Scale at a glance
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Type of Damage |
Pierce-sucking |
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Plant Symptoms |
Stunted growth, leaf drop, leaf curl, death |
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Favourite Plant |
Many trees and shrubs, citrus especially |
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Occurrence |
Year-round in areas with mild winters (SA) |
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Distribution |
Worldwide |
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Control |
Moderate: physical control and preventative measures preferable, biological control agents available |
Quick Intro
Scale insects are found across the world in all gardens, where both ornamentals and vegetables are vulnerable to infestation. Scales are often overlooked due to their small size but easily reach epidemic proportions that can kill off even the healthiest of plants. The best way to control scale is to prevent them from establishing on plants the first time round and to make sure that you control their ‘ant’ herders as well.
Science Stuff
Scale belonged to the previously obsolete Homoptera order, which were insects that had fully membranous wings. Now they have been allocated to the Hemiptera order, which include insects with half of their wings membranous and the other half covered in a thicker elytra. They are placed under the Sub-order; Strenorrhyncha and Superfamily; Coccoidae.
There are 23 families in total, of which 3 are of main concern to edible gardeners:
Coccidae – Soft scale
These are very small insects, 1-2 mm contained within a waxy shell. They can be a myriad of colours, but the most common are light tan – black. Adult females lose their legs, becoming sessile and remain feeding on the same place for the duration of their life cycle. Males are short lived, 1-2 days, winged and never feed. Therefore the main damage to the plants is induced by the immatures and females.
Monophlebidae – Cottony cushion scale
These are larger scale insects; immatures are 1-2 mm, whereas adults can be as large as 1 cm. They have soft, white bodies (do not have a waxy cuticle). These are hermaphrodites, meaning that no true males or females are present. They keep their eggs within their bodies and after hatching; bear live young. Adults retain their legs (unusual for scale insects), yet have limited mobility.
Pseudococcidae – Mealybugs
Again, small about 1-2 mm grey or white bugs. They do not have a hardened waxy shell, but secrete a waxy coating onto their bodies and eggs (hence the name mealybug). They can often be confused with mealy aphids that have similar appearance and feed on the same host plants.
Hosts
Woody trees and shrubs are most often targeted, this means any fruit trees or perennial herb are likely to be attacked. Citrus, and its relatives, such as curry plants are most often infested, especially during the winter months when trees become dormant and more susceptible to attack.
Life Cycle
Immature scale can be transported via the wind to new hosts. Afterwards it seeks out a suitable feeding place, such as safe crevices or joints of branches. It will likely remain in the same place throughout its entire life cycle. Depending on the species, there can be one or two nymphal (instar) stages. Most adult female scale, except for mealybugs and cottony cushion scale, lose their legs and become sedentary. Males die soon after fertilising females. Many scale species are parthenogenic, where unfertilised females give rise to ‘clones’ without any genetic contribution from the males. Some scale lay eggs outside their bodies, but most retain the eggs under their shell and once the eggs have hatched the adults ‘bear’ live young. Each generation the immatures, known as crawlers, move away from the adults and settle upon new terrain until the whole plant becomes occupied or the nymphs are swept away by the wind to start a new colony elsewhere.
Control
Multiple natural control remedies are available for scale, although several applications or rounds of scale removal will be necessary to ensure that the entire colony has been destroyed.
Firstly, physical removal is the most successful, but can be labour intensive. This means arming yourself with a blunt butter knife or twig or whatever tool is handy and scrape them from the plant bark. You do not require an excessive amount of force to remove them and once you have dislodged them they will die. I know of somebody who grabs a bucket of soapy water and a brush, then scrubs down her citrus trees when the winter infestation strikes!
Natural chemical control includes the use of horticultural white oil (see my Pest Control page for the recipe). Another option is to mix 50% v/v isopropyl alcohol with 1% w/v SDS, the alcohol dissolves the waxy cuticle and the SDS attacks the soft body beneath. Now I don’t think that these chemicals are too readily available to the public, so you can substitute it with ethanol and sunlight liquid – The exact quantities would be determined through some trial and error whereas the effect on the plant might be unknown (I would recommend avoiding the leaves of the plant should you wish to go this route).
Another alternative is diatomaceous earth, which contains small sharp particles that cut the bodies and leads to desiccation. This will control both the scale and ant herders, but might be a bit hard to come by or expensive.
