Pests

don't let them get your plants!

By Bridgette Saunders

Photo:Rabbit

Rabbit

Larry D Moore

Prevention is better than cure

Try to prevent troubles before they start, check that you have cultivated the ground thoroughly, making sure you have removed all perennial weeds (see last months bulletin on weeds) before planting as many of these cause pests and diseases. Adding organic matter will help to open up heavy soil where waterlogging in winter is one of the major causes of root-rotting diseases. Organic matter will also help with the water and food holding capacity on light soil, helping to prevent pests and diseases.

Make sure you choose the correct plants for the site. Don’t grow sun lovers in a shady spot, and make sure you pick hardy plants to grow in an area that is prone to frost. Make sure you buy good quality plant and bulbs; specialist nurseries are the best places to look, and don’t forget about disease resistant varieties, many of which are now available to the allotment grower. Ensure you know the soil requirements for your plants and then make sure you plant them properly, ensuring there are no air pockets when planting and that the roots will spread into the allotment soil as quickly as possible to support the plant. Remove weeds, rubbish etc as these really are a breeding ground for pests.

Pesticides

All your allotment enemies can be controlled either organically or with pesticides. There are as many pesticides as there are pests. However, there are many disadvantages in using chemicals too freely. Remember the term pesticide is the umbrella term used to describe insecticides for controlling insects, fungicides for controlling diseases, and herbicides for controlling weeds.

Some facts about the use of chemicals:

  • Expense. They are expensive especially when used in large areas such as allotments.
  • Organisms become resistant and consequently the chemicals have to keep increasing in strength and density.
  • Very few chemicals are specific enough; they affect everything they come into contact with, both enemies and friends.
  • Fruit crops need bees for pollination - these are harmed by too generous use of pesticides.
  • Unseen animals are killed that usually prey on pests; this will upset the balance of a natural community.
  • Chemicals can also contaminate vegetables and time is needed to elapse between spraying and harvesting; this is known as harvest interval.
  • The damage spreads through the food chain; birds die as a result of eating smaller animals that have fed on smaller insects.
  • Although a great deal of money is spent researching the toxic effects of chemicals there is no doubt that spreading poisons around the countryside has huge implications. Although chemicals are efficient there are several alternatives.

It is important to inform yourself about chemicals so you can make choices about the way you control pest, diseases and weeds. It is not enough to say you are organic without looking into the reasons why. Another thing to consider is that chemicals are constantly being reviewed and taken off the market as new research is carried out, therefore it can be difficult to keep up. For more information on chemicals look on the Department for Environment, Farming and Rural Affairs website. It is for many of the reasons stated above that I have chosen to be an organic gardener, and find that if you follow the alternative methods of crop protection listed below you should have a harvest enough for your family and friends, and at the same time learn to live with some of the pests that are around; for instance, if the birds eat some of your fruits and you have enough does it really matter?

Natural pest control

Beneficial insects and wildlife are really your best friends when it comes to controlling pests in your allotment. Planting simple annuals amongst your vegetables such as Californian poppies (Escholtzia) and marigolds (Tagetes) will attract a wealth of beneficial insects like ladybirds and hoverflies who will gobble up your aphids. Plant a few native shrubs and herbaceous perennials (e.g. hazel and hardy geraniums).

Maximizing air circulation by correct pruning and leaving just a little more space between your plants can help to control fungal diseases, for example powdery mildew in roses. If trying to remove a diseased branch from a tree, one with coral spot for example, cut into the healthy wood and always wash your tools in boiling water afterwards. Check your plants regularly so that any pest and diseases don’t get a chance to get a hold.

Use your diary to make a note of any pests and disease you see so that you can look out for them next year. For example, if you start checking the centre of your gooseberry bushes in April for sawfly eggs and larvae you can remove them and therefore prevent them defoliating your crop. Also be wary of accepting onion and cabbage plants from your friendly neighbour ; they may well carry the dreadful diseases of onion white rot and clubroot which will be discussed in the diseases section .

