Pan Fish vs. Weevils

Heather Zakrajsek, aquatic biologist with EnviroScience, in the EnviroScience Weevil Gazette reports: “Studies confirm that weevil cuisine is not a preferred menu item for sunfish”.

There has been quite a stir as to whether or not weevil populations could potentially be inhibited by a ravenous school of assorted sunfish such as bluegill or pumpkinseeds. One might think weevils would make an easy, satisfying meal on the go, but researchers are finding these tiny bugs may serve as nothing more than an incidental snack.

Studies conducted by Dr. Ray Newman at the University of Minnesota concluded that weevils were not a dominant prey item for sunfish. Gut content analysis revealed that other invertebrate species such as midges and caddis flies were more common than milfoil weevils. Other studies indicate that the density of Eurasian water milfoil (EWM) may also influence predation by sunfish. Results from one 1996 study revealed that invertebrate predation by sunfish declined in dense EWM beds due to the increase in time required to search for food in dense beds.

In 2006, EnviroScience conducted a study to examine this question.  Lake Bonaparte in northern NY has a large bluegill population and has been using weevils since 2002.  Fish were sampled by electroshocking. Survey results indicated approximately 75% of the fish population was bluegill.

Gut-content analysis of 60 fish confirmed that milfoil weevils were not common food, in fact only one of the bluegills was found to have had a single weevil among its stomach contents.  These observations have been supported by recent work by Cornell University that demonstrated a lack of relationship between weevil and sunfish abundance.

These results are supported by EnviroScience’s observations in nearly 100 lakes they successfully stocked over the past nine years. The vast majority of these lakes have large sunfish populations, and a number can be considered overstocked with small or stunted populations. Owing to their small size, inconspicuous nature, and because they spend a good part of their life cycle within the plant stem, milfoil weevils just don't seem to be “worth it” as a food source for panfish in most cases. 

Additional 2006 -07 research results now being completed confirm the experience of EnviroScience's studies.  Jennifer Jermalowicz-Jones, AES Environmental Technologies,  Ludington, Mich has promised to share the results as soon as they are released.

Eurasian Water Milfoil and The Weevil

Dr Chuck Cubbage

April 14, 2003

By 2000, it was clear that Eagle Lake had a Eurasian watermilfoil problem.  After careful consideration, in the summer of 2001, Eagle Lake - Paw Paw Association implemented the use of a native N. Am. insect (Euhrychiopsis lecontei), a weevil that feeds only on milfoil plants.  The weevil has less than 10 years of use, nation-wide, and so is not a fully tested approach.  On the other hand, biocontrol is about half the cost of the use of herbicides and has the advantage of zero health risks.  On the down side, results are not over-night.  So, where are we?  The Enviro-Science surveys show decreases in E. milfoil in some areas and increased native plants but anyone on the lake last September knows there is still plenty of E. milfoil.  In 2002 we restocked Trails End and Eagle Lake Drive sites and added an N shore site.

WHAT CAN WE DO?

Travel at a no-wake speed in any area where weeds are within reach of boat props or wash.  Some lakes have had spectacular control success while other lakes have experienced very limited control of E. milfoil.  What impacts the success or lack of it?  We already know that propeller damage from boats and PW craft can slow up the weevils’ control of E milfoil.  Multiple buoys will be set out to identify each stocking site.

Weevils over-winter on shore in the top 2” of soil and litter debris along the shore.  Give the weevils a chance to survive by not cleaning the shoreline between fall and spring.  When you rake the beach while the weevils are ashore you are throwing away the critters you paid for!!  Enviro Science suggested that we use the Labor Day to Memorial Day interval as an easy to remember marker for giving the weevils a chance.

More than you need to know, but for the curious, weevils prepare for winter in late August and show up on-shore by mid-September.  By mid-November most will be ashore¹.  After ice-out (usually mid-March) and through May weevils move back into the water to feed on the milfoil².  Since E. milfoil grows in cold water better than the other native plants, the weevils will find food.  So why not give them a break with the longest period free of disturbance that you can bear!! 

Want more detail yet?  Try the following web site:

www.fw.umn.edu/research/milfoil/milfoilbc.html

We are indebted to Dr. Raymond M. Newman, Fisheries, Wildlife and Conservation Biology, University of Minnesota for providing much of the detailed information on the life and times of the weevil (Euhrychiopsis lecontei)³.  Your board is sharing our information (1995 DNR fish survey & Enviro-Science weevil & weed surveys on our lake) with Dr. Newman to help increase the amount of understanding there is regarding the use of biological control over E. milfoil.

1.      Sheldon & O’Bryan 1996a. Life history of the weevil Euhrychiopsis lecontei, a potential biological control agent of Eurasian watermilfoil. Entomological News 107:16-22; Neman et al. 2001. Overwinter habitat and the relationship of overwinter to in-lake densities of the milfoil weevil, Euhrychiopsis lecontei, a Eurasian watermilfoil biological control agent.  Journal of the Aquatic Plant Management Society 39(1): 63-67.

2.      Newman et al. 2001 Overwinter habitat and the relationship of overwinter to in-lake densities of the milfoil weevil, Euhrychiopsis lecontei, a Eurasian watermilfoil biological control agent.  Journal of the Aquatic Plant Management Society 39(1): 63-67.

3.      Newman, Ray, Professor Fisheries, University of Minnesota: Personal communication (emails April 2003)

Dr. Chuck Cubbage            Ph: (269) 657-2122    email: charles.cubbage@comcast.net