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Winter Tree Care: Identifying Common Tree Insects in Winter, Part 6

This is the sixth part of a series on winter tree insects. This article examines Isabella tiger moth, Japanese Beetle, and knopper Gall Wasp.


Through the winter months, plants conserve their energy, often enduring inclement weather in anticipation of spring. Due to the frigid temperatures, many insects enter a state of dormancy, overwintering on or within their hosts. While some insects are visible during winter, others conceal themselves in bark crevices or beneath the soil surface. The following examines some of the most common insects to infest trees during winter, and how they may be observed.

Isabella Tiger Moth (Pyrrharctia isabella)

Isabella tiger moth is one of the most common insects in North America. It is recognized for its distinct coloration, and stiff bristles, which are soft to the touch. Isabella tiger moth can be observed throughout the growing season. It is considered a minor defoliator of many plants.


Isabella tiger moth consumes the leaves of asters, birches, clover, corn, elms, maples, and sunflowers, as well as a variety of grasses and weeds, including plantain, dandelion, and nettles. Adults drink nectar from wildflowers.

Symptoms of Infestation

Isabella tiger moth feeds on the leaves of host trees, causing minor defoliation. While defoliation can temporarily reduce the ornamental value of host trees, a second flush of foliage is generally produced by the end of the growing season.


  • Isabella tiger moth is considered a minor nuisance. It does not cause extensive damage to trees. As such, management practices are not required.
  • Isabella tiger moth has several natural predators that help limit populations. These include parasitic wasps, mantids, birds, and flies.

Japanese Beetle (Popilla japonica)

Japanese beetle is a long, green beetle that is native to Japan. It is considered one of the most devastating pests of turfgrasses, and urban landscape plants in the eastern United States. In the United States, the insect was first discovered infesting a plant nursery near Riverton, New Jersey. It is believed to have been introduced to the United States in a shipment of iris bulbs prior to 1912. The insect has since become widespread throughout the eastern United States, where it devours the leaves, flowers, and fruit of host plants.


Japanese beetle infests around three hundred species of plants. Trees that are commonly infested by Japanese beetle include American chestnut, American elm, American mountain ash, American linden, black cherry, black walnut, cherry, English elm, flowering crabapple, gray birch, hollyhock, horsechestnut, lombardy poplar, london planetree, peaches, plums, roses, rose of sharon, and sassafras. Japanese beetle also feeds on select weeds and non-cultivated plants, such as bracken, elder, Indian mallow, multiflora rose, poison ivy, smartweed, and wild grape.

Symptoms of Infestation

As the grubs feed within the soil, they consume grass roots, which reduces the ability of the grass to absorb water and nutrients. This causes large dead patches to form in grub infested areas. The adults feed on the leaves and flowers of host plants. Their persistent feeding causes the infested leaves to develop a skeletonized appearance. Dense beetle populations may completely defoliate host plants. The flowers on infested plants are often consumed entirely. In infested areas, the adults can be observed flying around, or hovering near host plants.


  • Soil insecticides can be applied to infested turf to control the grubs. Applications should be performed in summer to ensure success.
  • The grubs are susceptible to milky spore disease, which is caused by the bacterium Paenibacillus popillae. Milky spore disease is commercially available. It is sold in a powder form, and used for lawn application.
  • When planting, select resistant trees and shrubs. American elder, American sweetgum, boxelder, boxwood, butternut, common lilac, common pear, flowering dogwood, mulberry, red maple, scarlet oak, silver maple, white oak, and white poplar are seldom infested by Japanese beetle.
  • When beetle populations are low, they can be removed by hand, and doused in soapy water.
  • Select plants can be covered with cheesecloth, or other fine netting during the peak of beetle activity. This will prevent the beetles from feeding on the plant’s foliage and flowers. Roses in particular benefit from this method.
  • Various insecticides are registered for use on Japanese beetle. Applications should be performed in late spring to deter the adults. Applications should be repeated throughout the growing season to prevent additional infestations from occuring. Susceptible foliage and flowers should be thoroughly drenched.
  • Neem oil applications are effective at controlling beetle populations. Neem oil should be administered at the first sign of infestation. When applied to host plants, Neem oil reduces the beetles’ feeding.
  • Natural repellents include catnip, chives, garlic, and tansy. These herbs offer limited control, and are not recommended for use on dense populations.

Knopper Gall Wasp (Andricus quercuscalicis)

Knopper gall wasp is a gall wasp species that infests two types of oak trees: common oak, and turkey oak. Knopper galls form on common oak when the adults deposit eggs into the developing buds and acorns. Once the larvae hatch, they grow within the galls, causing a partial or complete distortion of the buds and acorns. While knopper galls are generally an aesthetic nuisance, severely infested trees may experience a drastic reduction in seed yield, inhibiting plant reproduction.


Knopper gall wasp infests Quercus robur, often referred to as common oak, English oak, European oak, or pedunculate oak, and turkey oak, also known as Austrian oak.

Symptoms of Infestation

The most visible symptom of infestation is the formation of the knopper galls on the buds and acorns of common oak. The galls are green to yellow-green at first, and sticky to the touch. The galls often function as a habitat for other microorganisms. Cynipid wasps are one of the most frequent occupants. The adult cynipid wasps lay their eggs in the galls. Once the larvae hatch, they feast on the surrounding woody tissue.


  • Knopper gall wasp is not considered a significant nuisance. However, severe infestations can be treated with a registered insecticide.
  • Applications should be administered in spring, when the adults first appear.
  • Several parasitoids, particularly chalcid and ichneumon wasps, inject their eggs into the grubs. The eggs hatch, and the larvae devour the grubs from within.

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