Insect Pests: Scale
By Heather Coste
Scale are a common garden variety pest in temperate regions. Scale are small insects of the order Hemiptera, generally classified as the superfamily Coccoidea. There are about 8,000 species of scale, with many of that rather large number considered pests on our plants. Scale are one of the most destructive insects in the United States for the home gardener, with many species invaders from other areas of the world. They can survive in harsh conditions, from the tundra to the tropics, and thrive on all parts of a plant, commonly the underside of leaves and on branches, but are not limited in their location habits.
There are three general categories scale insects fall in to: most common being armored scales; soft scales; and mealybugs. The armored scales are the ones with the most effective outer coating, and subsequently, the most difficult to deal with.
Anatomy of a Scale insect: Scale insects vary greatly in size, as well as appearance. They can be very tiny, at only a millimeter in diameter, or a little larger, though few are larger than about five millimeters across. Scale come in almost any colour, from brown to green, white to yellow, all depending on the species, and range in shape, including round, oval, pear shaped, oblong or threadlike. They are covered in protective coatings, ranging from shiny waxy covers to pearl-like, from woolly to oyster-like. These coatings are excreted by the insect for protection, the scale living and feeding under the protective coating. While there is a wide range of appearances, most resemble the scales of a fish or reptile, or like scale armor of the medieval ages (hence, the common name!). The armor of female scale are often larger and more obvious than that of the male. The appearance of the armor can vary not only with species, but also the host plant the scale is feeding on, gender, life cycle stage, time of year, and other environmental conditions, which can make identifying the particular species, or even genus, of scale by the layman, very difficult. They have piercing and sucking mouth parts.
Scale Habits: Most scale are plant parasites. They feed on the sap of a plant, usually pulled directly from the vascular system. While one or two scale may not be a cause for alarm, a host of these little critters feeding on a plant will quickly lead to problems. Almost every woody plant is a target for one scale species or another. Some scale feed exclusively on one type of plant. Others target a small number of species, while still others are happy to feed wherever they end up. Scale are around year round. They can over winter in any life stage (see Reproduction/Lifecycle below), but eggs and mated females have the best tolerance and survival of low temperatures. Once the weather starts to warm up in temperate climates, the new flush of growth we see in our plants coincides with egg hatching, though there may be later and earlier hatchers, depending on the scale species and host plant.
Soft scale and mealy bugs can excrete honeydew, sometimes in large amounts. Sooty mold happily grows on this stuff, and while it is generally harmless, it is unsightly. Ants are also attracted to the honeydew excreted. Often homeowners will notice the sooty mold or ant infestation without ever seeing the scale.
Scale Reproduction/Lifecycle: The sexual dimorphism (differences in appearance between male and female) is huge (relatively speaking) in scale insects. Mature female scale are sessile (which means they are unable to move), have no legs or antennae, usually much larger than the males, and are usually the ones we see when we notice a scale infestation. Mature males are smaller, typically have visible wings (one pair, thus making them resemble true flies), do not feed (they don’t even have mouth parts), and die in a day or two.
Reproduction varies between the different species, but can still be generalized. Eggs are laid beneath waxy coatings or the female’s body itself, and take about one to three weeks to hatch. Newly hatched scale are called ‘crawlers’, or more scientifically, first-instar nymph crawler stage. Phew, long name for crawly little bugs that eat our plants. These crawlers do exactly what it sounds like. They crawl around until they find a suitable place, and then plunk down and start feeding on the host plant. Wind can also spread these nymphs to other plants. As they get larger, they molt, with females losing their antennae and legs in the process of the first molt. These females molt a second time before reaching maturity with no middle stage (pupate) between juvenile and adult forms. The molted skins (also called exuviae) are often incorporated into the scale coating. Males molt several more times, and do have a pupal form between nymph and mature scale, pupating beneath the scale coating. Once they emerge they look more like small gnats than the larger female scale we are familiar with. In some varieties, the adult stage is reached in as little as five or six weeks, with several generations possible in each season.
How do you know if you have Scale: Scale come in all shapes and sizes, but the appearance of small, scale-like or woolly *things* on your plants, especially the trunks and undersides of leaves, is usually a good indicator. There may be no other signs until an infestation is serious. A serious infestation will be obvious by stunted growth, yellow spots on the top of foliage (caused by scales feeding on the bottom part of the leaves – these spots will get bigger and bigger the longer the scale feeds), premature foliage drop, and dieback of young twigs and even whole branches if allowed to continue untreated. An untreated scale infestation could easily be the death of a plant.
Fortunately however, these are easy to spot early just by keeping a keen eye on your plants. Also keep an eye out for sooty mould or ants, which can be a sign of both scale and aphids.
