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Biological Control Study

Background

Myriophyllum spicatum (Eurasian watermilfoil) is found throughout the United States.  In Connecticut, M. spicatum along with its close relative, Myriophyllum heterophyllum (variable-leaf watermilfoil), are two of the most common invasive aquatic plants.  It can grow rapidly to occupy disturbed areas and form a dense mat of leaves at the water surface.  This mat replaces native vegetation, destroys fish spawning areas, adversely affects recreational activities, impairs visual aesthetics, lowers property values, and provides breeding grounds for mosquitoes.  M. spicatum spreads very quickly because vegetative propagules (stem and root fragments and stolons) easily fragment and spread by currents, wind and human activities (such as motor boat propellers).

Dense beds of aquatic plants can be controlled by mechanical, chemical, biological or conservation control measures.  Mechanical and chemical control can be controversial for M. spicatum for many reasons.  Mechanical control methods such as harvesting can actually spread M. spicatum.  Chemical controls, such as herbicides, may kill plants, but leave excess nutrients in the water due to the plant decomposition. One promising alternative to chemical and mechanical control of M. spicatum is the use of biological control insects.  Several insect species can be used for biological control of M. spicatum. These include the moth, Acentria ephemerella (Lepidoptera: Noctuidae), the midge, Cricotopus myriophylli (Diptera: Chironomidae), and the native milfoil weevil, Euhrychiopsis lecontei Dietz (Coleoptera: Curculionidae) (Newman 2004).

The milfoil weevil larva (Euhrychiopsis lecontei) causes stem damage to the Eurasian watermilfoil by mining the plant. The larva consumes the entire cortex of the stem causing extensive damage down the plant through nodes causing a loss of buoyancy and ability to translocate nutrients and carbohydrates (From Newman et al. 1996). Stem damage by the milfoil weevil to the plant stem can be seen in Figure 2.

Current Research

We are currently working to determine the distribution and abundance of the milfoil weevil and milfoil-related invertebrates with the invasive watermilfoils, M. spicatum, M. heterophyllum, and the native watermilfoil Myriophyllum sibiricum.  We have surveyed 15 lakes and found the milfoil weevil to be present in 13 of those 15 lakes.  The average number of weevils per stem of milfoil is presented in Figure 1.  Within a lake, abundance of the milfoil weevil at a site can range from 0 to 4.6 weevils per stem, which could indicate localized control of the M. spicatum population by the milfoil weevil.

Future projects include: continuing to determine the distribution of the milfoil weevil in Connecticut; determining how to integrate the use of herbicides with biological control organisms; determining the development of the milfoil weevil on M. heterophyllum and its hybrid (M. heterophyllum X laxum); and investigating the role of plant chemicals in determining herbivore performance.

References

Newman, R.M., 2004. Biological control of Eurasian watermilfoil by aquatic insects:  Basic insights from an applied problem. Arch. Hydrobiol. 159, 145-184.

           Newman, R.M.  2006.  Biological control of Eruasian watermilfoil.  University of

           Minnesota.  http://fwcb.cfans.umn.edu/research/milfoil/milfoilbc.html

weevil abundance per stem graph

Figure 1.  Evidence of Euhrychiopsis lecontei on M. spicatum stems collected from 15 Connecticut Lakes.  Evidence includes the presence of eggs, larvae, pupae, adults or larval damage.  Approximately 200 stems per lake were inspected for weevil damage.  Red bars indicate lakes that were surveyed bi-weekly; black bars are lakes surveyed once this season.  Declines in milfoil populations might be expected in lakes with more than 0.5 weevils per stem.

Eurasian watermilfoil

Myriophyllum spicatum

Northern watermiloil 

Myriophyllum sibiricum

 

Mifloil weevil 

Euhrychiopsis lecontei adult

watermilfoil stem damage

Figure 2. Eurasian watermilfoil stem damage caused by mining by the milfoil weevil (Euhrychiopsis lecontei) larva.