Barriers are the best way of reducing pest damage. Simply by covering your vegetables with fine mesh you will stop them being attacked by flying pests. The best mesh to buy is Environmesh which last for years and this works well for carrot root fly and pea moth. Fine mesh is also an all-inclusive way of protecting your cabbages from just about everything. For example, flee beetles, leaf weevils, birds, cabbage white butterflies and white fly. If you use a barrier method with slugs remember that success depends on you being extremely generous with the chosen deterrent and not skimping.

Barriers can also be also be used to prevent disease. For example peach leaf curl is a devastating fungus that can simply be prevented by placing a barrier of polythene sheeting over a trained peach tree in winter. This barrier prevents the spores splashing up onto the plant.

Another popular method of protecting your plants is to use traps. This can be anything from beer traps for slugs to codling moth traps for you apple trees, or sticky traps for your glasshouse. A codling moth trap, for example, uses a pheromone placed on a sticky floor inside a small tent that is hung in the tree. The male is attracted to the trap as he thinks it is a female. When he lands inside the ‘tent’ he gets stuck in the glue. You can use similar traps for pear and plum moths. Another good reason for using this method is that you can monitor the number of moths that are around and when they appear ready to be on your guard for next time. Grease bands painted around the trunks of apple trees in autumn are a good way of preventing the wingless female winter moths from climbing up the tree to mate. Sticky glue is also very useful for glasshouse staging if you have an ant problem.

There are a whole host of pests that come in many varieties, the largest variety being insects. It is important to look at the life cycle of some insects, which go through a number of stages from egg to adult. The reason for this is that sometimes the larva (sometimes known as a grub, maggot or caterpillar) can do as much damage as the adult. Some insects such as caterpillars are munching insects and chew their way through vegetation and some such as aphids are sap-suckers. It is important to observe how the culprit works to identify who is responsible for the damage.

Many pests are plant-specific, for example Viburnum beetle only appears on Viburnums ; for this reason it is impossible to mention them all, but if you have a pest you are unsure of do ask other allotment holders as often pests colonize in one place. Failing that, consult a good pest and disease book (the RHS is always a reliable resource) to check out problem. If you are an RHS member you can send your pests to them for identification.

Birds

Let’s begin with birds; remember that they are protected by law in this country. Bullfinches are a problem from midwinter to spring as they go for the buds of trees, shrubs and soft fruit. Wood pigeons are always on the lookout for a nice lunch of tasty brassica leaves and will really enjoy your pea and bean seeds. They also damage young seedlings. They are on the go from spring to early summer and really can cause a huge amount of devastation. The best form of defence is to use buzz lines (special tape that is stretched between two posts and makes a buzzing sound) and netting, but make sure that this has very small holes in it as this will stop the cabbage white butterfly and also slugs and snails from attacking your Brassicas and other vegetables. Hanging up CDs above your vegetables is another method to try. There is also the potato bird, a potato stuck with feathers, which is supposed to scare the living daylights out of pigeons!

The netting method is my chosen method that seems to work well and I use a hazel pole the length of the bed with willow arches and drape the netting over in and weigh it down at the sides. This makes weeding easy as when you want to weed all you have to do is to roll up the netting on the hazel pole. You may sometimes see starlings pecking at grass areas but they are generally feeding on lawn pests such as leatherjackets and this outweighs any damage they may cause.

Rabbits

It is soul-destroying when your crops are ruined, and rabbits are a real culprit. They are particularly difficult to control due to the typical allotment layout. Leafy vegetables are an obvious risk and they will strip bark from young trees, potentially killing it. Check out the rabbit problem and find out how the other allotment owners deal with this issue, preferably before taking on your plot.

Controlling rabbits can be achieved through several methods:

  • Rabbits can be stopped burrowing underneath ground by surrounding the area with 1.24-m (4ft) wire fence 45cm (18in) sunk into the earth and angled outwards. Make sure that the gauge is small, or baby rabbits may still slip through!
  • For protection of individual plants: 90cm (36inches) high netting can be put round the plant without the need to bury any underground.
  • Tree guards around young trees prevent rabbits stripping the bark
  • Research plants that are more resistant to rabbit attack, and grow them around plants that rabbits like! They tend to nibble their way along the edge of the bed, so this method should help. The best plants are those with very aromatic leaves, prickles or spines or tough leathery leaves.

