Eggs of the flower head weevil. Photo by Ben Puttler

The rosette weevil buds have been active all last fall through the winter and early spring. Eggs have been laid and hatching larvae are developing in the crowns of the rosettes, feeding on the meristematic tissue and also secondary buds. This produces a necrotic area that enlarges as the weevil continues to develop.

Infestations of rosettes are now quite evident in many areas of the state and will continue for the next two weeks. Larval feeding can kill a rosette outright, or it can change the growth pattern of the thistle causing a shorter, bushier plant that produces few flower heads, which in turn are smaller and contain less seeds. These plants are still susceptible to attack by the flower head weevil, the other beneficial weevil. Due to the prolonged egg-laying period, new adults of the rosette weevil occur on the plants from mid-May to mid-June.

Rosette weevil damage. Photo by Ben Puttler

The flower head weevil adult will start emerging the last week in April and begin feeding in crowns of the thistle. From May 5-15 as plants start to bolt, more weevils will become evident, and the adults will start laying eggs on the bracts of the developing flowers.

These eggs are covered with a secretion of chewed plant material that gives the eggs a dirty scale-like appearance. Larvae hatch from the eggs and tunnel into the flower receptacle where they feed on the developing seeds. The weevil larvae become very conspicuous in late May and June and can be readily seen by breaking open the flower head, especially the earliest ones formed.

For more detailed information, refer to MU publication IPM 1010, "Biological and Integrated Control of Musk Thistle in Missouri" at http://muextension.missouri.edu/explore/agguides/pests/ipm1010.htm.

Ben Puttler
Entomology
573-882-1457