Top Ten Garden Pests

Every organic growing area will have its share of pests and diseases – they are a part of the natural way.  But there are times you need to take action to save your precious plants.  Saying no to poisonous chemicals is the organic way.  This means you can be creative and thoughtful in the way you treat pests - and careful when dealing with diseases.
So here below are some tips to help, including a five point plan to keep your growing area healthy! 
And then we list the top 10 pests, from carrot fly to caterpillar, and the top five diseases, including blight, mildew and clubroot.  By identifying the problem, we find ways to prevent and treat it.

To keep your growing area healthy, work on the principle of prevention, rather than cure.

We’ve drawn up a 5 point plan. Follow this, and you’re on the organic way to preventing pest and disease problems:

  1. Maintain a healthy, active soil with plenty of nutrients.  Add homemade compost and well-rotted manures. This will in turn provide healthy robust plants.
  2. Work with nature. Mix flowers with vegetables to attract beneficial wildlife, such as caterpillar-eating birds and aphid-eating insects.
  3. Choose plants that are suited to the site and soil. Otherwise you will get spindly unhealthy specimens which are vulnerable to attack.
  4. Move your veg crops around each year to prevent harmful diseases building up in the soil. We call this crop rotation.
  5. Be observant! Constantly check your plants for infestation. Also check barriers, traps and covers which you’ve used to keep out pests such as slugs, caterpillars and pigeons.

Why not toxic chemicals?
The organic growing area is full of life – in the soil, above it, and beyond.  Using pesticides and weedkillers can destroy a variety of life forms.
You might think you’re targeting a snail, instead you could be killing a valuable earthworm. Sprays can harm bees, butterflies and other helpful insects.   And many chemicals can cause serious pollution, either in their manufacture, or in the residues – which get washed into the soil or water bodies.

The top ten pests
Slugs and snails – these munching molluscs live above ground as well as in the soil. 
See here  to help identify them, as well as full details on how to deal with them.
Here are our top tips:

  • Don’t plant out young vulnerable plants – wait until they are a little tougher and able to withstand attack.
  • Some growers put out decoy plants – spare lettuces, for instance, to protect their main crop.  “One for you, one for me!”
  • Be vigilant.  If you have put down traps or barriers, check them regularly.  Especially after rain or damp conditions.
  • Never use slug pellets that haven’t been approved for organic use.  Even these will have some dangerous chemicals therein – so use them sparingly and store them wisely.

Aphids - these little greenfly and blackfly will suck the sap out of a plant.  They like young fresh growth. So avoid using too much nitrogen-rich feed which encourages the soft leafy growth.
The best way to deal with them is to squish them by hand (you may want to wear gloves.)  Birds feed on aphids, a blue tit family will eat over 100,000 in their lifetime, so make your growing space a haven for birds.  Insects such as ladybirds, lacewings and hoverflies will also eat aphids.  Encourage them into your garden with flowering plants such as the Umbelliferae family which produce tiny flowers arranged in umbrella shaped clusters - fennel, dill, angelica, carrot and coriander; and the Compositae family - flat open daisy-like flowers such as calendula, sunflowers and asters. Studies have shown that fennel, blooming from July to September, can attract around 500 different species of insect.

Cabbage whitefly - these are small white insects on the underside of brassica plants, often leaving scales (their young) on the leaves.  They can also leave a sticky honeydew, which in turn grows a black mould, which makes the leaf unappetising.
Do not panic: assuming your plants are in a healthy soil, and not short of water, they can tolerate quite high populations of this pest. Remove infected leaves (you can put them on the compost heap.) Plant fennel, coriander and cow parsley to attract parasitic wasps who feed on the scales. In this way the life cycle of the whitefly is broken.  Lacewings also feed on whitefly, and are attracted by dandelions, yarrow and shasta daisy.

