Lecanium scale numbers building on chestnuts

Chestnut growers in some areas of Michigan are experiencing high levels of Lecanium scale infestation. Growers with young, susceptible trees should be particularly diligent in scouting for this pest.

Lecanium scale on chestnut. Photo credit Dennis Fulbright, MSU
Lecanium scale on chestnut. Photo credit Dennis Fulbright, MSU

Lecanium scale is a soft scale pest that attaches to and feeds on many deciduous plants in Michigan, most notably hardwood trees. Lecanium scale populations can build to extremely high levels over a series of favorable years before crashing due to natural enemies and disease. When present in low numbers, lecanium scale are often overlooked and are of little economic or aesthetic significance.

Lecanium scale has one generation per year. Mated females overwinter on the host plant and appear black in the spring before they molt into larger, tan scales. These larger, mature scales lay eggs under their turtle-like exoskeleton before dying. The eggs hatch and the immature scales known as crawlers disperse from the protective covering to find new places to feed and develop, returning to woody material where they mate and attach to the host for the winter. Dead scales that have already laid eggs may persist on the host for an extended period of time and are dark in color and appear dusty.

Growers need to check chestnut branches for evidence of this pest. Scales may look a lot like leaf scars or bud scales, so a close inspection is important. Some infestations have only a few scales per branch while others are well-covered. One tell-tale sign of scale activity is the shiny film of honeydew that the scales excrete, similar to aphids, onto surfaces beneath the scales. Increased ant activity is also associated with scale insects as they collect the honeydew.

As mentioned above, the scale’s feeding creates honeydew that can act as a substrate for sooty mold. On agricultural crops, sooty mold can become an issue when it ends up on the fruit or nut being produced. Additionally, the scales removes sap when feeding which can weaken shoots and even cause shoot death in some cases. The rule of thumb is that vigorous and healthy trees and plants can tolerate some scale infestation, but if high populations of Lecanium scale are found, control programs should be considered, particularly on small trees.

Natural enemies usually regulate scale populations and prevent outbreaks of these pests, but growers with high populations this 2014 season may need to consider chemical control options. Dormant oil applications that smother overwintering scales are the preferred method of control, but it is too late for oil applications at this time. Most other insecticides that are active on scales are used at the crawler stage, after the insect has emerged from the waxy covering of the mature female. Crawlers are much more vulnerable to insecticides than the mature scales and may be active at this time, generally emerging in June and remaining active through July in Michigan. Growers can scout for crawler emergence by placing some double-sided sticky tape near the scales on infested shoots and checking with a hand lens until you see tiny dots (the young crawlers) on it.

There are a number of insecticides labeled against scale for chestnut growers in Michigan. Refer to “Pesticides Registered for Edible Chestnuts, 2014” by Michigan State University Extension for more information.

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