By Stephanie Hudak
Along with our beloved magnolias and honeysuckle, those of us who live in the South love our gardenias, both for their brilliant, white waxy flowers and the heavenly scent that wafts around the entire garden. But with all that pleasure comes trouble – in the form of whiteflies and the black sooty mold that grows on the honeydew they produce. Make no mistake though; whiteflies don’t only visit your gardenias. They are happy to take up residence on many different plants. And if that isn’t confusing enough, these pests come in colors other than white.
Most people are familiar with ones that have the white, powdered-covered wings which create a large cloud when infested plants are disturbed. They are known as a citrus whitefly – and of course, will be found on your favorite Meyer Lemon tree that you have been babying for years. There is also a citrus blackfly, which is still a “whitefly” even though its color is black. Last year when I placed verbena in some of the city containers I had an infestation that I didn’t recognize at first. Turns out it was the banded winged whitefly, where the adults have two dark gray bands across each forewing. You’ll also find them on okra and ornamental hibiscus. They are welcome to all the okra they can eat but I’d prefer they left my ornamentals alone. As if having pesky native whiteflies isn’t enough trouble, we also have a non-native species that is a serious problem.
The silverleaf whitefly first appeared in this country in the 1980s and because it has such a large host range it became a nursery industry nightmare. It now impacts row crops such as cotton and soybeans in addition to visiting home gardens, where it can transmit dozens of serious plant diseases, including tomato yellow leaf curl virus. Their toxic saliva, which is injected as they feed, causes irregular ripening in tomatoes. To make matters worse, this species has a strong potential to develop insecticide resistance; which brings us to how we can keep these friendly little critters away from our beloved plants.
But first we have to understand the life cycle of whiteflies. Even though the fluttering white cloud that rises up into your face is annoying that isn’t the real threat to your plant. Despite their name – whitefly – they are not really flies but scientifically related to aphids and scales insects, which means they will have piercing and sucking mouthparts that feed on the plant sap. The life cycle of whiteflies is a major reason they are difficult to control. Sorry folks, you can’t just spray that bush one time and expect to be rid of the problem. And you can’t keep using the same spray regularly or they will develop a resistance to it – but back to the life cycle.
To begin with, all life stages spend most of their time on the underside of the leaf which makes it difficult to effectively spray them; and then there are two non-feeding stages, the egg and nymphs, which are not susceptible to systemic insecticides – more on systemics shortly. The female lays her eggs on the underside of the leaf, which then hatch into crawlers – they don’t go far before inserting their mouthparts into the leaf and start to eat. The eating doesn’t stop once the crawlers molt into their next phase. They sit there and feed much like scale insects until they turn into adults. Now here is the big news – depending on temperature and the host, this cycle can be repeated every three to five weeks. Now, are you getting the picture why just one spraying won’t work?
So how do you control this problem? The first step is to be sure you don’t bring plants home that have insects on them. Carefully look at all plants you purchase, especially the undersides of the leaves; don’t overwinter susceptible plants like poinsettias indoors where the environment is ideal; and you might have to turn down that offer of free plants from your friend if they are sickly.
Systemic insecticides which are applied to the soil and then drawn up into the sap of the plant work well for those cycles that are feeding. Check to see that they contain imidacloprid or dinotefuran. When using foliar sprays, look for acetamiprid or azadirachtin as an ingredient. To be successful in controlling whiteflies you need to put your efforts in controlling the immature stages and that means using a systemic and/or ensuring that foliar sprays drench the undersides of leaves.
Beneficial insects like parasitic wasps, lady beetles and lacewings help keep whitefly populations in check outdoors so proper use of insecticides is important. Worse than doing nothing at all is to use the wrong insecticide. Malathion, carbaryl and pyrethroid insecticides are very destructive to beneficial insects. We need all the help we can get with these pests so let’s take care of the good guys.
So do you have a better understanding why you haven’t been successful in ridding your gardenias of whiteflies? It isn’t going to be easy but it can be done with patience and a good understanding of what you are dealing with. I love gardenias – they were my first prom flower and in my wedding bouquet – so I’ll do whatever to ensure they grow in my yard – happily without whiteflies.
“Adopt the pace of nature; her secret is patience.” – Ralph Waldo Emerson
Book recommendation for this week is Insects and Gardens by Eric Grissell. Excellent photos and it discusses both good and bad insects. Hugs will help lower your blood pressure – hug someone and get healthy.