Lecanium Scale in Highbush Blueberry
By Greg Welfing
Pest Profile is a recurring feature in this magazine. Periodically we pick an agricultural pest to feature and learn more about its biology and control methods.
Lecanium scales are a group of closely related scale insects. There are multiple species present in Fraser Valley blueberry fields. They used to be all grouped together in one genus (Lecanium spp.) however they have since been grouped into multiple genera. Scales are hemispherical in shape, brown or dark red in color, and small (~5mm in diameter) when mature. All species have one generation per year. They overwinter as immature forms on blueberry plants and many other hosts. They mature through early spring and mate. A female will lay eggs starting in May-June underneath her shell-like protective covering. The eggs will hatch into nymphs (crawlers) in late June – early July. The nymphs will move to the underside of leaves and feed on the plant sap throughout July and August and then return to twigs and stems to overwinter. Once settled on an overwintering location, they begin to secrete their protective waxy covering. They will continue to feed through early fall.
Scales are a sucking pest. They penetrate the bark with their long mouth parts and feed on plant sap. The resulting damage takes the form of stunting, distortion, or wilting of the host plant. Scales can also cause indirect plant damage. As the pest feeds, it secretes a sweet sticky substance called honeydew. This secretion is then fed upon by fungi and sooty molds. When the honeydew secretion gets on the fruit, it will be a harvest contaminant and lead to decreased yields.
There are three main methods of control for this pest. The first is biological control. Scales are fed upon by many beneficial insects including ladybug beetles, parasitoids, and green lacewings. The majority of this predation occurs against the crawler stage (mid-summer). In many seasons, this biological control is enough to keep the scale populations in check and economic damage to a minimum.
Mechanical control via pruning is the second method. When performed properly, management of scale by pruning is the most effective option at reducing pest numbers within your field. Scales tend to thrive on older, less vigorous wood with more bark and not on younger vigorously growing stems. The older wood should be pruned out yearly anyway, so pruning out this insect does not cause significant crop loss or increased cost.
During some years, the beneficial insects cannot keep up with the scale population and an outbreak may occur. If this happens, chemical control is an option. The general consensus on the best time to spray is in the late dormant period (February) in order to protect the natural predators. The most common product used for control is horticultural oil/dormant oil. Dormant oils kill insects by suffocation. Scales like to hide in the flaky bark on older branches—even on the main stem—so thorough coverage of the whole plant is essential. A few other insecticides are registered for use during the summer when the crawler stage is present, however toxicity to bees and other beneficial insects is a concern at this time.