Southern Europe
Primarily diffuse knapweed (Centaurea diffusa), occasionally spotted knapweed (C. stoebe subsp. micranthos), and squarrose knapweed (C. virgata subsp. squarrosa)
Eggs are flat and white when first laid, but change to dark bluish-purple after five days. Larvae have an enlarged head and a long, thin, cylindrical body that tapers to the end. They are whitish with inflated segments and dark brown head capsules. Pupae are white initially but later darken. Adults can be up to 10 mm long. They are a metallic bronze color and somewhat flattened, with their bodies tapering towards the narrowed abdomen tip.
Adults emerge in summer when knapweed flowers, and they feed on knapweed leaves for 2–3 days before mating. During late summer, females lay multiple eggs between the bases of rosette leaves. Leaf stems with diameters of 3–6 mm are preferred over smaller leaves. Females lay an average of 50 eggs during their lifetimes. Larvae hatch after two weeks and feed between leaf stalks. As knapweed sets seed, second-instar larvae mine into the upper root; their feeding creates swollen galls and tunnels often filled with frass. Larvae overwinter in roots. After the third instar, pupation occurs in nine days within the feeding chamber during early summer the following year. There is one generation per year.
Adults feed on knapweed foliage, leaving characteristic circular and oval feeding holes over the entire leaf. Feeding larvae consume roots which, in turn, may kill plants outright, prevent rosettes from bolting and flowering, or decrease the reproductive output of flowering stalks.
Randall, C.B., J.E., Andreas, and J. Milan. 2023. Knapweed Biocontrol Agents: History and Ecology in North America. In: R.L. Winston, Ed. Biological Control of Weeds in North America. North American Invasive Species Management Association, Milwaukee, WI. NAISMA-BCW-2023-34-KNAPWEEDS-A. https://bugwoodcloud.org/resource/files/27451.pdf