What’s That on My Kale? Whiteflies in Your Garden
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This article was written by Meredith Farve, Summer Intern for North Carolina Cooperative Extension in Lee County.
It is getting sticky out there. Real sticky. Summer is almost here in North Carolina, and Mother Nature has brought out the insects with her. Since many of us are spending more time at home these days, it is a great opportunity to enjoy your garden and see which six-, eight-, or more-legged critters are sharing your plants. Whether you have a few plants on your patio, a couple flower beds, or a large food garden, a garden is a wonderful place to relax. You can really keep weeds in-check this year!
On my last outing to the garden, I found some unsavory characters on the underside of my kale plants. At first, it is hard to pinpoint what is happening. Is it a fungus? Upon closer inspection, you find these white circles with little specks arranged in a semi-circle inside; if you are lucky, you will find the culprits for these mysterious circles, which are easier to identify: whiteflies!
What are whiteflies?
Whiteflies are insects which look like tiny moths covered in waxy powder, but actually belong to the same insect group as aphids, stink bugs, cicadas, scales, and many others. All these insects belong to the insect order Hemiptera and have sap-sucking mouthparts which they use to feed on plants. When they find an appetizing plant, they pierce into the plant and inject enzymes which digest the tissue. They then drink up the plant slurry, like a green smoothie.
Although whiteflies feed on our plants, we can appreciate their interesting life cycles. Adult females lay eggs on the underside of leaves, hidden from view and protected from the elements. The eggs change color from white or cream to dark blue or purple as they mature. When they hatch out, the immature or nymph stages (also known as “crawlers”) look nothing like the adults; they more closely resemble scales with fringes of waxy filaments. Despite their name, crawlers only crawl around for a few hours after hatching. Once they find a nice spot on a plant’s leaf, they begin to feed on the phloem, the plant’s sugary vascular fluid. They will not move again until becoming adults. As adults and crawlers of different species look very similar, entomologists use the last nymph stage to identify species.
The entire whitefly life cycle takes approximately 3 weeks when environmental conditions are favorable, such as in greenhouses. This quick life cycle allows them to reach high numbers quickly and thus be difficult to control.
What does whitefly damage look like?
Although there are 1500 species of whiteflies globally, only a few species are major agricultural pests. Being fed on by whiteflies reduces a plant’s vigor. if insect pressure is high enough, infestations can cause yellowing of leaves, chlorotic spots, leaf loss, and stem blanching. In addition to feeding damage, whiteflies excrete honeydew, a sugary liquid which promotes sooty mold growth on leaves. Sooty mold reduces a plant’s aesthetics and its ability to perform photosynthesis.
Most significantly, whiteflies have been found to transmit over a hundred types of plant viruses. Plants vary in their responses to these viruses, ranging from showing no symptoms to being completely unmarketable. Examples of viruses that whiteflies vector which impact cucurbits and solanaceous crops include:
- Cucurbit leaf crumple virus
- Cucurbit yellow stunting disorder virus
- Squash vein yellowing virus
- Tomato yellow leaf curl virus
- Tomato chlorosis virus
On which plants should I look for whiteflies?
Whiteflies are not picky on which plants they like to feed on. Whiteflies have been documented to feed on over 500 different plant species of vegetables, fruits, and ornamentals. Some examples include woody perennials like roses and azaleas, trees such as redbud, ash, dogwood, and sycamore, and herbaceous plants like lantana, verbena, salvia, and hibiscus. Regarding vegetables, whitefly-vectored viruses can be a significant threat to tomatoes, cucurbits like squash and pumpkins, and cole crops such as cabbage, broccoli, and kale. When populations of whiteflies are high, this poses a higher risk of viral infection in plants.
If you find whiteflies, what should you do to control and manage them?
Whiteflies are easy to miss because they are so tiny! Fortunately, whiteflies need to reach high population levels to cause serious damage to their host plants. If you have a small population of whiteflies in your garden, often beneficial insects like predators and parasitoids will help you control your whitefly population. Chemical control treatments are also available for whiteflies and work best when population levels are low.
Here are some tips to help monitor for whiteflies, prevent introducing them into your garden, and to keep infestations small:
- When in the garden, inspect the undersides of leaves for whiteflies. Use a magnifying glass or hand lens to check for whitefly eggs, nymphs, or adults.
- When bringing new plants home, check to ensure they are free of both whiteflies and viruses. If possible, quarantine new plants from your garden for 1-2 weeks. This will allow larger nymphs to develop if present, which are easier to see than the minute eggs
- Plant host plant varieties which are resistant to plant viruses
- If you find whiteflies on your plants, remove and dispose of the most heavily infested leaves
- Remove old infested plants from the garden as soon as the crop finishes
- Remove weeds around previously infested plants, as they act as alternate hosts for whiteflies
- Clean up dead plant material and fallen leaves in autumn to help reduce locations for whiteflies to overwinter in warmer climates
- Insecticides are available for whitefly control. Contact your local extension agent for recommendations, as certain whitefly species are resistant to certain chemicals
For information specific to managing whiteflies in greenhouses, see these links:
For more information, contact the North Carolina Cooperative Extension, Lee County Center at 919-775-5624.
Meredith Favre is the 2020 Summer Intern for North Carolina Cooperative Extension in Lee County.