I've recently noticed that my basil and other herb plants are infested by tiny white flies. They spread out like snow-flakes when I spray the plants with water, and there are scary looking number of what appear to be the eggs of those flies on the underside of the leaves.
What would be the best way to grid rid of (and prevent) those flies? I want to stay away with any chemicals or insecticide if I can. I wonder how professional organic growers deal with those nasty flies.
I believe those are aphids and I had the same issue. My solution was to take the pot of basil and put it outside where native predators (tiny green grasshopper-type insects) had a feast. They also attacked my parsley and cilantro, but vanished once the plants were moved to the great outdoors. I assume the same predators took care of the problem as organically as possible. ;-)
My understanding is that ladybugs are a good predator of aphids and they can usually be purchased at a garden center. You might also check out the answers to "What plants will keep ladybugs happy?"
Another place to try would be the answers to "What is an effective organic pesticide...?".
I have a similar problem with these tiny white flies. It considerably weakened some Shrubby Cinquefoil (Potentilla) seedlings I was growing in pots in the garden and is still feeding on them. Although you haven't posted a photo, given your description, I'm still confident that the insect that attacked your basil is the same as the one I have been doing battle with, whitefly (almost certainly the Greenhouse Whitefly variety, Trialeurodes vaporariorum), particularly as basil, together with tomatoes and cucurbits, is one of its favorite foods.
Whiteflies are difficult to control, both chemically and biologically. I have tried spraying with a soap solution, which is fairly effective, but only for a day or so, after which they return and the plants have to be sprayed again. There is an excellent article about them here, and a very useful organic pesticide chart here.
I've done some research in an attempt to find out whether there is a more effective method of controlling this pest;
Characteristics
The flies [1-2mm in length] can be found on the leaves of plants - both on the upper and lower surfaces, but mainly on the latter. These are the adults, which feed on growing shoots and lay eggs that hatch into tiny white scales [larvae] that remain attached to the underside of leaves. Like aphids (greenfly, blackfly), whitefly are sap-sucking insect pests.
This sap sucking weakens the plant and may introduce plant viruses that further weaken the plant, and may even kill it. Unfortunately, whitefly don't go round in ones or two - they go round in hordes of hundreds and a severe attack can severely weaken a plant. You will often see a 'cloud' of whitefly flying away from a plant whose leaves have been disturbed.
Source: Garden Forum - Pest Watch
You say the flies “spread out like snow-flakes” when you spray your plants, and this is exactly what happens when I spray or disturb mine.
Control
Here is what I suggest. You could take only one or two or, preferably, all of these measures, depending on how bad the infestation is and how much time and money you're prepared to spend:
Buy some Yellow Sticky Aphid Whitefly traps, or make your own, plunge your plants in a bucket of water for a minute or so, drain them and then place the traps around them.
Vacuum the plants with a small hand-held vacuum cleaner, preferably in the early morning when it is still cool and the flies are sluggish.
Place your plants in a mini-greenhouse, and then introduce a parasitic wasp called Encarsia formosa (it doesn’t sting) which, it seems, is highly effective in controlling whitefly in a confined space, and is used by commercial growers. Live Encarsia formosa eggs are available here. I have just bought a mini-greenhouse and this is a method I intend to try myself. Although my plants are outdoors, native predators have failed to control the whitefly and, as their numbers have increased, spraying with a soap solution has proved less and less effective.
I regularly bring home whiteflies on organic farm stand greens. I used to wash those greens in the kitchen sink, and the aphids would fly up to the potted herbs in my kitchen window, and so my problems began. Eventually I discovered that if I place my affected herbs in a breezy window for a while, and gently shake the plant every once in a while, the aphids are sucked against/through the screen. I also make sure to wash my herbs, one at a time, in the laundry room, so when the aphids fly off, there's no other plant for them to go to. I also started washing my greens outside, before I bring them in, then put them straight into the fridge (in a container or bag). The next day I give them a more thorough washing in the sink, and the (dead or stunned) flies wash right off, instead of clouding. All these measures haven't eliminated my problem, but it allows me to keep the herbs healthy enough for me to continue using them.
