LETTER OF THE WEEK

My once magnificent but old Euphorbia mellifera suffered badly in the past two winters but rallied nevertheless. It has recently started to look pale and drop most of its leaves on one side. Close inspection revealed some fine silken threads around the pointed shoot tips. Is this the final straw, or can I save it? BRENDAN WILLIAMS

– VIA EMAIL

This sounds ominously like an infestation of a nasty mite usually referred to as glasshouse spider mite, although it is no longer only a pest of greenhouses and conservatories. The sap-sucking mites are microscopic, only detectable with the help of a lens, but the webbing they create on curled-up leaves and shoot tips on a wide variety of plants is highly distinctive.

The (relatively) good news is that spider mites only run amok in extremely sheltered gardens during an exceptionally hot, dry summer, when they can produce several almost indestructible generations within weeks. They “windsurf ” via their webbing, and bad infestations can drift across overcrowded gardens, creating a visible trail of damaged, browning foliage.

While hosing the leaves of plants slows down the mites’ breeding rate, organically approved insecticides just can’t keep pace with them, and systemic chemicals are not much better. The best treatment is to “slash and burn” affected herbaceous plants – with any luck their new growth will be “clean” and appear after the mites’ breeding frenzy has abated.

As temperatures drop, female mites go off to hibernate in cracks and crevices nearby, to emerge the following year if another hot summer follows. Which it seldom does. The last time I saw a serious garden infestation was at the end of 2006. I am not sure what to say about your poor old euphorbia. It may indeed be curtains.