Whitefly is one of the pests greenhouse gardeners know best. The adults are tiny, white and winged. Numbers can build quickly under cover, where it is warm and sheltered. A few on a tomato plant can turn into a serious problem over a summer.

The good news is that whitefly can be managed without reaching for a spray at the first sign. Regular checks, good hygiene and natural enemies all help. This guide covers how to tell the main species apart, what damage to look for and how to keep numbers down.

The two main greenhouse whiteflies

Two species cause most of the trouble in greenhouses here. They are the greenhouse whitefly and the silverleaf whitefly. The NSW Department of Primary Industries describes both as common and potentially major pests in its guide to whitefly management in greenhouse vegetable crops. It also notes they can become a problem in summer or under dry, warm conditions.

The two look similar, but there are clues. Greenhouse whitefly adults hold their wings flat over the body. Silverleaf whitefly adults are smaller and more yellow, with wings held at a steeper angle like a tent. The Queensland Government puts silverleaf whitefly adults at only 0.8 to 1.2mm long.

Both species are usually found on the undersides of leaves. Silverleaf whitefly females lay their eggs randomly, mostly on the lower leaf surface. Telling the species apart with certainty is hard for beginners. A plant diagnostic service can confirm which one you have.

What whitefly damage looks like

Whiteflies feed on plant sap. Under very heavy infestations, plants lose vigour and leaves can wilt. They also leave sticky honeydew behind. Sooty mould grows on the honeydew and reduces photosynthesis.

Queensland Government advice on silverleaf whitefly lists it as a serious pest of tomato, eggplant, cucurbits, brassicas and beans. It notes fruit discolouration in cucurbits and beans, and white or yellow tissue inside tomato fruit. Pumpkin, zucchini and squash leaves often turn silver. Whiteflies can also spread plant viruses, including tomato yellow leaf curl virus, which is carried by silverleaf whitefly.

How fast numbers build

Life cycle speed explains why whitefly gets away from gardeners. The Queensland Government says greenhouse whitefly breeds through the warm months, with a life cycle of about five to seven weeks. Silverleaf whitefly is quicker, taking 18 to 28 days in warm weather. Whiteflies also tend to gather in hot spots, where temperatures suit their breeding.

Checking plants and using sticky traps

Early detection makes everything easier. Check plants regularly, and more often in warm weather or when numbers are high. Look closely at the undersides of leaves, where whitefly usually sit. Stunted or pale plants deserve a closer look.

Yellow sticky cards are a cheap way to see what is flying. Queensland Government advice includes yellow sticky traps as part of regular monitoring. It also suggests checking adults in the early morning, between 7 and 9am. In a backyard greenhouse, one or two cards will show when numbers start to climb.

Keeping the greenhouse clean

Good hygiene stops whitefly carrying over from one crop to the next. Remove old plants and debris at the end of each crop. Clear weeds from inside the greenhouse and around the doorways. Weed matting or a plastic floor makes this easier.

New plants can bring whitefly in with them. Inspect anything new before it goes into the greenhouse. Where possible, choose resistant varieties. If you have more than one growing area, work in the cleanest one first and the dirtiest last.

Plants grown in lightweight garden pots are easy to carry out into the light for a close look. A badly affected plant can also be moved away from the rest. That gives you time to deal with it before the problem spreads.

Screening vents and doors

Insect screens on vents and doors keep adults out. The mesh needs to be very fine, as the adults are only about a millimetre long. Check that screened vents still let enough air through on hot days. A small greenhouse with one door and a roof vent is easier to screen than a large open structure.

Natural enemies and biological control

Biological control uses living predators and parasites to keep pests down. The best known option for whitefly is Encarsia formosa, a tiny parasitic wasp. It parasitises whitefly nymphs, which turn dark as the wasp develops inside them. The Queensland Government says these wasps usually provide effective biological control.

Other helpers include lacewing larvae, ladybirds and small predatory bugs. Broad spectrum insecticides such as pyrethroids and organophosphates can harm them, so avoid those if you want them around. A garden that welcomes beneficial insects outside can support the work inside. Ideas for wildlife friendly landscaping can help bring them closer.

Using sprays carefully

Sometimes numbers get too high for traps and wasps alone. For moderate infestations, Queensland Government advice suggests an oil or insecticide treatment that works on adults. Spray thoroughly and pay attention to the undersides of leaves. Adults are the main target.

Resistance is a real concern with whitefly. Silverleaf whitefly was first recorded in Australia in 1994. It is now widespread in Queensland, New South Wales and Western Australia. The Queensland Government says it is difficult to control because it has developed resistance to conventional insecticides. Rotating between chemical groups helps slow resistance, and the label should always be followed.

Planning ahead

Prevention is easier to build in from the start. Hard floors and clean benches leave fewer places for weeds and debris. Leave space between plants so you can get in to inspect leaves. Our guide to sustainable materials for your garden may help when choosing benches and flooring.

Frequently asked questions

How do I know if I have whitefly?

Look for tiny white winged insects on the undersides of leaves. Sticky honeydew, sooty mould and wilting leaves are other signs. Yellow sticky cards will catch the adults.

Can whitefly be controlled without chemicals?

Numbers can often be kept down with hygiene, screening, sticky traps and biological control. The parasitic wasp Encarsia formosa usually provides effective biological control. An oil treatment is another option if a spray is needed.

Why does whitefly get worse in summer?

NSW DPI says whiteflies can become a problem in summer or under dry, warm conditions. They tend to gather in hot spots where temperatures suit their breeding. Checking those spots first each week makes sense.

Does whitefly spread plant diseases?

Yes. Whiteflies can transmit plant viruses. Silverleaf whitefly can spread tomato yellow leaf curl virus, which affects tomatoes, capsicums, beans and many ornamentals.