Insect Description

Larvae are creamy-white c-shaped grubs with brown heads, up to 3.5 mm in length, found only inside the toadflax stems. Adult weevils are small, only 2.4-3.4 mm long, metallic bluish-black in color with long snouts, usually found in early spring on newly emerging toadflax shoots, then later on shoot tips and leaf axils of growing plants.

Field Identification

Adults typically gather in small groups while feeding along the length of new shoots, and on new leaves growing on the upper part of yellow toadflax stems.  Adult feeding usually produces a few small round holes chewed in the interior parts (not edges) of yellow toadflax leaves. Plants under heavy attack from within by feeding larvae are stunted, have yellow colored foliage and are twisted in appearance, similar to plants with herbicide damage. Their small size, 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, and 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.  Mecinus janthinformis and M. janthinus are difficult to distinguish; however, M. janthinis has such a strong preference for yellow toadflax that it will usually only be found on this toadflax species.

Life Cycle

Adults emerge from stems in the early spring to feed and mate.  M. janthinus adults emerge from the stems where they overwintered approximately one month earlier in spring than the adults of its close relative, M. janthiniformis.  Females select the widest part of the host stem, usually in the lower half, to chew holes where they deposit their eggs, which are cemented in place and protectively covered by fluids secreted during egg-laying. Each female produces up to 45 eggs, depositing at most 2 per shoot, which hatch within a week or two after deposition.  Larvae typically create tunnels from 10-20 mm to as much as 100 mm in length within the stems while feeding, which are back-filled where the stem tissue was eaten away with tightly packed sawdust-looking frass (powdery, white-to-light brown colored excrement). Pupation occurs by late summer wherever the last larval stage stopped feeding within the mined out stem.  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.


There is limited availability of this weevil in Montana.  If you are interested in obtaining these insects, contact the MT Biocontrol Coordinator.  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 should be released at sites where yellow toadflax infestations are large enough (over five acres) and immediate eradication is not the goal.  Adults are collected off of toadflax stems and leaves either by aspiration or tapping the plants over a tray on warm, sunny and non-windy days relatively early in the growing season.  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.  Due to this insects limited availability, we currently do not suggest utilizing management methods that could potentially limit or reduce populations.  However, it is possible to use an herbicide on the perimeter of the infestation to reduce spread, while using biocontrol agents within the infestation with limited impact to the insects.



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.