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Scale & Mealy Bug

Cottony Scale

Cottony Scale

In the Pacific Northwest, scale and mealy bug are notorious attackers of orchids. There are thousands of species of scales and mealy bugs but they are all close relatives, hailing from the order Homoptera (along with aphids). Four distinct families make up the scale/mealy bug section (aka, the “superfamily” Coccoidea) :

  • Coccidae – soft scales
  • Diaspididae – armored (or hard) scales
  • Margarodidae – cottony cushion scale
  • Pseudococcidae – mealy bugs

All of these families of scale and mealy bug can be found in the Pacific Northwest and can be detrimental to the health of orchids but one strain of scale is of particular note since it is the main nemesis of orchidists: Boisduval’s scale (Diaspis boisduvali). This form of scale is NOT from the Pacific Northwest but has spread from tropical America via commercial trade to infest orchid collections around the world.

Male Boisduval Scale

Male Boisduval Scale

A confusing factor in identification of any scale is that they have different stages to their development and it is important to be able to recognize those stages. Scales are so called because they have a stage in which they are under a hard protective shell that sits on the leaves or pseudobulbs of the plants. This is done when the female is producing eggs. After the eggs have been laid under this hard shell the female dies but in due time the shell will break open and the eggs will hatch. Treatment in this stage is very difficult.

After the scale stage there is a massive explosion of nymphs because under one scale are thousands of eggs. Making things even more difficult for the orchid grower the motile scale insects actually have wings and, while they are not good fliers, they can travel from plant to plant. If you’ve been good to your orchids you’re also giving them some decent air movement – but this can also serve to spread scale throughout your greenhouse. If left untreated scales (and mealy bugs) will destroy an orchid collection and easily become untreatable without drastic measures.

Young Scale and Female Scale

Young Scale and Female Scale

Scales reproduce in exponential numbers and the rate of reproduction is accelerated in a warm greenhouse setting. It is imperative that the minute you see any sign of scale on your orchid(s) that you treat it.

In the crawler stage insecticides will kill some scale, but their sheer numbers will likely make your efforts in vain. A multifaceted approach that includes insecticidal soaps and horticultural oils, which smother the crawlers, is needed and should probably be used as a preventative.

Also, pay attention to the dark crevices on your plant as well as the undersides of the leaves. These bugs like to reproduce in the darker, cooler spots. When plants are crowded there are more dark places with stagnant air so a good preventative measure is to keep ample space between your plants.

Scales and mealy bugs are fond of certain orchids and the Boisduval scale was probably introduced to temperate regions via their favorite hosts: Cattleya alliance orchids. Some orchids seem to be impervious to Boisduval scale, but weaker non-Cattleya alliance orchids can succumb to infestation if the scale population is not controlled.

If you have a scale-infected plant, you should quarantine it. It is wise to treat the infected plant with alcohol (item 1 below) and to follow up with your whole collection, including the infected plant, with items 2 and 3.

Treatment Details Indoors?
Isopropyl Alcohol Spray any visible scale (in any stage) with a mixture of rubbing (isopropyl) alcohol and water with a few drops of dish soap included as an emulsifier. 60-80% water should be an appropriate amount. Spray the scale directly and use cotton swabs to rub off the pest. Hard scale may take extra effort in this regard, but the alcohol should loosen or dissolve the shell. Don’t soak plants in alcohol for extended periods of time as this will dehydrate the plant and kill more sensitive plants. Safe indoors but labor intensive. Doesn’t get to scale in the nooks & crannies of the plant.
Systemic Use a good systemic on the plant and in its water. A good chemical to look for is Imidacloprid, which is a chlorinated analog of nicotine. While Imidacloprid is safer than many insecticides, it is NOT a non-toxic chemical, so handle it with caution and avoid any ingestion. Furthermore, there is a potential link between Imidacloprid and colony collapse disorder in honeybees, so restricting its use to the greenhouse is recommended. It can be applied topically as a spray (be sure to get the undersides of all leaves), but when it’s included in watering, the plants will absorb the chemical and prevent biting insects (like scale) from attacking. If you have a severely infected plant you may need to pull it out of its pot and soak it in a bath of Imidacloprid and water. You can use Imidacloprid in conjunction with a mild insecticidal soap for a more intensive treatment.

WARNING! Many products that include Imidacloprid also contain fertilizer and are intended for much hardier plants than orchids, so look for products where the only active ingredient is Imidacloprid as regular plant fertilizers will burn your orchids!

Careful indoor use is ok – avoid use in the yard.
Oil A good quality horticultural oil can be applied. Most orchids can handle only a lightweight, or fine horticultural oil. The oil works primarily in suffocating insects by blocking the spiracles through which they breathe. The oils can also disrupt the metablolism of insect eggs and adults. Many orchid resources recommend neem oil, probably because it is less toxic than the petrolium-based horticultural oils people otherwise use, but neem can be too heavy for more delicate orchids such as those with thinner leaves. Cattleyas can handle neem fine and the results should be positive.

WARNING! Never apply horticultural or neem oil when the temperature is high (nearing 100o) as this will damage the plants.

Safe indoors and very effective as a long-term treatment.
Predatory insects Depending on your growing environment, you may also opt to introduce a predator, such as ladybugs, into your growing space. This, of course, is the most natural approach but is difficult for those that grow in homes. Of course, if you are taking this natural approach, be sure that you haven’t just sprayed any chemicals that are toxic to your predator. You can find more info on preditors at an organic garden store or at some hydroponic shops. Would be difficult to use indoors

Reapply the Imidacloprid (when watering) and spray with oil on a weekly basis. Watch for recurrence of scale. If a plant is continually being a host for scale and there seems no way to make it happy, you may need to discard of the orchid in order to keep your collection safe.