15:54 - 11 June 2010
The warmer weather brings much more than fair weather gardeners out into the garden and greenhouse. It’s the perfect conditions for pests to thrive and multiply.
Indoors and out our plants are under constant threat by a number of sap sucking bugs. Already you may have seen an explosion in aphids around the soft sappy shoots of roses, clematis and more. A healthy population of ladybirds and lacewings, as well as fledgling birds will help to keep your aphids under control.
Another more problematic summer pest is the whitefly, those that affect outdoor crops such as cabbages, rhododendrons and even viburnum are not the same species as those annoying clouds of ghostly flies that waft around the greenhouse.
Both are called whiteflies but they are specific to their host plants and have different modes of control.
In the garden
The most commonly affected plants outdoors are leafy members of the cabbage family such as cabbage, kale, sprouts and broccoli. Give your plants a helping hand and keep them in good health. Avoid overfeeding them with high nitrogen fertilisers that will encourage soft and sappy growth, which is far more susceptible to pest attack. Young cabbage plants can also be covered with horticultural fleece to keep flying insects at bay. This will provide a protected environment as the plants develop. Avoid chemical use as much as possible. They are designed to kill insects and do not discriminate between good garden insects and the problem pest. If absolutely necessary, heavy whitefly infestations can be controlled with a suitable insecticide that is approved for use on edible plants. Spray after dusk when the bees have gone back to their hives and follow the instructions to the letter.
Under glass
Glasshouse whitefly poses a different problem. Over the years this pest has evolved to develop resistance to most chemical controls. It’s a lesson to us all and simply mimics the problems with overuse of antibiotics too.
Tomatoes and cucumbers are a favourite delicacy of the greenhouse whitefly (Trialeurodes vaporariorum) but they also attack many other greenhouse plants including Gerbera, fuchsias, pelargoniums and begonias. The whitefly lay their eggs in batches on the underside of plant leaves and these hatch into larvae a week later. These then develop into immobile scale-like nymphs that suck plant sap, until they become adult whitefly.
You can control greenhouse whitefly using an integrated approach. First identify whitefly as the pest that is present. Hang yellow sticky traps at the growing tips of your plants to catch passing bugs, these will give a clear indication of what flying pests are present. You need to act as soon as they are seen. If you plan to use biological control then take the traps down when you introduce your control method as the traps may catch your beneficial insects.
Then it's decision time. Make a decision whether you intend to use chemical control or biological control, this is important, as the former will destroy the latter. If you start to use chemical control and then decide to use biological control you will need to get advice from your biological control supplier: tell them exactly what chemicals you have used and they will advise you of any delay period needed. For some chemicals this can amount to several weeks. And don’t forget that many greenhouse whiteflies are resistant to chemicals anyway so it may not even work.
For the safest, greenest and most effective route use biological control. There are two main bug busters that you can use: Encarsia wasps lay their eggs in the whitefly scales and Delphastus beetles eat all stages of the whitefly.
Encarsia are the best choice when the whitefly population is first developing and should be introduced at the very first sign of whitefly. Delphastus beetles need large populations of whitefly to survive and also need to be contained within the greenhouse with window netting screens.
Don’t forget that greenhouse whitefly can be deterred using Tagetes (marigold) plants. Grow these in close proximity to your tomatoes and cucumbers to reduce pest infestations.
