In companion planting jargon Dill and fennel are called a "trap" plant, due to the fact the aphids will go to it first. Monitor your gardens regularly, hose off aphids at the first sign. Unlike egg bearing aphids the rapidity with which these dang live-baring pests can proliferate is astonishing.
White flies are a form of aphid, far more difficult to control and the best way is to hard-hose plants off a couple times a week, in the evening or twilight.
While the soap sprays are very effective and I highly recommend them, if you hit beneficial insects the spray will kill them too.
The year I was diligent about pouring a little soapy water right down the center stalk of my kale and dill, once a week, I had no problem with aphids and enjoyed the kale and dill instead of the aphids having a feast.
Some of the beneficial insects are (many insects are predators of aphids):
Assassin Bugs — juvenile and adult (middle picture of juvenile going after aphids
Praying Mantis (above right - one of best known garden good-guys)
Lace wings — juvenile and adult (not pictured)
Place a quarter teaspoon of original dawn dish detergent into a gallon jug - add some water, shake, let the suds settle down a bit and slowly fill up the jug the rest of the way with water. In the evening, turn the jug upside down once to make sure it is still mixed, and pour about ½ cup right down the center stalk of the plant, so that the water gets into the base of the stalk and bottom leaves — that is where the aphids usually start their work. Do this once a week, or every 5 days if you initially see a lot of aphid activity.
To 1 quart of water add one teaspoon each of dawn and any edible vegetable oil — I keep old olive oil, which is no longer good for cooking, for these sprays. Put in a spray bottle and make sure you shake-spray, shake-spray to keep the mixture homogenized while spraying. Get the underside of the leaves of the plant, stalks, and basal leaves. Do this once every 5 days for a total of 3 times. Watch for further infestations, and spray accordingly. Doing the spraying in the twilight or the evening prevents sunburn from the oil in the spray. The 5 days repetition interrupts the pests' life cycle.
When harvesting for the dinner table, just rinse off any residue spray, or soapy water — and enjoy your undamaged garden edibles!
Want to learn more about the bugs in the garden? It is a great learning opportunity for the children too! Also a note of caution — a bug that preys on other bugs is a carnivore — which means they can bite you. While many are not know to readily bite when gently handled — like praying mantis, the assassin bugs will and do bite.
Visit what's that bug.com an amazing site begun by a couple of professors which has turned into THE source for families to learn about bugs.
Consider getting a great book on insects: The Kaufman Field Guide to the Insects of North America is a great one — it is a little more technical than some other books, but their inclusions are massive with pages of similar insects to allow you to better ID the critter you found on the back the rose bush. Amazon link in our sidebar.
-- Catherine, The Herb Lady