The Milfoil Weevil (Euhrychiopsis lecontei)
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.
Distribution
and Abundance
(click on images for more detail)
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.
Egg
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).
Larva
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).
Pupa
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
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).
Host Plant Choice
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.
