Last weekend on the GardenLine shows, I got hit up with some questions about what I assumed were whiteflies. Unfortunately, without seeing them with my own eyes, it was sort of a guessing game.
I’ve written a whitefly control tip sheet or two over the years, but the topic is worth another visit if there are whiteflies out there, because they need to be controlled quickly.
Let's start with some basics: These small, sucking insects have developed a resistance to many synthetic pesticides. That's why I recommend alternating at least two spray products, synthetic or organic. I've given out this advice for over 15 years, and I've never had anyone tell me it didn't work. The key to success is breaking the egg cycle with multiple applications. If you follow the protocol, you will get a handle on your whitefly problem.
In fact, if you don't get control quickly at this time of year, you could lose an entire vegetable crop in a matter of days. And you could wind up fighting black sooty mold on ornamentals through to our first cold snap.
So, how do you know if you have whiteflies? If you rustle the plant in question, the tiny white flies flutter up and around, then soon settle back down. When heavily infested plants are rustled, a great cloud of winged adults fly into the air. Both the nymphs and the adults damage the plants by sucking juices from new growth, resulting in stunted growth, yellowing leaves, reduced yields and, ultimately, black sooty mold. That's because whiteflies, like aphids, secrete honeydew which leads to mold. And on some plants, especially vegetables, whiteflies transmit several viruses.
So, let’s not ignore them … let's control 'em!
If you are not opposed to using chemical products, one regimen I recommend is alternating malathion and any other synthetic pyrethroid, such as bifenthrin, cypermethrin, resmethrin, or permethrin. Use two pump-up sprayers - one with malathion and one with the synthetic pyrethroid. Spray one of the controls, wait a few days, then spray the other. Then, do it again. And again. Try to get down at least four applications over about two weeks, alternating between the two controls. That will break the egg cycle and kill any adults. Be careful, though ... malathion is not approved for some plants like hibiscus. Read the label to learn about those on which it should not be used.
Now, what if you want to remain completely organic? I have good news and bad news. I've noted that whiteflies can develop a resistance to chemicals, and there are a few organics they'll thumb their noses at, too. However, you can still use a great many liquid organics in the same alternating procedure. But previous experience has shown that it can take twice as long to control the egg cycle with organics. So, whether you use soapy water sprays, insecticidal soaps, spinosads or neem oils, you'll need to keep using them for as long as three or four weeks. And when using organic liquids, my advice is to alternate between three of them, not just two.
Another great idea for whitefly control, whether you do it organically or synthetically, is adding "sticky traps." You can purchase them at garden centers and feed stores, but you can make your own with some yellow plastic drink cups. (Flies and aphids are attracted to yellow.) Pick up some sticky substances like Tangled Foot or Tangle Trap organic to spread on the outside of the cups. You can also use plain old Vaseline.
If you've got another homemade trick for whitefly control, send it to me or call the show this weekend.
By the way, there are also some predatory insects you can purchase to control whiteflies, if they're detected early. Aggressive, non-stinging parasitic wasps are the most well-known. But once a whitefly population is full-blown, in my opinion such “beneficials” are a waste of money. Plus, while they may sound like the ultimate organic control, they are hard to come by. And in many cases, they're ridiculously expensive.