Plant Doctor Archive

Pesky whitefly - 2

I HAVE a real problem with whitefly in my outside garden. It all started when I transferred a tomato plant from my glasshouse into the garden. The whitefly followed and multiplied in a big way. At the end of March I pulled up the badly infested plants, put them in a plastic bag and took them to the dump. However, the whitefly that got away immediately took residence in a display of annuals (Salvia 'Victoria Blue'). I sprayed them with a mixture of Target and Conqueror Oil and eventually reduced the numbers, but forgot to check under the leaves for some weeks. When I did, I found they were everywhere, even under the leaves of weeds. I want to replace these annuals with winter ones. How do I get these insects under control?

DON'T despair, there is hope. Unless your garden is exceptionally warm and sheltered, the adult whiteflies will die off in winter, leaving their eggs under the leaves to survive until the weather warms up again in spring. So if you pull up the salvia and dispose of them, you should get rid of the problem, for your winter annuals at least.

You can put the salvia in the compost heap. As the plants rot down, the eggs and any surviving whitefly nymphs should die off.

To help avoid the problem next season, get rid of any weeds around the place before the weather warms up, as they could be harbouring eggs. Also, next season check your plants regularly, both outdoors and in the greenhouse, especially under the leaves, for early signs of infestation. The sooner you take action the better, before the whitefly population gets well established.

Some whitefly may survive as adults all winter in your greenhouse, so if you still have susceptible plants in there you'll need to keep up the spraying programme. For a bad infestation in warm conditions, such as in a greenhouse, you need to spray every five days or so for at least three sprays in order to really disrupt the whitefly life cycle of egg, nymph and adult.

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