Original source for
U.S. release was Greece. First U.S. releases were made in 1980.
Now established in California, Idaho, Montana, Oregon, and
Washington. First documented release in Stevens County was in
The larvae overwinter
in the root. The whitish larva are distinctive in that their head
is noticeably wider than the rest of their body. Pupation into an
adult occurs inside the root in late May and June. The peak adult
emergence coincides with flowering, usually in July. Adults are
somewhat flat, metallic bronze in color, and about 10 mm long.
Females need 5 hot days (86 degrees Fahrenheit or more) before
they lay eggs. The flat white eggs are placed between tightly
packed rosette leaves in July and August. When the eggs hatch the
larvae tunnel into the plant and down into the roots. Usually only
one larva develops per root. The larval activity causes a rather
large swollen gall on the root. Dry soil conditions favor larval
Larva damage the plant
by tunneling within the roots. Surviving plants are stunted and
produce fewer flowers. Adults do a lesser degree of damage by
feeding on leaves.
Use a sweep net or deep sided plastic dish pan to collect adults
in July-August an hour or so before dusk.
The population of this
agent remains low in Stevens County. The reason may be predation
of the larva. Outside of Stevens county there are sites where the
collapse of knapweed population is attributed to this agent. Sites
that are dry, and have some bare ground between plants spaced one
to two feet apart are said to favor establishment. Though an
excellent agent where established this agent appears to play a
minor role in Stevens County.