Several insects parasitise or predators prey upon scale. Those that make up the front line in biological control are:
|
Biological Control Agent |
Parasite or Predator |
Prey species |
|
Metaphycus helvolus, Encyrtus spp., Encarsia citrinia, Aphytis spp., Coccophagus spp. |
Coccus hesperidum, Saitettia coffeae |
|
|
Cryptochaetum iceryae |
Icerya purchasi (cottony cusion scale) |
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Chilocorus spp., Hyperospisspp., Rhyzobius spp. Rodolia cordinalis (Vedalia ladybug) |
Various species Icerya purchasi (cottony cusion scale) |
The tiny parasitic wasps are most effective in greenhouses as they are simply swept away by the wind in outdoor gardens. Ladybeetles are more readily available, but will only arrive and remain in the garden if the scale populations are very high. Ladybeetle larvae feed beneath the scale insects and some of the adults resemble scale insects, such as the Mealy bug destroyer, so be careful not to harm them during your scale control efforts. Please refer to my Ladybird and Wasps Biological Control Posts for more detail information about each insect.
Preventative Tips
The old adage that ‘Prevention in better than Cure’ certainly applies to scale insects. This comes down to two crucial points:
1) Prevent scale attachment
2) Limit ant access
This can be done by applying Petroleum jelly, such as Vaseline, to the base of the tree or shrub BEFORE infestation. Scale insects cannot move past this barrier which effectively keeps them off the plants. BUT, ants – AKA herders – seek the honeydew (sugary secretion) that scale and other pests produce. Ants will actively tend, protect and transport scale to hosts plant to keep them like cattle and harvest the honeydew. This is the biggest problem, because the ants do figure out how to manoeuvre over the Petroleum jelly and bring along their herds.
I have found that fresh Vaseline (still sticky) works very well until it becomes clogged with dust or develops a crusted layer. So depending on the season and weather you would need to replace the Vaseline once every few months. Also any support ties between the plant and stakes or other supports (such as ties to a nail in the wall or I tie the trees to some of our burglar bars) also need to be Vaselined or else the ant tightrope walk along the ties to gain access to the host plant once more.
The Vaseline should be applied about 20-30 cm from the ground on the stem if possible in order to prevent it from becoming dirt-clogged too quickly due to soil back splash when it rains.
Another option is to get double-sided tape and stick them onto the stems and other access point. I haven’t tried this yet, but I am planning to in the near future. The same applies to the double sided tape – once enough dirt has settled onto it, it becomes useless.
I have Vaselined all my fruit trees and it makes the world of difference to keep the ants and associated pests (aphids and scale) off the plants in the first place. The honeydew is not only attractive for ants to eat, but the affected leaves soon become breeding grounds for black sooty mould that weakens the plants even more.
NB! Just a quick note on the Vaseline barrier on the tree trunks. Some trees do not seem to appreciate this Vaseline collar (such as the Brush cherry) and don’t form any new bark at the collar site (likely too moist). This means you will end up with a restricted trunk at the collar and a weakened trunk. Therefore, I suggest checking up on the collars just to make sure this doesn’t happen. If you notice restrictions occurring remove the Vaseline from the afflicted tree.
Alternatively, you can use the white oil or bicarb/sunlight solutions along with a soft bristle brush to scrub off the nasties – no need to take off any bark in the process, only enough to dislodge the scale.
Something Interesting: Carmine Dye
One would believe that scale insects are merely an agricultural pests and that no good has ever come from them, but surprisingly a certain group of scale insects have been used to make dye since the middle ages.
Cochineal insects, a type of mealybug, are soft-bodied, oval shaped and use their beak-like mouthparts to feed on members of the Opuntia cacti (this includes the Prickly Pear). As with many other scale, female cochineal insects are wingless, but have limited mobility and form the bulk of the infesting colony.
They originate from Mexico and South America where they have been raised on cacti for the red substance they produce within their bodies. This is carminic acid (19-22% of a mealybug’s body), which is a predator deterrent and when mixed with either aluminium or calcium salts it becomes a red carmine dye. This dye is one of the few naturally derived dyes that is a water-soluble colourant yet remains resistant to degradation, light, heat and oxidation. It is more suited to the dyeing of animal derived fabrics and is also used in cosmetics (lipstick, blush) and food colouring (labelled E120), since it is less allergenic and more stable than many synthetic colourants.
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