Japanese Beetles on Ferns

Japanese beetles are common garden pests that effectively destroy plants through consumption and proliferate infestation. They affect gardens from Maine to Georgia, Massachusetts to Iowa and, in rare instances, areas as far west as Colorado. Ferns are a group of plants from numerous genera and families that occur naturally throughout the world. Despite a number of common natural habitats from the United States to Japan, ferns and Japanese beetles are rarely bedfellows.

Japanese beetles (Popillia japonica) were introduced to the United States from Japan via New Jersey. The pest was first found affecting growth in New Jersey in 1916. In the years since, the Japanese beetle has spread north, south and west. Ohio State University etymologist David J. Shetlar describes the species as "the most abundant and important landscape pest in Ohio." Japanese beetles are known to infest more than 300 species of plants.

Adult Japanese beetle are small and metallic green. They reach a maximum size of a quarter of an inch long. Female beetles lay eggs underground. After hatching, larvae push their way to the surface to feast on plants, damaging root systems as they do so.

Ferns are a large and diverse group of plants belonging to the phylum Pteridophyta. Species of fern belong to various classes, orders, families and genera. Ferns are grouped by common traits; they do not flower, they reproduce by spores and they generally have green, frond-like serrated foliage. Author Sue Olsen writes in "Encyclopedia of Garden Ferns" that species are grown for their elegant foliage.

Ferns can be found everywhere from northern Europe to central and eastern Asia and North America. Mountain male fern (Dryopteris abbreviata) grows in locations as disparate as Scandinavia and Pakistan.

Species of fern commonly used by US gardeners include lady fern (Athyrium filix-femina), northern maidenhair fern (Adiantum pedatum ssp. aleuticum), autumn fern (Dryopteris erythrosora), Boston fern (Nephrolepis exaltata) and western sword fern (Polystichum munitum).

Japanese beetles and ferns have a neutral relationship. The pests are not commonly attracted to ferns. In fact, Sue Olsen writes that pests in general are not a threat to ferns. However, no fern species appear on lists of plants known to repel Japanese beetles.

University of Illinois Extension natural resources educator Brenda Roedl writes that Japanese beetles avoid ferns. However, they are not known to keep Japanese beetles away. Thus, while Japanese beetles may not take to fern in the way that they do to Japanese maple or apple trees, fern will not keep Japanese beetles out of the garden in the way that species such as northern red oak and hemlock will.

Various methods of Japanese beetle management are available to gardeners who find the pests affecting ferns or other plants. These methods include the introduction of repellent plants, predatory pests and the use of insecticides.

Species known to keep Japanese beetles out of gardens include red maple (Acer rubrum), tulip poplar (Liriodendron tulipifera), sweetgum (Liquidambar styraciflua), northern red oak (Quercus rubrum) and species of the Boxwood (Buxus), Hemlock (Tsuga), Juniper (Juniperus), Yew (Taxus), Pine (Pinus), Spruce (Picea), Magnolia (Magnolia), and Lilac (Syringa) genera.

Natural predators of the Japanese beetle include insects (spiders, praying mantids, robber flies), birds (grackles, meadowlarks, starlings, cardinal, chickens, ducks, and geese) and parasitic insects (tachinid flies and two tiphiid wasps).

Washing plant leaves with soap and water will deter Japanese beetles, along with insecticides such as marlate, dymet, orthene and sevin. Cythion-based insecticides are known to damage Boston and maidenhair fern.