It’s that time of year again, when wafts – nay, clouds – of whitefly lay siege to summer favourites like tomatoes and cabbage. They appear from seemingly nowhere and colonise plants fast – they’re certainly contenders for the most efficient-at-reproducing-and-spreading pest!
How to Identify Greenhouse Whitefly
For those unfamiliar with whitefly (lucky you!) here’s a quick description of what you’re looking for. Greenhouse (or glasshouse) whitefly are closely related to aphids but a touch smaller, at around 1-2mm (a twelfth of an inch) long. They have a triangular shape, like tiny, anaemic cheese wedges. The real clincher for quibble-free identification is their habit of swarming away when approached or disturbed.
Just like aphids they produce a sticky ‘honeydew’, which is in fact…their faeces. Yes, it’s all very unpleasant stuff. And if that’s not enough, honeydew can attract diseases such as black sooty moulds on plants already weakened by the whiteflies’ persistent sap sucking.
Whiteflies are great at making more of themselves. Each adult lays hundreds of eggs which hatch out into larvae with an insatiable appetite for whatever plant they’re sat on. Severely affected plants can eventually turn yellow and wilt, producing lacklustre growth at best. As well as tomatoes and cabbage family crops, particular whitefly favourites include peppers, aubergines, strawberries, cucumbers, pumpkin, okra and sweet potato. They will attack plants growing outdoors as well as in the greenhouse.
Greenhouse Whitefly Organic Controls
Whitefly can strike any time from the middle of summer. The first frost will put paid to their plans for crop-wide domination but, be warned, in mild areas or in warm greenhouses they can happily remain active throughout the winter. Needless to say, the first line of control is to take action as soon as they appear; the early gardener catches the whitefly, so to speak. Here are five tactical techniques to help you banish the bugs:
1. Blast off. Begin with a good, strong blast from the hose to knock the blighters off your plants and onto the ground where they’ll perish. Blast all areas of the leaf, especially the undersides where they mostly congregate.
2. Organic insecticidal soap. With an initial assault complete, it’s now time to apply an organic insecticidal soap, again taking great care to cover all areas of the leaf including, crucially, those undersides. Spray at a cooler time of the day and follow up with one or two more sprays a few days later.
Recipes abound for homemade insecticidal soaps. For example, add a few drops of dish washing liquid and a squeeze of lemon juice into a spray bottle then top up with water, shake and spray. The trouble with homemade sprays is that their effectiveness varies dramatically, and they aren’t always as harmless as they seem, potentially knocking back whitefly predators too. They are perhaps best reserved for use within a closed environment such as a greenhouse, but be aware that results may disappoint and potentially could do more harm than good.
3. Vacuum. What, really? Yes, really! In a bad outbreak a handheld vacuum can be very effective at sucking up clouds of whitefly from disturbed leaves. Repeat over several days to begin to bring an infestation under some semblance of control.
4. Sticky traps. You can buy sticky traps, or make your own. Simply paint or colour in cards yellow, then smear with a concoction of petroleum jelly cut with a little dishwashing liquid. Hang up close to affected plants. Whitefly love the colour yellow and will flock to the cards, only to get stuck and meet their end. Hanging up sticky traps is also a great way to monitor populations of whitefly early on in the season – act immediately when you spot them.
5. Parasites. Encarsia formosa may sound like the Latin name of a beautiful vine, but it’s actually a special type of parasitic wasp that lays its eggs inside whitefly larvae. Then, when the eggs hatch, the young Encarsia feed on the larvae from the inside out. If that all sounds a bit grim, it is.
And it gets worse. The whitefly larvae are still alive at this point and it’s only when the parasite is almost ready to hatch out as an adult that it then sets to work on its host’s vital organs. Once these are devoured, the whitefly turns black and just over a week later, adult Encarsia hatch out to go on a morbid hunt for their next victims.
Gross as it is, Encarsia formosa are formidable at controlling whitefly within a greenhouse. You can buy them on cards primed with ready-to-hatch pupae to hang up near infected plants. They need a relatively warm temperature of at least 21°C (70°F) and can really only be used within enclosed environments.
Other Greenhouse Whitefly Predators
If the Alien-like approach of Encarsia is too much, consider one of the whitefly’s many other natural enemies, including hoverflies and ladybirds. Flowers and flowering herbs such as calendula, thistles, oregano, fennel, parsley, poached egg plant and buckwheat are all easy to grow and will draw in whitefly-eating predators by the crowd. They’ll feast on aphids too.
Grow some of them next to the greenhouse door to tempt these beneficial bugs closer. A few pots of choice favourites placed inside will almost certainly seal the deal.