How to control and spot the symptoms of glasshouse whitefly

Learn what to look out for and how to treat glasshouse whitefly.

Scientific name for glasshouse whitefly: Trialeurodes vaporariorum.

Glasshouse whitefly are an all too common pest found in our greenhouses and conservatories. It as a sap-feeding insect which attacks houseplants, greenhouse plants and many vegetables.

If left uncontrolled, they can cause damage to a number of plants, feeding by sap-sucking and reducing the plant’s vigour.

Let’s look at the glasshouse whitefly a little more closely and what symptoms to look out for, and how to control them.

Facts about glasshouse whitefly:

• They are most often found in warm conditions, which is why they aren’t (usually) a problem for outdoor plants.
• The whitefly found on cabbages, kale and brussel sprouts (Brassicas); Rhododendrons; and Lonicera are a different type of whitefly altogether - a whitefly specific to those particular plant groups.
• Unfortunately, glasshouse whitefly are active all year round.
• They can remain active right through winter in an unheated glasshouse, so long as there is a suitable host plant present.
• They usually do not survive winter outside.
• The adults are roughly 2mm long with white wings - hence their name.
• Female, adult whitefly produce rapidly, laying greyish-white, cylindrical eggs on the underside of leaves - and each one can lay more than two hundred during its lifecycle!
• The eggs hatch into scale-like nymphs, which pupate and become adults - this transformation is called “complete metamorphosis” (butterflies also undergo complete metamorphosis).

Plants most affected by glasshouse whitefly:

• Glasshouse whitefly affects many greenhouse and houseplants, including; Pelargonium, Lantana, Poinsettia and Gerbera.
• They also infest some edible crops, such as; cucumbers, melons and peppers.

Signs that you may have an infestation of glasshouse whitefly:

• Whitefly “clouds” - Whitefly are usually easy to spot, so this is an obvious symptom to look out for. Disturb or brush past an infested plant and you will see a cloud of whitefly fly up.
• Sticky leaves - Glasshouse whitefly feed on plant sap and excrete a sticky honeydew. This drops onto the lower foliage.
• This allows the growth of an unsightly, black mould to develop.
• Scale-like nymphs - Look closely and you may see the whitefly’s flat, oval, creamy-white, scale-like nymphs on the underside of your plant’s leaves.

Control methods and treatment of glasshouse whitefly:

• Biological control - Biological controls often give better, more effective control on greenhouse plants, than insecticides, as the glasshouse whitefly reproduce so rapidly. The best biological control is to introduce the parasitic wasp (Encarsia formosa) to the glasshouse. The tiny wasp will lay its eggs in the whitefly scale-like nymph - then the parasitised nymph will turn black (so it’s easy to monitor its effectiveness). Use this natural control method before the infestation becomes too heavy.
• Physical control - Hang sticky yellow sheets/traps to physically trap the adult whitefly - hang them above or among the plants.
• Cultural controls - Generally, good, “cultural” habits like these ensure a hygienic greenhouse and environment for growing your plants.
• If possible, before bringing any newly purchased plants in amongst the rest of your plants, quarantine them - this allows any glasshouse whitefly eggs, that may be present, to hatch and allow themselves to be recognised.
• At least annually, clean your greenhouse itself to reduce overwintering populations of glasshouse whitefly.
• Remove weeds from in and around the greenhouse, as weeds can act as hosts for the glasshouse whitefly. Chickweed often grows outside as well as under greenhouse staging, and is a particular favourite for overwintering them - so keep on top of it.
• Ensure there is good ventilation to help you to check the growth of developing sooty moulds.
• Chemical control -
• Fruit and vegetables - Spray with a naturally occurring or pyrethrum-based insecticide that kills on contact. Spray thoroughly.
• Ornamentals - Spray with a systemic insecticide that is absorbed into the plant’s tissue and taken up by the sap-sucking whitefly when they feed. This type of spray is the more effective of the two, but you must check the product label if you are using it on crops.

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