14:38 - 24 May 2011
The greenhouse environment can create the perfect place for pests to multiply especially in the absence of predators. However it can also protect plants from some problems by keeping them isolated from garden plants and also by blocking the transmission of diseases such as blight and peach leaf curl.
There are however several pests that can become a problem within the greenhouse. How you deal with them very much depends on your gardening stance. Some gardeners think nothing of grabbing a sprayer of pesticides and soaking their plants as soon as they see any insect or pest. The danger of this is that you can affect beneficial insects, especially if you aren’t too good at identification and, depending on what you choose to use you can also leave residues on your plants. If you are growing edible plants such as herbs and salads, you may not want them dressing with chemicals and poisons. If chemicals are your choice of control then use them with great care, they are by their very nature designed to kill things and they can have a knock on effect. Read the label, follow the instructions to the letter and never, ever use anything that has been kicking around the shed for years.
If you are new to gardening or greenhouse growing it makes more sense to employ pest control methods that work with nature. These can be safely used on all plants including fruit, vegetables and herbs so there are no harvest intervals to observe and you can eat your produce confident that they are free from pesticides.
Whichever method you choose you need to be vigilant. At the first sign of a problem you need to take action as one or two pests can quickly become an epidemic. It’s really important to be able to properly identify the problem. Some creatures that look pretty menacing are the larvae of some very useful creatures; ladybird and hoverfly adults and larvae look horrible but eat shed loads of aphids and are extremely useful in the garden and greenhouse. Pesticides, even organic or natural ones will not be selective and will kill the goodies and the baddies and upset the natural balance. These days the Internet is a brilliant way to ID garden and greenhouse pests accurately. Once armed with the correct information you can then make an informed decision on what to do next. Small clusters of aphids can be removed or squashed manually. Constant vigilance in the greenhouse can control slugs and snails without the need for pellets, however organic slug pellets can be used safely in a greenhouse and are now deemed to be as effective as their metaldehyde-based alternatives.
Greenhouse plants can succumb to the voracious appetite of caterpillars, so keep a careful watch on your plants and act when trouble first appears. The other major greenhouse pests are red spider mite, which can be deterred by increasing the greenhouse humidity, and greenhouse whitefly, which is often resistant to pesticides. The most effective way to control both of these common greenhouse pests is to use a biological control. These are carefully selected predators or parasites for the pest problem, which means it is vital to correctly identify the pest first otherwise they won’t work. It’s a bit like adding ladybirds to the greenhouse to eat aphids, they will only eat aphids, and so if you’ve got whitefly it won’t help.
There are biological controls for most garden and greenhouse pests including vine weevil, slugs and snails, ants, caterpillars, leatherjackets, aphids, whitefly and more. Used correctly they can keep pest populations to a minimum without using poisons. Don’t use biological controls as a last resort, especially if you have already sprayed a systemic insecticide inside your greenhouse as these chemicals persist for weeks and sometimes months and will actually kill any friendly bio controls that you then introduce. Basically you need to make a choice to either use chemical controls or work with nature. Personally I think the natural way should always be the first course of action, it’s kinder to your greenhouse, kinder to your plants and kinder to you and the environment.
