Whiteflies!

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Q. Last year, whiteflies appeared for the first time and absolutely destroyed my vegetable garden. I went to a local nursery and spent a lot of money on products that did not work. The pests have come back this year and are destroying my garden again. I am desperate. What would you tell me to do?

---William Rogers from McCalla, Alabama

Whiteflies are swarming on my eggplant, tomatoes, basil, tropical hibiscus, and even mint. I suspect they got to the hibiscus in the greenhouse this winter and then spread to my plants outside. Hope you have some ideas.

---Bill Rogers in Oklahoma City, OK.

I have discovered white flies on my dahlias, and am very concerned. What do I do?

---Pat in Groton, MA

What are the best ways to control whiteflies? They're on most of my garden. I feed with Miracle Grow. I don't have any mulch down yet, but plan on using some later this year.

---Hubert in Houston, TX

A. Ah, yes-whiteflies; I remember them well-if not fondly.

We used to stage major exhibits at the Philadelphia Flower Show back when I was editor of ORGANIC GARDENING magazine. At the end of one, I took home some flowering tobacco (Nicotiana sylvestris) that we had used outside the Native American suburban home we had 'imagined' as being in an America where the Natives were never displaced by those original "illegal immigrants".

The fragrant, shade-loving white tubular flowers looked lovely in my garden. The thousands of tiny white insects that flew into the air whenever I rustled up the leaves did not. I feared I had invited a Biblical plague to move in with me forever, but everyone said that the whiteflies would not be able to survive my winter. So, I tolerated them for the rest of the season, and as assured, never saw them again. I don't miss them one bit.

My plants had originally become infested in a greenhouse. This is such a common origin that most sources use "Greenhouse Whitefly" as the common name of this pest. So, our Bill Rogers #2 is almost certainly correct in assuming they came into his garden via that hibiscus. Probably the same story with those dahlias, as they would have to have been over-wintered indoors up in chilly ol' Maine. Yet another reason to quarantine and carefully wash all plants before bringing them into your horticultural day care center.

Although considered by virtually all experts to be more of a nuisance than anything else, large numbers of whiteflies can stress the plants they infest. The biggest problem is that their sap-sucking can transmit diseases. And their black, sooty "honeydew" (how's THAT for a euphemism? I may never call my wife "honey" again!) can make a big mess.

'Houston Hubert' has the most to worry about here as his area rarely freezes, making these pests a possibly perennial problem. So, Hubie: No more Miracle-Grow or other chemical fertilizers; the fast, weak growth you get from these wretched things attracts these pests like mad. Build up your soil with lots of compost and other organic matter, get a nice non-wood-based mulch down, and invest in yellow sticky traps.

Whiteflies can't resist the color yellow, and so most garden centers and catalogs sell packaged yellow sticky traps for houseplant and greenhouse protection. These are great for all indoor situations, and even for a few potted plants outside. But if you need to protect large areas outdoors, you might want to make your own bigger ones. Paint sheets of plastic or cardboard bright lemon yellow and coat them with an "insect trapping adhesive" (sticky stuff), like Tanglefoot. Hang the traps around the affected plants and the adults will get stuck to them.

Now, you have to replace the traps when they become full of felons, so it's best to try and knock the population down a bit first. One way is to have a helper disturb the plants while you suck the flying hordes into a canister vacuum or Shop-Vac. Don't laugh; giant vacuums were being used for pest control on Texas farms just a few years after the home version was invented.

Or use a spray to reduce the population before you put your traps in place. Many gardeners write in saying they've found sprays of all kinds to be ineffective against this pest, but that's often because they're spraying the wrong part of the plant. They're hitting the part that they can see, the tops of the leaves. But whitefly adults and nymphs-and the next generation's eggs-spend virtually all their time on the undersides of leaves. So spray that hidden area well with insecticidal soap or a light 'summer spray' (vegetable-based) horticultural oil; this will smother the eggs and nymphs, and maybe get some of the adults. (See the question below for the USDA's 'home made' whitefly spray recipe, combining soap and oil.)

Now these creatures reproduce fast; the eggs hatch in just a few days and every time a bell rings, a whitefly nymph gets its wings (or a couple of weeks after it hatches, whichever comes first). That makes weekly sprays a good idea, especially early in the season, before their numbers get out of hand. You can also spray the plants preventatively next Spring with a garlic-based repellant or Neem, a natural pesticide that also inhibits feeding.

Q. Mike: I have a small nursery in the basement of our house. It appears that somewhite flies came in with my spider plants and vines for the winter. What is your recommendation? And what is the correct ratio of soap to water for spraying? Thanks,

---Ed in Green Lane, Pa.

A. In indoor situations, your main weapons are yellow sticky traps and beneficial insects. Two exceptional whitefly foes are sold for greenhouse use; a specialized ladybug and a tiny parasitic wasp, Encarsia formosa. If the infestation is outdoors, you'll get better results by attracting native beneficials to prey on the pests. (To learn how, see this previous Question of the Week on attracting beneficial insects.)

And I generally recommend that people buy packaged insecticidal soap instead of trying to make their own; there is often a VERY fine line between creating a helpful spray and an unfortunate herbicide with home-made. But USDA researchers have come up with a 'home remedy' specifically for use against whiteflies that seems worth passing along:

Add one tablespoon of liquid dishwashing soap (the regular kind; NOT one of these awful anti-microbial things) to a cup of vegetable oil (peanut, corn, soybean, sunflower or safflower) and shake well. DON'T USE THIS AS YOUR SPRAY; IT MUST BE DILUTED! Mix two tablespoons of this 'master solution' into a cup of water, shake well and spray on those infested undersides.

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