  
Left: Larinus minutus adult weevil.
Center: L. minutus receptacle gall", caused by
larval feeding.
Right: L. minutus larval feeding damage in spotted knapweed
seedhead.
R.Richard (all)
Larinus minutus
Coleoptera: Curculionidae
by R.F. Lang, USDA-APHIS-PPQ, Bozeman Biocontrol Facility, Forestry
Sciences Laboratory, Montana State University, Bozeman, MT 59717- 0278.
Larinus minutus (seedhead weevil), a native weevil of Europe,
was cleared and first released in the United States in 1991. The weevil
has been released in Arizona, California, Colorado, Idaho, Minnesota, Montana
(established), Nebraska, Oregon (established), South Dakota, Utah, Washington
(established), and Wyoming (established) as a part of a biological control
program to control spotted and diffuse knapweed (Centaurea maculosa
Lam. and C. diffusa Lam.).
Appearance
L. minutus is a brown-grey weevil with a very large, bulbous
snout. L. minutus measures 4-5 mm in length. They are strong fliers
and disperse throughout the entire knapweed patch in several years. When
the adults first emerge from the knapweed seedhead they are light grey
with some yellow fuzz on their bodies.
Habitat
 Spotted
and diffuse knapweed are weed species that can be found throughout the
northern tier of states and as far south as Nebraska and Virginia. These
highly competitive weed species favor and establish quickly on disturbed
sites and overgrazed rangeland. Both weeds will invade well established
grassland communities and out compete the native vegetation. The release
of L. minutus is part of a program to introduce a complex of spotted
and diffuse knapweed enemies to help control these weeds.
Pests Attacked
L. minutus is host specific and limited to a few species in the
genus Acrolophus and Calcitrapa. L. minutus was host tested
with plants in the subgenera that are native, endangered or of economic
importance before being cleared for release in the United States. L.
minutus attacks both spotted and diffuse knapweed with a slight preference
for diffuse knapweed.
Life Cycle
Adult L. minutus weevils emerge in late spring from the ground
litter where they have been hibernating. The female weevil must feed on
the flowers of the knapweed for ovary development. Therefore egg laying
begins after the knapweed has started to bloom.. The eggs hatch within
three days and the larvae begin feeding on the flower and migrated to the
seeds and commence feeding. The seeds are consumed and when development
of the larvae is complete the larva pupates and emerges for the seedhead
as a new adult. Development from egg to adult takes about four weeks. The
newly emerged adults feed on the plant foliage and eventually go into the
soil and ground litter to hibernate for the winter.
Pesticide Susceptibility
Not yet known
Commercial availability
In some states, L. minutus adults may be obtained at no cost
from state weed management agencies.
Effectiveness
L. minutus larvae destroy up to one hundred percent of the seed
in an infested knapweed seedhead. This weevil along with other seed feeders
will further reduce the seed that spotted and diffuse knapweed are dependent
on for reproduction, dispersal, and survival.
Conservation
Sites that are chosen for L. minutus release should be considered
for long term availability and should be left undisturbed by development
or pesticide use for at least ten years. The release site should be dry
with some bare ground and knapweed plants should be one to two feet apart.
Frequently this will be the outer edges of the knapweed infestation.
References
Groppe, K. 1990. Larinus minutus Gyll. (Coleoptera: Curculionidae),
a suitable candidate for the biological control of diffuse and spotted
knapweed in North America. Intl. Inst. of Biol. Control, DelÇmont,
Switzerland Final Report. 30 p.
Groppe, K., R. Sobhian, and J. Kashefi. 1990. A field experiment to
determine host specificity of Larinus curtus Hochhut (Col., Curculionidae)
and Urophora sirunaseva Hg. (Dipt., Tephritidae), candidates for
the biological control of Centaurea solstitialis L. (Asteraceae),
and Larinus minutus Gyllenhal, a candidate for biological control
of C. maculosa Lam. and C. diffusa Lam. J. Appl. Ecol. 110:
300-306.
Jordan, K. 1995. Host specificity of Larinus minutus Gyll. (Col.,
Curculionidae), an agent introduced for the biological control of diffuse
and spotted knapweed in North America. J. Appl. Entomol. 119: 689-693.
Lang, R. F., J.M. Story, and G.L. Piper. 1996. Establishment of Larinus
minutus Gyllenhal (Coleoptera: Curculionidae) for biological control
of diffuse and spotted knapweeds in the western United States. Pan-Pacific
Entomol. 72: 209-212.
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