Tips & Tricks

Pests and Diseases On Orchids

Here at Kev’s Orchids, we try to grow as organically as is realistically possible. As we all know, even in an indoor setting such as ours, it is inevitable that any chemicals used will find their way into waterways by some method which is damaging to the environment.

The best way to avoid pest outbreaks is to be scrupulous about quarantining incoming plants for at lest two weeks, regardless of where they came from, but this is hardly practical for most people, us included. I’ll go through the pests most frequently encountered in my private collection a bit later. It is a mistake to try to eradicate every living thing in sight; balance is important, and each potted plant is an ecosystem. Pests are there to survive, not to spite us, though it can be difficult to keep sight of this when there are mealy bugs everywhere! Having grown orchids for 30 years, I have come to view pests as a fact of life and have taken the decision to not let them worry me too much. They might enter your collection with a prized new plant that you couldn’t live without, or with your weekly vegetable shop, on your clothes, or just through an open window.

Why are pests a problem? Well, a significant infection can weaken or even kill a plant and is very disfiguring as orchids are slow growing and damage takes a long time to grow out. Not only this, but they are also vectors for various bacterial and fungal diseases, as well as the rather more serious (but thankfully comparatively rare) viral infections.

If you aren’t sure what you’re dealing with, or you have trouble seeing what’s going on, I highly recommend either buying yourself a jeweller’s loupe (a small folding type of hand lens) or using the flashlight and camera to zoom in on the critters so you can see and describe them properly. This is especially useful for thrips and spider mites where the bugs themselves are so small you’ll probably notice the damage they cause before you spot the culprits.

The pests most often seen in indoor collections are as follows:

Mealy Bugs. Possibly the most hated of all pests. Most often seen on new growth, hiding on the undersides of leaves, in leaf axils, beneath leaf sheaths (the dead papery bit that often surrounds pseudobulbs) and they also love flowers. They are a type of aphid and are quite persistent.  They are a greyish or pinkish colour and are covered with a powdery substance, so they look white or grey.  They don’t fly, but the females and nymphs do crawl around from plant to plant, and they can be very difficult to control. They are very common and every indoor grower of any type of plant will be very familiar with them. There is even a variety of mealybug that attacks roots but luckily for us, that type doesn’t go for orchids (except maybe terrestrials). You can find mealy bugs on almost any type of orchid, they aren’t fussy. They’ll leave disfigured growths in their wake and seem to appear as if by magic.

Scale Insect. There are a whole host of species of scale insect, but I have only seen one species on my collection, and that is Boisduval scale, a known and widespread pest of orchids. It will latch onto almost any orchid, but it has a definite preference for Cattleya and allies, as well as some types of Dendrobium. It is rarely seen on Coelogyne, Bulbophyllum or Phalaenopsis. This pest also resides under leaves, in leaf axils, underneath sheaths, but is also found on pseudobulbs and it particularly likes to hide on the rhizomes of Cattleya. This pest can build up to significant numbers before being noticed so is hard to control. The scale itself is small, only a couple of millimetres in diameter and is generally a yellowish colour with a raised centre. You will also see some that have gone fuzzy at the edges, and these are the nymphs running off to find new sites to feed. You can see the damage they cause in pitted leaves, yellow spotting and distorted growths, especially on Cattleya and relatives. The scales themselves are also large enough to be quite conspicuous unless they are really hiding.

Thrips Again, there are as many species of thrips (singular – also thrips) as I’ve had hot dinners, and they are so small it is hard to know what species you’re looking at. Thrips generally aren’t too much of an issue for orchids but one type (Echinothrips americanus) can cause massive issues on flowers. They don’t seem to bother much with growths, but they love flowers, especially those of Cattleya and relatives, some Dendrobium and the fleshier types of Bulbophyllum. I have occasionally seen them on Coelogyne, but this is rare. They are small insects, about 1mm in length, white or tan when very young, orange when a bit older and the adults are brown with two spots on the back which you can see with a hand lens. Unlike other species of thrips, they aren’t really airborne as such so the blue or yellow sticky traps sold to trap them don’t work. The damage they cause is limited to flowers and occasionally root tips and this is really frustrating because flowers are the main reason we grow orchids. Spraying flowers is never a good idea so I mostly just remove infected flowers and dispose of them well away from the rest of the collection.

Aphids Almost all gardeners will know what aphids look like, whether they are black, green or white. We don’t discriminate here – they are equally horrid. Winged aphids are small enough that they can drift around on currents of air during the warmer months in Britain and they easily waft in through open doors and windows. They can sneak in with vegetables, flowers, on your clothes or via infected plants. They aren’t a major pest on orchids, mainly latching onto developing new growths or sappy flower spikes where they are easily squished. Still, they can be quite disfiguring on open flowers where they seem able to absorb the pigment from whatever they are feeding on to camouflage themselves. If you keep plants outside during summer (particularly Cymbidium, some Dendrobium and some Coelogyne), you may well pick up an infestation then, and they in turn attract ants which farm them. Indoor growers generally don’t have issues with them. They can be prevented access with cleanliness and by putting fine net curtains up against your windows, and mosquito netting works very well across growroom doors.

