Red Spider Mite


by John McDonald

First published by the BFS in 2004

I can think of nothing more soul destroying than getting Red Spider mites on your plants. Horror of horrors… how could anyone who has been growing plants for a couple of years not know of this terrible affliction? Easy, Very Easy! I have, therefore, done some research, which members may find of interest. I hope so. It may help to get people talking to me again.

What are Red Spider mites?

They are tiny, crawling, wingless, insects that have 6 or 8 legs depending on the stage of the development. Juvenile mites have 6 legs and the adult 8. They are so small that they are invisible to the naked eye, and a magnifying glass is needed to see them. If you imagine the full stop that I have just used they are more difficult to see than that.

The BIG question… what colour are they?

Most people would say RED of course. Some say they are black, some say green. Therefore if you are checking your plants you will know what you are looking for… or will you? This has got to be one of the biggest misnomers guaranteed to mislead the unwary. A RED SPIDER MITE is only RED at a certain stage of the year, after hibernation and in the springtime. The newly hatched mite is almost white (off white/buff coloured); the adult can be almost white (but in varying degrees through to green) with two spots on its back, looking like a saddle. These spots are said to be very dark green or very dark red, so dark that it could be taken for black. But these are only spots and not the whole insect. It is also known as the Two Spotted Spider Mite. There is also another mite who is sometimes found on plants indoors and that is the Carmine Spider mite, but I haven't seen that one yet, so I don't know how it develops.

What is the Breeding Cycle?

Adults can lay eggs from only 36 hours old!!! The eggs are more easily seen with the glass as they are laid in small clusters, usually close to the veins of the underside of the leaf. You may also see the Adult two-spotted female close by, as she will lay about 5/6 eggs every day. However, as I mentioned it speeds up dependent upon temperature. One account that I have read suggests that at 60 degrees she produces 20 offspring, at 70 degrees she and her offspring number 13,000 and at 80 degrees she represents a potential 13,000,000 individuals, and all within one month! This is clearly a major problem in the making, as the short breeding cycle (as little as 8 days from an egg to an adult breeding pest) combined with the early ability for the young to procreate, means that a massive infestation can arrive in a very small space of time. Hence the reason for the paranoia surrounding this pest!

How does the hot dry dusty atmosphere increase the likelihood of infestation?

Simply speaking, the heat of the greenhouse speeds up the life cycle. It can be as short as three days in hot places, and as long as a month in cool weather. So clearly temperature is key to the proliferation, along with a dry dusty atmosphere. As Greenhouses tend to be warming up considerably from April it is about this time that the breeding really gets going. It will go on until autumn given the conditions when is tends to slow down as plants begin to take on dormancy. The remaining females will now turn RED (described by some of being more Orange than red) and find places to hibernate. This is likely to be in the soil or compost of the host plant, or in the bark of mature plants, or in the wood or brickwork of the building itself. Bear in mind they are extremely tiny so no crevice is too small for a winter home for them. They will re-emerge in the spring and will be bright red (or Orangey red). A cooler temperature and moist atmosphere is thought to slow down their metabolism and slow the reproductive rate so misting and capillary matting may help in control.

Where should you look for them?

As with most pests they will be found on the underneath of the leaves so they have to really be looked for regularly.

But you can't see them can you? On close inspection you will see the effects of the eggs from above. The hatching young will eat away at the underside of the leaf, sucking the life out of the plant and this sometimes leaves a visible mark through and onto the top of the leaf. This may be yellowish or silvery and will be a small cluster of spots, each about the size of a full stop, and only by the fact that they are clustered does the mite give itself away. Most of us don't notice mites until the infestation is established and the plant has been damaged. Leaves of affected plants seem to be drying out very quickly, which they are, as the mites suck the very life giving sap from them. Dead and dried out leaves fall and the plant begins to fade away. As the adults colonise the plant they may also spin a very fine web over the leaf and from branch to branch, and alas from plant to plant. At this stage you must take drastic action. This also enables them to walk down the host plant across the compost, over the bench or floor and up onto the next victim.

Real serial killers these are. The webbing also tends to act as a shelter for the young underneath the leaf so that the effectiveness of sprays is considerably reduced.

Where do they come from, you may well ask?

I trust most people know about the Birds and the Bees, so I won't go into that except to say that insects bring in more disease and infestation than any other method, and big bees can do considerable damage to your carefully tended show plants in a very short space of time. You may be aware that the wild honeybee is itself under threat of extinction by a "mite" which is attacking them. Therefore as the bees fly from plant to plant they are well capable of spreading other mites or viruses from infected to clean plants. Personally, I believe that white fly and aphids are the biggest cause of the spread of rust among plants, along with bees and wasps.

