The aquatic weevil, Euhrychiopsis lecontei, has the potential to
serve as an agent of biological control against the aquatic weed, Eurasian
watermilfoil (Myriophyllum spicatum). Eurasian watermilfoil is an
exotic species of submersed aquatic plant that arrived in North America in
the mid 1900ís. Since its introduction, Eurasian watermilfoil has become a
major nuisance plant, spreading to 45 states and at least3 Canadian
Provinces. Thick beds of Eurasian watermilfoil plants clog canals, lakes
and ponds all across North America while boats continue to carry Eurasian
watermilfoil fragments to new bodies of water where they grow into new
Eurasian watermilfoil infestations. Lake managers try to control Eurasian
watermilfoil by harvesting the plants, adding benthic barriers, drawing down
water levels during the winter, and applying herbicides. These methods are
costly, rarely specific to watermilfoil, and often require frequent
re-application. Using herbivorous insects as a means of biological control
may prove to be a cost-effective and environmentally sensitive alternative.
E. lecontei is a weevil species which is native
to North America. This weevil, a watermilfoil specialist, traditionally
feeds on the native northern watermilfoil (Myriophyllum sibiricum),
and now feeds on Eurasian watermilfoil as well. Many scientists believe
that large weevil populations contribute to Eurasian watermilfoil decreases
in some lakes in Vermont, New York, and Minnesota.
E. lecontei adults
are smaller than most terrestrial weevils, measuring only 3 mm in length.
This weevil is generally dark-colored with a pattern of dark brown/black and
yellowish stripes on the dorsal half, fading to a lighter, yellow-beige
underbelly. However, some weevil individuals vary in color from almost
completely black to mainly tan or beige. The eggs are round and opaque
yellow in color, with a diameter of 0.5 mm. The larvae are a translucent
greenish or whitish color with a dark purple-black head capsule. As the
larvae age, they begin to turn a purplish gray color, indicating that they
are nearing pupation. Pupae take on an immature adult form: a head with a
long snout and closely jointed thorax and abdomen with wings (elytra),
although they are immobile and the exoskeleton is still soft. The pupae
develop mature color patterns just before emerging from the pupal chamber as
E. lecontei inhabit temperate lakes and ponds where watermilfoils
(Northern and/or Eurasian) are abundant. Both species of watermilfoil (and
the weevils that feed on them) thrive in lakes with varying sediment types,
water chemistries, and flow rates. Weevil populations are largest in small,
shallow lakes where watermilfoil grows near the shore. Each fall, adult
weevils migrate to shore where they spend the winter in loose soil, plants,
and leaf-litter. Because of this yearly migration, weevil populations may
be restricted in lakes where watermilfoil beds are far from the shore
(increased fish predation), or where shorelines are highly developed (no
E. lecontei attacks the invasive weed,
Eurasian watermilfoil (M. spicatum). Eurasian watermilfoil is an
aggressive species, which can quickly dominate an aquatic community, shading
out desirable native species by forming a canopy on the surface of the
water. These canopies of Eurasian watermilfoil also limit human
recreational uses of the water (boating and swimming), and cause shoreline
property values to fall.
E. lecontei adults
swim and climb from plant to plant, feeding on leaflets and stem material.
After mating, the female lays an average of 1.9 eggs a day, usually one egg
per watermilfoil apical meristem (growing tip). One female may lay hundreds
of eggs in her lifetime. The eggs hatch, and the larvae first feed on the
apical meristem, and then mine down into the stem of the plant, consuming
internal stem tissue. Weevils pupate inside the stem in the pupal chamber,
a swelled cavity in the stem. Adults emerge from the pupal chamber to mate
and lay eggs. In the autumn, adults travel to the shore where they
over-winter on land. In the laboratory, E. lecontei take from 20 to
30 days to complete one life cycle, depending on water temperatures. For
complete development, weevils require about 310 degree-days with
temperatures above 10 degrees C. In the field, we generally observe 2 to 4
generations per year.
In laboratory and lake-enclosure experiments, as well as in natural lake
systems, we find high levels of damage to Eurasian watermilfoil plants
associated with large numbers of E. lecontei. However, the high
levels of seasonal damage that we observe in the field, may not translate
into long-term declines in Eurasian watermilfoil populations (measured as
plant biomass, stem density, and/or plant height). E. lecontei seem
to be contributing to some short-term declines in Eurasian watermilfoil in
some Vermont, Minnesota, and New York Lakes, but not in others. Current
research is focusing on the roles of fish predation and over-wintering
habitat on E. lecontei populations and their possible biological
The direct effects of herbicides on E. lecontei are unknown, although
any effective herbicide may harm E. lecontei indirectly by
eliminating their host plants.
Mechanical harvesting, herbicide applications, benthic barriers, and water
draw-downs all remove either E. lecontei individuals or their habitat
(aquatic plants) from waterways. In doing so, these popular control methods
lessen the ability of the weevil to control watermilfoil growth
E. lecontei populations
rely on natural shorelines with dry shelter (leaf-litter) for overwintering
habitat. Loss of natural shorelines may reduce winter survival of E.
For more information about E. lecontei biology please visit:
Eggs and larvae are available from EnviroScience, Inc.at:
Thanks to Dr. Ray Newman and Dr. Bern Blossey for reviewing this page.
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