Identifying Whiteflies is easy because they congregate on the undersides of plant leaves and fly up in a cloud of white when disturbed. The adults are typically found on the top parts of plants and the nymphs will be found lower down on the undersides of plant leaves. The eggs are a grayish or yellow cone shape and are the size of a pinpoint.
Whiteflies occur in groups on the undersides of leaves. Their name is derived by the mealy white wax that covers the adults wings and body. The adults are tiny with yellowish bodies and whitish wings. Some whitefly species have distinctive wing markings. Many species are easily distinguished in their last nymph (immature) wingless stage.
Whiteflies develop rapidly in warm weather, and populations can build up quickly in situations where natural enemies are sparse and weather is favorable. Whiteflies normally lay their tiny oblong eggs on the undersides of leaves. The eggs hatch and the young whitefly increases in size through four nymphal stages called instars.
Whiteflies suck more plant juice than they can digest, as such they excrete the excess as a sweet, sticky substance called honeydew. The honeydew covers leaf surfaces and acts as a growth medium for a black, sooty mold. Between the removal of plant juices and the presence of the black, sooty mold growing on the honeydew can interfere with photosynthesis.
Nymph Stages of Whiteflies
- The first nymphal stage (crawler) is hardly visible even with a hand lens. The crawlers move around for several hours, then they settle down and remain immobile.
- This nymphal stage they are oval and flattened like small-scale insects.
- The legs and antennae are greatly reduced, and the older nymphs do not move.
- The winged adult emerges from the last nymphal stage, sometimes called pupa for convenience purposes.
All stages feed by sucking plant juices from the leaves and excreting excess liquid as drops of honeydew as they feed.
Adult whiteflies are about 1⁄10 to 1⁄16 inch long and have four broad, delicate wings and are covered with a white powdery wax. Adult females usually lay between 200 and 400 eggs. Sometimes the eggs are deposited in a circular pattern in groups of 30 to 40 because the female will often keep her mouthparts in the plant to feed while moving her abdomen in a circle.
Within about a week, the eggs hatch into flattened nymphs, called crawlers, that wander on the plant. Soon, they insert their mouthparts into the plant and begin to feed. After their first molt, the nymphs lose their legs and antennae. They attach themselves to the undersides of leaves with several wax-like rods coming from their bodies, giving them the appearance of small white oval scale. The nymphs remain fixed to the plant and feed for about four weeks. After a pupa stage, the adults emerge and live for about one month.
Whitefly Feeding and Plant Survival
Whiteflies are tiny sap sucking insects. Their feeding behavior, done by piercing-sucking, plant-feeding insects is a much more complex plant-insect interaction than the feeding by herbivores with chewing mouth parts. The mouth parts are modified having a long, hypodermic needle-like structures called stylets that insert deep into the plant tissue. Because they have very thin stylets that penetrate they can pick and choose which plant cells to feed from.
Whiteflies also specialize on the types of plant tissue utilized (for example: parenchyma, xylem or phoem sap). The physiological interactions between the feeding insect and the defense responses of the plant can determine whether or not the insect will be successful in feeding on the plant.
Whiteflies are frequently abundant in vegetable and ornamental plantings. They develop rapidly in warm weather and their numbers can build up quickly in situations where there are no natural enemies and the weather is favorable. They excrete a sticky honeydew which a sooty black mold can grow over on the tops of leaves. This honeydew can also cause yellowing of plant leaves and cause leaves to drop-off. Either condition can cause plant death. Outbreaks of whitefly can occur when the natural biological control is disrupted.
Whitefly Damage to Plants
Whiteflies suck phloem sap and large populations can cause leaves to yellow, appear dry, or to fall off of plants. Due to the excretion of honeydew plant leaves can become sticky and covered with a black sooty mold. The honeydew attracts ants, which interfere with the activities of natural enemies that may control whiteflies and other pests.