Slugs and snails

If I had a pound for every time I have been asked what to do about slugs and snails I would be very rich; they are the bane of the gardener’s life. They seem to eat just about everything, including vegetables, herbaceous plants, seeds, bulbs, and of course slugs even enjoy a good feast on main crop potatoes. They are more active at night, especially after a heavy downpour. They leave irregular holes in leaves and other plant parts, and always leave slime trails behind them so it is easy to identify them. I think that one of the biggest attracters of slugs and snails is untidy and unhygienic areas in the allotment and this is often a problem if you have a neighbour who is not very vigilant. Try not to leave empty pots around and piles of material for them to hide in. Raised beds help as you can get old copper pipe from scrap yards and put this round the top of the bed. Petroleum jelly is also a helpful means of defence, if you put it round the rims of your pots. Another effective form of control is to put lots of grit around things like sweet peas and other vulnerable plants. You need to make sure that there is at least 5cm (2in) around the plants with no gaps for the snails to get through. It is an expensive way of controlling them but worth it on special plants. You can also use beer traps, using a plastic bottle cut off the end that you drink from, about a third of the way down the bottle, turn it round and put the drinking end into the bottle, then fill with cheap beer - that way once the slug is in the slug pub it can’t escape. Using a torch after a rainfall at night is a good time to go out collecting, and sometimes it is a great incentive to children for earning a penny a snail! It can get quite competitive to see who gets the most snails.

You can buy organic slug pellets from hardware stores and the organic gardening catalogue. These are not harmful to pets, wildlife or humans and you only need four or five pellets around your plants. Like all slug pellets, they need reapplying after a period of time.

There is also a biological control available in the form of nematodes, (microscopic worms) which are naturally present in the soil but you can buy them by mail order and then mix them with water and apply to the soil. You must make sure that the soil is at the correct temperature (see packet for instructions) for the nematodes to work. This is an excellent form of control but may be expensive for the allotment gardener if used on a large area. The nematodes get inside the slug and eat it from the inside out - not a very appealing end but as these nematodes are present in the soil anyway, but in smaller quantities, it seems preferable to slug pellets containing metaldehyde.

Aphids

Aphids, commonly known as greenfly, black fly, etc., are one of the commonest garden pests. They may also be yellow, pink, grey or woolly. Some are winged and can travel long distances on the wind. There are over five hundred species in Great Britain, with various life cycles. They feed on the sap of plants by inserting sylets into the plant tissue and sucking out the sap from the cells if not controlled they can cause severe damage. They also spread virus diseases from plant to plant.

They breed very prolifically, females giving birth to live young which mature in about a week and consequently one female could have millions of descendants in a season.

Symptoms of aphids include obvious infestation, particularly at the buds or tips of plants, curled and distorted leaves and honeydew and possibly associated sooty mould, which is a blackish fungal infection. You may find aphids on many different vegetables. Black bean aphid is the most serious of all broad bean pests as it stunts growth and damages the flowers, which in turn distorts the pods. They also attack many other plants including fruit; cherry foliage can be severely distorted by blackfly. They over-winter as eggs which hatch when the weather is warm enough around the end of March.

Ways of control include insectivorous birds such as tits, which you can encourage onto the allotment by feeding them in winter and putting up nest boxes. Ladybirds and lacewings are also big aphid-eaters. They can be encouraged by avoiding insecticides, which would otherwise harm them; if you have aphids it is likely you will get ladybirds and lacewings. I generally blast them off with a powerful hose. I also make a mixture of a tablespoon of soap flakes, dissolved in some hot water and one teaspoon of cooking oil in a plant spray bottle topped up with water. This can be sprayed on safely and save you money on products from the garden centre. Do not use washing-up liquid as it will burn the leaves. The oil helps the mixture to stick to the plant.

