Scale Insects - Infestation and Control

Summer is good time for growth in orchids as well as for orchid pests. Pests (like aphids, mealy bugs, scale insects and mites) are more noticeable in orchids during spring and summer. High temperatures and humidity are congenial conditions for pests to multiply.

Out of all the pests of orchids, scale insects are probably the most persistent and hence destructive. These insects infect orchids groups like Angraecums, Cattleyas, Cymbidiums, Oncidiums, Stanhopeas etc. Once any orchid gets infected, it is quite an uphill task to get rid of them. 

Indications of infection
• Presence of whitish, circular and translucent scales on the plant. There are three stages of development of the insect; egg, larva and adult. Eggs are protected under the scale covering formed by the female. Adult females are recognised by tiny whitish circular waxy scales. Immature males are white, while adult males are orange in colour. 
• Male insects aggregate together to form a white cottony mass, which looks similar to an infection by mealy bugs.

How do they spread?
• Crawling insects are blown by wind to healthy orchids. Closed environments like greenhouses, make orchids more susceptible. Sometimes, these insects fall down from overhanging infected orchids. 
• Buying an infected plant. 
• When an infected plant comes into physical contact with a healthy plant. If the orchids are kept in close proximity to each other, the insects can easily crawl from one orchid to another. 

Damaged caused by the insects
• Scale insects feed on cell sap.
• Scales cause circular patches of damaged tissue, forming necrotic (yellowish) spots on various parts of orchids.
• These can be quite damaging in seedlings and young plants; adult plants become weak and unstable. It takes orchids several growing seasons to recover. In case of serious infection, scales can even kill a fully grown plant.

Controlling the insect infestation
• Prevention is always better than cure, so regular inspection of the orchids is an important step to keep these insects in check. As the name suggests, scales can occur anywhere on the infected orchid; from leaves to the roots. Check along the leaf veins, ridges/crevices on the stem or the pseudobulb. Sometimes, these insects hide away under the leaf sheaths. 
• When buying, always inspect the new plant thoroughly. When you bring it home, keep it in isolation for sometimes. This period of quarantine is necessary to confirm whether the new plant is free of pests, or not.
• After emerging out from the protective white covering, scale insects are effectively controlled when they start crawling. 
• You can manually remove the insects, by rubbing with isopropyl alcohol. Check the whole plant carefully, especially hidden or covered areas. Scales contain eggs, so remove each and every scale as well as female insects, where ever found.
• You can also use the dishwashing detergent to wash out the insects. 
• Commercially available insecticides is another option. Before using the insecticide, check the usage instructions. Also check whether it can be used on orchids (ornamental plant) or not. Please be judicious in use of pesticide. Heavy application of pesticide is not only ineffective; it can also be harmful to insect populations which eat these scale insects. 
• Limonene, a safe natural insecticide, is also effective in controlling scale insects. Recommended dosage is 1% full strength.
• It is better to isolate or destroy a highly infected orchid or an orchid which does not show improvement despite repeated applications of insecticides.

Precautions
• When using alcohol, use only isopropyl alcohol, as other alcohols can do more damage than good to the orchid.
• After removing the insects, with any of the above methods, wash (to remove the chemicals) and dry the orchid and plant it in a new potting medium.


Sources: 
• Johnson, P.J. 2010. Biosduval scale on orchids. Orchid Digest, (July, Aug, Sept), 74(3): 170-177. 
• Hollingsworth, Robert G. 2005. Limonene, a Citrus Extract, for Control of Mealybugs and Scale Insects. Journal of Economic Entomology 98(3):772-779.