The milfoil weevil (Euhrychiopsis lecontei) is a small, herbivorous aquatic beetle, belonging to the family Curculionidae, that is native to North America. It is a watermilfoil (Myriophyllum spp.) specialist, meaning that it feeds and develops only on plants in this genus. Its original host before the introduction of Eurasian watermilfoil (Myriophyllum spicatum) was the native northern watermilfoil (M. sibiricum). The weevil completes all life stages fully submersed, feeding and developing on milfoil, and the larvae are stem miners. These characteristics make it unique, as specialist herbivores are very rare among aquatic insects. These characteristics are why the milfoil weevil has shown the most promise as a potential biocontrol agent for Eurasian watermilfoil and why it has been the subject of much research. Several other herbivores, such as the lepdidopterans Acentria ephemerella and Parapoynx spp., and the midge Cricotopus myriophylli, can damage and control milfoil and may be important in some locations, but have not shown as much potential as the milfoil weevil.
Milfoil weevils affect Eurasian watermilfoil mainly by boring through the stem and consuming the cortex. This results in reduced plant buoyancy and accumulated carbohydrate stores, which may ultimately decrease overwinter survival and competitive ability. Because milfoil declines have often occurred over winter and in early summer or have persisted over several years, it is likely that longer term effects, such as reduced overwinter survival or reduced competitive abilities are important to sustained control of Eurasian watermilfoil. Our observations suggest that plant community response, i.e., the ability of other species to occupy space left by damaged milfoil or the stress imposed by competition with other plants, is also important to successful biological control.
Research on the operational use of the milfoil weevil is continuing and incomplete. Although we know milfoil weevils can control milfoil, we are currently unable to predict when and in what lakes this will occur. More work is needed to determine what controls milfoil weevil and other milfoil herbivore populations and what role plant competition plays in successful control.
Where they're found
The milfoil weevil appears widespread across the northern U.S. and southern Canada and occurs in many lakes in Minnesota and Wisconsin. It is found in lakes that have Eurasian watermilfoil and lakes with the native northern watermilfoil that have not yet been infested with Eurasian watermilfoil. Recent surveys in Wisconsin indicate that the milfoil weevil likely occurs in most lakes that have northern or Eurasian watermilfoil.
In the Midwest, milfoil declines have been associated with the milfoil weevil in Fish Lake, Wisconsin and a less well-documented decline in McCullom Lake, Illinois. In Minnesota, documented declines have been associated with the weevil in sites at four lakes (Auburn, Cenaiko, Otter and Smith’s Bay of Lake Minnetonka); in some cases the declines persisted and in other cases suppression only lasted a few years.
In several lakes of the Minneapolis Chain of Lakes, milfoil weevil densities are very low and milfoil is not controlled. Experimental evidence suggests that a density of 100-200 weevils per square meter of bottom may be required to affect a decline. At many sites in Minnesota, weevil densities have failed to reach such levels, although 25-75 weevils per square meter (or less) may be adequate to cause declines if they persist through the summer. We are currently investigating factors that influence weevil density; fish predation appears to be an important factor at sites with low weevil populations. Mechanical harvesting of milfoil over large areas can reduce weevil abundance, but harvesting was not occurring at many sites we have studied.
Like many insects, the milfoil weevil develops through 4 stages: egg, larva, pupa and adult. The complete life cycle of the milfoil weevil takes 21-30 days at 20-25 degrees C, with survival ranging from 20-70%; there is a positive linear relationship between development rate and temperature.
The milfoil weevil lays its eggs on milfoil plants, usually on apical meristems within a few feet of the water surface. Female weevils lay single eggs that they stick to the plant, and generally lay one to several eggs on a meristem. However, up to 29 eggs have been laid on a single apical meristem when no other options were available. Eggs are a yellow-cream color, elliptical, and approximately 0.5 mm long. The egg stage lasts about 3-6 days at typical mid-summer temperatures of 20-25 degrees C. Hatching success has been reported to be between 65-100%.
After hatching, first instar larvae feed on meristem tissues for 3 to 5 days, while older larvae spend the majority of their time inside the stem where they feed on cortical and vascular tissues. Larvae are generally found in the top 1 m of the plant. Stems that have been hollowed-out by weevil larvae appear darkened, are less buoyant due to gas leakage, and are weaker than undamaged stems. Occasionally, larvae will bore out of the stem, travel up or down the stem in a spiral path, and bore back into the stem. This behavior is most common when a larva reaches the end of an internode. Late instar larvae reach a length of about 4.5 mm. Development time through the larval stage ranges from 7-10 days at 20-25 degrees C and larvae damage about 15cm of stem from their feeding. Survival through the larval stage has been recorded at 78-90%.
The milfoil weevil pupates (metamorphoses) inside of milfoil stems in a pupal chamber. Generally, they are found further down the stem than larvae (> 0.5 m), possibly because a larger diameter stem is preferred for pupation. However, successful pupal development has been recorded on stems as narrow as 1 mm. Typical development times through the pupal stage are 8-12 days at mid-summer water temperatures (20-25 degrees C). Survival through the pupal stage has been recorded at 69-80%.
Adult milfoil weevils are usually located on the upper 1 m of milfoil plants. They are weak swimmers, and will usually remain on a plant even after it has been disturbed. Adult weevils primarily eat milfoil leaves, but will also consume stem tissues. This is the only stage of the weevil that can exit the water. However, they appear to only leave the water in fall when they migrate to shore and over winter terrestrially in leaf litter near the shoreline. It remains unclear if the weevil swims or flies on this short migration to shore, but many raft in on milfoil fragments. They possess wings, but weevils are rarely observed in flight, which appears restricted to the terrestrial phase. Adult milfoil weevils are approximately 2-3 mm in length and have lived as long as 162 days in captivity. Females lay an average of 1 to 5 eggs per day (3/day over lifetime), and total egg production by captive females ranged up to 562 eggs.
Water temperature, host plant, and host plant quality have been shown to affect development time and success. This life cycle period allows for three or more generations per summer. Adults leave the shore in spring and return to the water after ice out, between mid-April and mid-May in Minnesota; they do fly at this time and may disperse to other lakes in the spring.
Host plant choice
The milfoil weevil is highly specific to milfoil plants (Myriophyllum spp.) Because the weevil is endemic to North America, and Eurasian watermilfoil probably was not established in North America until the 1940s, it is evident that the original host was northern watermilfoil on which it is commonly found. However, with the introduction and spread of Eurasian watermilfoil across much of North America, the milfoil weevil was exposed to a novel plant that is closely-related to its natural host. When both plants are present, weevil densities are usually higher on Eurasian watermilfoil and host choice tests indicate a strong preference for Eurasian watermilfoil and performance (survival, time to complete development, and size) is generally better on Eurasian than northern watermilfoil. It is clear the milfoil weevil has undergone a host range expansion to include and prefer Eurasian watermilfoil.