There are undoubtedly more orchid pests than there are species of orchids. A lot of them are only
found in specific areas. Others, such as the few more likely ones you might encounter, are common and prevalent all over the globe.

If you only have a handful of orchid plants growing in your home, you may never have such unwelcome guests. However, if you grow many plants in one area such as a greenhouse, and  especially under crowded conditions, keep an eye out for these:
 

  Scale
There are a variety of scale insects that attack orchid plants and you need to take immediate measures to eradicate any one of them. One of the two most common scale found on orchids is soft brown scale. Initially, when in the crawler stage, they look like tiny flat crabs, but quickly mature to resemble little shiny brown, tan or greenish bumps which are attached to leaves and pseudobulbs. This pest is readily visible in the adult stage, when they form a hard domelike dark brown shell. They also excrete a sticky substance which attracts ants and a an ugly grey mold-like fungus.

The other common scale is boisduval scale, also known as white or  wooly scale. They also have several stages of development which are difficult to see at first. Boisduval scale first appear as small round raised spots on the leaf or pseudobulb. These are the females, which later develop a thin protective cover. The larger white males are more conspicuous and you will usually find them in clusters, especially on the underside of leaves. Most likely there are many more hiding in large fuzzy colonies around and under the rhizome as well as under bracts and dry sheathing. This is a very persistent critter and seems to be able to survive repeated treatments by appearing again perhaps even a year or so later.

First aid for both of these is to use an old toothbrush which has been dipped in water containing a few drops of dishwashing liquid and gently scrub to dislodge as many of them as you can see. Rinse the entire plant thoroughly with clean water and let it dry. After that, a systemic insecticide specifically geared towards scale should be used according to directions. Applications may have to be repeated several times as recommended.

  Slugs and Snails
We hope you don't ever encounter slugs in your greenhouse. They absolutely love orchids and will travel up the spikes to reach and chomp on tender new buds. They are difficult to get rid of and even harder to locate during the daytime since they are nocturnal and spend the day hiding out in the sheaths and axils of the plant.

One method of control are slug pellets distributed on the floor of the  greenhouse. You don't want to put the stuff in the pots, because moisture will degrade the poison and the remains will become a source of harmful molds. There are a couple of deterrents you might try.

Slugs love beer almost more than new orchid buds! Place a saucer filled with beer near the affected plants, you will find the drunken offenders in the morning. Another method is to discourage slugs and snails from dining on your newly developing blossoms. Wrap a small piece of fluffy cotton around the stem of the inflorescence, below the first buds. Snails won't cross over the cotton and don't appear to have much interest in the stem itself.

  Red Spider Mite
Red spider mite is a serious and devastating pest in the greenhouse. They attack mostly the softer leaved varieties of orchids such as phalaenopsis, dendrobium and cymbidium. They are so miniscule in size that they are difficult to detect with the naked eye. This is why infestation most often is only noticed when leaf damage is obvious.

The first indication is when the leaf color develops a silvery white sheen which soon turns yellow and the leaf will drop off. In severe cases, you will actually see dirty grey webs. Rub your thumb over the leaf of a suspect plant. Red spider mite leaves a reddish/orange residue. You can do another test by tapping a leaf over a piece of white paper. A good magnifying glass will bring them into disgusting focus.

Red spider mites like hot and dry conditions and are therefore most often encountered and active during the summer months. Try to keep up humidity by daily misting to help ward off this pest. If caught early, fairly good results can be achieved with liquid systemic miticides applied several times in succession.

  Roaches, Flies and Ants
You will encounter any one or all of these at one time or another. Even if you think you have a completely sealed greenhouse, there are many ways these critters will find a way in. Look around and you will spot a few, such as fan vents, doors, cooling pads and more.

Keeping a clean environment, along with a regular pesticide spraying program will keep them down to manageable populations, if not eliminate them altogether.