Although one of the smaller pests, red spider mite is one of the biggest problems for plants grown under glass and indoors.
Anybody with a conservatory or greenhouse will have more than likely come across the mottled leaves and light webbing caused by this little troublemaker and, in the often-dry conditions of the home, they can also thrive on houseplants.
What is red spider mite?
Glasshouse red spider mite is somewhat distantly related to true spiders and, like spiders, they produce fine silk webbing.
Unlike their fly-chomping cousins, they are vegetarian.
During the winter they appear orange-red in colour (main image), but during the spring and summer they’re more yellowish and have two dark dots on their sides (leading to its alternative name of two-spotted spider mite).
These two females with an egg are exhibiting the spider mites' summer colours.
One mite alone is not an issue – they only grow up to 1mm in size – but they breed very rapidly and soon can cover ornamental and edible plants alike.
In the perfect storm of a hot and dry environment, mites can complete development in 12 days.
The first signs of red spider mite usually appear on the leaves.
The upper side will have pale mottling and, if you turn the leaf over, the underside will appear ‘dusty’ with tiny mites, their eggs and cast skins. If in doubt, grab a magnifying glass, as they can be difficult to see with the naked eye.
In heavier infestations, a fine web will appear over the plant, particularly the growing tips, and growth will be stunted with the leaves appearing a pale silvery-yellow before dropping off.
It is often advised to keep humidity levels up to prevent red spider mite. This does reduce the risk, but will not prevent it altogether.
The mites can easily pass from plant-to-plant on the fine silk they spin.
Grouping plants together will increase humidity, but can also make it easier for the mites to spread. Any plant that is suffering from a severe infestation should be destroyed to contain the infestation.
If you have had a suspected outbreak over the summer, clean thoroughly around the growing area to kill overwintering females during autumn and winter.
Although predominant in glasshouses and on houseplants, red spider mite can affect garden plants during hot dry weather.
If you haven’t found your preventative measures effective, then a biological control could be the answer.
Always check the requirement of the predator, as they may not be as robust as the pest and require certain conditions for optimum impact.
The rove beetle is a voracious predator that feeds on fungus gnat larvae and red spider mites in both its adult and larval stages.
Predatory mites, Phytoseiulus and Amblyseius are quite readily available for purchase online and are often used instead of chemical control.
There is also a midge species (Feltiella acarisuga), whose larvae predate upon red spider mite.
You should not use chemical controls if you have predators on a plant, however, the use of fatty acid/plant oil sprays just prior to introducing biological measures can help thin out a heavy infestation with no knock-on effects.
The light mottling on this leaf is a sure sign that red spider mites will be lurking on the underside.
Before buying chemicals, it's better to use a spray that contains plant extracts/oils and fatty acids.
This will kill the red spider mite without affecting the plant and, should you then decide to introduce biological controls, there are no lasting effects, such as you might find with a systemic pesticide.
If you are growing anything edible, such as citrus, then this is definitely the best solution
You can also sponge the leaves of houseplants with water, but this is very time-consuming.
To make your own soap solution mix:
- 2 x tablespoons of gentle (preferably natural) soap. Avoid using detergent soaps, like washing-up liquid.
- 1-2 x tablespoons of cooking oil.
- One gallon of water.
- Spray plants thoroughly every 4-7 days until the mites disappear. It is advisable to test on a small area of the plant first.
- Don’t spray in direct sunlight and be aware the solution will also kill beneficial insects, like predatory mites, lacewing larvae and ladybird larvae.
There are pesticides on the market but always read the label in full. Unfortunately, red spider mite has developed a resistance to many chemicals used in the past and so this is not always an effective method of control.
Predatory mites, like this Phytoseiulus, are often more effective than branded pesticides due to resistance in red spider mites.
Almost all houseplants will be susceptible to red spider mite in hot, dry conditions, with the exception of cacti and thick-leaved succulents. These plants listed below are notorious for being on the spider mite menu:
This list is not exhaustive, so keep an eye out on your plants for the symptoms, particularly during summer months or in warm winter conditions.
The webbing, silvery-yellow mottled foliage and mites on this pepper plant are symptomatic of a severe infestation.