Plant Doctor Archive

Two-spotted mite

CAN you help me with my dahlia problem? Six plants in the back garden were the first to show signs, followed by those in the front garden, and now the ones in the side garden have it too. I have had various suggestions on what the problem is, from red spider mite to soil deficiencies. What is your opinion?

THE leaf samples you sent had a mild infestation of two-spotted mite, a tiny pest that feeds by sucking cell contents. They live mostly on the undersides of leaves and thrive in hot, dry conditions, so symptoms are worse in midsummer.

The first signs of this mite on your plants are pale patches appearing on the leaves. When infestation is advanced, you may notice fine webbing on new shoots and leaves, with older leaves drying out and dying. The adult mite is just visible with the naked eye, pale orange in colour with two dark patches on the lower half of its body, hence the name two-spotted mite.

They attack a wide range of plants in summer, including citrus, roses, runner beans and many annuals and perennials. They dislike damp, moist conditions, so this is one situation where watering in the evening and leaving the foliage wet for as long as possible is actually a good thing to do. Mite infestations vary depending on the weather, so in dry summers they are worse than when there's regular rainfall.

You could do nothing and next year they might not be a problem. If you want to try spraying, it pays to choose carefully what you use, as many insecticides have little effect on mites. Spraying oils, such as Conqueror and Super Spraying Oil, are effective, but they can damage tender foliage of some plants, particularly in hot, sunny conditions, so try on one dahlia first and wait for a few days to see the result. Mavrik, Mite Killer, Super Sulphur and Nature's Way Insect Spray can also be effective. With all these, you need to make sure to get thorough spray coverage on the undersides of the foliage so the spray actually hits the mites. You'll probably have to apply three or four sprays at 7-10 day intervals to really knock down the population.

Mite eggs survive over-winter on the bark of many trees, shrubs and roses, so to help avoid another infestation next season, you could give all deciduous shrubs in your garden a good spray with Lime Sulphur or oil.

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