GREENFLY (aphids) can attack almost any plant, but their favourites are ones with plenty of soft new growth. Look on the underside of young leaves and around the growing points. Greenfly suck sap from the plant and to do this they aim for the easiest points. Viruses can be introduced directly by greenfly, or disease can get in through the wounds that they leave where they suck on sap. The first signs might be new leaves that are small and curled, or extended growing points with little sign of new leaf growth. If you leave greenfly unchecked, they can spill over on to flowers and fruit. Plants become sticky from the honeydew that the flies produce. This sweet substance is sought after by ants, which will guard colonies of greenfly from attack by predators.
What can you do about greenfly?
• If there are only one or two, squash them! Do this regularly until none are left.
• Spray with a jet of water aimed into troublespots. This will wash plenty of wingless females off the plant. Winged females could fly straight back, but repeat spraying will keep the problem under control. Plants in pots should be taken out of the polytunnel for spraying so that any squirted greenfly fall outdoors and won’t so easily find their way back.
• Buy a biological control. Aphidius is a small black insect that kills greenfly. One insect can lay as many as 100 eggs. Each egg is laid into one immature aphid.
• Introduce ladybirds to eat the aphids. You can carry these into the tunnel from other parts of the garden or purchase them as larvae or adults.
• If legislation permits, sprays of soft soap and nettle tea are also effective controls.
• Plant nasturtiums to help keep greenfly away.
Greenfly
BLACKFLY are another type of aphid, but they don’t attack such a wide range of plants. Look out for them around the tips of broad beans. The whole top of a plant may be smothered until stems look black. If left to their own devices, blackfly will spread down the stem and disfigure the whole plant, including pods. Attacks usually start in June, and the beans might well have finished cropping in the polytunnel then.
What can you do about blackfly?
• At the first signs of attack, simply cut off all growing points of all the plants, whether affected or not, since this is where blackfly always starts off. Either burn them or put the tops in a sealed polythene bag for a few weeks until all the flies are dispatched.
Blackfly
WHITEFLY isn’t actually an aphid, although the two are related. Whiteflies also suck sap and produce honeydew, which encourages moulds. These small, waxy, white, winged insects can attack a wide range of polytunnel plants. Immature forms are green and scale-like, but these can also damage the growing plant. Strawberries, tomatoes and cucumbers can be particularly affected. If a cloud of white specks rises up in the air when you brush against a plant, check further. Turn a leaf over and look for the characteristic white insects. These are clear to spot against the green foliage.
What can you do about whitefly?
• If there are only one or two, squash them between your finger and thumb.
• Hang up sticky strips to trap flies. This may not control them, but it will indicate when they are present.
Note: you must remove sticky traps before releasing any biological control.
• Use the biological control Encarsia.
• Plant a row of French marigolds to help repel whitefly.
RED SPIDER MITE can be a real problem in a hot, dry summer. These tiny mites look like minute crabs on the underside of leaves. Even though the name says ‘red’ they may well be green in the summer months. Leaves can start out with bleached spots but turn a mottled brownish-red in colour and there might be visible webs at the growing points. If you use a magnifying glass you can spot mites scuttling around the webs. Several plants can be attacked, but some seem to be favourites: aubergine, French bean, cucumber and melon are top of the list. If plants are severely affected, they will stop cropping.
What can you do about red spider mite?
• Spray overhead and keep everything damped down in hot weather. The mites don’t like wet conditions.
• Use the biological control Phytoseiulus. This is a smaller mite, which feeds on the red spider ones.
• Scrub down frames, benches, etc. to destroy overwintering adults.
Red spider mite
CATERPILLARS will happily munch their way through most leaves and fruit that they find themselves on. The adult butterflies and moths come in through open tunnel doors and look for the most appropriate place to lay their eggs. A tomato moth will choose a tomato plant, whereas a cabbage white butterfly will choose a brassica (even if it isn’t totally specific as to which one). You may notice caterpillars in serious numbers around midsummer, but there will be far more in August and September. If there are large holes in leaves, look underneath and pay attention to any curls or folds in the foliage: caterpillars often hide in such places during the day. Droppings can be a telltale sign. These are dark, wet, and smell of boiled cabbage. Look out for rows of eggs glued to the underside of leaves. If these are dispatched before they hatch, damage can be avoided.
What can you do about caterpillars?
• Squash the eggs before they hatch.
• Hunt out the pests as soon as you notice damage. Pick off individual caterpillars and dispose of them (use tweezers or rubber gloves, because some caterpillars can cause a skin reaction).
• Cover low-growing crops with a fine mesh, so that butterflies can’t get through to lay their eggs.
• Catch any butterflies and moths that find their way into the polytunnel. Pick them up and throw them out! They probably won’t find their way back.
• Put screening material across the doors to allow air through but keep butterflies out.
