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.
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 make 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, 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.
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 three gallons of Superior 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 at 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
Published in Grower, Vegetable and Small Fruit Newsletter. February 1993. Volume 93-2. p. 4-5.
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