The Imported Willow Leaf Beetle (Plagiodera versicolora) is a small metallic-blue-green beetle with an insatiable appetite for willow and poplar trees. Native to Europe, the beetle was introduced to North America in 1915 and is predominantly found in the eastern U.S. and south/eastern Canada.
Identification and Damage
Adults: beetles are small, about 4 or 5 mm long ( 1/8″), metallic blue-green in colour, with an oval convex shape. They chew small holes in the leaves and notches in the margins. They prefer to feed on new foliage and tend to feed together in groups.
Larvae: are about 6mm (1/4″) in length, blackish in colour with shiny black head capsules and are elongated and oval in shape. They have rows of bumps that run down and across their bodies and 9 pair of glands that run down their bodies. These glands are filled with secretions that the insect releases when threatened, to deter it’s predators. The younger larvae feed on the underside of leaves while the older larvae feed on both sides, they skeletonize the leaves, leaving behind only the leaf veins. Although the larvae prefer to feed on new foliage, they are more likely to be found on older foliage. This is because the mother lays the eggs on older leaves, lower down in the canopy where there are fewer predators and the conditions are less harsh. They tend to feed together in groups.
Eggs: are a shiny yellow colour, oval in shape and laid in clusters, typically on the underside of older leaves, lower down in the canopy.
Other signs to look for: Leaves are covered with frass and tend to look dirty and ragged. Heavily infested trees may turn brown and defoliate prematurely.
Life Cycle
The adults overwinter, near the host tree in loose bark, in the soil or in leaf litter, even sod. Come spring (late April/early May) they emerge from winter dormancy and briefly feed on new foliage before laying their eggs on the underside of leaves in groups of about 50 eggs. The eggs hatch in a few days and small larvae begin feeding on the underside of leaves in groups, then as they mature they move also to the upper surface of the leaves. They feed for about about 3-4 weeks then find a place to pupate, on the underside of a leaf. Adults emerge and the cycle begins again, with the second generation appearing about early June through July. There can be additional generations, depending on length of season. Come fall the adult female beetles mate with several males then look for a place to overwinter.
Management
While damage can be extensive and unsightly it is rarely fatal to the tree. A healthy tree can usually with stand 2-3 years of defoliation before it’s health is in serious jeopardy. There are several things you can do help manage their populations however.
- Begin scouting for the adults and eggs in May. Eggs clusters can be destroyed. Scout for eggs again in June through July.
- There are a few beneficial insects that feed on Imported Willow Leaf Beetles such as Asian lady beetles, that will eat the eggs; and assassin bugs, that will eat the larvae; as well there are a few parasites and parasitoids that will lay their eggs in or on the larvae, eventually killing them.
- Horticultural oil or insecticidal soap can be applied at the larvae stage, to suffocate the insect.
- Bacillus thuringiensis var. tenebrionis, is a naturally occurring bacteria that is effective against the larvae.
- As a very last resort Pyrethrin, a natural, organic insecticide made from the Dalmatian daisy flower (Tanacetum cinerariifolium) can be spot sprayed on adults and larvae. The insecticide acts as a nerve toxin and is effective against many soft bodied insects.
Preferred Hosts:
The preferred hosts are poplar (Populus spp.) and willow (Salix spp.) trees. The preferred species within these 2 genre are: “Salix nigra (Black Willow), Salix lucida (Shining Willow), Salix alba (White Willow), Salix interior (Long Leaf Willow), Salix bebbiana (Bebb’s Willow), Salix discolor (Pussy Willow), Populus balsamifera (Balsam Poplar), Populus deltoides (Eastern Cottonwood)” (Cheung D.K.B., Llewellyn J., Marshall S.A., (n.d.)).
Other Species of Willow Leaf Beetles:
In addition to Imported Willow Leaf Beetle (Plagiodera versicolora) there are other willow leaf beetles species that also feed on willow trees. They include:
- Altica subplicata are slightly bigger than Plagiodera versicolora with adults mature length being about 5-7mm (1/4″). They are oval in shape and have a dark metallic blue color. You will find them feeding on the sandbar willow (Salix interior) in “sand dune habitats along the Great Lakes and on sand bars in rivers” from Northern Mexico and across North America (Leasia, M. and B. Scholtens (2013)). They will also feed on Salix amygdaloides and Salix cordata (Leasia, M. and B. Scholtens (2013)).