What can you do about the problem?

1) It is about this time that a pair of kneeling pads becomes useful, as you may have to refer to praying before the problem is solved! Just kidding…

2) Most importantly prevention is better than cure, as I have found to my cost. If you can cut off the source of supply then you are on your way to preventing and outbreak of whatever sort in your greenhouse. Place "bug screen" over open windows and doors to stop ingress of pests, whilst still allowing air to circulate. You will almost certainly become aware very quickly of how effective this is in keeping down the number of insects getting at your plants in the first place. I replace whole sections of glass with this screen and this helps to keep the temperature down and the airflow up! Replace the glass in the greenhouse door with screen and you can ignore the advice of "leave the greenhouse door open on hot days". This will only let the blighters in. Keep the bees out and there is less chance of getting a Red Spider infestation.

3) Remove any infected plants immediately… and if you can afford to lose the plant then burn it or get rid of it…don't throw it into your compost bin as this may lead to re-infestation. You may be fortunate enough to get a "clean cutting or two" before disposing of the plant, but let that be a lesson to you, and think of it as part of the learning curve.

4) Check your plants regularly for any tell tale signs… Pick the plant up and look underneath the leaves using your magnifying lens and you may see the Mites moving about, or eggs clustered. You will be on the lookout for other insects anyway so now use the magnifying glass and check for Red Spider as well. It may seem obvious but don't buy infected plants. And DON'T bring infected plants to the plant sale. Do not be afraid to get out your magnifying glass and really check out any plants you are about to buy! It will save you a lot of time, effort, and expense in the long run.

5) Keep a generous space between plants to prevent mites dropping from one plant to the next… This is true for most crawling pests too… This is probably the biggest fault that us "space restricted" amateurs have. We always cram too many plants into our small greenhouse, or growing space, and thereby bring most of our problems upon ourselves. So how many plants is it possible to have in a 12 x 8 greenhouse? Good question. How does about 30 or 40 sound? One "Expert" I know tells me that you shouldn't have more than a dozen, and perhaps that is too many. As a broad rule of thumb, one 5" pot will contain a plant 15" in diameter. If you leave a 3" gap between plants you will need about 18" square for each fully-grown specimen. Hence a 12 ft Greenhouse will take about 8 x 5" pots along the length. A 6" pot therefore requires 21" square, and a 7" pot will need 2ft square, and so on… Easy to see how the space is quickly used up…

6) If you are lucky, you will find the problem early and have a good chance of eradicating the problem… by simply squashing the mites and the eggs between finger and thumb, but I find that this always tends to damage the leaf.

7) You could try washing them off with a strong water sprayer. Some people use soapy water for this…but you should cover the surrounding area under the plant to catch any mites that fall from the plant as they could infect other plants.

8) Improve the conditions in the greenhouse with more misting and cooler temperatures… this may be a bonus for Fuchsia growers as the harder you can grow the plants the better they seem to like it. If you can afford it, install a watering and misting system. Together with capillary matting & increasing humidity by hosing down the hot greenhouse floor you will almost certainly keep mites at bay. Be careful about increased humidity as this can lead to botrytis if allowed to cool down too much, particularly at night. A cold humid atmosphere will certainly lead to increased botrytis problems. You need to strike a balance with regard to heat and humidity.

9) Introduce Biological controls - predatory mites. These can eradicate red spider mites PDQ provided they are introduced early i.e. before the infestation is out of control. Other predatory mites feed directly on the Red Spider mites and will devour them at a rapid rate. The predators should gain control in about 4 weeks by which time they will have eaten all the Spider mites and then they become cannibals and eat each other until they too have all gone. A bit expensive I understand, but worth it if it puts you back in control. The secret here appears to be to introduce the predators before the problem gets out of hand. If you do use predators you will almost certainly have to stop using insecticides, as they will kill your precious beneficial insects as well. There is also a period to wait between your last spray and your first introduction of predators.

10) It may be possible to place sticky paper traps cut into strips around the pots to prevent the mites emigrating from plant to plant.

11) How many times have you ignored a mark or blemish on a leaf because there was no easily identifiable cause? I have. Sometime the culprit has gone and is now doing damage elsewhere. Or is he… Perhaps he is just too small to spot. If you have a marked or damaged leaf take it of without too much disturbance. Chances are you will be getting rid of a major headache about to unfold. Lessons I have learned: Prevention is better than cure. Don't just pick up a plant and bring it to the show to help fill the benches. If you haven't properly prepared the plant - leave it at home!

Take your magnifying glass with you when you are going to buy plants - no matter where from - and check them thoroughly.

Keep the Bees out of your greenhouse.