Spider Mites Red spider mites are the bane of the lives of houseplant growers. They love dry air and underwatered plants so they are rife in our centrally heated homes. How do they get in? It could be via infected new plants, or they could just drift in on the breeze through an open window. They are almost microscopically small, and you’ll likely see the damage before you see them. Tell tale signs are fine silver pitting and spotting on some leaves followed by yellowing and premature leaf drop. You might also spot webbing between leaves and stem if you look closely. If you’re suspicious, use a spray gun of water on the leaves and the extent of the webbing will become apparent. You might also notice fine brownish dust on some leaves which is the frass (droppings) and shed skins of the mites. Take a piece of white paper, hold it under a leaf and give the leaf a shake. You’ll soon spot it. If you’re a glutton for punishment, use your hand lens or your phone’s camera zoom and you’ll see the creatures themselves crawling around. Luckily for orchid growers, they only go for a few types, and they tend not to be an issue in humid places with well watered plants. If you grow Cymbidium, Catasetum and allies, Calanthe or a few others you might see spider mites occasionally. They mostly go for thin leaved deciduous orchids. They are easily deterred by keeping the humidity up or spraying your plants.

Fungus Gnats Fungus gnats (or sciarid fly) are another massive pet hate for houseplant growers. They are the denizens of overwatered and poorly drained soils. I include them here for completeness, but they aren’t too much of an issue for orchids. They might be seen if you grow Bulbophyllum or other moisture loving genera in Sphagnum moss. The flies are mostly an annoyance rather than an actual problem, and the larvae which are often accused of eating roots are eating fungi in saturated soil and it’s the wet soil that damages roots. They are easy to deter by using well drained potting media and avoiding peat-based composts (as if we needed another reason to exclude peat from our growing media). Consider mulching your plants. You can use sticky traps to catch the flying adults, but it is better to just tweak your watering and growing medium. You’ll have less pests and better plants.

Slugs and Snails.  Every grower, inside and out, will have horror stories to tell about slugs and snails. They are a nightmare for greenhouse growers of orchids. They emerge at night from who knows where and chomp on leaves, roots, new growths and their favourite – new inflorescences. They love humidity and are incredible at hiding themselves away from even the keenest of eyes. As an indoor grower they aren’t too much of a problem, and they are best left for a blogger who grows in a greenhouse.  However, I grow on the first floor, and they are quite capable of climbing in through open windows, and they are frequent hitchhikers in newly acquired plants. You will usually know they are about because you’ll find slime trails in the morning round your plants and if you’re unlucky, stuff will have been eaten. You can often trace the slime trails back to their source and find the hiding places so the pests can be disposed of. Once again, vigilance is important here as one slug left unattended can do quite a lot of damage. If you put plants outside over summer, you’ll inevitably bring them indoors with your plants during autumn. I don’t generally recommend people put orchids outside for exactly this reason.

Pest Control

But don’t worry! It is rare for a pest outbreak to result in the death of a plant. Sometimes it is best to cut your losses and chuck the plant out, but that’s the exception, not the rule. The key is vigilance, using your chosen method of pest control promptly and targeted properly. It is about population management, not eradication. An obvious but usually overlooked tip is to make sure your plants don’t touch each other and to wash your hands and tools regularly during your maintenance routine.

Take time to pick up and inspect each of your specimens thoroughly every fortnight. Look under leaves, between pseudobulbs, check the base of the pot. Remove papery leaf sheaths to take away hiding places. You can also consider putting your plants in the shower for a good wash. This will clear dust from them as well as dislodging quite a wide range of pests and drowning them. If you’re brave enough to join your plants in the shower, your soapy suds will help.

I have tried every chemical control under the sun in my effort to eradicate pests and I have found that none of them work particularly well, many of them damage the plants more than the pests do, and they are becoming increasingly expensive. I have found through experience that a very small arsenal of relatively harmless household chemicals will do the trick for most pests.

Methylated spirit It is always a good idea to have a bottle of methylated spirit/rubbing alcohol on hand for sterilisation of tools and to dip cotton wool in for hard-to-reach areas on plants. This method is very good at killing pests and the alcohol evaporates away, desiccating the pests with it. Critters such as mealy bug and scale insect are very good at hiding in hard-to-reach areas, but alcohol is even better than water at getting into those places. Use it with caution though, because it can mark plants unless properly diluted.

Soapy water Soapy water is by far and away the most effective form of pest control in my arsenal. It lowers the surface tension of the water and drowns critters. You can use any reasonably unscented soap for this. I did read that detergents should be avoided but I see no reason for this. Washing up liquid works well. You want the water to feel ‘soapy’ between your fingers. You can buy specialist products that are soap based to do this job, but they are expensive and to my mind, not so effective. Most recently I am using unscented washing liquid, such as is used for clothes. It comes in a little dissolvable sachet and will fill two spray bottles with solution of the correct strength.  You must be persistent with it, though, and spray every time you see evidence of pests, and you should also be thorough and spray every part of the plant that you can until it drips. For persistent attacks of scale, you can use an old toothbrush dipped in soapy water to gently clean rhizomes and pseudobulbs – very useful on Cattleya and other more robust plants. Be careful not to spray flower buds or open flowers with anything as even water can mark them.

Carnivorous Plants. A novel way of controlling some pests (and fuelling our plant habit) is to grow a few carnivorous plants alongside our orchids. They are very good at attracting and consuming flying insects, especially fungus gnats and I imagine they would be useful for some types of thrips, too. There will be no control of mealy bugs or scale via this method. If you grow warm as I do, consider varieties of Nepenthes or Pinguicula which will enjoy similar conditions to your orchids. If you grow cooler, you can grow Dionaea, Sarracenia or Drosera. Be aware that these types may require a winter rest period, but this should coincide nicely with that required by your orchids.

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