- Lochmaea capreae is predominantly found in the wet lands of Europe and Asia. The beetle measures about 4-6 mm in length, is oval in shape and a yellow-brown colour with a black head.
- Tricholochmaea decora (Grey Willow Leaf Beetle or Pacific Willow leaf beetle) is native to Canada and the U.S.A. predominantly in the prairies. They feed on Elm, Fruit trees, Poplar and Willow. The beetle measures about 5 mm in length and are greyish-brown in colour.
References:
Cheung D.K.B., Llewellyn J., Marshall S.A., (n.d.). Plagiodera versicolora. The Canadian Journal of Arthropod Identification. https://dkbdigitaldesigns.com/clm/content/plagiodera-versicolora
coleoptera.org.uk, (n.d.). Lochmaea caprea (Linnaeus, 1758). U.K. Beetle Recording. https://www.coleoptera.org.uk/species/lochmaea-caprea?msclkid=d2e636d1ad1411ec8f8b690ff1ae8492
Dellinger T.A. and Day E., (2015). Department of Entomology, Virginia Tech. Imported Willow Leaf Beetle. Virginia Cooperative Extension. https://vtechworks.lib.vt.edu/bitstream/handle/10919/70778/ENTO-139.pdf?msclkid=7be82478ac7411ecb39016aecfeaac0e
Gov. of Canada ( updated 2020-01-24). Gray Willow Leaf Beetle. https://agriculture.canada.ca/en/agriculture-and-environment/agricultural-pest-management/diseases-and-pests/gray-willow-leaf-beetle?msclkid=8f52dc5aad1511ecb7dd77f1453ddc62
Klass C., Johnson W.T., & Weston P.A., (1973 & 2003 Updated 2012) Dept. of Entomology, Cornell University. Imported Willow Leaf Beetle. INSECT DIAGNOSTIC LABORATORY. http://idl.entomology.cornell.edu/wp-content/uploads/Imported-Willow-Leaf-Beetle.pdf?msclkid=a546a817ac7111ecb117f35191db0699
Krischik V., (University of Minnesota) and John Davidson J. (University of Maryland), Last modified on March 06, 2013. © Regents of the University of Minnesota. IPM of Mid-west Landscapes. Section IV. Pest of Trees and Shrubs. Pg. 154. Imported Willow Leaf Beetle. http://cues.cfans.umn.edu/old/Web/154ImportedWillowLeafBeetle.pdf
Leasia, M. and B. Scholtens (2013). “Altica subplicata” (On-line), Animal Diversity Web. https://animaldiversity.org/accounts/Altica_subplicata/?msclkid=871e0834ad1411ec99e4edef5c6ec377
Maine Department of Agriculture, Conservation & Forestry, (2000). WILLOW FLEA WEEVIL AND IMPORTED WILLOW LEAF BEETLE Rhynchaenus rufipes (Lec.) and Plagiodera versicolora (Laich.). https://www.maine.gov/dacf/mfs/forest_health/insects/willow_flea_weevil_willow_leaf_beetle.htm?msclkid=9aa05f65ac7211ecb3dad073b91ef4f8
Ostry, M.E.; Wilson, L.F.; McNabb, H.S.; Moore, L.M. A Guide to Insect, Disease, and Animal Pests of Poplars. United States Department of Agriculture, Forest Service. Agriculture Handbook 677. January 1989. 118 p. https://wiki.bugwood.org/Archive:Poplar/Plagiodera_versicolora?msclkid=a5481732ac7111ec922819e0fe2d9ab3
University of Wisconsin-Milwaukee, (2013). Imported Willow Leaf Beetle (Family Chrysomelidae). College of Letters and Field Station. https://uwm.edu/field-station/imported-willow-leaf-beetle/?msclkid=7be7e0daac7411ec8ef78e717f5a9c94
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