Biology
Adult R. conicus are dark brown in color and 10 to
15 mm long. Each female lays about 100 to 200 eggs on the
bracts of thistle heads. Eggs hatch in 6 to 9 days and newly
hatched larvae feed through the bracts into the receptacle.
Developing larvae feed on the receptacle and the young seeds,
reducing or preventing the production of viable seeds. There
are four larval growth stages which last about 4 to 6 weeks.
Pupation occurs in a black oval cell inside the thistle head,
and requires another 7 to 10 days. Adults emerge from seed
heads in July and August in more northern locations and in
June and July in Texas. They remain inactive during the summer
and then hibernate through the winter. The following spring
adults emerge from winter shelters to lay eggs on the developing
buds. There is one adult generation per year.
History
Thistle head weevil has been released and established in
many parts of the United States over the last 20 years. Research
in other states shows a 50 to 95 percent reduction in the
number of thistles after weevils have become established.
The decline in thistle populations is gradual over several
years, and some resurgence of the weed may take place. Resurgence
of thistles can be caused by dumping soil containing seeds
onto a site, exposing soil containing thistle seeds during
tillage, overgrazing that creates bares patches of soil where
thistle seeds can germinate, or temperature fluctuations during
the winter which kill many overwintering weevils. Cool temperature
also inhibit weevil activity. Resurgence of thistles is temporary,
however, established weevils eventually gain control.
The first weevils were released in Texas, near Kerrville
in 1987. Establishment of thistle head weevil was first confirmed
in 1992. At that time they had dispersed only about 1.5 miles
from the release site. This site in Kerrville is the most
southerly location where this weevil is established in North
America. Establishment at Kerrville is particularly significant
because the thistle blooming period is about one month earlier
there compared with most other locations in the United States.
Since 1992, weevils have been released at other sites in the
Hill Country, near Dallas, and at Hereford. (See
complete report.)
Status
R. conicus had been highly successful in controlling
musk thistle after an establishment period of 5-6 years. In
locations where weevils can multiply rapidly, a dramatic decline
in musk thistle may be possible in as little as 2 to 3 years.
This successful biological control of musk thistle is partly
due to the good synchronization of plant growth and insect
activity.
Management
Biological control of musk thistle can best be implemented
by taking the following steps:
1. Select release sites. An appropriate site is any dense
patch of thistles that will not be disturbed for 3 to 5 years
after the release. Select a site where cattle will not be
grazing because cattle interfere with thistle head weevil
activity.
2. Obtain starter colonies of thistle head weevils. Spring
emerging R. conicus can be collected at established
sites in April before they complete egg laying. Use a beating
sheet and an aspirator to collect adult weevils as they feed
on thistles and lay eggs in the heads. Contact your county
Extension agent or the Extension entomologists for information
about local release and establishment efforts. Thistle head
weevils are also available from a few commercial sources.
3. When weevils are received, keep them in a cool place (less
than 80 degrees F) because high temperatures will kill them.
Paper cartons with tight lids are appropriate for holding
and transporting weevils. Release them as soon as possible.
4. Release a minimum of 300 to 500 weevils per site depending
on the size of the thistle infestation. Starter colonies are
best released when thistles have elongated and buds have formed.
Release adult weevils among dense thistle patches away from
traffic and in non-windy areas near trees or shelter. Release
weevils in the evening, not during the hottest time of the
day. For widespread thistle infestations, release weevils
at a number of sites.
5. Monitor weevil establishment. Make additional releases
only if weevils fail to become establishment. Make additional
releases only if weevils fail to become established.
Along with biological control:
1. Establish a good stand of an adapted grass.
2. Avoid overgrazing or unnecessary plowing that would expose
barren land.
3. Cut small infestations of thistles before they produce
seed.
Weeds Affected
The thistle head weevil primarily attacks musk thistle and
other members of the genus Carduus. However, strains
of the weevil are known to attack milk thistle, Silybum
marianum (Boldt and DeLoach 1985). Recently concern about
Rhinocyllus attacking other thistle species has become
controversial. When use of this weevil became a concern, the
relocation efforts were discontinued in Texas.
Literature
Boldt, P. E., and C. J. DeLoach. 1985. Evaluating Rhinocyllus
conicus Froel. (Coleoptera: Curculionidae), an introduced
weevil for the biological control of Carduus and Silybum
thistles. Bull. Entomol. Soc. Amer. 26: 355-358.
Boldt, P. E., and J. A. Jackman. 1993. Establishment of Rhinocyllus
conicus Froelich on Carduus macrocephalus in Texas.
Southwestern Entomologist 18(3): 173-181.
Jackman, J. A., P. Boldt, J. W. Stewart and T. W. Fuchs.
1992. Biological Control of Musk Thistle in Texas. L-5067.
Biological Pest Control. Texas Agricultural Extension Service.
Texas A&M University System, College Station, TX.
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