Southern Europe and the Mediterranean
Diffuse knapweed (Centaurea diffusa), spotted knapweed (C. stoebe subsp. micranthos), squarrose knapweed (C. virgata subsp. squarrosa)
Eggs are yellow ovals covered with dark egg caps. Larvae are white with brown head capsules, C-shaped, and up to 8 mm long. Pupae are white and up to 5 mm long. Adults are small (≤4 mm long) with short snouts, mottled gray/brown, and covered in small hairs.
Overwintering adults emerge from soil and plant litter in spring and feed on knapweed foliage prior to egg laying. Eggs are laid from late spring through summer, individually on the underside of leaflets or on stems below the developing flower head. Eggs are covered with masticated plant tissue (which forms a black egg cap; and hatch in 8–12 days. Depending on egg placement, hatching larvae either mine into the midrib of the leaflet or into the stem prior to tunneling into the flower head. Larvae develop through four instars and feed on developing seed tissue throughout summer. Pupation occurs in the flower head within a chamber made of frass and fused seeds. Adults emerge in late summer or early fall when knapweeds are senescing. Adults drop to the ground to overwinter. There is one generation per year.
At high densities, defoliation by adults can be severe, which can stunt affected plants. Larvae feed on seed head tissue and developing seeds. Seed consumption does not kill existing plants, but reduces seed production which may help decrease the rate of spread of knapweed populations and may reduce infestations in some habitats.
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