Insect Description

Larvae are creamy-white c-shaped grubs with brown heads, up to 5 mm in length, found only inside the toadflax stems. Adult weevils are small, only 3.2-6.0 mm long, metallic bluish-black in color with long snouts, usually found in spring on toadflax rosettes, then later on shoot tips and leaf axils of growing plants.

Field Identification

Adults typically cluster in groups while feeding on new leaves and flower buds growing on the upper part of the toadflax stem. Adult feeding produces in a shot hole pattern on flower and leaf buds, and in the interior parts (not edges) of Dalmatian toadflax leaves. The tip of the main or primary stem of plants under heavy attack from within by feeding larvae is usually curved downward, and the foliage has a shredded appearance due to the large number of holes chewed by the adults in the upper leaves. Their bluish-black color and elongated rectangular shape distinguishes them from other insects found on Dalmatian toadflax, for example, the longer and oval shaped Brachypterolus pulicarius, which is dark grey to black with a light grey stripe across the back (wings), or the triangular shaped, light to dark brown Rhinusa weevils, which are noticeably broad across the back end and very narrow or pointy-looking toward the front of the body due to the small size of the head.  Splitting open stems during the summer, fall, and winter will reveal larvae, pupa, or adults if they are present.  However, looking for insects with this method will likely result in their death.  Previous years’ stems may also be examined for emergence holes or sawdust-like frass inside the stem to confirm that weevils were present.  M. janthinformis and M. janthinus are difficult to distinguish; however, M. janthinformis prefers Dalmatian toadflax and will usually only have an impact on this species.

Life Cycle

Adults emerge from stems in the early spring to feed and mate. Female weevils orally excavate shallow divot-like holes in the surface of the toadflax stem where their white oval-shaped eggs are singly deposited. Each female weevil produces up to 45 eggs, which hatch within a week or two after deposition.  The host stem responds to egg laying or larval feeding by producing a short (10 mm long) swelling (also known as a semi-gall) in the stem where all larval development takes place. Larvae feed exclusively on the swollen inner stem tissue. Pupation occurs by late summer in the larval chamber, the area hollowed out through larval feeding inside the semi-gall.  All M. janthiniformis adults remain inside the stem where they began life as a deposited egg, overwintering there before finally moving out to feed and mate in the following spring.


Adult feeding stunts shoots and suppresses flowering and seed production.  Larval mining damages vascular tissue, reducing water and nutrient transport leading to reduced root carbohydrate storage and above ground growth.


This weevil is widely available in Montana.  If you are interested in obtaining these insects view the biocontrol vendor list for possibilities.  You will want to obtain the insects in the early spring when they emerge as adults and are collectible.


Mecinus janthinus (yellow toadflax stem weevil) is very difficult if not impossible to distinguish from M. janthiniformis (Dalmatian toadflax stem weevil) in the field. These weevils are also species specific, meaning that the yellow toadflax weevil will only impact yellow toadflax and the Dalmatian toadflax weevil will only impact Dalmatian toadflax. It is therefore very important to make sure that the insects are released on the same toadflax species that they were collected from.

Using the Agent

Biocontrol agents should be released on sites where Dalmatian toadflax infestations are large enough (over five acres) and immediate eradication is not the goal.  Adults can be collected using a sweep net or by tapping plants over a tray or into a bucket on warm, sunny and non-windy days.  Impacts can be observed at some sites in two to five years.  Integrated weed management is always encouraged but with this biocontrol agent any method that prevents stem growth during the growing season will reduce or eliminate biocontrol populations.  It is possible to use an herbicide on the perimeter of the infestation to reduce spread while using biocontrol agents within the infestation.  Good results have also been observed when using grazing and biocontrol together.  Typically, goats and/or sheep are used and actively managed so that flowers are the majority of the grazed material, resulting in a reduction of seed production and limiting the amount of stem grazing, which the weevils need for maturation.



Sing, S.E., De Clerck-Floate, R., Hansen, R.W., Pearce, H., Randall, C.B., Toševski, I., Ward, S.M. 2015. Biology and Biological Control of Dalmatian and Yellow Toadflax. USDA Forest Service, Forest Health Technology Enterprise Team, Morgantown, West Virginia. FHTET-2015-03. 139 pp.

Winston, R., C.B. Randall, R. D. Clerck-Floate, A. McClay, J. Andreas, M. Schwarzlander.  May 2014.  Field Guide for the Biological Control of Weeds in the Northwest.