Caterpillars - the very hungry caterpillars of the Cabbage White butterfly will munch their way through your precious brassicas - and any other greenery you care to provide.  At surprising speed!
To prevent butterflies laying their eggs, cover young plants early in the season with a fine mesh.   Make sure it is well pinned to the ground and has no holes.
If you’re too late to prevent egg laying, you need to constantly check every leaf of the plant and pick off caterpillars by hand.

Pigeons - these birds love brassicas, especially in the winter when food sources are scarce. They can happily strip a plant.
If you can’t bear to share your winter greens, then the best way to stop pigeons getting amongst them is string.  Lines of the stuff, criss-crossed, will prevent the birds being able to land and take off. 

Carrot fly – these flies can infest your carrot roots, creating nasty rusty brown tunnels where they have fed.  They are attracted by the smell of the bruised carrot foliage, usually caused when thinning out plants after sowing seeds too thickly. 
To stop the fly getting to the carrot, you can use a fine mesh.  Either by covering the plants, or by setting up a wall a metre high around the plants (these pesky flies are weak flyers and stay low to the ground.)  They usually fly during the day and in bright sunlight, so do any weeding on a dry evening with no wind when the scent of the carrots will not spread so far. Pull carrots for eating in the evenings too, for the same reason. Some say growing onions near the carrots will deter them, but research has shown you need at least 4 rows of onions either side of every one row of carrots for this to succeed.

Allium leaf miner - the larvae (maggots) of the tiny allium leaf miner (Phytomyza gymnostoma) can devastate allium crops, particularly leeks. It can also attack onions, shallots, chives, garlic and ornamental alliums.  Typical symptoms include lines of white spots on leaves where maggots have been feeding, split leaves and distorted plants which lead to rotting.
The best way to prevent the leaf miner, is to cover susceptible plants (as early as seedling stage) with an ultra-fine mesh cover (the fly is tiny) or horticultural fleece. This prevents adult flies laying eggs on the crop. Make sure the cover is firmly fixed so it will not blow off in the wind. Clear all plant debris, and soak infected plants in a bucket of water for a couple of weeks before adding them to a compost heap, or bury them in a deep hole. Do not put them straight into your compost heap. Lightly dig over the soil to disturb over-wintering adults and pupae.
Flea beetle - they are most commonly seen on brassicas (the cabbage family), rocket, radish and particularly Chinese greens – as well as flowers such as nasturtium and stocks. Tiny pin holes appear where the beetle has fed, and the small glossy beetles jump when disturbed. Flea beetles are especially active during dry weather.
To avoid infestation, don’t grow susceptible crops (oriental brassicas and rocket) before July. Late summer sowings will still give a productive crop, as at this time of year they produce leaves for longer before flowering.

Sciarid fly – if you have a heated greenhouse or conservatory, you may get an attack of these tiny flies which feed on decaying plant material, especially in plant pots and containers. The flies are just a few mm long and tend to fly in jerky movements or even run along the surface of the compost. By the time the first flies are seen there is a good chance that several hundred eggs will have already been laid in the compost, creating larvae (maggots) which eat the seedlings roots, stopping them from accessing water in the soil, thus wilting and dying.  Plants with large seeds in particular, such as cucurbits and beans, attract sciarid flies.
Sciarid fly is mostly a problem in the heated glasshouse rather than outside, so move plants out as soon as possible (beware the risk of frost). Loam (soil) based compost is best; and cover the surface of pot plants with 1cm layer of horticultural sand or grit to prevent adults from laying eggs. Practise good hygiene by a) using new bought compost each year, putting the last year’s onto the soil outside (or being sure to seal your bag of old compost)  b) remove any dead plant material around plants or in greenhouses  c) clear up any spilt compost and mossy areas on mats. Yellow sticky traps placed close to infected plants will catch adult flies.
Gooseberry sawfly – these will strip the leaves off gooseberry and red currant bushes, considerably weakening the plant. Adult sawflies lay small batches of eggs on the underside of leaves in late spring. When the larvae hatch, they feed voraciously on the leaves for a while, before dropping to the soil and pupating around the base of the plant. The best way to deal with them is to keep a close watch on the plant from leaf bud onwards, and remove the larvae as soon as you notice the damage has started (small pin holes, usually starting at the centre of the bush). It is essential to break the sawfly life cycle. Hoe around the base of the plant to expose larvae for birds to eat.

