Wednesday, 19 July 2017

Return of the red spider mite

They're back.

Spider mites are the peskiest of pests. There are two sorts of red spider mite, those which suck the blood of chickens and those which suck the life out of plants in the polytunnel. Neither red spider mite is red! Both are very destructive and an absolute pain to get rid of.

I first identified red spider mite in my polytunnel four years ago. These tiny arachnids multiply rapidly and in that first year weakened my Yardlong beans so much that I got virtually no crop.
One standard treatment for red spider mite is to import a biological control predator, but it's not cheap. I don't mind spending twenty quid if it gets rid of the problem, but not if it merely controls the numbers to come back every year.

In my ignorance I hoped the spider mites would vanish over the winter, but over the following two years they came back stronger each spring. Last year I'm pretty sure they got outside too as my whole bean crop was disastrous with the plants making no headway whatsoever.

So this winter I blitzed the polytunnel, spraying with Jeyes fluid (I had previously tried less aggressive solutions such as Citrox) and treating the wood with creosote. I also removed all possible overwintering homes such as bits of wood, support ropes etc. I've also been using the overhead irrigation regularly, as spider mite do not like humid conditions. The trouble is, these conditions encourage moulds too, which can be a problem when the vegetation in the tunnel inhibits air circulation.

All of this brought me success in my battle... or so I thought. My plants have grown well with none of the tell-tale signs of spider mite damage.
Luxuriant growth in the polytunnel

Growth trimmed back to allow air circulation

But then, a couple of days ago, I went to check if any of my aubergines were beginning to grow fruits and I came face to face with leaves that looked like this.
This mottled effect is caused by a
multitude of bite holes which
significantly weaken the plant and can kill it.
On very close inspection, for spider mites are absolutely tiny, I could just about see the pesky little blighters crawling along the stems.
Look very closely!

In preparation for this I had brought in some Pyrethrum, newly available as a group of producers clubbed together to pay the huge cost of getting it approved for use. I also made some rosemary oil earlier in the year, as this is supposed to be the active ingredient in some hideously expensive commercial sprays.

The pyrethrum is to be sprayed at seven day intervals. I spray it in the evening, after the bees and hoverflies have gone to bed, making sure to drench the plants and get right under the leaves. Fortunately I have caught it early so only need to do a small area of plants.
In the morning I turn on the irrigation to wash the leaves before the sun comes up and to keep conditions humid during the day.
But I am taking no chances. On the days I don't spray with pyrethrum I will use a weak rosemary oil solution. I am praying that my actions have a significant impact, as last year it was a tad demoralising.
If I don't see a change very soon the aubergine plants will go - they rarely give me a crop anyway, though I suspect that is because for some reason they seem to be extremely prone to spider mite.
Other plants which get hit are beans (I don't grow these in the tunnel any more), cucumbers and melons and, in a bad year, even the pepper plants. So far the tomato plants have seemed largely unaffected. Removing all the leaves below the developing fruits probably helps too.

Meanwhile, the harvest has started in earnest. Tomatoes are ripening and cucumbers are coming thick and fast. It looks like a very good year for melons too.

Cucumbers galore

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