Pests & diseases

I am often asked the question ‘what do I do about bugs?’ Or ‘what organic product replaces xyz that I use for controlling this bug or that?’ There really is no answer to either of these questions. What I do say is that organic gardeners do not rely on killing bugs when they become a problem but that it is ALWAYS better to prevent a problem rather than trying to cure it.

Here are a few things you can do.

Cabbage Caterpillars
There are three types of caterpillars which cause considerable damage to the brassicas. The small white butterfly, the cabbage moth and the cabbage white butterfly.

The smallwhite lays eggs singly on the underside of beans. The cabbage white lays clusters of bright yellow eggs on the underside of the leaves making them difficult to see. The cabbage moth lays clusters of eggs on the underside of leaves and they hatch out into small light green caterpillars. All the caterpillars eat a lot of leaf until they are ready to pupate.

– Crush between the fingers any eggs found. Once a week is often enough. Take care not to mistake the elongated eggs of the ladybird with the bullet shaped eggs of the cabbage white.

Coddling Moth
Larvae of this moth eat into the flesh of fresh fruit. Eggs are laid later in the season, July/August. Eggs are laid on the fruit near the eye. Once there, the fruit is doomed. No sprays are effective once the grub is inside the fruit. Grease bands around the tree stop the grub reaching the fruit or use the very effective coddling moth trap. Natural pheromones (the attractants given off by the female moths) lure male moths into a sticky trap. If there is no mating there will be no maggots. This pheromone is only attractive to the coddling moth male, therefore harmless to all other beneficial insects and bees.

Carrot Fly
In carrot growing areas, such as East Anglia, and on allotments, carrot fly can frequently be a serious pest limiting the growth of carrots. The creamy-white larvae, up to 10mm long, feed on and in the roots often making the crop unusable. Eggs are laid in late May and June, with a second generation in August and September.

Sow thinly to avoid thinning out the young plants as the smell from the damage attracts the fly. Lay Enviromesh netting over the crop from late May onwards. Burying the edges in the soil, or covering the edges with stones, bricks etc. Alternatively, make a tunnel type clothe with wire hoops. The Enviromesh could also be used to form a half metre high barrier around the crop by cutting it into four equal strips lengthwise (sealing the edges with a flame, such as a gas lighter or blow lamp). Posts should be driven in around the plot, which should be about two metres wide and the mesh stapled or nailed onto them. Allow an inch of the net to be buried into the soil. The barrier prevents the carrot flies from ‘homing in’ on the crop as they fly close to the ground causing them to fly up and over the crop. This method is widely recommended by organic growing advisers using polythene, but Enviromesh has the advantage that it can be reused over a period of at least five years and its greater rigidity makes it easier to erect and fix into the posts. Enviromesh is also less vulnerable to wind damage than polythene, particularly in exposed areas. (This cover will also protect against the carrot-willow aphid which spreads a carrot virus.)

If Enviromesh netting is placed over the beds after sowing, soil capping and paddling from heavy rain can be reduced, which can lead to improved germination and crops.

When celery and parsnips are covered with Enviromesh, they will also be protected against celery fly (also known as celery leaf miner). These are small flies, the pupae (maggots) of which tunnel into the leaves.

Vine weevil
This is a pest of numerous plant species, both outside and under cover. Fairly indiscriminate, the vine weevil will attack almost anything.

In summertime damage is mainly caused by the adult, a nocturnal beetle which feeds on the foliage, cutting notches around the edge of the leaves. This type of damage, though unsightly, is not normally fatal to the plant. The larval stage of the beetle lives in the soil, and feeds on the roots of plants during spring and autumn. If enough root feeding takes place, and it usually does, the plant withers and dies. Peat based compost is the ideal environment for vine weevil larvae, allowing it far more freedom of movement than in ordinary garden soil. Improving the soil structure of ones garden by digging in compost and manure therefore actually encourages the vine weevil. Unless bought in plants are from a known and reliable source, one can bring the pest in also.

Under normal conditions the life-cycle of the pest is quite predictable. In the spring the warmer soil temperatures “wake up” the overwintering grubs, which start feeding and growing. When fully grown, the grub burrows deeper into the soil and enters a pupal stage, which lasts about 20 days. When this is completed an adult beetle emerges and works its way to the surface and is active throughout the summer until October. At night adults feed on foliage and lay eggs near the base of the plant. During the day they hide under pots, in crevices or beneath the soil. The adult beetle is about 10mm long, greyish black, with dull yellow spots. It cannot fly, but can walk and climb very efficiently.

The eggs take 2-3 weeks to hatch depending on the temperature. The grubs are very small at this stage, resembling very small maggots. When fully grown they can be 10mm long. They are a creamy white colour with a brown head, and do not have legs. At rest they form a curved position like the letter ‘C’. The grubs burrow into the soil and start feeding on the roots of the plant.

Very young grubs feed on the fibrous root hairs, but larger grubs attack the larger roots and even the stem below ground. As autumn progresses and temperatures drop, the grubs stop feeding and remain inactive during the winter. As spring temperatures warm the soil, the life-cycle continues.

Outside, usually only one generation occurs each year, though there is evidence to suggest that a warmer than normal winter can produce two generations. If the adult beetle manages to survive the winter, egg laying can begin as early as March. This will produce grubs active in the soil during April, May, June and July that will become adults that same year. Grubs that have overwintered will have all pupated by the end of June, but egg laying from overwintered adults can result in active soil borne grubs throughout the summer.

In heated conservatories and greenhouses, the grubs may not enter the inactive winter stage, and the life-cycle can be continuous.

Parasitic nematodes, which occur in minute numbers naturally in the soil, are watered into the growing medium in very large numbers. The nematodes carry a bacteria with which they infect the vine weevil grub to kill it. This bacteria is very specific, and has no effect on mammals, reptiles, or earthworms. For the nematodes to be able to infect the vine weevil with this bacteria, the soil temperature must be above 10°C.

In Autumn, treat all pots and containers containing plants that will be kept. Dispose of all compost from plants that are discarded at the end of season. Do not put this on the compost heap. In beds and borders, treat plants on an individual basis. If you have a greenhouse or conservatory, treat everything in it, even bare soil if it was planted during summer but has now been cleared.

Apply the nematodes from the middle of August to the end of September. Soil temperature can be guaranteed to be warm enough, as can the presence of the grubs.

Later applications are more sure of total success as they will be present during these months. If damage from grubs is causing unacceptable plant damage in July, treatment will cure it, but another application in September may be needed.