Plant Doctor Archive

Invasion of thrips

Question SEVERAL years ago a landscaper planted a hedge of Chilean cranberry in my garden. It grew well at first though didn’t produce much fruit. But now all the leaves have turned silvery and it seems to be dying back in places. A few green shoots appear at times, but they eventually turn silvery too. Any tips?

Answer  YOUR Chilean cranberry (Ugni molinae, sometimes sold as Myrtus ugni) is suffering from a severe invasion of thrips.

These tiny pests live on the undersides of the leaves and feed by rasping at the cell walls and sucking out their contents. If you look underneath a leaf you may see dark tar-like blobs — these are thrips’ droppings, which are almost as large as the pests themselves.

Adult thrips are usually black, about 1–2mm long and very narrow in shape. Young thrips are even smaller and tend to be yellowish or honey-coloured.

You can spray to control them, but your cranberries are so infested I recommend you prune them right back to within 10cm or so of the ground. Burn the prunings or put them out with the rubbish. Water the plants well and feed with a general fertiliser to encourage fresh clean growth.

There’s obviously a source of thrips in your neighbourhood, so you’ll need to try to prevent them invading the new growth. Keep the plants well watered and spray water over the plants, as thrips don’t thrive in damp, humid conditions. At the first sign of damage (silvering of some leaves) you could spray with an organic pyrethrum-based insecticide such as Garlic & Pyrethrum or Nature’s Way Pyrethrum, but you’ll need to make sure you get good spray coverage under the leaves. Or use a systemic insecticide like Confidor or Rogor 100 which is translocated through the leaves. Make sure you read and follow the label instructions, particularly with regard to withholding periods if you get some fruit next season.

Weekend Gardener, Issue 144, 2004, Page 26