SYMPTOMS OF WIREWORMS

The symptoms of wireworm vary depending on the type of vegetable affected. Common symptoms by vegetable are listed below. In general though, root crops will appear unaffected until you dig them up. Other crops will suddenly wilt and die.

POTATOES

Potatoes will appear unaffected until the time comes to dig up your crop. Each affected tuber will have several small holes in them. The holes will have a black edge with a slightly larger brown ring around the edge. You may or may not find signs of the wireworms themselves still present.

The damage caused by the pest will have damaged the potato and may make other diseases more likely causing the potato tuber to rot. The longer the potatoes are left in the ground the more extensive will be the damage. The crop can still be eaten but cutting around the damage is likely to be a very tedious process and not worth the effort.

To distinguish wireworm damage from slug damage in potatoes, cut an affected tuber into quarters to expose the tunnelling. Wireworm damage will consist of thin holes whereas slug damage will have hollowed out large parts of the potato tuber.

CARROTS

There will be few symptoms, if any at all, above ground if carrots are attacked by wireworm. When the crop is dug up however, the carrots will have small black-edged holes in them. Similar to potatoes, the longer carrots are left in the ground the worst the damage will be. The crop is still edible if the affected areas are cut out.

TOMATO PLANTS

Tomato plants are normally attacked by wireworms when they are young emerging plants. The symptoms are plants which are initially healthy and then within a matter of days they wilt and die suddenly.

In the case of tomatoes the wireworms eat the most tender roots and then burrow up the stem to eat that as well. You may even see them on the top of the soil surface eating out from the main stem.

LIFE CYCLE OF WIREWORMS

Most wireworm larvae in the ground will emerge as click beetles in late spring to early summer. However some larvae will remain in the ground for up to four years before emerging as adults. During this time the larvae are constantly feeding on roots and vegetable matter although feeding is more intense from spring to autumn. There are three species of wireworm in the UK and their technical / Latin names are Agriotes lineatus, Agriotes obscurus and Agriotes sputator but for all practical purposes they can be treated exactly the same by gardeners. The full life cycle is:

1. April to June – adult click beetles burrow their way to the surface and begin feeding.
2. May to June – click beetles lay their eggs just below ground level to a depth of about 3cm to 15cm (1in to 6in). They lay about 250 eggs over a week or two with the eggs being spread over a wide area. The eggs are tiny, white and are unlikely to be noticed even if the soil is dug up.
3. June to September – the larvae feed on roots and other vegetable matter. As the larvae mature they increase in length to a maximum of about 3cm and gradually turn to a deep brown colour.
4. September to October – the larvae pupate into fully grown adults. Note that larvae generally live in the soil for two years before pupating but this larval stage can last anything from one to five years.
5. October to April – adults click beetles and overwintering larvae hibernate in the soil until activity begins in April the next year.

Because the larvae take between one to five years to pupate into adults there will be several generations of wireworm in infested soil at the same time.

Wireworms prefer cool and moist soil, in warm and dry weather they tend to migrate down deeper in the soil. Their activity spans a range from just below the ground surface to 30cm / 1foot below it. In the UK they have increased in numbers significantly over the past few years.

HOW TO GET RID OF WIREWORMS

There are no chemical treatments available to UK gardeners which will kill wireworms at any stage of their lifecycle. Managing wireworms can be achieved in several ways, each of which is described below.

EFFECTIVE DIGGING OF THE SOIL

Wireworm larvae will be eaten by birds, especially when other sources of food are not available. Digging the soil to a depth of 30cm / 1ft in autumn after the crops have been harvested will bring many of the larvae to the surface where the birds can see them. Many of the larvae will also be killed by digging because they thrive best several centimetres below the soil surface.

MAKE CONDITIONS UNFAVOURABLE FOR WIREWORM

Wireworm prefer moist, cool conditions especially overgrown grassy areas. The first priority should be to keep grass well cut near areas where vegetables are to be grown. If it can be removed entirely then even better.

At the end of each season remove all traces of crops grown in the ground. This will take away a food source of the wireworm. In areas where wireworm are a problem try to grow crops which they don’t like, peas, beans and broad beans are a good choice. For a year or two crops which wireworm like can be grown in grow-bags or containers. They are almost never found in containers.

Turn any nearby compost heaps frequently. Compost heaps are an ideal food source for wireworm and turning them will disturb the pests and bring them to the surface for birds to eat.

GROW WIREWORM RESISTANT VARIETIES?

A search on the internet reveals that there are no varieties of potatoes, tomatoes or carrots which are resistant to wireworm, they love to eat all of them! There is some circumstantial evidence that potato varieties King Edward, Nadine and Maris Piper are less likely to be affected but the evidence is slim indeed.

What is emerging from recent research is that wireworms do have very definite preferences for the conditions in which they live best. So, if you have a choice, unlikely we understand, always grow crops attractive to wireworm on north facing land in preference to south facing land, they don’t like cold soil!.

Another, more practical, nugget of advice is to grow early or second early potatoes and early carrots. Harvest both as soon as they are of a reasonable size. This will definitely minimise the damage done by wireworm and also reduce their source of food. The key period for wireworm damage is August and September.

What is also clear is that one patch of land may be affected by wireworm whereas another patch very nearby may well be almost free of wireworm. This brings us to the often mentioned subject of using bait to trap wireworms. The idea is simple, place a single vegetable (a potato or carrot) cut in half a few inches / 10cm below the soil surface and it will quickly begin to rot. Rotting carrots / potatoes are very attractive to wireworms and they will be eaten in preference to healthy growing vegetables. Pull up the vegetable in a week or so and with a bit of luck it will have several wireworms in it which can be disposed of. Fine in theory but almost useless in practice because the wireworm population will be almost unaffected by the removal of a few of their number.

However, this practice can be used to decide which particular areas of a plot of land are badly infested and which are relatively free. Using this information you will know where to cultivate wireworm prone crops and where not to. You can also concentrate your digging and removal of grass on the areas where it will be most beneficial.

The easiest way we have found to do this is to buy some long carrots from the supermarket, cut them in half, peel them and then push them into the ground with the top just poking out so that they can be found in a week or two. Pull them out after a couple of weeks and compare the damage done by wireworms around different areas of your plot.