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Pests & Problems Part 4 - Red Spider Mite

Red spider mite is one of the most troublesome pests of greenhouse and house plants. It is a basically a sap sucking mite that attacks the foliage of plants which can lead to leaf loss and at worst, kill the plants.

Red Spider Mite

It attacks many common houseplants and greenhouse plants, both ornamentals and edibles, including peppers, tomatoes, cucumbers, vines etc

During the spring and summer these pests are a yellowy-greeny colour with 2 black dots on their backs. They are also known as the "two-spotted spider mite".  They become orange and red later in the season. 
They overwinter in cracks in the soil and in nooks & crannies amongst the greenhouse paraphanalia. They become active in early spring and like warm/dry conditions. Spider mites emerge from hibernation during March and April and begin laying tiny, spherical eggs on suitable host plants. As autumn approaches, females stop laying eggs and begin to seek out places away from the plant to overwinter. They'll usually choose places, such as cracks in walls, fences and old plant material.
Webs formed in severe infestations
The mites are very small - only 1mm long and barely visible to the naked eye. However as infestation gets larger, the undersides of infected leaves become covered in a fine webbing, which the mites use to crawl from stem to stem.  The web also acts as a blanket to protect the colony and its eggs.

The presence of spider mites will cause leaves to become mottled and curl downwards. Damage to plants is spotted more easily than looking for the mites themselves.  Using their mouth-parts they pierce the leaves and suck out the plant sap.

Growth is restricted on infected plants and can lead to death of the plant if not treated.


Control

  • The most commonly used biological control is Phytoseiulus Persimilis which is a predatory mite.   It  feeds on the eggs and active stages of the red spider mite. They work very effectively and will clean up infested plants within a week or so, if conditions are right.Biological control
  • Another biological control is Feltiella acarisuga or "Predatory Gall Midge". This is a type of midge that works by laying its eggs amongst colonies of spider mites. When the eggs hatch the larvae will eat the spider mite eggs and adults. This control is not recommended for the hot dry conditions of mid summer but these predators work will in damp/humid conditions.
  • There are many sprays available but a natural fatty-acid organic spray can be used. This suffocates the pests without damaging the plants. The problem with sprays is that you can also wipe out the good guys aswell as the mites/aphids you are trying to control.    Natural Rhubarb/Garlic sprays are also good for deterring mites without removing the beneficial predators.
  • Other biological controls include Amblyseius andersonii and Amblysieus californicus.
  • Ladybirds and their larvae love aphids but will also munch on Red Spider mites, so well worth encouraging them into your greenhouse/polytunnels.


Prevention

  • Spider mites like dry, warm conditions. So misting a greenhouse or windowsill makes your plants a less desirable location. Make sure to spray under the leaves. Mites hate humid conditions. Also, always remove infested leaves, buds and stems because this will reduce numbers. 
  • At the end of a growing season, make sure to clear away as much plant debris as you can and if possible disinfect the greenhouse.  Garlic candles are a good way of doing this.