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Original source for U.S. release
was numerous sites in Europe. First U.S. releases made in 1969. Now well
established in the Northwest, northern plains states, and Stevens county
north of Hwy 20 (where most of our Plumeless, Musk, and Scotch Thistle
are).
LIFE CYCLE
Starting in May overwintered
adults mate, lay eggs, and then die. A single female may lay close to 200
eggs. Eggs are laid on the flower bud bracts and stem, and covered with
chewed plant material. This covering turns tan and looks like warts. When
the eggs hatch the larvae tunnel into the flower bud and feed in the
receptacle and on developing seeds. Larvae in the seed head fashion
individual hard protective pupal chambers from chewed plant material and
feces, larvae in the stems do not. Development from egg to emerged adult
takes anywhere from 40 to 60 days. Adults averaging about 6mm long, emerge
from inside the seed head by chewing a hole in their pupa chamber, and
then the receptacle face. These new adults look like they are covered in
pollen, but this soon wears off and they are almost black to the eye. When
they are not feeding, the new adults hide. These new adults do not lay
eggs until the following spring. Adults overwinter in sheltered places.
EFFECT
Adults may slightly defoliate
the Thistle. The larval stage is more destructive. This is an effective
agent against Musk Thistle. It is much less effective against Plumeless
and Canada Thistle, even though there may be many larva in a particular
seed head.
REDISTRIBUTION
In Stevens County
collect the egg laying adults
in May and June. Shake the agents off the plant into a sweep net or deep
sided plastic dish pan. A funnel assembly can also be used. Overwintered
adults need to be released as soon as possible. Some new adults can be
collected in the summer and fall if you can find them when they come out
of hiding to feed. Release 250-500 per new site. This agent requires sites
with less heat and some moisture. Hot dry places are not good.
COMMENTS
This agent is present at most
thistle sites in Stevens county. Redistribution will be undertaken only
where the need is clear. In Stevens County the agents impact on Plumeless
Thistle is helpful, but is not providing control.
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