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

This is the sixth part of a series on spring tree insects. This article examines Japanese beetle, knopper gall wasp, and pine needle scale.


In spring, deciduous trees break dormancy, and resume the growing process. During this period, many insects become active, emerging from their overwintering sites to plague their hosts. The following describes some of the insects that may be commonly observed in spring, and how they can impact susceptible trees.

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. 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 can be used for lawn application.
  • When planting, select trees and shrubs that exhibit an increased resistance to Japanese beetle. 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. The word knopper is derived from the German word ‘knoppe’, which is defined as a swelling or protuberance. It also refers to a type of helmet worn during the 17th century. 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.

Pine Needle Scale (Chionaspis pinifoliae)

Pine needle scale, also called white scale, is a tiny armored insect that infests various species of pine. The insect is notable for the distinct wax covering the adults produce to conceal their eggs. Severe infestations can cripple host trees, causing significant foliar dieback, and reducing tree vigor.


Pine needle scale attacks a slew of hosts, including Austrian, eastern white, mugo, red, and Scots pine. Scots pine is especially prone to infestation. Infestations of pine needle scale also occur on Douglas-fir, eastern red cedar, and spruce, albeit with less frequency.

Symptoms of Infestation

The feeding of the larvae, or crawlers, causes the needles of infested trees to become discolored. Infested needles will turn yellowish-brown in the area surrounding each sessile scale. When trees are heavily infested, needles, shoots, or entire branches may die back. Significant foliar dieback can reduce the growth rate of the tree. This often results in affected trees developing sparse sections of the crown. The foliage of severely infested trees may appear gray or white-washed, due to the prevalence of waxy coverings on the host’s branches.


  • Registered insecticides may be applied to combat the two generations of crawlers that develop. Initial applications should begin in May, with subsequent applications performed from mid-July to August at three to four week intervals.
  • When the crawlers are stationary, they become vulnerable to several natural predators, including lady beetles and wasp parasitoids. The life cycle of two species of lady beetle, the twice-stabbed ladybird, and Microweisia misella, is synchronous with that of pine needle scale. When present, these two species of lady beetle may drastically limit crawler populations.
  • Horticultural oils can be utilized to smother the insects. This method will not harm any beneficial insects, and is safe for application to organically grown crops.

Photo courtesy of Katja Schulz CC-by-2.0

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