Southern Europe and the Mediterranean
Primarily diffuse knapweed (Centaurea diffusa), occasionally spotted knapweed (C. stoebe subsp. micranthos)
Eggs are black and oval-shaped with a slightly depressed center. Larvae are translucent white with inflated segments and have brown head capsules. Pupae are enclosed in “chimney tubes”. Adult moths are ≤8 mm long. Their wings are a speckled light brown, sometimes exhibiting a silvery sheen. Wingspans are up to 20 mm.
Adults emerge from late summer through early fall, mate and lay eggs during their short (15–20 days) life span. Eggs are laid singly or in small groups on the lower surfaces of rosette leaves or the bases of leaf stems. A single female may lay 140+ eggs in her lifetime. Larvae hatch within 12 days and mine down the root, feeding on the woody central portion of the root or the soft tissue near the outer edges, causing galls to form. There are five larval instars; third instars typically overwinter within the root and resume feeding the following spring. Larvae construct silken “chimney” tubes that extend from the galls upward to 20 mm above the soil surface, where they pupate. The chimneys provide easy exits for the emerging adults. Pupation usually lasts 15 days and occurs in early summer. There is one generation per year.
Larvae feed on roots, which interrupts the vascular flow of nutrients, thereby decreasing the plant’s biomass and flowering ability. Damaged roots become spongy and fragile and easily break apart; they are also more susceptible to soil-borne pathogens which move into the roots and provide secondary attack.
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