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A Closer Look - Bonsai Pests
Part 2, Scale
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.
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.
The protective cover of the Calico
scale (Eulecanium cerasorum), an
invasive species, is readily apparent.
Photo by R. Gill. From the USDA
website, used under open source
permissions.
scale (Eulecanium cerasorum), an
invasive species, is readily apparent.
Photo by R. Gill. From the USDA
website, used under open source
permissions.
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 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.
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 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.
These invasive scale insects, Citrophilus
mealybugs (Pseudococcus calceolariae),
when disturbed, secrete a red liquid as a
means of defense. Note the two droplets
on the large mealybug in the center.
Photo by Q. Holdman. From the USDA
website, used under open source
permissions.
mealybugs (Pseudococcus calceolariae),
when disturbed, secrete a red liquid as a
means of defense. Note the two droplets
on the large mealybug in the center.
Photo by Q. Holdman. From the USDA
website, used under open source
permissions.
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.
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 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 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.
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 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 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.
With a careful eye, entomologist Gary
Miller takes a closer look at a fern scale
(Pinnaspis aspidistrae).
Photo by Stephen Ausmus. From the
USDA website, used under open source
permissions.
Miller takes a closer look at a fern scale
(Pinnaspis aspidistrae).
Photo by Stephen Ausmus. From the
USDA website, used under open source
permissions.
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.
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.
Scale insect under a laural leaf.
Photo by Ramiro Barreiro, from
wikicommons, used under open source
permissions.
Photo by Ramiro Barreiro, from
wikicommons, used under open source
permissions.