Pests and Diseases : Red Spider Mite

What are Red Spider Mites

Red spider mite are so small (less than 1mm) they are almost invisible without a magnifying glass but they can wreak havoc in a greenhouse or on houseplants.

Spider mites reproduce rapidly especially at in conditions with high temperature and low humidity.

At 25°C, a freshly laid egg will hatch, grow into an adult mite and lay more eggs after only 10 days - At 30°C, it's  only 7 days!

Females can produce up to 150 eggs in their life, laying around 10 eggs per day. These are 0.14mm in diameter and transparent at first. Eventually, they turn white to light yellow.

What do they look like?

Confusingly, for most of the year of the year they aren’t red in colour at all as their name suggests.

They begin life as a greenish-yellow, and only turn red in the late summer/early autumn. The first you know of them may be what looks like sandy coloured ‘dust’ moving around the growing tips and undersides of new leaves.

What do Red Spider Mite Do?

Red spider mite suck sap from plants through foliage.

They will attack almost anything in the greenhouse, in fact, very few plants are fully resistant to this mite. They will feast on fruit and vegetables, such as tomatoes, peppers and cucumbers, as well as flowering plants, like fuchsias, pelargoniums and orchids, and many more besides.

How to prevent and treat Red Spider Mite

  • Keep a regular eye on all you plants to look out for the early signs. On leaves, you’ll see subtle marks and mottling, and, if you have your reading glasses on you may be able to see the mites themselves underneath the leaf
  • You may also spot white shedded ‘skin casts’, and sometimes tiny round eggs as well. In severe cases, you will see very fine webs near the top of the affected plants
  • It is important to be vigilant and look out for bleached, unhealthy looking leaves, often with yellow or brown speckles. If under continuous attack, leaves will dry up and fall off and the host plant can become severely weakened
  • Red Spider Mite like hot, dry conditions so keeping the greenhouse well ventilated will help to deter attacks
  • Damping down can help to keep the atmosphere humid – which the Spider mite hate. Spray water from a hose or can onto the greenhouse floor and under the staging to keep the air moist – the plants will enjoy it too!
  • Washing leaves with a mild soap solution can be very effective if done regularly
  • Encourage ladybirds and other predators into your garden – they will feast on mites and aphids in your greenhouse. Other biological controls include the predatory mite phytoseiulus persimilis Which can be introduced once temperatures in the greenhouse reach 21 degrees
  • Spider Mites can remain dormant once the temperature drops for up to a year so good hygiene and thorough washing of pots and equipment at the end of the season is a must
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