Top five diseases:
Potato blight – this appears as dark brown blotches on the leaves, which then develop a yellow halo.  The leaves eventually rot and die.  The fungal spores can then settle in the soil and affect the tubers, causing them to rot. Warm, wet and still weather causes a rapid spread, especially on dense growing areas like allotments.
Plant good quality, certified, seed tubers. It is not good practice to save your own potato tubers as diseases get passed on from generation to generation. Choose a variety with blight resistant foliage and tubers.  Mulch the soil with a layer of straw to prevent the spores penetrating the soil. Dry weather can halt the disease, so it’s worth removing the first few infected plants as soon as you spot the disease. Cut off the potato tops and burn them. Don’t harvest any tubers for 3 weeks, this allows the skin to set.

Tomato blight - dark brown/ blackish round patches appear in the foliage, often surrounded by a pale yellow halo that quickly spreads to rot the whole leaf. The underside develops a downy white coating of spores. Fruits develop dark markings, quickly developing a dryish brown rot and sometimes a whitish-grey mould. Even though the fungus is not poisonous to humans, fruit are not pleasant to eat and they won’t ripen or store.
To prevent or control, keep the plant leaves dry by watering the soil not the leaves. Infection occurs in warm, moist, airless conditions. Increase air flow between plants, particularly in greenhouse and polytunnel. Growing earlier maturing and smaller fruiting varieties might allow you to harvest fruit before blight strikes, and where possible, choose resistant varieties.

Powdery mildew – mildew can strike in beans, courgettes and pumpkins, apple and pear trees, as well as the herbaceous border (phloxes in particular.) A grey powdery coating appears on leaves and stems, particularly in hot dry weather.
In leafy veg, such as spinach, cut out or destroy the infected leaves. This will stimulate new growth. In general, it is important to improve your soil’s ability to hold water by adding organic material such as compost. Keep watering throughout dry periods, and use a mulch of grass cuttings or compost around the plant to keep the moisture in the soil. 
In apple or pear trees, the mildew appears white on stems and leaves. Cut the affected stem out straight away.  It usually signifies hot humid conditions, so during the annual prune be sure to cut out dense overcrowded growth, giving the tree an open aspect.
Brassica clubroot – the first sign of this cabbage family disease can be wilting of plants, particularly during dry weather. Despite being well-watered, plants may appear stunted or sickly and the foliage develops a purple-red tinge. Infected roots swell and distort, often producing either a single large gall (‘club’), or a cluster of smaller galls. This disease can last for 20 years or more in the soil without you growing a cabbage. It often arrives on infected plants, and there is no known cure.
Where possible, choose resistant varieties. Adding lime to the soil occasionally will help. To give plants a healthy start, raise plants in 7 cm pots, then transplant. They will still be partially affected but may reach maturity. Ensure good hygiene, don’t spread the soil with your boots or tools to other areas.
Rose blackspot - is a common fungal disease of roses particularly in the warmer South West England and South Wales. It appears as dark, irregular, brown/black blotches on leaves, most noticeable on the upper surface. The blotches may eventually join up so that large areas of leaf become blackened. A bad attack can cause leaves to fall.
Blackspot survives the winter on fallen leaves, and in infected shoots and buds. Clear the soil of all fallen leaves, and prune away infected parts.  Clean secateurs after pruning.  Cover soil to prevent spores landing in it - compost, leaf mould, or straw would all be suitable.
To prevent a buildup of damp conditions, plant and prune roses to encourage good air flow, and water the soil not the leaves. Choose resistant varieties if possible.

For other pests and diseases see here.