This is the seventh part of a series on summer tree insects. This article examines elongate hemlock scale, and red pine scale.
As spring transitions into summer, temperatures gradually rise, and plants enter the next phase of their development. This period coincides with the appearance of numerous insects, many of which infest vulnerable trees and shrubs. When infestations occur, they can be detrimental to plant health. The following discusses some of the insects that commonly infest plants in summer, and how they impact their hosts.
Elongate Hemlock Scale (Fiorinia externa)
Fiorinia externa, also referred to as elongate hemlock scale or fiorina scale, is an insect native to Japan that infests eastern hemlock, northern Japanese hemlock, Carolina hemlock, fir, and spruce. Elongate hemlock scale may also be observed on cedar, Douglas-fir, pine, and yew, which tend to grow adjacent to infested hemlocks. Elongate hemlock scale has become a problem in North America, where it sometimes occurs with two other exotic pests: the circular hemlock scale Nuculapsis tsugae, and the hemlock wooly adelgid Adelges tsugae. Mixed infestations of scale insects and adelgids can cause hemlocks and other affected plants to decline rapidly.
Elongate hemlock scale develops and reproduces on forty three plant species, representing seven genera of native and exotic conifers, including fourteen species that are native to the United States. Though hemlocks are common hosts, spruce and fir tend to be more susceptible to infestation.
Symptoms of Infestation
Elongate hemlock scale populations typically build slowly on healthy trees. They accumulate more rapidly on stressed, or wounded trees. As the scale feed, they cause the infested foliage to turn yellow, and drop prematurely. Dieback of major limbs may occur, progressing upwards through the tree. Dieback generally occurs when populations exceed more than ten individuals per needle. Severe infestations can decimate trees, with many retaining only a sparse amount of foliage at the top of the crown. Successive defoliations over several years can contribute to a general decline in tree vigor. Some trees may succumb to the infestation, eventually failing. Weakened trees are more susceptible to invasion from secondary pests and fungal pathogens. Outbreaks often intensify following infestations from secondary pests, drought, and other stresses.
- To discourage the accumulation of scale populations, maintain tree vigor through sound cultural practices.
- Hemlocks have shallow root systems, which renders them susceptible to drought stress. As such, ensure that hemlocks and other plants are sufficiently watered, especially during dry periods.
- Avoid excessive applications of nitrogen fertilizer, and insecticides. These chemicals can contribute to increases in scale populations. Nitrogen enhances the survival, and development of the scale, resulting in dense scale populations on fertilized trees. Inadequate, or improper insecticide applications can eliminate the scale’s natural predators, leading to a resurgence in scale populations.
- Horticultural oil can be applied to ornamental plantings during early spring, when trees are dormant. A second application should be made during the growing season. This will help limit scale population density.
- Aspidiotiphagus citrinus, an aphelid parasitoid, eradicates more than 90 percent of each generation of elongate hemlock scale in Japan.
- A few coccinellid beetles, the twice-stabbed ladybird, Chilocorus stigma, and Microweisea misella prey on the scale in North America, though their impact is not severe enough to significantly alter scale populations.
- In forests, declining hemlocks should be salvaged to prevent the accumulation and spread of scale populations.
Red Pine Scale (Matsucoccus resinosae)
Red pine scale is a small exotic insect that infests several species of pine. The insect’s preferred host is red pine. Red pine scale was first discovered in 1946, infesting red pine plantations near the Hemlock Reservoir in Easton, Connecticut. The insect is believed to have been introduced to the United States on exotic pines planted at the New York World’s Fair in 1939. By 1950, scale infestations had spread to New York. Additional infestations has since been discovered across much of New England, and several neighboring states. Dense populations are capable of decimating host trees.
Red pine scale infests red pine, Japanese pine, Japanese black pine, and Chinese pine in nurseries, landscapes, and forests.
Symptoms of Infestation
The foliage on infested trees will gradually change from olive green to light yellow, before finally deepening to a brick red. Discoloration initially appears on individual branches on the lower part of the crown, before progressing upwards. New growth may be stunted. When populations are dense, masses of cottony white filaments will appear on the affected branches. Trees weakened by red pine scale are more susceptible to invasion from disease pathogens and secondary pests, such as bark beetles. Red pine scale, coupled with diseases or bark beetles, can result in rapid tree mortality. Tree mortality occurs soon after the foliage has turned a dark reddish-brown. The bark on infested trees will appear swollen and cracked. If the bark is stripped away, dead tissue will be found beneath each feeding scale.
- Red pine scale has several natural predators, including the anthocoroid bud, Elatophihus inimica, the coccinellid beetle, Mulsantina picta, a hemorobiid, Hemerobius stigmatus, and a chrysopid, Chrysopa. Though each of these insects prey on red pine scale, they are often not abundant enough to effectively limit scale populations.
- There is no effective chemical control for red pine scale. Two applications of a two percent oil emulsion can help prevent infestations on ornamental plantings. The first application should take place in early June, with the second administered in early September.
- Maintain tree vigor through sound cultural practices. Ensure susceptible trees are sufficiently watered, especially during periods of drought. Apply a layer of organic mulch around the base of pines to moderate soil temperature, improve soil quality, and preserve soil moisture.
- Avoid fertilizer applications, as they create favorable conditions for red pine scale.
- Winter harvests of infested trees will reduce the incidence of outbreaks. Logs produced by harvested trees should be pealed. The branches and bark of harvested trees should be burned.
Photo courtesy of Eric R. Day, Virginia Polytechnic Institute and State University CC-by-3.0