Scale can live on any part of a plant, so beside checking the most common locations (the stem and under sides of the leaves) be sure to also check in leaf axils, buds, the tops of leaves, along the midveins of leaves and anywhere else you can reach. A magnifying glass can be useful if in doubt, as some scale can look less like bugs and more like naturally occurring bumps on a leaf or stem surface. Crawlers can be detected by wrapping double sided sticky tape around a branch and seeing what pops up over a day or two. Pay careful attention to old wound scars on trees or shrubs. The bark may be thinner here, especially if the wound is only a few years old, and the folding of the healing bark can hide scale effectively from view. The colour differences between bark and scale can be very subtle. Wetting the bark can often help differentiate between normal bark bumps and scale.
Examine plants for live scale insects by crushing the wax cover. Dead scales are dry inside. Live ones… well…. are not.
Scale prevention: The best prevention for scale is healthy, happy plants and early detection. When plants aren’t stressed, they have better defenses against infestations. Don’t over plant an area, leaving the right amount of space between plants to ensure good airflow. Keep plants well watered, but not over irrigated. Scale thrive in moist conditions, so be certain plantings are being watered at the base, rather than soaking the entire plant regularly. Be certain of the light requirements (sun or shade) for new plants, and locate them accordingly. Don’t over fertilize. Chemical fertilizers high in nitrogen produce large amounts of just the right kind of growth that attracts scale. Scale will also lay more eggs on plants receiving more nitrogen. Slower acting, organic fertilizers are better for helping prevent these infestations, especially if they have been a repeated problem in your landscape.
Scale Removal: If despite your best efforts, you notice a scale infestation on one of your trees or shrubs, don’t panic! There are several steps you can take. Scale is a difficult insect to get rid of using common insecticides. Because in mature scale (which is usually when we notice an infestation) their waxy coating protects them from most insecticides, careful treatment is necessary.
In cases of only a few scale, the best bet is to just remove them with your finger nails or a stiff bristled brush. Use a strong jet of water afterward to wash off the plant and any possible eggs that might have been left behind. Continue to check back every few days or so, just in case.
Scale are preyed upon by parasitic wasps. Check for tiny holes in the outer armor which is a sign that the beneficial wasps have already been on the job. Some other beneficial insects, such as lady bugs, green lace wings and praying mantis feed on the nymphs (crawlers), but not on mature scale. These beneficial insects will stick around as long as there is food (i.e. scale crawlers, aphids, etc), but you can encourage them to stick around longer by planting certain things that attract them (yarrow, cilantro, parsley and sweet alyssum are all plants that lady bugs in particular like). Lady bugs and praying mantis eggs can be purchased at most garden centers now, as an alternative to chemical pesticides. If you are going to try beneficial insects, make certain you don’t use any pesticides, as these will kill the good bugs as well as the bad.
Insecticidal soap or horticultural oil (like neem oil or dormant oil) can be used against all stages of scale growth, including mature scale (it suffocates them), and is generally safe for most types of plants, but check the label, and if in doubt, do a test on a small portion of the plant to be certain it does not cause an ill effect. An even coat that gets every part of the plant (especially the underside of the leaves!) is necessary. These only work on contact, and do not provide long term prevention. If a spot is missed on the plant, the infestation can balloon again once the oils have dried, and applications may need to be repeated. Do not use oils on water stressed plants (those that have been under watered and have wilted, or those that have been over waters and may have root rot), or if the weather exceeds 90F (30C), as this can harm the plant. Supreme- or superior-type and dormant oils will kill overwintering populations when applied in late autumn and again in midwinter. These can be safe to use in conjunction with beneficial insects.
Most insecticides that list scale on their labels are really only useful during the crawler stage, or on mealy bugs. If use of an insecticide seems necessary, use the double sided tape method to watch for crawlers and spray then. If possible, prune off some of the infested foliage before treating, not only to remove the scale, but also to allow deeper penetration of the insecticides. Follow label instructions, coating the plant thoroughly on all surfaces (especially under the leaves!). Some systemic insecticides may be useful, especially when plants are too large to spray effectively. Repeat the process if needed (which is likely) one to three weeks later. General guidelines are to follow the instructions on the bottle for usage, including timing and dosage. If in doubt, test it on a small area of the tree. If there are no negative affects (dying leaves, blackening bark), you can probably proceed with the rest of the tree.
Even once scales are dead, they will not always fall from your plant. You can tell a dead scale from a live one by crushing the outer coating. Dead scale are dry on the inside. Live scale will squish. There is no way to remove dead scale, other than manually. Use your fingernails or a stiff brush to remove them from the bark and leaves, or leave them be.