Wireworm

family: elateridae
species: ctenicera destructor
common names: click beetles, wire-worms, wireworms, c.aeripennis, c.lobata, limonius canus, l.infuscatus, l.californicus
hosts: most vegetables, beans, carrots, cole crops, potatoes
notes: Minute to medium-sized, with elongate. somewhat flattened bodies. dark-colored or metallic (in tropics). Antennae filiform or serrate. Prothorax large with pointed posterior angles. Larvae: long, slim, cylindrical and hard-bodied. Larvae destructive to the roots of vegetables, cereals, and forage crops

Agriculture and Agri-Food Canada Insect Identification Sheet No. 50 March 1981

Wireworm

Elateridae

There are over 800 species of wireworms in North America, nearly half of which are found across Canada. While most are harmless, several species are serious pests of a wide variety of crops including corn, potatoes, wheat, barley and onions.

Injury:

Larvae of pest species feed by boring into seeds and underground portions of plants. Seeds are often hollowed out and seedlings killed. Later in the season they feed on the small roots of host plants, stunting the plant's growth or killing them. Disease organisms often enter plants damaged by wireworms. Tubers such as potatoes are burrowed into, rendering them unmarketable. As larvae tend to remain in the field they hatch in, infestation may vary considerably from field to field.

Life History:

Adult wireworms are known as click beetles because of the distinct clicking sound they make as they flip into the air after being overturned. Pest species are usually brown or black, ranging in length from 8 to 1 5 mm. They overwinter both as adults and larvae. Early in the spring, the beetles become active, mate and lay their eggs around grass roots. The larvae, hatching in 3 to 10 days, spend the next 2 to 6 years feeding on the roots of host species. Larvae of pest species are usually hard, smooth and dark colored, though some are yellow or white. They range from 1.2 to 4 cm in length when fully developed. They pupate late in the summer. Adults overwinter in the ground in the pupal cell. There are usually wireworms in all states of development in an infested field as the larval stage takes several years.

Pest Management:

For information on the control of this insect consult your provincial Department of Agriculture.

Elaterid larvae (wireworms) require up to five years to mature in their soil habitat. Damage to tubers and grain roots may subject the plants to secondary infection by pathogens. Rotation minimizes threat.

THE FOLLOWING (UNTIL OTHERWISE NOTED) IS FROM: GERBER, H.S. 1983.
MAJOR INSECT AND ALLIED PESTS OF VEGETABLES IN BRITISH COLUMBIA.
MINISTRY OF AGRICULTURE AND FOOD.

Wireworms

• Agriotes sparsus LeConte
• Ctenicera aeripennis (Kirby)
• Ctenicera destructor (Brown)
• Ctenicera lobata (Eschscholtz)
• Limonius californicus (Mannerheim)
• Limonius canus LeConte
• Limonius infuscatus Motschulsky

Remain active in the soil for two to five years, attacking a wide range of plants, including vegetables. Damage is more prevalent in crops planted in newly broken land following permanent sod. There are about 150 species in British Columbia, but only seven are of economic importance.

Vegetables Attacked:

Most vegetables: bean, carrot, cole crops: broccoli, brussels sprouts, cabbage, cauliflower, kale, kohlrabi, radish, rutabaga, turnip

Injury:

In the spring, wireworms bore into seeds or underground stems of seedlings and transplants, causing the tops to show stunting and abnormal growth or shriveling. Tops remain attached to their roots. Damage is distinct from that caused by cutworms, which sever emerging plants at or below the soil surface. Later in the growing season, wireworms feed by boring tunnels up to 3 mm in diameter and 4 cm deep in developing roots, tubers or bulbs. Feeding is most damaging at this time and reduce yield or makes root, tubers or bulbs unmarketable. Mid- and late-season damage in root crops cannot be detected until harvest.

Insect:

The adult or beetle is 10 to 16 mm long, hard shelled and flat, black, tan or dark brown. Beetles fly and feed on the foliage on blossoms of trees and shrubs. They are called click beetles because of the clicking sound made when flipping over from their back to their feet. Eggs are white and round. Newly hatched larvae or wireworms have three pairs of legs, are white and 1.5 mm long. Later they become slender, hard-bodied, shiny yellow-brown and when mature are up to 3 cm long. They move slowly in the soil and remain motionless when disturbed. Several developmental stages of one species may be found in the soil at one time. Pupae are white but are rarely seen.

