Southeastern Europe
Prefers Dalmatian toadflax (Linaria dalmatica), but also found attacking yellow toadflax (L. vulgaris)
Eggs are white and oval-shaped. Larvae are C-shaped, creamy-white with brown head capsules, and are up to 5 mm long. Pupae are typically creamy white with conspicuous appendages and a long snout. Adults are bluish-black (rarely greenish-black), elongated, and have long snouts and elbowed antennae. Newly emerged adults may have a subtle metallic sheen that becomes more dull black with age. Mecinus janthinus adults are smaller (2.4–3.4 mm vs. 3.2–6.0 mm long), and their snouts are less abruptly curved than M. janthiniformis. Otherwise, these two species are nearly identical.
Adults emerge from overwintering within the previous year’s toadflax stems in early spring and immediately begin feeding on new toadflax shoot tips. In the native range, Mecinus janthinus emerges a few weeks earlier than M. janthiniformis. The timing of spring emergence of adult weevils in North America varies significantly due to the influence of site-specific characteristics such as latitude, elevation, aspect, and residual snow cover. Adult feeding on toadflax shoot tips and leaves produces a distinctive shot hole injury pattern, which tends to be more conspicuous on Dalmatian toadflax. Females chew holes into toadflax stems and lay eggs singly (60–160 in a lifetime under lab conditions), covering eggs with chewed plant tissue. Larvae feed through three instars in short tunnels chewed inside the toadflax stems. Mecinus janthinus tends to mine lower in toadflax stems than M. janthiniformis. Pupation occurs in mid-summer in chambers located in the larval feeding tunnels. Development to adulthood occurs by late summer, with adults overwintering inside the pupal chambers. There is one generation per year, with all development from egg deposition to adult emergence the following spring taking place in the same toadflax stem.
Adult weevils consume/chew on the growing tips of toadflax shoots, stems, and leaves, producing a distinctive shot hole injury pattern. This damage weakens the plant, can suppress flowering and seed production, and severely stunts shoots. Large aggregations of M. janthiniformis adults feeding on new toadflax shoot growth can result in significant injury to shoot tips and leaves; heavily attacked shoot tips may turn brown and shrivel. Larval mining severs water/nutrient-conducting tissues, causing desiccation and death. Larval feeding also likely depletes nutrient reserves in the roots, which is important for the year-to-year persistence of individual plants. Repeated, yearly attack by both adults and larvae can lead to striking reductions in toadflax plant density.
Sing, S.E., R.A. De Clerck-Floate, C.B. Randall, and I. Toševski. 2022. Toadflax 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-2022-29-TOADFLAXES-A. https://bugwoodcloud.org/resource/files/27419.pdf