Why a Polytunnel Becomes a Pest Paradise

A polytunnel earns its keep. It gives you weeks of extra growing time at both ends of the season, protects crops from wind and driving rain, and lets you grow peppers, aubergines and melons that would sulk outdoors. It also creates near-perfect conditions for a small group of sap-sucking pests.

Think about what is missing under cover. There is no rain to wash aphids off the leaves, no wind to disturb whitefly, and far fewer natural predators finding their way in. The air is warm and often still, the plants are growing soft and fast, and one generation of aphids can be followed by another within a fortnight because there is no cold snap to interrupt them. The result is that a handful of insects you would barely notice on the outdoor plot can build into a serious infestation inside a tunnel in three or four weeks.

The good news is that tunnels are also easier to manage than open ground. You control the ventilation, the watering and the planting plan, and predators introduced under cover tend to stay put and work harder. Dealing with pests here is mostly about being early and consistent rather than reaching for a spray.

Ventilation Is Your First Line of Defence

Most problems start with stagnant air. Still, humid conditions suit aphids and whitefly beautifully, and crowded, soft growth gives them plenty to feed on. Getting the air moving is the single most useful thing you can do.

• Open doors at both ends early in the morning on any day that looks warm, and close them again in late afternoon to hold the warmth overnight.
• Aim to keep daytime temperatures below about 25°C. Above 30°C, plants become stressed, pollen fails and red spider mite multiplies at speed.
• Water in the morning so that foliage dries through the day rather than sitting wet overnight.
• Space plants generously and keep paths clear. If you cannot see the soil between rows by midsummer, you have planted too tightly.
• Remove yellowing lower leaves regularly, and pinch out the soft, sappy side shoots that aphids colonise first.

There is a balance to strike. Whitefly and aphids prefer warm, humid, still air, while red spider mite thrives in hot, dry conditions. On a baking afternoon, damping down the path with a hose raises humidity and slows spider mite, but do not make a habit of it if whitefly is already present.

Rotate Crops, Even in a Small Space

Rotation matters more under cover than outside, because pests and their eggs survive in the soil and on crop debris over winter. Split the tunnel into three or four zones and move crops between them each year.

• Never follow tomatoes with aubergines, peppers or potatoes. They are all in the same family and share the same pests and diseases.
• Follow a heavy-feeding fruiting crop with leafy salads, then with a legume or a green manure such as grazing rye or field beans.
• Clear all plant debris at the end of the season, including roots and fallen fruit, and take it away rather than composting it on site.
• Leave the doors open through a hard frost if you can spare the crops. A proper cold snap does more against overwintering whitefly than anything else you can do.
• Wash tools, gloves and reused pots, and give the polythene a scrub in late winter to let more light through.

Know Your Enemy: Aphids, Whitefly and Red Spider Mite

Aphids cluster on new growth, flower buds and the undersides of young leaves. Look for curled, distorted tips and sticky honeydew that turns black with sooty mould. Squash them by hand where numbers are small, or knock them off with a firm jet of water. Planting French marigolds, basil and nasturtiums near the doors brings in hoverflies and ladybirds.

Whitefly announce themselves as a cloud of tiny white insects when you brush past tomatoes or peppers. The adults are a nuisance, but the flat, pale larvae on the leaf undersides do the real damage. Yellow sticky traps hung just above the crop give you an early warning and catch adults, though they will not solve an established problem on their own.

Red spider mite is the one people miss. The mites are barely visible, but the damage shows as pale speckling across the upper leaf surface, followed by fine webbing on the undersides. It thrives when the tunnel is hot and dry. Keep a hand lens on the bench and check the undersides of older leaves weekly. Regular misting of the foliage and damping down the paths slows it considerably.

Bringing In Predators

Biological control is made for polytunnels. A predator introduced under cover cannot blow away, and the sheltered warmth keeps it active. The key is to introduce it at the first sign of trouble rather than waiting until the crop is white with insects.

• Encarsia formosa for whitefly. It needs warmth and good daylight, so introduce it from late spring onwards when daytime temperatures are reliably above about 18°C.
• Phytoseiulus persimilis for red spider mite. It hunts best in humid conditions above roughly 20°C, which is another reason to damp down.
• Aphidius colemani and the predatory midge Aphidoletes aphidimyza for aphids, particularly in a developing hotspot.
• Stop using broad-spectrum sprays for at least a fortnight before introducing anything, or you will kill the very helpers you have paid for.
• Keep a few flowering plants near the tunnel doors to support native predators between introductions.

A Simple Weekly Routine

Half an hour a week, ideally on a warm morning when insects are active, will keep you ahead of most problems.

• Walk the tunnel with a hand lens and check the undersides of leaves on your most valuable crops first.
• Inspect sticky traps and note whether numbers are rising or falling.
• Pick off and bag any infested leaves. Bin them, do not compost them.
• Adjust ventilation and watering according to the forecast, not habit.
• Keep a short notebook of what you see and where, so you know which beds to watch next year.

A tunnel with a few aphids and a couple of ladybird larvae in it is a healthy tunnel, not a failing one. Nudge small populations early, lean on airflow, rotation and predators, and you will spend far more of your summer harvesting than fighting.