A Closer Look – Bonsai Pests Part 2, Scale
Scale are another ubiquitous garden variety pest in temperate regions. Scale insects 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, 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 Diaspididae, or armored scales; Coccidae, or soft scales; and Pseudococcidae, commonly known as mealybugs. The armored scales are the ones with the most effective outer coating, and subsequently, the most difficult to deal with.
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 tree 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.
Scales 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 (black) mould happily grows on this stuff, and while it is generally harmless, it is unsightly. Ants are also attracted to the honeydew excreted, though they do not herd scale the same way they do aphids. Armored scale do not excrete honeydew.
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.
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.
All scale have (or had) antenae and six legs, though mature females lose (or almost lose) these features completely. They have piercing and sucking mouth parts.
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. An untreated scale infestation could easily be the death of a tree. Fortunately however, these are easy to spot early just by keeping a keen eye on your trees. 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 so common to bonsai. 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. Keep a good distance between your trees to keep good airflow, adequately water your bonsai, and make sure they are getting the right amount of light for their species preferences. Don’t over fertilize. Chemical fertilizers high in nitrogen produce large amounts of just the right kind of growth that attracts aphids and scale. Scale will also lay more eggs on plants receiving more nitrogen. Slower acting, organic fertilizers are better for helping prevent these infestations. While we go for good growth in our trees, this is another reminder for us that balance is the key to all things in bonsai. Scale can also thrive in thick inner canopies of trees, so keep your trees from getting too over grown in the interior. Allowing light and fresh air to penetrate will help prevent other diseases as well. Because they thrive in warm, moist areas, adding additional fans to increase airflow may help. Keep new plants isolated for a week or two before putting them near your other bonsai. This way, if there are scale nymphs (harder to spot than mature females) on the tree that you may have over looked at purchasing, you have a chance to catch it before it spreads to your other trees.
Scale Removal: If despite your best efforts, you notice a scale infestation on one of your trees, don’t panic! There are several steps you can take, ranging from the simple to the more complex. 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, other courses
must be followed.
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 bonsai, 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 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 as much of the infested foliage as possible before reating, 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 as well. Repeat the process if needed (which is likely) one to three weeks later. Some insecticides are safe for bonsai. Others are not. 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.
Want to know exactly what kind of scale you are dealing with? Below are two online resources that may help further identification of your scale infestation. There are too many types to go over here, and it would be a disservice to everyone involved if I tried.
Scale Keys – http://www.sel.barc.usda.gov/ScaleKeys/index.html
Scalenet – http://www.sel.barc.usda.gov/scalenet/scalenet.htm