Plant Doctor Archive

Plague of thrips

CAN you identify the tiny black creatures on the undersides of the leaves on my camellia 'Fairy Wand'?

I thought it might be spider mite, but can see no sign of any webbing. The plant is in a part of our garden facing south, partially shaded by a water tank and the leaves in the deep shade where the growth is longest seem more affected by the pest. I have trimmed some of them back, but the rest of the plant is covered in flower buds.

THE samples you sent were quite dried out, but from the black spots and the silvery appearance of the leaves it is obvious the culprit is thrips, not mites. Adult thrips are tiny black insects - the young are even smaller and yellowish in colour. The black shiny blobs that are easy to spot on the undersides of the leaves are their excretions.

Thrips feed by rasping away at the cells on the leaf surface and sucking up their contents. The dead cells cause the characteristic silvery appearance of badly affected leaves. Thrips thrive in warm, dry, sheltered conditions and attack a wide range of shrubs. Badly affected leaves will eventually die and drop off.

Cut back badly affected shoots to encourage fresh new growth which should be relatively free of the pest, for a while. Dispose of the prunings by burning or put them in the rubbish. Prune surrounding plants to allow more exposure to fresh air and rain, if possible. In hot, dry periods, if practicable, set up an irrigation system to spray water over the plants.

Garden thrips are relatively susceptible to many of the commonly available sprays, so once flowering is over, you could give the plant a thorough spray, especially the undersides of the leaves, with a product like Mavrik, Confidor or Rogor. You can add spraying oil, such as Conqueror or Clear White Oil to the mix to enhance its effectiveness, but make sure to spray on an overcast day or some foliage damage may occur.

Reproduced with permission from the former Weekend Gardener magazine. The views expressed here are not necessarily those of the RNZIH.