Biological Control Agent Links      See: Larinus minutus for photos and more info.

Origin: Europe and the Mediterranean. (5)
Life Cycle:
Over Wintering Stage:
  Winters as adult beetle in the litter or in the soil cracks near the host plant.
Egg Stage: Eggs are laid among the inner florets (central area) of newly opened flower.  They hatch 3 days later at about 77 degrees F.  The eggs are yellowish in color, and range from an oval to a round  shape and measures 0.05 in. long to about 0.03 in. in width.  There are up to 5 eggs in a cluster, but a female lays between 28 and 130 eggs (1). They develop between June and September. (3)
Larval Stage: Larvae feed in the seed head on pappus hairs and developing seeds. They are fully developed in about 17 days. (5)

Pupil Stage: Pupation happens in a chamber in the seed head, constructed from seeds and pappus hair. They develop over 9 days. (5)

Adult Stage: After the pupil stage the beetles chew their way out of their chamber.  They over winter as adults in leaf litter and other "nooks and crannies" in the environment.; Their wintering state ends at the end of May, and they reach peak population in July as the adults gather around flowering knapweed plants to mate and lay eggs. The 1st generation of beetle attacks seed head in late July and August.  They are black in color with yellow hair on back and they range from 0.2 in. to 0.28 in. in length. (5) They are often covered in pollen and this lightens their "color." [Breitenfeldt, Todd].

Damage to Host:  1-2 larva can destroy all the seeds in a seed head.  Larva can consume the entire content of immature knapweed seed heads, including the seeds, florets, and portions of the receptacle. They can also cause gall-like structures in the seed head causing the plant's resources to be further depleted. They destroy nearly one-hundred percent of the seeds in infected seed heads.  Adults feed on the leaves, stems, and florets but prefer flower heads when available. (5,4,1)

Plant Species: They like spotted knapweed best but also feed on diffuse knapweed. (5)

Location: They like somewhat moist areas but will adapted to any area that contains spotted knapweed or and other weed that grows in that area. (5)

Host Impact: Will reduce seed production and plant heath then eventually kill the knapweed. (4)  In Montana, they do not usually kill spotted knapweed but are needed to reduce the soil seed bank so that the knapweed root borers can then go on to eventually kill the plants.  It is this combination of seed head AND root feeders that seem to be working. In other states, they have been known to severely damage or kill diffuse knapweed. [Breitenfeldt, Todd].

Release Habitats: Will develop anywhere there is a good source of food (knapweed). (5)

Collect, Transport, and Release: During the months of July to August you can collect adults by taking a sweep net to an area covered in knapweed, or you can take the seed heads of the knapweed while the larvae are present for transport to another area [be careful as you are bringing some seeds along as well]. When you sweep for any kind of weevils you make sure that you have a container to put them in (such as a paper cylinder) but once you have all the weevils you want to collect you put them in a cooler with an ice pack.  Do not let them freeze!

They are a very hardy insect and can be stored in a fridge for up to a week if they have some knapweed  foliage to munch on. After all, they over winter as adults, in Montana! [Breitenfeldt, Todd].

Purchase: Unavailable for purchase at this time. (5)  You may purchase Larinus minutus from several sources.  These two insects look so much alike that only a few experts can tell them apart. Be sure and look for holes in the seed heads [evidence you have them] of your knapweed infestation BEFORE you purchase to be sure you do not already have these insects.  They are strong fliers so dispers over long distances on their own.  You very likely have them already in many locations in Montana. [Breitenfeldt, Todd].

Info: They were first released in 1993 in the United States. They have been released in Idaho, Minnesota, Montana (established), Nebraska, Oregon, and Washington (established). Prefer the spotted knapweed but will also attack diffuse knapweed. (3) They are well established in many sites in Montana. [Breitenfeldt, Todd].

1. Ronald Lang. Larinus obtusus. [online] Available: larinus_obtrusus.html Sept. 1, 1999.

2. Knapweed Weevils [online] Unavailable: http://  Sept. 3, 1999.

3. Division of Plant Industry  Biological Pest Control Section, Spotted and Diffuse Knapweed
Biological Control Plan [online] Available: Sept. 3, 1999.

4. Biological Control- Stop/ Diffuse Knapweed, USDA, Dec. 1994.

5. Rees Norman, et. al, Ed., Biological Control of Weeds in the West, Weston Society of Weed Science, in cooperation with USDA, ARS, Mt, Dept. of Ag, and Mt State Univ., Color World Printers, Mt, Fed., 1996.

By: Amber Hacker.  Published By: Jessica Beal 11/23/99.  - Updated By: Anthony Reed  3-2-09.

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