Feeding by the immature Whiteflies can cause plant distortion, silvering of leaves and possibly serious losses in some vegetable crops. Some whiteflies even transmit viruses to certain crops. Low levels, or concentrations, of whiteflies are usually not damaging, however, it is best to address a small issue early on so an infestation doesn’t hit you. Generally, adult Whiteflies themselves will not cause significant damage unless they are transmitting a plant pathogen, still keep an eye out to avoid a huge problem.
Management of Whiteflies
Heavy infestations can be difficult to manage. The best strategy is to take a pro-active approach and prevent problems from developing.
Frequently natural enemies will provide adequate biological control. However, the use of pesticide and insecticides, dusty conditions, or interference from ants, can disrupt any natural process.
Trying these simple techniques may help get Whitefly populations under control:
- Removal of infested leaves (during early stages of development).
- Hose down the plant(s) with a fine but strong blast of water.
- A reflective mulch, such as aluminum foil can help repel Whiteflies (make by spray painting clear plastic sheeting with silver spray paint). Note: plastic mulches do require drip irrigation to be installed before laying down of plastic.
- Yellow Sticky Traps (these can help monitor Whitefly numbers as well as catch most of them). Whiteflies do not fly far so many traps will need to be placed area the garden or greenhouse. Place trap so the sticky side faces the plant and not direct sunlight.
- Vacuum the Whiteflies off of plants with a Dust Buster during mornings or cool evenings when insects are sluggish.
5 Additional Tips:
- Try companion planting Nasturtiums to help ward-off Whiteflies and Squash Bugs. However, do not plant Nasturtiums next to your brassicas, corn, or potatoes. The Nasturtium flowers are also edible and bring a colorful splash to salads and other recipes as a garnish.
- Using insecticidal soaps and Neem may reduce but will not completely eradicate a Whitefly problem.
- Always rely on multiple tactics when dealing with Whiteflies as usually no one organic or IPM (Integrated Pest Management) method will eradicate the problem particularly if there is an infestation.
- Insecticides are typically not a good method of treating Whitefly issues because the it will only kill the Whiteflies that come into contact with the spray.
- Insecticidal Oil, such as Neem, could be used as a spray for the Whitefly Nymphs, be certain to heavily spray the undersides of leaves and do so only when plants are not drought-stressed and when temps are below 80 degrees F to prevent damaging your plants.
- Avoid using pesticides as these are not terribly effective in garden situations and are very harmful to the beneficial insects. Additionally, Whiteflies quickly build-up resistance to pesticides.
- Interplanting tomato rows with cabbage rows can provide some protection from Whiteflies and Cabbage Flea Beetles, in one study; researchers theorized that the smell of the tomatoes hid the smell of the cabbage. Clover, lettuce, and weeds also help protect from infestations by making the cabbage hard to find.
- Keep in mind that when you use Nasturtiums or Calendula to deter insect pests that it is likely those plants will become a magnet for the pest and you will need to pull the plants at some stage. Think of it as they’re performing their job well and making a sacrifice for the sake of your gardens health.
How to make your own Yellow Sticky Trap
Paint a piece of plywood bright yellow and attach a stake.
Or, if your garden or greenhouse is small, use index cards painted bright yellow – you will want to make small stakes for these too as you do not want them blowing in the wind.
Use one part Vaseline and one-part dish soap detergent, smear evenly onto board.
Your homemade Yellow Sticky Trap can be and should be cleaned regularly with soap and water, and a fresh Vaseline/detergent mixture reapplied.
Whiteflies, Aphids and other insects are attracted to the yellow color because they think the yellow color looks like new leaves.
Controlling Whiteflies with Biological Methods
Insects that feed on Whiteflies:
- Lady Bugs / Lady Beetles
- Bigeyed Bugs
- Minute Pirate Bugs
Parasites that feed on Whiteflies:
Encarsia formosawasps (good for greenhouse situations but not outdoors)
Have Whitefliess been a problem in garden or greenhouse? Please share below what you have done for control and your results.