Milfoil Weevil (Euhrychiopsis lecontei)
The milfoil weevil is a small aquatic beetle belonging to the family Curculionidae. It is a milfoil (Myriophyllum spp.) specialist in that it feeds on and completes its juvenile stages and most of it adult it life cycle only on plants in this milfoil genus. All life stages of the life cycle are completed submersed. The larvae are called "stem miners" since they burrow into the milfoil stem and strip the tissues, a behavior that ultimately kills the plant. Specialist herbivores are very rare among aquatic insects (Solarz and Newman 1996) thus making the habits of the milfoil weevil rare. These habits account for why the milfoil weevil has promise as a potential biocontrol agent for Eurasian watermilfoil (Myriophyllum spicatum) and why it has been the subject of much research.
Eurasian milfoil probably was not established in North America until the 1940's (Smith and Barko 1990) because of the presence of the milfoil weevil. Since the weevil is endemic to North America (Colonelli 1980, O'Brien and Wibmer 1982) it is thought that the original host was northern milfoil (Creed and Sheldon 1994, 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 milfoil exhibited extensive weevil damage. This indicates that the milfoil weevil has undergone a host range expansion (Bernays and Chapman 1994) to include Eurasian milfoil.
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.
The life cycle of the milfoil weevil takes 21-30 days at 20-25 degrees C, with survival of all stages ranging from 20-70%. Development rate is directly related to temperature and (Mazzei et al. 1999) with all life cycle stages developing successfully from 15-31C.
Egg: The milfoil weevil lays its eggs near the apical meristems near the water surface (Sheldon and O'Bryan 1996a). Eggs are a yellowish-cream color, oval, and about 0.5 mm long (Sheldon and O'Bryan 1996a). The egg stage lasts about 3-6 days at 20-25C. Mazzei et al (1999) reported 90% survival at 21-25C while Sheldon and O'Bryan (1996a) and Newman et al. (1997) reported hatching success between 65-100%.
Larva: The neonate larvae feed on meristem tissues for 3 to 5 days, while older larvae prefer the inside of the stem (Sheldon and O'Bryan 1996a) where they feed on the plant's tissues. Larvae are usually found in the top meter 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). Larvae tend to bore out of the stem, move up or down the stem in a spiral path before re-entering 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 20-25C (Sheldon and O'Bryan 1996a, Newman et al. 1997) or 8.9 days at 21-23C (Mazzei et al. 1999). Survival through the larval stage has been recorded at 78-90% (Newman et al. 1997) and 68.3% (Mazzei et al. 1999).
Pupa: The milfoil weevil pupates inside of milfoil stem. Pupae are found further down the stem than larvae (> 0.5 m) (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 through the pupal stage ranges from 9-12 days at 20-25C) (Sheldon and O'Bryan 1996a, Newman et al. 1997). Survival through the pupal stage has been recorded at 69-80% (Newman et al. 1997) and 87% (Mazzei et al. 1999).
Adult: Adult milfoil weevils are weak swimmers usually remaining on the plant even after disturbance. Adults are usually located on the upper 1 m of milfoil plants. Adult weevils eat milfoil leaves, but will also consume stem tissues (Sheldon and O'Bryan 1996a). The adult can exit the water whereas all other stages are always submersed. Generally, the adults only leave the water in fall when it migrates to shore and over winters terrestrially in plant litter and debris near the shoreline. Adults possess wings but have rarely been observed in flight. Adult milfoil weevils are approximately 2-3 mm in length. They are known to 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) and over 600 eggs (anom).
The complete life cycle lasts from 17 to 30 days at 20-27C, with survival ranging from 20-70% (Newman et al. 1997, Mazzei et al., 1999). Water temperature (Mazzei et al., 1999), host plant (Newman et al. 1997) and the health of the host plant (Sheldon 1997) have been shown to affect development time and survival. The multivoltine life cycle produces three or more generations per summer. Sheldon and O'Bryan (1996a) 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 until fall when the adults stopped laying eggs. Shortly after all sub-imago life stages were absent in fall, the adults disappeared from the submersed plant habitat, presumably to over winter on shore (Sheldon and O'Bryan 1996a). In spring the adults leave shore and return to the water between mid-April and mid-May in northern temperate areas (Minnesota) (Newman, Ragsdale and Biesboer 1997).
--Bernays, E. A., and R. F. Chapman. 1994. Host-plant selection by phytophagous insects. Chapman & Hall, New York, NY.
--Colonnelli, E. 1980. Notes on Phytobiini, with a key to the New World genera (Coleoptera: Curculionidae: Ceutorhynchinae). The Coleopterists Bulletin 34: 281-284.
--Creed, R. P., S. P. Sheldon, and D. M. Cheek. 1992. The effect of herbivore feeding on the buoyancy of Eurasian watermilfoil. Journal of Aquatic Plant Management 30: 75-76.
--Creed, R. P., and S. P. Sheldon. 1994a. Aquatic weevils (Coleoptera, Curculionidae) associated with northern watermilfoil (Myriophyllum sibiricum) in Alberta, Canada. Entomological News 105: 98-102.
--Mazzei, K.C., R.M. Newman, A. Loos, and D.W. Ragsdale. 1999. Developmental rates of the native milfoil weevil, Euhrychiopsis lecontei, and damage to Eurasian watermilfoil at constant temperatures. Biological Control 16:139-143.
--Newman, R. M., M. E. Borman, and S. W. Castro. 1997. Developmental performance of the weevil Euhrychiopsis lecontei on native and exotic watermilfoil hostplants. Journal of the North American Benthological Society 16: 627-634.
--Newman, R. M., and L. M. Maher. 1995. New records and distribution of aquatic insect herbivores of watermilfoils (Haloragaceae: Myriophyllum spp.) in Minnesota. Entomological News 106: 6-12.
--Newman, R.M., D.W. Ragsdale and D.D. Biesboer. 1997. Can Eurasian watermilfoil be managed in Minnesota by biological control with native or naturalized insects? Fourth Progress Report to the Minnesota Department of Natural Resources, Ecological Services, St. Paul, MN.
--O'Brien, C. W., and G. J. Wibmer. 1982. Annotated checklist of the weevils (Curculionidae sensu lato) of North America, Central America, and the West Indies (Coleoptera: Curculionidae). Memoirs of the American Entomological Institute 34: 1-382.
--Sheldon, S. P., and R. P. Creed. 1995. Use of a native insect as a biological control for an introduced weed. Ecological Applications 5: 1122-1132.
--Smith, C. S., and J. W. Barko. 1990. Ecology of Eurasian watermilfoil. Journal of Aquatic Plant Management 28: 55-64.
--Solarz, S. L., and R. M. Newman. 1996. Oviposition specificity and behavior of the watermilfoil specialist Euhrychiopsis lecontei. Oecologia 106: 337-344.
For an excellent review see: R.M. Newman, http://fwcb.cfans.umn.edu/research/milfoil/milfoilbc.html