I recently visited two landscapes in southwest Ohio with medium-sized red maples (Acer rubrum) there were heavily infested with cottony maple leaf scale (Pulvinaria acericola). However, I would not have known the trees were infested had I not walked beneath their canopies.
It's amazing how well these highly visible soft scales can remain concealed awing to their location on the underside of leaves. This is particularly true with the drooping leaves of red maples.
The cottony maple leaf scale is so named because the females exude their eggs in an elongated, white, cottony structure called an ovisac which are stuck onto the underside of leaves. A close examination of the ovisacs reveals the eggs are held inside a sack-like matrix of sticky, silk-like material.
The ovisacs are elongated because the female crawls forward as she's laying her eggs. She dies once she finishes and her body remains visible as the end of the ovisac appearing as a shriveled, reddish-brown husk. On the average, her ovisac may contain more than 2,500 eggs. Small wonder she's shriveled!
The closely related cottony maple scale (P. innumerabilis) also produces ovisacs but they are attached to stems. The only time they appear on leaves is when populations are exceptionally high. Both of the cottony scales have life cycles that are very similar; however, the cottony maple scale appears to have a wider host range.
The cottony maple leaf scale prefers maples which isn't surprising given the scale's common name. They are most commonly found on red (Acer rubrum) and silver maples (A. saccharinum); however, they may also produce damaging populations on sugar maple (A. saccharum) and boxelder (A. negundo). Other occasional hosts include Andromeda (Pieris Japonica), flowering dogwood (Cornus florida), hollies (Ilex spp.), and sourgum (Nyssa sylvatica).
Cottony maple leaf scale eggs begin hatching when the accumulated Growing Degree Days (GDD) reach 1,216. The full bloom of goldenrain tree (Koelreuteria paniculata) is a pretty good phenological indicator of the event.
The first instar nymphs (crawlers) that hatch from the eggs are cream-colored to pale green and very small making them difficult to see with the unaided eye. Pieces of double-sided sticky tape are highly effective with helping to detect egg hatch and dispersion of the crawlers.
The crawlers scatter over several leaves and settle along the leaf veins where they insert their piercing-sucking mouthparts into phloem vessels to suck sap. Their primary target is the various amino acids that are dissolved in the sap; however, they must suck-up a large volume of sap in order to extract the small amounts of amino acids contained within the sap.
Once they process the sap, the excess liquid is discharged in the form of sticky, sugary, "honeydew" that drips from their anus onto branches, leaves, understory plants, lounging gardeners, etc. It's common for the honeydew to become colonized by black sooty molds to produce an unsightly mess on anything located beneath the canopy.
The nymphs molt into second instars as they feed and eventually migrate from the leaves to twigs in early fall. In the spring, the male and female nymphs mature, mate and the females crawl to the underside of leaves to lay eggs starting the life cycle all over again. There is one generation per season.
Although high populations can occasionally cause premature leaf drop and branch dieback, the overall impact of the cottony maple leaf scale is strongly related to plant health. Impacts are much greater on trees that are stressed. Thus, the first line of defense against this and other soft scales is to focus on maintaining tree health.
Control is seldom required particularly since there is a wide range of beneficial insects such as predators and parasitoids that can keep scale populations in check. For example, lady beetles belonging genus Hyperaspis are commonly found chowing-down on this and other soft scales.
Hyperaspis lady beetles are collectively referred to as sigil lady beetles. This the second largest coccinellid genus with 94 species found in North America north of Mexico. These small, round beetles sport markings of various shapes and colors on a black background.
However, it's the larvae that are the real show-stoppers! In a twist on "you are what you eat," the larvae are typically covered in white, flocculent material making them blend with the white scale ovisacs. Of course, these are wolves in sheep's clothing. Unlike the immobile ovisacs, the lady beetle larvae are highly mobile and can be observed moving around to feed on the scale flock.
If control is deemed necessary, systemic insecticides will have less of an impact on beneficial insects compared to topical contact insecticides. NC State has an excellent fact sheet on cottony maple leaf scale that includes insecticide recommendations that are compatible with beneficials.
You can access the NC State Fact Sheet by clicking this hotlink: