Pretty Guy for a Whitefly

By Brett Kerley

Do you have whiteflies in and around your crops? These pesky little blighters can be found all over a whole host of plants, with their populations often the highest towards the end of summer.

Both the adult whiteflies and larvae feed on plants by injecting enzymes, dissolving the living tissue (phloem) and removing it. This reduces the vigour of your plant, and a severe infestation can kill the host. The impact of direct feeding and honeydew excreta (sugary poop) that favours sooty mould production on leaves of infested plants can affect your plants yields and make the aesthetics of your plant less appealing. The most obvious whitefly feeding damage symptoms are stem blanching, chlorotic spots, leaf yellowing and shedding. Basically, it significantly reduces a plant’s ability to photosynthesize and further reduces plant health. In many plants, the damage caused by whitefly can be indirect (for example, by transmitting disease-causing viruses) and not necessarily by the act of feeding itself.

The whitefly life cycle (egg to egg) consists of 4 instar phases (stage in the life of an arthropod; such as an insect) before finally emerging as an adult. An adult female usually lays between 200 to 400 eggs. Whitefly eggs are small and oval in appearance, typically laid on the underside of leaves, often in a circular pattern. The transition from one stage to the next is somewhat temperature dependent. At 21C eggs typically take 6-10 days to hatch.

Upon hatching, the whitefly emerges as a flattened nymph and its first instar is referred to as a crawler. Crawlers wander over leaves for about a week in search of feeding sites. Instar phases 2 to 4 resemble small scale insects that are immobile and lack legs. Each phase lasts between 3 and 5 days. Instar phase 4 is typically referred to as the pupal stage, during which whiteflies stop feeding and remain like that for 6-10 days. The adults hatch through a T-shaped slit and soon mate and reproduce. Adult whiteflies can live 30-40 days. However, without plant material to feed on, they will succumb to starvation in as little as 3 days.

Do they overwinter here in Alberta? The simple answer is they’ll most likely die off if temperatures go below -6c, unless they can find a nice warm area to hide out during those long cold winters, such as a greenhouse. I know some of you keep your greenhouses just above freezing, that’ll be fine for them to survive in, and especially if you keep some nice exotic plants over for next spring, it would be like having ‘skip the dishes’ for whiteflies. They can also survive on outdoor plants brought into your house or garage to overwinter!

So how do we control these bothersome pests? Firstly, make sure you’ve correctly identified the insect you want to control. Use those megapixel cameras/phones so that you can zoom in. If you’re still unsure, post a photo to the EHS Facebook page, the Alberta Insect Identification Facebook Page, or ask at your local greenhouse or garden centre, usually they’ll have someone there that can help.

Throughout the season you should be inspecting your plants, making sure to turn the leaves over to spot them. Heavily infested leaves can be picked off and thrown into your garbage. DO NOT put them in your or the City’s compost!

I can’t stress enough that a garden with many pollinators will help your cause. Lady beetles, lacewings and spiders are just a few garden heroes that love to munch down on the gourmet whitefly eggs and larvae. There are also several predatory wasp species that feed on whiteflies in all their life stages.

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In greenhouses you can use the yellow sticky traps. These can also be useful to gauge the density of your pests. I would avoid using chemical pesticides as whiteflies become immune to it, then you’re basically killing off the beneficial insects that would do a much better job anyway. You could try using a neem oil solution on infestations but make sure you spray both sides of the leaves. The trouble with this that is as soon as you start spraying, the adults take off, and return when all is calm again. The last method I have for you is to buy or grow some plants that deter them; Marigolds and calendula repel whitefly so it may be worth investing in them to place around your prized plants.

Good luck with the fight and happy gardening!