Stevens County Noxious Weed Control
Board This web site will help
you identify & control noxious weeds. Conventional & biological control
(BioControl) information and photographs are here. Maps & related
information are included.
GENEALOGY
Original source for
U.S. release was Switzerland. First U.S. releases were made in
1980. Now established in Idaho, Montana, Oregon, and Washington.
In Stevens county 10,600 adults were released between 1986 and
1990. This agent is established in Stevens county, but is not very
important.
LIFE CYCLE
Larvae which have
overwintered inside the seed head pupate into adults in May.
Adults are small moths about 7 mm long. They only fly at dusk and
are rarely seen. Starting in June females lay between 60 and 100
eggs on the flower head. Eggs are reddish-brown, oval, less than
1mm long, and turn yellowish as they mature. Hatched larvae crawl
into the opening flower head and begin to consume the contents.
Larvae are aggressive and kill each another, and other larva in
the seedhead. Generally, only one larva survives per seed head to
start overwintering. Good snow cover increases the chances of
larval survival for the winter.
EFFECT
In a single seedhead a
young larva will consume a few seeds and reduce the viability of
surviving seeds as it mines the seedhead receptacle. The older
larva will web seeds together, preventing dispersal.
REDISTRIBUTION
Adult moths in the
field are seldom seen or collected. The better method is to
collect seed heads in early spring from sites where you have
confirmed the presence of pupa and/or larva.
COMMENTS
Although Metzneria
larvae kill one another, and other larval species in the seed
head, it is thought the best possible impact occurs when Metzneria
shares the knapweed patch with two gall fly species: Urophora
affinis and Urophora quadrifasciata. Adult Urophora gall flies are
everywhere in Stevens county. The distribution and density of
Metzneria in the county is unclear. It is likely they are being
supplanted by the much more important agent Larinus minutus. If
other knapweed agents are absent, the impacts of Metzneria and the
two gall flies are minimal.
