Scale Control on Highbrush Blueberry

Literature reviews report three insect scale pests of highbush blueberry: Putnam scale, Lecanium scales, and terrapin scale. In my surveys of Connecticut highbush blueberry plantings, I have observed that Putnam scale is the major cause of scale infestations. On two occasions, I have observed oystershell scale on blueberry branches. Dr. Cynthia Wescott, in The Gardener’s Bug Book, lists 12 additional scale species attacking blueberry. They are: Azalea bark, cottony maple, European fruit Lecanium, Japanese wax, oak Eriococcus, parlatorialike, and red bay. European fruit Lecanium has been reported on blueberry in Connecticut; but these other species do not appear to be pests at this time. The most common scale in Connecticut and Southern New England is the Putnam scale, which can cause excessive defoliation, decline, and eventual death of blueberry plants.

Putnam Scale Biology and Control

In the last few years, we have seen an increase in Putnam scale (Diaspidiotus ancylus) infestations in Connecticut blueberry plantings. These usually occur in older plantings where irregular or improper pruning has allowed excessive old wood to remain on the bush. Stems and old canes are most likely to be infested, but scale colonization of new growth under heavy infestation and fruit infestations have been seen in some cases. Any planting more than six years old may be a candidate for Putnam scale invasion.

The Putnam scale is an armored scale insect that has a life cycle typical of this large group of pests. The scales pass the winter in Connecticut as fully developed adult insects, which appear as grey waxy dots about 1/16-inch in diameter. With old grey bark and wood that is scaly, these scale encrustations blend in with the color of the bark and are often invisible to the naked eye. The aid of a 10x to 20x hand lens is needed to detect infestations on wood. However, on leaves and fruit the detection is easier. The small grey dots stand out on waxy green leaves and even more so on coloring berries. On fruit, the scale appears to be surrounded by a circular red discoloration. Often the fruit is dimpled at the site of scale attachment. Even a single scale attachment per fruit can completely distort the fruit at harvest.

Life Cycle

If the waxy scale covering is carefully lifted, the immobile yellow insect can be observed. Under this waxy covering, the female feeds, mates and produces living young, called crawlers. The adult male is a tiny winged insect, which emerges from the scale covering and mates with the female. The adult male does not feed. In the spring, female scales lay a mass of eggs under their scale coverings. Around mid-May, the young scale insects, called crawlers, hatch. This stage is highly mobile and can migrate to leaves and fruit. Crawlers are mite-like in size, six-legged, yellow, with two antennae and flattened. As it matures, it stops moving, then settles down to feed. It molts, shedding legs and antennae to become flattened yellow sacs attached to the bark. It then begins to form the typical grey waxy scale covering over its body. These scales also secrete honeydew while feeding. Honeydew can drop onto leaves and fruit below. Black sooty mold may grow on this honeydew.

Putnam Scale Control

This scale is a much greater problem if bushes are not frequently pruned. The best strategy for management of scale insects is an annual pruning of old wood. Putnam and other scales that attack blueberry are principally stem feeders and do not thrive on strong, vigorous wood. Dormant pruning of old, weak canes and scale-encrusted wood prevents the scales from increasing. This should be followed by dormant oil application. Treat from March 1 to first bloom with horticultural oil. Thorough spray coverage of all stems and branches is essential. Large volumes of spray, 200 to 300 gallons per acre, are needed under heavy scale infestations. Use high pressure so that the plant is well soaked. Do not apply oil sprays within twenty-four hours before or after temperatures dipping to 32oF or below. It is best to wait for the temperature to rise above 50oF and the wind to be calm. Oil and lime sulfur, if used for the disease Phomopsis, should be put on as separate sprays and not combined. Sulfur is physically incompatible with superior oils.

By: Norman L. Gauthier, University of Connecticut, Cooperative Extension Educator, Entomologist

Updated by: Mary Concklin, UConn IPM. 2012.

Published in Grower, Vegetable and Small Fruit Newsletter. February 1993. Volume 93-2. p. 4-5.

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