Controlling pest in an organic greenhouse

Longer days and improving temperatures means plant growth is rampant, especially in the greenhouse. But the plants are not alone. Armies of aphids, mealy bugs, spider mites, scale insects and whitefly are just waiting in the wings to take advantage. So what can the organic gardener do to help protect crops? The answer is to set a thief to catch a thief. In my organic greenhouse, I use a range of cultural techniques, natural predators and specific biological control measures to keep the most troublesome pests at bay.

Organic controls for greenhouse pests

• Aphid - squash small colonies as soon as they are noticed and encourage natural predators, such as ladybirds, lacewings and hoverflies, that will devour the larvae.
• Caterpillar - pick off individual caterpillars and squash webbing in leaves to kill pupae, or try a biological control.
• Leaf miner - squash any larvae seen and pick off badly affected shoots or introduce a biological control.
• Mealybug - remove dead leaves and pick off badly affected growth where practicable and introduce a biological control - a predatory beetle that eats mealybugs.
• Scale insect - flick off individual scales with a fingernail or introduce a parasitic wasp biological control.
• Sciarid fly - put up sticky traps to catch flying adults and introduce a biological control -nematodes or predatory mites that kill the larvae.
• Red spider mite - pick off badly affected growth and introduce a predatory mite biological control which eats the red spider mites.
• Thrip - put up sticky traps to catch flying adults and introduce a biological control predatory mite to kill the larvae.
• Vine weevil - tip out rootballs of permanent container plants and remove any grubs found or apply a nematode biological control that will kill vine weevil larvae.
• Whitefly - suck up flying adults using a car vacuum and put up sticky traps, or introduce a parasitic wasp biological control that targets the whitefly pupae.

Dealing with common pests In the greenhouse, the pests may have a veritable garden of Eden in which to breed and multiply, but they are a captive audience to any control measures you care to try. A few pests can be deterred by cultural means, for example populations of thrips can be thwarted through regular watering and careful shading, so that plants are not put under stress. The spread of spider mites, on the other hand, can be slowed by spraying the crops each day with water to increase humidity. But most pests will need to be tackled directly. Even then, you can control the numbers of many pests by hand. For example, caterpillars can be picked off as they are seen or have their webbed cocoons squashed. Many flying pests, including sciarid flies, thrips and whitefly can be reduced using sticky traps or, in the case of whitefly, sucking up clouds of disturbed flying insects with a car vacuum is said to work well. In addition, many of the most damaging greenhouse pests can be combatted using a specific biological control.

Using biological controls This is a method of introducing one of the pests’ natural enemies to actively seek out and kill it. These biological assassins are perfectly safe to use, because they attack only the pest, leaving beneficial insects and other creatures alone. In the greenhouse, this method of control can work very well, because the pest and biological control are held within a confined space. The higher temperatures found in the greenhouse, also suit the rapid breeding of biological controls. Like miniature secret agents, they come in the form of tiny insects, mites or microscopic worm-like nematodes that can be predatory or parasitic. You can get a mixed pack, containing a blend of nematodes, making it effective against a wide range of pests including cabbage root fly, carrot fly, caterpillars, codling moth, cutworms, leatherjackets, onion fly, sciarid fly, gooseberry sawfly, thrips and even ants.

Biological controls are also very easy to use but you have to wait until you see the first signs before you can set the biological control into action. Therefore, you’ll need to be prepared to tolerate a low level of pest activity, since the biological control will die out if they are released where there are no pests on which to prey. I use yellow sticky traps as an early warning system in my greenhouse. Hung at around the tops of the crops they will trap a proportion of the flying insects and make an effective monitor of pest levels. I also inspect the crops each time I water and have a hand lens ready if I suspect the presence of an interloper. For the most effective deployment of biological controls, always follow the supplier’s instruction on the packaging. Bear in mind they are living creatures and so must be used as soon as possible after they arrive. Some, such as predatory mites and parasitic insects, need a daytime temperature of over 21oC to be effective, while nematodes will work with temperatures down to 12oC. Once you have introduced the biological control, don’t use chemicals to control other pests since you may unwittingly kill your secret agents.