I noticed the Black Russian tomato had some curling leaves. On inspection I found Whiteflies and eggs under some of the leaves, on the Black Russian, and the Tommy Toe next to it. None of the other plants seem to have been targeted so hopefully we've caught it early.
Whiteflies are not actually flies, but 3mm long white flying sap-suckers, closely related to aphids. They can breed up quickly, particularly in areas that have been sprayed with pesticides (not in our case) where there are few predatory insects, and they also quickly become resistant to any pesticides used against them. Smothering sprays such as soap or oil mixes are recommended, and you can buy or make sticky traps.
I sprayed the plant with Eco-Oil - 5ml to 1 litre which seems like a very weak solution but I followed the instructions. A few Whitefly have settled back on to the plant. I'll check it in the morning, and if it isn't looking better I have some Natrasoap I can try.
UPDATE: 15/01/09 Thismorning the Whiteflies seem to have cleared off the tomatoes but have settled on the nearby mint. I'll continue with the Eco-Oil for now, and may even try getting the vacuum cleaner out to suck them up, which some gardeners have recommended. Buying some beneficial insects is also an option: http://www.bugcentral.com.au/products/index.php
15/01/09 Found eggs on the back of Nasturtium leaves and several other tomato plants. Sprayed all thoroughly with Natrasoap - 15ml to 1L.
16/01/09 No signs of Whitefly in the garden today. Yay!
18/01/09 Uh oh, whitefly on some of the zucchini leaves. I gave the colonised leaves a quick squirt with Pyrethrum - something I avoid, but I kept it localised and it did kill them immediately.
9/02/09 The odd whitefly ends up on the sticky yellow trap, but they're not causing any problems.