Many people have asked how to get milfoil weevils (Euhrychiopsis lecontei) or if they have milfoil weevils in their lake. EnviroScience of Stow, Ohio is marketing milfoil weevils and a control process but there are legitimate concerns about moving weevils and milfoil within and between states. Currently the best advice is to determine if you have weevils in your lake and take steps to conserve or promote weevil populations and native plant communities. We do not advocate moving weevils or milfoil at this time - even though the weevil is native to Minnesota a particular strain may not be native to your lake.
This is what you are ultimately looking for:
The milfoil weevil (about 3mm or 1/8" long) appears to be widely spread throughout northern North America (distribution map); it has been found in most lakes surveyed in Minnesota (table with incomplete list) and a survey in Wisconsin (Jester et al. 1997, 1998) indicates that the milfoil weevil has been found in 29 lakes and in all but two of the 27 lakes sampled in 1996-1997. It is likely that the weevil occurs, although perhaps at very low density, in most lakes in the upper midwest that have native or exotic watermilfoils.
General search approach
To search for weevils you need to be able to locate watermilfoil plants in your lake: northern and Eurasian are the two most common species in the upper midwest:
To more photos of watermilfoils
Once you have located a milfoil bed the simplest things to look for are signs of weevil damage - weevil damage is more quickly spotted than adult weevils. Searching is easiest by snorkeling, followed by wading and lastly by boat. Milfoil weevils live in the top meter, more typically top foot, of healthy milfoil plants - avoid flowering plants and look for plants that are submersed, several inches to a foot below the surface. Search in 3-8 ft of water. Examine the top foot of plants under the water or grab clumps of milfoil a foot or two beneath the surface for examination.
Weevil-damaged plants will have darkened and hollowed stems contrasting, as seen below, with healthy light or green stem.
Occasional small holes should also be present in the darkened stem. The larvae will mine from the meristem (growing plant tip) downward; depending on the extent of damage, only meristems may be missing, a few inches may be mined and darkened, or several feet of a stem may be damaged. Weevils almost always cause darkening of the entire stem circumference - damage by other insects such as chironomids usually only results in darkening of one side of the stem.
The plant below shows a missing meristem (dense green clump of leaves at the tip of the plant) and extensive mining damage (mining hollows out the stem resulting in darkening and lack of structural integrity).
Note that the mining is not continuous. Also note the darkened
circular areas lower on the stem. These are pupation sites - finding
these sites are fairly certain signs of milfoil weevil activity.
Pupae and pupal holes (shown below) will typically be located 2-3ft down the milfoil stem and generally are several inches or more below the closest stem mining damage. The pupa below is metamorphosing from a larva to adult.
Larvae are small and non-descript - occasionally they venture out of the stem, but mining damage, pupae or adults are better to search for in the field.
The meristems should also be examined for eggs. Eggs are about 0.5mm in diameter, yellowish and most typically are adhered to the meristem. The scurrying female right below has recently oviposited two eggs.
If you spot damage or eggs in an area you can then search for adults. Adults can be positively identified to species and either adults or very advanced pupae are required for positive identification. As indicated in the above photos, adults are typically near the tops of submersed milfoil and are small. All weevils have a characteristic snout, which in the milfoil weevil looks like an elephant trunk.
If you find weevils in your lake and wish to be certain they are the milfoil weevil, you should preserve several adults in alcohol (70% or stronger rubbing alcohol will suffice) and include a paper label with your name, lake name and county, nearest city and date. The specimens can then later be verified by an expert.
The milfoil weevil undergoes a complete life cycle - egg, larva, pupa and adult, underwater during the summer. In Minnesota, weevils will lay eggs from late May to early September; in October adults migrate to shore where they spend the winter in leaf litter at dry sites right along shore. Populations seem to go in pulses - late June through early August seems to be the best time to locate adult weevils. If you spot damage but no adults, return to the same site in two weeks and check again; repeat if necessary.
A more detailed desciption of the milfoil weevil life history is available here: other information on Eurasian watermilfoil and biocontrol can be found at the home page by clicking below.
updated 06/22/01
Copyright 1997-2001 by the Ray Newman, University of Minnesota, Department of Fisheries and Wildlife. All rights reserved. Original content and current HTML by Ray Newman; HTML-ized originally by Jeremy Trexel. The University of Minnesota is an equal opportunity educator and employer. The views and opinions expressed in this page are strictly those of the page author. The contents of this page have not been reviewed or approved by the University of Minnesota.