Whiteflies tend to be a problem in glasshouses where they attack tomatoes and cucumbers. They enjoy the warm conditions that a glasshouse offers. Putting up sticky traps will help you to identify if you have whitefly and indeed many other pests. If white fly is a problem you can use a biological control, which is very effective if used correctly. This comes in the form of a parasitic wasp called Encarsia Formosa and if conditions are right, e.g. there are plenty of whitefly for the Encarsia to eat and the temperature is right, it can be an excellent control measure.

Another form of whitefly is the cabbage whitefly which is hardier and survives outside on Brassicas. The females can lay up to 200 eggs, which hatch and pupate as scales before the adult emerges. They reproduce in 3-4 weeks. It is usually on the underside of leaves and when you disturb the leaves they will fly off in a cloud. Because the white fly is often on the underside of leaves this is another good reason why spraying with chemicals is not a good idea as often the spray will not reach the affected part of the plant. Derris dust is an organic product and can be used outside to control the problem but this must be done frequently to be effective.

Red spider mite

Red spider mite are very small and not red at all! They attack a whole range of fruit, vegetables and ornamental plants. They are very small and difficult to see but mites develop colonies on the underside of leaves and spin a very fine web that is sometimes visible.

There is a biological control for the glasshouse mite called Phytoselulus persimillis, but it would not work outside as conditions are not right for it so try derris if spider mite is a problem. You can spot spider mite damage as the leaves develop a yellowish mottling which turns bronze and the leaves may wither.

Caterpillars

Leaf-eating caterpillars are the larvae of moths and butterflies and there are many different species that are plant specific in their attack. Others however are not so discriminating and munch their way through a range of plants. They attack annuals, perennials and also of course love vegetables. There is the green velvety angle shades moth and this attacks dahlias, gladiolus and many other perennials. The cabbage white is a slightly hairy caterpillar and the females can lay several hundred eggs and there may be many generations within a season. These caterpillars skeletonize the leaves of Brassicas and other annuals and perennials. The colourful and hairy vapourer moth is a pest of trees and shrubs. It is easy to spot the damage they cause and the best way to deal with them organically is to pick them off when you see the first sign of them or use fine netting.

Vine weevils

Vine weevils are the curse of the gardener because they can really devastate your plants and it is hard to protect your plants against attack. The adults are nearly all female and reproduce in such a way that they do not need a male partner. The bites that they take from the leaves do very little harm but it is upsetting because it shows that they are around and may have laid their eggs by the roots of your plants. Each weevil can lay hundreds of eggs. It is the grub stage of the vine weevil that is so destructive since they live off roots. These wrinkled white grubs are extremely destructive both outdoors and under glass. Plants in containers are one of their favorite targets. The effect on your plants will depend on the size of the root system and the number of grubs eating them. To start with the growth will slow down and the plant may then begin to wilt and then die as it no longer has enough roots to keep it alive.

Adult vine weevils cannot fly and they walk around at night so you can often see them they are easy to destroy. You can also get a biological control in the form of nematodes that lay their eggs in the vine weevil grub, which then hatch to feed on the vine weevil. The nematodes come in powdered form and are watered on in late summer, while the soil is still warm (the soil temperature needs to be above 5ºC in the day time), but the grubs are, hopefully, still small enough not to have caused too much damage. If damage is a problem, it is possible to remove plants from their pots, squash the grubs and repot. There is also a type of compost available that contains nematodes that will deter these pests.

Flea beetle

I think it is important to mention flea beetle as this is a real problem for the allotment holder. It tends to be a pest of seedlings, especially attacking those in the cabbage family, which includes swedes, turnips and radishes. They seem to be more of a serious problem during warm periods when it is also dry around April and May time. Small round holes appear in the leaves and the plant may grow very slowly. You can see the beetles as they jump when the plant is disturbed. To avoid this make sure you water well when weather is try and you can spray or dust with derris which is organic if you want to.

It is worth noting that Derris is one of the strongest and longest lasting of the organic pest controls. It also controls caterpillars, aphids, thrips and sawfly. The active ingredient is rotenone, which is derived from a number of tropical plants. You can use it up to one day before harvesting, but it is best used on dry days as it is not soluble in water.

This page was added by Bridgette Saunders on 29/06/2011.