The cottony maple scale, Pulvinaria innumerabilis (Rathvon), is a highly modified insect pest that commonly attacks silver and red maples in Ohio. The scales are usually first noticed when the females produce an egg sac which appears as a 1/4 to 1/2-inch long ball of cotton. Heavy infestations can result in branches being turned completely white with the egg sacs. Like most scale insects, the nearly mature insects, the adults and the eggs are resistant to pesticides and the anxious tree owner must wait until the correct time for management.
Cottony maple scales reach epidemic numbers on silver maple but noticeable populations can occur on red maple. It is also known to be able to survive on other species of maple, honey and black locust, white ash, euonymus, oak, boxelder, dogwood, hackberry, sycamore, beech, elm, willow, basswood, and poplar.
Normally, this scale is a mere curiosity and nuisance. The white egg sacs easily attract attention and the developing scales produce honeydew. Honeydew is the excess water and sugar excreted by many plant sap-feeding insects. Honeydew is commonly mistaken for "plant sap" being dropped on cars, sidewalks and lawn furniture lying under trees. When honeydew collects on leaves and branches, bees, wasps and ants are attracted to the area. If the honeydew is allowed to remain, molds called "sooty fungus" grow on the material, turning the surface a gray-black color.
Occasionally, heavy outbreaks of this scale occur, usually on weakened or stressed trees. These outbreaks can cause the death of numerous small branches and occasionally the death of a tree.
Mature cottony maple scales are small, flat, oval, brown insects without obvious legs, antennae or wings. They are firmly attached to the twigs and branches of various trees and may be 1/4 to 3/8-inch in diameter. At maturity, the females produce the white, cottony egg masses (called ovisacs) over a period of several weeks. The "cotton" is really waxy threads and the ovisac may contain over 1,500 eggs. These eggs hatch from mid-June to August and the young nymphs are called scale crawlers. These microscopic crawlers are small, flat, oval insects with two distinct eyes, short antennae and tiny legs. The crawlers walk onto the leaves and tend to attach alongside the major leaf veins, usually on the underside. Here, the nymphs produce copious amounts of honeydew and grow by molting once. By this time the scales look like two different kinds, a translucent white to pink form and a larger, flat, tan form. These are males and female scales, respectively. In September, the male scales emerge as tiny winged gnat-like insects that move around on the leaves in search of females. After mating, the males die and the females soon withdraw their mouthparts and crawl back onto small twigs and branches. Here, the reinsert their mouthparts and settle down for the remainder of their life. These females first appear greenish with a white powdery coating and they are about 1/8-inch long. By winter, they have turned buff in color and in the following spring and early summer they turn a chestnut-brown. When the sap begins to flow in the spring, the females continue to grow and they again produce considerable amounts of honeydew. By late May to early June, the females have matured and they begin to produce their ovisacs.
This pest has numerous parasites and predators that normally keep its populations in check. However, every few years, these natural controls seem to fail and the cottony maple scale can reach epidemic proportions. Unfortunately, weak trees can be severely damaged during these outbreak years. Healthy trees often loose a few small branches and have no apparent long term problems. What makes management of this scale more difficult is the fact that insecticide applications may actually cause the scale problem to continue longer than normal. This is caused by poor timing of a pesticide application that kills most of the parasites and predators but allows survival of the resistant scale stages.
Strategy 1: Cultural and Biological Control - Silver maples in restricted street lawns or suffering other stresses from drought, compacted soils, poor fertilization, etc. are the ones most likely to exhibit major branch or crown dieback. Improving the tree growing conditions will significantly help the trees survive the cottony maple scale attacks until the natural predators and parasites build up to effective levels. The most important predators are several black lady beetles that have two red spots on the upper surface. The most common one, Hyperaspis signata, has a larva that looks like a mealybug. Both the adults and larvae enter ovisacs to dine on the egg masses. This causes the ovisacs to have a very tattered look by mid-July. All of these lady beetles continue to dine on the nymphs that settle on the leaves. The settled nymphs are also attacked by several tiny wasps. These lay eggs in the nymphs and their developing larvae devour the pest from the inside out. Affected scales often turn brown or black and have tiny round emergence holes on their back surface.
Strategy 2: Use "Soft" Pesticides - Soaps and horticultural oils can be very effective in managing the freshly settled crawlers. These materials also have a minimal adverse affect on the adult lady beetles and parasites already in settled crawlers. Insecticidal soaps or 1.5% horticultural oil must be applied thoroughly to the leaves, both to the underside and upper surfaces, in order to kill the scales. Soaps and oils only kill the pests on contact. Application of soaps or oils should be made in mid-July and again in early August, if additional crawlers are found.
Strategy 3:Standard Insecticide Applications - Several insecticides are registered for control of scale crawlers and newly settled crawlers. These pesticides, again, often need to be applied in sufficient spray quantity to wet both the leaf upper and lower surfaces. Apply registered products in mid-July and again in the second week of August for best control. See OSU Extension Bulletin 504 for insecticides currently registered for scale crawlers.
Strategy 4: Dormant Oils Sprays - Dormant oil sprays have been traditionally used to manage many scales on ornamental trees. Unfortunately, many maples are sensitive to these oil sprays and significant small branch dieback or spring leaf drop can occur, if the dormant oil was applied after the maple sap has begun to flow.
NOTE: Disclaimer - This publication may contain pesticide recommendations that are subject to change at any time. These recommendations are provided only as a guide. It is always the pesticide applicator's responsibility, by law, to read and follow all current label directions for the specific pesticide being used. Due to constantly changing labels and product registrations, some of the recommendations given in this writing may no longer be legal by the time you read them. If any information in these recommendations disagrees with the label, the recommendation must be disregarded. No endorsement is intended for products mentioned, nor is criticism meant for products not mentioned. The author and Ohio State University Extension assume no liability resulting from the use of these recommendations.
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Keith L. Smith, Associate Vice President for Ag. Adm. and Director, OSU Extension.
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