in subsequent years.
Due to the lack of knowledge regarding establishment and impact they should not be relied on as a sole strategy for managing Russian knapweed on a landscape level. They are best used in tandem with other management strategies including additional biocontrols, chemical and mechanical methods if eradication is your goal and to reduce spread while insects are establishing. Consult your local weed district or Extension Service for Russian knapweed management advice and/or recommendations.
References
Winston, R., C.B. Randall, R. D. Clerck-Floate, A. McClay, J. Andreas, M. Schwarzl änder. May 2014. Field Guide for the Biological Control of Weeds in the Northwest. http://www.ibiocontrol.org/westernweeds.pdf