The milfoil weevil is a small, herbivorous aquatic beetle, belonging to the family Curculionidae. It is a milfoil (Myriophyllum spp.) specialist, meaning that it feeds and develops only on plants in this genus. The weevil completes all life stages fully submersed and the larvae are stem miners. These characteristics make it very unique, as specialist herbivores are very rare among aquatic insects (Solarz and Newman 1996). These characteristics are precisely why the milfoil weevil has shown the most promise as a potential biocontrol agent for Eurasian watermilfoil (Myriophyllum spicatum) and why it has been the subject of much research.
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The milfoil weevil has been reported from three distinct areas in North America. These include the northeastern U.S. (Connecticut, Massachusetts, New York, and Vermont), upper midwestern U.S. (Michigan, Minnesota, Wisconsin, Illinois, and Iowa), and western Canada/northwestern U.S. (Alberta, British Columbia, Idaho, Colorado, Saskatchewan, and Washington) (O'Brien and Wibmer 1982, Creed and Sheldon 1994a, Newman and Maher 1995, Sheldon and O'Bryan 1996a, Creed 1998, Waltz et al. 1998). However, widespread efforts to find the milfoil weevil have not been made, and it seems very likely that it has one large range across northern North America, where native watermilfoils occur. Newman and Maher (1995) located the milfoil weevil at 13 sites in 10 lakes out of 25 sites in 19 lakes sampled in Minnesota and Wisconsin. All but three of these site contained the exotic Eurasian watermilfoil, and these three sites contained the native northern watermilfoil (Myriophyllum sibiricum) (see also, Creed and Sheldon 1994a). They concluded that the weevil is patchy in distribution, both temporally and spatially. Several sites had average densities of >20 adult weevils per m2.
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 linear relationship between devlopment rate and temperature (Mazzei et al. 1999). Mazzei et al. (1999) provide a table with development times for each stage over a range of water temperatures.
The milfoil weevil lays its eggs on milfoil plants, usually on apical meristems near the water surface (Sheldon and O'Bryan 1996a). Female weevils lay single eggs that they stick to the plant, and seem to prefer to lay eggs on meristems where no other eggs are present. However, up to 29 eggs have been laid on a single apical meristem when no other options were available (Sheldon and O'Bryan 1996a). Eggs are a yellow-cream color, elliptical, and approximately 0.5 mm long (Sheldon and O'Bryan 1996a). 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% (Sheldon and O'Bryan 1996a, Newman et al. 1997).
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 (Sheldon and O'Bryan 1996a) where they feed on cortical and vascular tissues. Larvae are generally found in the top 1 m of the plant (Sheldon and O'Bryan 1996a). Stems that have been hollowed-out by weevil larvae appear darkened, are less buoyant, and are weaker than undamaged stems (Creed et al. 1992). 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 (Sheldon and O'Bryan 1996a). Late instar larvae reach a length of about 4.5 mm. Development time through the larval stage ranges from 8-15 days at 20-25 degrees C (Sheldon and O'Bryan 1996a, Newman et al. 1997). Survival through the larval stage has been recorded at 78-90% (Newman et al. 1997).
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 (Sheldon and O'Bryan 1996a). However, successful pupal development has been recorded on stems as narrow as 1 mm (Newman et al. 1997). Typical development times through the pupal stage are 9-12 days at mid-summer water temperatures (20-25 degrees C) (Sheldon and O'Bryan 1996a, Newman et al. 1997). Survival through the pupal stage has been recorded at 69-80% (Newman et al. 1997).
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 (Sheldon and O'Bryan 1996a). This is the only stage of the weevil that can exit the water. However, it appears to only leave the water in fall when it migrates to shore and over winters terrestrially in organic matter near the shoreline. They possess wings, but weevils are rarely observed in flight. It remains unclear if the weevil swims or flies on this short migration (Newman and Ragsdale 1995). Adult milfoil weevils are approximately 2-3 mm in length and have lived as long as 162 days in captivity (Sheldon and O'Bryan 1996a). Females lay an average of 1.9 eggs per day, and total egg production by captive females ranged up to 562 eggs (Sheldon and O'Bryan 1996a).
The complete life cycle takes from 17 to 30 days at 20-27 degrees C, with survival ranging from 20-70% (Newman et al. 1997, Mazzei et al., 1999). However, water temperature (Mazzei et al., 1999), host plant (Newman et al. 1997) and host plant quality (Sheldon 1997) have been shown to affect development time and success (see also Watson and Newman) This life cycle period allows for three or more generations per summer. In addition, Sheldon and O'Bryan (1996a) reported cyclical patterns in abundance of each life stage. They observed that a peak in egg abundance was followed by a peak in larvae abundance, followed by pupae abundance, and finally adult abundance. This pattern was repeated several times until fall when the adults stopped laying eggs (see also results from Minnesota). Shortly after all sub-adult life stages were no longer found, the adults disappeared, presumably to over winter on shore. Adults leave the shore in spring and return to the water after ice out, between mid-April and mid-May in Minnesota (Newman, Ragsdale and Biesboer 1997).
The milfoil weevil is highly specific to milfoil plants (Myriophyllum spp.) (Sheldon and Creed 1995, Solarz and Newman 1996). Because the weevil is endemic to North America (Colonelli 1980, O'Brien and Wibmer 1982), and Eurasian watermilfoil probably was not established in North America until the 1940's (Smith and Barko 1990), it is evident that the original host was northern watermilfoil (Creed and Sheldon 1994a, Newman and Maher 1995). 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. Newman and Maher (1995) reported finding milfoil weevils on northern watermilfoil only in lakes where Eurasian milfoil was absent or, in one case, where Eurasian watermilfoil exhibited extensive weevil damage. This indicates that the milfoil weevil has undergone a host range expansion (Bernays and Chapman 1994) to include Eurasian watermilfoil.
Several laboratory experiments have demonstrated that adult milfoil weevils prefer Eurasian watermilfoil for feeding and oviposition. Sheldon and Creed (1995) showed that adults reared on Eurasian watermilfoil have high feeding preferences for watermilfoils, particularly Eurasian watermilfoil. Solarz (1995) showed that weevils are attracted to substances released by Eurasian watermilfoil into the water. Solarz and Newman (1996) demonstrated that weevils are specific to watermilfoils for oviposition. However, weevils reared on Eurasian watermilfoil highly prefer it over the native northern watermilfoil, whereas adult weevils reared on northern milfoil select northern and Eurasian watermilfoils equally (no preference). Furthermore, development times from egg to adult were 1-2 days longer and survival was lower on northern milfoil than on Eurasian milfoil for captive weevils (Newman et al. 1997). These results indicate that natural populations of the milfoil weevil will shift from the native northern watermilfoil to the exotic watermilfoil when exposed to both. In addition, weevils may benefit from this shift through faster development and increased survival.