Life History:

The life cycle ranges from three to six years with two to five years as actively feeding larva-e. Beetles emerge from the soil in early summer when the soil temperature has warmed to 18ø or 21øC. Following mating, each female beetle lays 50 to 300 eggs in the soil 2.5 to 15 cm beneath the surface. Larvae hatch in three to four weeks and begin feeding. When the surface temperature reaches about 27øC in mid-summer, wireworms move downward in the soil until suitable temperature zones are found. When summer temperatures modulate, they return to the surface and continue feeding until cooler fall temperatures cause them to move downward again, where they remain for the winter. In the spring when surface soil temperatures reach about 10øC, they again move upward. This cycle is repeated until the wireworms are mature. Mature worms pupate for three weeks in earthen cells 7 to 20 cm below the soil surface. Beetles emerge in early summer to repeat the cycle.

Control:

Controls are directed against the larvae. Recommended insecticides must be incorporated into the soil to depths of at least 15 cm to be effective. Fumigants are expensive and reliable only if properly applied when the surface temperature is over 10øC and the soil is not excessively wet. Once wireworms have been controlled, reinfestation to economic levels may not occur for up to 10 years. Insecticide seed coatings protect direct-seeded crops but only slightly reduce the wireworm population in the field.

New Ways to Manage European Wireworms:

Bob Vernon, Ph.D., Agriculture and Agri-Food Canada PARC, Agassiz. V0M 1A0

European wireworms, notably Agriotes obscurus and Agriotes lineatus, are gradually spreading in Canada. In the Lower Mainland, these species are the most important insect pests of many crops in the Fraser Valley. They cause serious damage to potatoes, corn, ornamental crops and small fruit, such as strawberries. Since 1990, potato crops have sustained heavy damage resulting in heavy losses in some years. For example, the Delta and Surrey areas alone suffered $500,000 to $800,000 in crop losses in 1994. Wireworm populations appear to be increasing each year and pose a serious threat to B.C. farming sustainability.

Wireworms are the larval stage of click beetles. They live in the soil for 3 to 6 years, depending on the species, where they feed on plant roots and other organic material (we have even seen them feeding on dead birds!). Their favourite habitat is grassland or pasture, where populations can build up to enormous levels over time. When pasture is replaced with another crop such as potato, the wireworms left behind in the soil can cause severe damage for several years.

Procedures for managing wireworms in plantings of strawberries and potatoes are non-existent in BC since all previously registered granular insecticides for wireworm control have been withdrawn from use. Few new effective insecticides are expected to be registered for wireworm control, therefore alternative control strategies must be developed. To address this concern, a research program was initiated at the Pacific Agri-Food Research Centre, in Agassiz in 1996.

Potential For Using Trap Crops Several species of wireworms are known to move through the soil toward pieces of plant roots or germinating wheat; studies have also found that wireworms could follow a CO2 gradient for distances up to 20 cm. Since wheat is a preferred host of the two wireworm species, and CO2 is produced by germinating wheat, we wanted to find out whether freshly sown wheat could act as an effective trap crop.

Our studies were conducted in a farmer´s field in Agassiz, BC that had been in pasture for over 10 years. It had been plowed the previous year and had very high numbers of wireworms (A. obscurus).

First, we studied how wireworms move in the soil, both with and without a trap crop present. Our studies confirmed that wireworm populations are near the surface of the soil in April and May, but move further down later in the summer. In the spring, the majority of wireworms were actually within the top 8 cm (3 inches) of soil. This means that we would expect trap crops to be more successful at attracting wireworms in the spring months than in the summer or fall.

We also studied how wireworms move horizontally in the soil using plots with rows of wheat, seeded 1 meter apart. These were compared to control plots without any vegetation. The results showed that the number of wireworms found in the wheat rows was significantly higher than in the bare soil regions in between the wheat rows during the first 3 sampling periods in the early spring. We estimated that on May 23, from 68% to 80% of the resident wireworms in the soil had moved to the wheat rows. The rest of the population would have been in between the wheat rows, or further down in the soil. The number of wireworms in the samples declined steadily after this date, probably because they were beginning to move deeper into the soil by this time. The results of these studies showed that rows of wheat, planted one meter apart, will attract wireworms from unplanted areas at least half a meter away.

To find out how much wheat seed should be sown in the rows we also looked at rate of seeding in relation to wireworm attraction. One week after planting, the number of wireworms aggregated at the untreated wheat rows increased as the density of the wheat increased. Doubling the seeding rate about doubled the number of wireworms caught. Two weeks after seeding, however, there was no significant difference between rows, probably because most of the wheat in the untreated rows had been killed by wireworms by that time.

