Whiteflies - Vegetables

Life cycle/habits

• Whiteflies are often introduced to outdoor plants from infested greenhouse-grown transplants. Cold winter weather used to prevent their ability to survive Maryland winters. However, they are overwintering in warmer parts of the state, including Central MD, due to climate change and the increased production of leafy greens through fall and winter under protective plastic and fabric covers.
• Eggs are typically laid on the newest leaves.
• Newly hatched nymphs move to a desirable feeding spot, where they fasten on and begin feeding; the other 3 nymph stages are immobile.
• Both nymphs and adults aggregate, feeding together on leaf undersides by piercing and sucking out cell contents.
• Nymphs are inconspicuous and easily overlooked, but adults fly up in swarms when disturbed, and then quickly re-land.
• Whiteflies excrete a sticky substance, known as honeydew.
• There are several generations each summer.

Host plants

• Most common greenhouse vegetable hosts are cucumber, eggplant, and tomato.
• In gardens, there are numerous hosts, the most common are bean, cucumber, eggplant, lettuce, okra, potato, tomato, squash, the cabbage family, and sweet potato.

Signs/symptoms

• Piercing-sucking method of feeding produces stippling of leaves, followed by yellowing; drying and distortion, and premature leaf drop in heavy infestations.
• Heavy feeding may wilt and stunt plants.
• A black fungus called sooty mold grows on the honeydew excreted onto leaf and fruit surfaces. Sooty mold can be washed off produce, but interferes with leaf photosynthesis and weakens plants.

Monitoring

• Before purchase, tap transplants (especially tomato, eggplant and cucumber) and watch for whiteflies taking flight. In the garden, be alert for whitefly swarms, as well as sticky honeydew or the resultant sooty mold on leaves.

Prevention/control

• Do not introduce infested transplants: inspect before purchasing by disturbing leaves and watching for whitefly flights.
• Yellow sticky traps will catch some adults.
• Whiteflies have many predators and parasitoids that suppress numbers as the season progresses. Early problems may clear up. Parasitized whiteflies look like pepper specks.
• For heavy infestations, insecticidal soap can be sprayed on leaf undersides and repeated as necessary. Avoid spraying in very hot weather or on very stressed plants. Read label carefully.
• Horticultural oils, neem, and insecticidal soap can be used for nymphs on leaves; pyrethrins are more effective against adults.
• Control weeds because some may serve as alternative hosts.
• Avoid continuous planting of favored crops to extend season/harvest, as it enables whiteflies to move from older to younger plants.