Polydrusus Weevils

Sarah Lisak, Brian Aukema, and Hailey Shanovich
Department of Entomology, University of Minnesota

Background

The green immigrant leaf weevil (Polydrusus formosus) and the pale green weevil (Polydrusus impressifrons) belong to a group of beetles known as weevils or snout beetles.  There are 188 species of Polydrusus weevils, with four native to North America. These two species were introduced to North America from Europe in the early 1900’s and can now be found throughout much of North America. The pale green weevil is found to exist throughout the northern part of the US and southern Canada whereas the green immigrant weevil is only established in the northern half of the eastern US and into southeastern Canada. Both species are present in Minnesota. The adult weevils feed on the leaves and buds of a variety of hardwood and fruit trees, and occasionally on selected crops such as raspberry and strawberry (Table 1.) In Minnesota, both species can be found on ornamentals, urban trees, and occasionally on fruit crops (however, these weevils are not an economic concern).

Table 1. Trees and shrubs on which these two species have been found

Alder (Alnus)

Dogwood (Cornus)

Mountain Maple (Acer)

Red Maple (Acer)

American Hazel (Corylus)

Elm (Ulmus)

Oak (Quercus)

Strawberry (Fragaria)

Apple (Malus)

Hawthorn (Crataegus)

Pear (Pyrus)

Sugar Maple (Acer)

Aspen (Populus)

 Hazel (Corylus)

Pine (Pinus)

Thimbleberry (Rubus)

Basswood/Linden (Tilia)

Hop-hornbeam (Ostrya)

Plum (Prunus)

White Birch (Betula)

Beech (Fagus)

Ironwood (Olneya)

Poplar (Populus)

Willow (Salix)

Cherry (Prunus)

Leatherwood (Dirca)

Raspberry (Rubus)

Yellow Birch (Betula)

Identification

Polydrusus
Figure 1.  Left: adult green immigrant leaf weevil (Polydrusus formosus), Right: adult pale green weevil (Polydrusus impressifrons) (Photo by Sarah Lisak)

Both weevils are known for their brilliant iridescent green scales that cover their bodies, though color vibrance and shade can range and color depends on lighting (Figure 1). Black patches may appear on weevils’ bodies where scales have been rubbed off from foraging in plants or handling.

Green Immigrant Leaf Weevil (Polydrusus formosus)

  • Color: reddish-green scales
  • Size: 0.2-0.27 in (5.3-6.8mm)
  • Body: narrow face, lined pattern on back

Pale Green Weevil (Polydrusus impressifrons):

  • Color: blueish-green scales
  • Size: 0.15-0.22 in (3.7-5.5mm)
  • Body: broad face, alternating line-dot pattern on back

Life Cycle

Both of these species have one generation per year in the Great Lake region. Adults begin to emerge from the soil in late May/early June and feed on plant buds and leaves through August. Females lay clusters of small, white cylindrical eggs into grooves of bark or into the surface of the soil. Generally, females will lay about 800 eggs but this number can reach up to 2,000. Once the eggs hatch, the larvae burrow into the soil where they feed on the roots of plants and overwinter. Larvae continue feeding into the spring, and then pupate and emerge as adults from the soil.

Damage

Adults feed on young leaves and buds but this feeding does not typically damage the appearance of larger trees and shrubs. However, large numbers of weevils can damage very young trees. Larvae are known to feed on roots but are not known to cause problematic damage. 

Management

If you observe large numbers of Polydrusus weevils and severe defoliation on young trees, pick off adults by hand and consider covering young trees with a fine netting. Use of insecticides is generally not necessary, but spraying the plant with a soap and water solution can offer further protection as desired. The following pest management strategies have been used in the past for large numbers of weevils and severe defoliation on young trees: spraying insecticides (e.g., indoxacarb, chlorantraniliprole), soaking new cuttings and applying arsenicals. A parasitoid wasp (Diospilus polydrusi Gahan) (Hymenoptera: Braconidae) has been introduced in an attempt to control eggs of the green leaf weevil.

References

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Coyle, D. R., W. J. Mattson, and K. F. Raffa. 2008. Invasive Root-feeding Insects in Natural Forest Ecosystems of North America. In Root Feed. Ecosyst. Perspect. eds. Scott N. Johnson, Philip J. Murray.

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Majka, C. G., R. S. Anderson, and D. B. McCorquodale. 2007. The weevils (Coleoptera: Curculionoidea) of the Maritime Provinces of Canada, II: New records from Nova Scotia and Prince Edward Island and regional zoogeography. Can. Entomol. 139: 397–442.

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Parrott, P. J., and H. Glasgow. 1916. Leaf Weevil (Polydrusus impressifrons Gyll.) Technical Bulletin of the New York Agricultural Experimental Station, Geneva, New York.

Pierce, W. D. 1916. Notes on the Habits of a Dangerous Genus of Weevils. J. Econ. Entomol. 9: 424–431.

Pinski, R. A., W. J. Mattson, and K. F. Raffa. 2005a. Host Breadth and Ovipositional Behavior of Adult Polydrusus sericeus and Phyllobius oblongus (Coleoptera: Curculionidae), Nonindigenous Inhabitants of Northern Hardwood Forests. Environ. Entomol. 34: 148–157.

Pinski, R. A., W. J. Mattson, and K. F. Raffa. 2005b. Composition and Seasonal Phenology of a Nonindigenous Root-Feeding Weevil (Coleoptera: Curculionidae) Complex in Northern Hardwood Forests in the Great Lakes Region. Environ. Entomol. 34: 298–307.

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