Our next step was to determine if wireworms aggregating in the wheat rows could be killed there using wheat seed treated with insecticide (i.e., Agrox DL plus or Vitavax Dual). In the treated wheat after 1 week, the number of wireworms aggregated at the wheat rows also increased with the density of the wheat planted. The main difference, however, was in the ratio of dead to living wireworms in these treatments. At the highest seeding rate there were more dead than living wireworms.

Conclusions from these studies:

1. In early Spring, between April and May, the majority of wireworms in soil are near the surface, and it is at that time that trap crops would be most effective. Trap crops planted after May and as winter crops in the fall would likely not be as effective.
2. In properly fallowed fields, most wireworms in the spring can be lured to trap crops such as wheat. The number of wireworms attracted increases proportionally with increased number of wheat seeds per foot of row. From the 1996 studies, an optimal seeding density for a lethal trap crop of wheat will be about 4 treated seeds per inch, with adjacent rows being about one half meter apart.
3. Finally, the results indicate that once wireworms have aggregated to a trap crop, they can be killed by insecticides used as a seed treatment. It appears as though lethal trap crops might be a less expensive and possibly more effective method of reducing wireworm populations.

Practical Applications:

Given the results of the studies described above, we have worked out how trap crops could be used in agricultural systems. Trap crops of wheat, for example, could be planted in advance of other susceptible crops such as strawberries. The earlier planted wheat rows would theoretically aggregate the wireworms and keep them away from later planted crops. We have shown this is feasible by planting wheat 1 week before we set out strawberries between the rows of wheat. Wireworm populations aggregated at the wheat rows and did not harm the strawberries. Unprotected strawberries, on the other hand, sustained severe wireworm damage.

For field crops, such as potatoes, we are in the process of studying the following procedure to significantly reduce wireworm populations in a field before planting:

1. The field to be planted should be fallowed overwinter to ensure that wireworms would not be distracted to other food sources in the spring.
2. The lethal wheat crop (i.e., seed treated with insecticide) would be planted into fallow ground at least 10 days before the potatoes were planted. The wheat would then be destroyed. If this an be done on more than one consecutive occasion prior to planting, wireworm control would likely be improved.

Environmental Impact of Wireworm Pesticides:

Before 1970, the use of organochlorine (OC) insecticides such as aldrin, heptachlor and chlordane was common. A single applications of heptachlor to soil could control wireworms for more than 10 years. Unfortunately, many OCs were also quite persistent in the environment, deleterious to certain wildlife species, and as a result were eventually withdrawn from use. They were replaced in the 1970s by other classes of insecticides, such as the carbamates (i.e. carbofuran; aldicarb) and organophosphates (i.e., phorate, terbufos and fonofos), which were shorter lived, but more toxic than the OCs.

In recent years, the use of some of these insecticides has been linked to mortalities in wild duck and geese populations as well as eagles and hawks, especially in the Delta region. It is thought that the problem originates with the use of certain granular materials in the soil for control of wireworms and other insect pests of potatoes. It is hypothesized that:

1. granular materials become exposed on the soil surface when potato rows are opened at harvest,
2. that ducks eat the exposed granules later in the autumn when flooding occurs in the fields,
3. that they become sick and die in the field, and
4. that eagles and other raptors eat the crop contents of the dead birds and in turn become sick or die.

One OP product, phorate (Thimet) was associated with several eagle mortalities in 1993 and 1994, and as a result was withdrawn rom use in B.C. by the parent company. In a study presently underway by Agriculture and Agri-Food Canada and the Canadian Wildlife Service, it has been determined that insecticide residues do, in fact, persist on the granular carriers of some insecticides long enough in certain types of soil to pose a threat to wildlife. In another study in 1996, it was determined that granular insecticides applied in-furrow at planting can be brought to the surface at harvest where they would be exposed.

" Fatal Attraction" (from http://res2.agr.ca/parc-crapac/english/3electronic_publications/growidea/
giv5a/fatalwireworm.htm) Wireworms (inset), the voracious larvae of click beetles, are quickly becoming the most troublesome insect pests of many vegetable and small fruit crops in the Fraser Valley. Conventional insecticides have not been reliable in controlling wireworms, and for many crops there are no registered products available. Recently PARC researchers discovered that wireworm populations can be lured to migrate en masse through the soil to rows of wheat (below), sown as a trap crop between rows of a main crop. The wireworms are attracted to the CO2 released by wheat at the germination stage. Once they reach the trap crop, the wireworms indulge their voracious appetites, but are killed by an insecticide which was applied to the wheat seed. This method reduces wireworms effectively and uses a fraction of the pesticide previously required.