• Use the biological control Steinernema feltiae directly on the caterpillars.
Caterpillar eggs stuck to a leaf
CUTWORMS, also called surface caterpillars, will eat through the stems of plants at ground level. Top of the list are lettuce. If a plant suddenly collapses in a row, check the stem. If it’s cut through at ground level, a cutworm is almost certainly to blame. These pests feed at night and hide just under the surface of the soil during the day.
What can you do about cutworms?
• Dig around in the soil near where a plant has collapsed. The culprit is a greyish coloured grub about 4cm/1½in long. There is usually only one at a time, so dig it up and dispatch it.
WIREWORMS are most often a problem in newly cultivated soil and so may affect the polytunnel for the first couple of years after you put it up. The thin, segmented, orange-coloured larvae are up to 2cm/¾in long. They feed off grass roots if they hatch in a field, but if the field has been turned into a polytunnel border, they are happy to burrow into any root, bulb or stem that presents itself. The result can be a disheartening loss of a crop.
What can you do about wireworms?
• The problem decreases in subsequent years, so try sowing a sacrificial crop to get rid of most of the pests in one go. Potatoes are usually a favourite sacrificial crop. The tubers will be riddled with wireworm and may not be fit to eat, but the pests can be physically evicted along with the spuds.
Wireworms
SLUGS AND SNAILS must be some of the most annoying pests that a gardener faces. They creep out of hiding places when it turns dark and slither back at daybreak. In the meantime, they munch their way through pots of seedlings and eat holes in the leaves of larger plants. They can climb 2m/7ft up a climbing bean plant and hide in the leaves all day, just waiting to munch the beans at night. Not all varieties of slug are destructive and some eat only decaying vegetation, but nonetheless most gardeners have to use slug and snail controls.
What can you do about slugs and snails?
• Go out after dark with a torch and collect them up.
• Use organically approved slug pellets based on ferric phosphate. These won’t harm other wildlife and they won’t wash away in the first rain shower.
• Use barriers of copper, crushed egg shell, wood ash, bran, etc.
• Drown them in beer traps sunk into the soil.
• Stand pots above moats of water.
Slugs hide between leaves
VINE WEEVIL grubs will eat through roots, causing plants to collapse. They are a particular problem for plants in pots and containers. The grubs are fat, C shaped, white and about 1cm/½in long. They are found in the compost or soil surrounding a plant. Strawberries in pots seem to be a particular favourite.
What can you do about vine weevil?
• Tip the contents of the pot out and shake the compost loose around the roots. The short fat grubs can be picked out and squashed.
• Remove any adult beetles if you find them.
• Use the biological control Steinernema kraussei to destroy the grubs.
• Prolonged periods above 28˚C/82˚F and below –2˚C/28˚F will kill grubs.
WOODLICE, EARWIGS AND ANTS will all be present to a greater or lesser degree. Woodlice and earwigs will nibble seedlings and they can cut through stems. They make holes in the leaves of larger plants. Ants will ‘farm’ aphids in order to get the honeydew and so they help preserve a more damaging pest. Ants will also eat into strawberries, spoiling the fruit.
What can you do about woodlice, earwigs and ants?
• Stuff a jam jar with straw and turn this upside down on a stick. Earwigs will crawl up into the straw at night and can be removed during the day.
• Put a pile of decaying organic material on the ground. Woodlice will feed off this, but they will also hide underneath it during the day. Remove the pile a little at a time and collect the woodlice before they run away.
• Ants like dry soil. Soak everything and they will look for a drier place to make a nest.
BIRDS can be a help to the gardener. Some will eat insects and destroy lots of pests. However, birds can also be pests themselves. Blackbirds and starlings will squeeze under doors and peck through netting in order to get at ripe strawberries and grapes. Birds will also scratch about in the soil, burying seedlings and displacing mulch.
• Raise netting on hoops above strawberry pots. Weight it down well at the sides so that birds can’t get underneath. You may lose one or two fruits if they are close to the netting, but most of the crop should survive.
• Hang netting across polytunnel doorways so that birds can’t get in but fresh air can.
MICE AND RATS are a problem in the winter months. They seek a bit of shelter as the weather worsens and are always on the lookout for an easy source of food. Both rats and mice will dig up and eat pea and bean seeds, but they will also nip through the stems of these newly sprouted plants. All the young pea shoots might disappear overnight, only to be found hidden somewhere like in a pot or under a bit of polythene. A mouse will have made a store to feast on later and the potential pea crop will be lost.
What can you do about rats and mice?
• Get a cat!
• Set humane traps to catch them. Lethal traps might catch a cat instead of a rat.
• Poisoned bait will kill them, but please don’t use this in the polytunnel. Rats will carry the poison and bury it, possibly in the compost heap, or somewhere close to an edible crop.