Wireworms may be lurking in the ground beneath your feet. Learn how to recognize wireworm damage and use bait to get a good sample
Wireworms are omnipresent insect pests that can attack many different crops including annual and perennial grasses, wheat, barley, corn and potatoes. They are becoming increasingly problematic, especially in areas where grassland or pasture has been broken up for cropping, due to their long life cycle and the lack of effective control methods.
Wireworm life cycles
Perpetual populations of click beetles (the adult stage of wireworms) can be found in many undisturbed areas, such as grasslands or pasture. Click beetles emerge from the soil where they have overwintered in late April or early May, in search of new areas to colonize.
Female adults prefer to lay their eggs in grasses or cereal crops, but the wireworm larvae — which hatch about three to seven weeks after the eggs are laid — will feed on other crops, including canola.
Wireworm larvae generally take four to five years (yes, years!) to complete their life cycle. They spend all of this time underground, so it’s often hard to detect them until there is evidence of crop damage. The larvae are hardy, and hibernate 25 centimetres or more below ground level. If they survive their first winter they can go at least two years without any food other than humus.
From late May through June, the female click beetles can lay 200 to 1,400 eggs in loose or cracked soil and under lumps of soil. The young wireworms hatch in the early summer, and initially feed on the roots of germinating seeds, but cause little damage at this stage. As wireworms grow over the years, it’s common for them to feed on germinating seeds, and they may burrow into stems, causing above-ground parts of crops to turn brown, wilt and die.
A large wireworm can kill two or more seedlings, and high populations can result in thin stands. These thin stands are often mistaken for poor germination, especially during dry conditions.
Larval activity is governed by temperature and moisture conditions. Wireworms are most active at soil temperatures between 10 C and 20 C in the spring, at which time they move closer to the surface to find food. As temperatures increase and the soil dries out in July and August, wireworms are forced to move deeper into the soil.
Cool spring weather can restrict adult activity and lengthen the egg-laying period. Eggs are laid near the soil surface or in compact soil, and newly hatched wireworms are subject to high mortality (between 92 and 98 per cent) when moisture levels and temperature fluctuate rapidly.
When fully grown, after four to five years, the larvae pupate in July and August, about five to 10 centimetres below the soil surface for about a month. At this time, the adult wireworms are formed, but they will not emerge from the ground until the following spring.
More damage is generally seen on crops planted in silty, medium- textured, well-drained soils and in soils that have been under previous continuous pasture for at least five years. There is usually less damage on crops grown in heavy or very light soils.
Crops grown in newly broken sod can suffer great losses for one to two years, then the damage will decrease rapidly as long as there are no crops planted that are favourable for egg laying, such as cereals.
“Any year that a cereal crop or grassy cover crop is planted will give rise to considerable egg laying,” says Bob Vernon, an Agriculture and Agri-Food Canada (AAFC) research scientist at Agassiz, B.C. Vernon has done considerable research into wireworms and is considered the foremost Canadian expert on the pest. Vernon says that recent work from the U.K. has suggested that wireworm populations prefer fields under reduced tillage, since regular cultivation practices are known to reduce wireworm populations.
Sometimes, wireworm damage is confused with cutworm damage. With wireworms, the plant is wilted and discoloured but remains attached to the root. With cutworms, the plants are usually cut off completely at or near the soil surface.
Scouting for wireworms
Scouting for wireworms can be quite variable, depending on the method used, the time of year, field conditions, their activity cycles and the wireworm species present. There are over 80 species of wireworms on the Prairies, but only three of these species are of major economic importance.
“There are no sampling methods currently available to growers that can be fully relied upon to accurately predict whether or not an economic wireworm population is present,” says Vernon. There are ways, however, to improve the efficiency of sampling.
One way to sample the wireworms in a field is to use baits to attract them. Vernon says that farmers doing this should keep in mind the following:
Wireworms are attracted to sources of carbon dioxide in the soil, such as germinating crop seeds, existing crops or decomposing plant material (such as plowed-under pasture).
Baits such as whole potatoes, carrots, wheat seed, or preparations of wheat flour or whole bran in gauze packets (about one cup with or without a sugar or honey additive to promote microbial growth) also attract wireworms by producing carbon dioxide.
For baits to work well, they should be placed in the field when temperatures are above 10 C (when wireworms are active and carbon dioxide production is high), and in sites where there are no competing carbon dioxide sources (such as green manure or other living plants) within one metre of the bait.
“Bait attraction can be quite variable and even ineffective if these conditions are not met,” says Vernon.
“In determining the number of baits to use per field, keep in mind that wireworm populations can be spotty, so you should cover as much of the field as possible,” says Vernon. “The more baits you put out (determined by how much time you want to spend), the more accurate the sampling, but at least 20 baits should be put out in fields between four and 10 hectares.”
Baits should be placed at a maximum depth of 15 cm, and in the case of living baits (potato, carrots or wheat seed), checked in about 12 to 14 days. Prepared baits (wheat, bran, rolled oats in packets) should be checked in seven to 10 days. Baits left too long will begin to repel wireworms.
Finding any wireworms in baits may indicate a problem. To assist in the development of wireworm control programs, you can put wireworms you collect in pill vials (in soil) and send them to Dr. Wim van Herk at AAFC in Agassiz, B.C. for species identification. Please contact Dr. van Herk for mailing instructions ([email protected]).
Wireworms are difficult to control, partly because there are multiple species across Canada and many are specific to a particular area and environment. Control products that are effective in one province against one species may not be effective in another province against another species.
Wireworm larvae do have some natural enemies, including parasitic nematodes. And, if soil is disturbed and the larvae are brought to the surface, they are easy prey for birds and small rodents. Larvae are also susceptible to a number of bacterial diseases, which can be especially prevalent in soils with high moisture content.
Wireworm control is limited to the use of seed treatments, which means management decisions and preparations can only be made prior to seeding.
There are fewer options for control than there was in the past. Before it was banned in 2004, Lindane was a seed treatment used on cereal crops every three to four years to control wireworms. It was very successful, killing 60 to 70 per cent of wireworms resident in the field and also the baby wireworms that hatched later in the season. This gave effective control both in this current crop and in subsequent crops.
Newer insecticides, like Raxil WW or Cruiser Maxx, which are registered for use as wheat seed treatments, will provide damage suppression, but they don’t reduce wireworm populations in the same way. These products contain neonicotinoids and work by knocking out the resident wireworms long enough to allow the crop to establish. Later, the wireworms recover.
“They call it an intoxicant,” says Harry Brooks, a crop specialist with Alberta Agriculture and Rural Development. Wireworms are usually close to the surface and feeding at the same time that the new crop is emerging.
“The product stops them from feeding for about a month, after which the soil has warmed up and that drives the wireworms down, so it’s fairly effective to prevent damage of the seedlings.”
The problem is that newly hatched baby wireworms are not killed, and with a life cycle of at least three to five years populations can continue to grow after treatment.
Some agronomists recommend that farmers treat seed for two consecutive years after breaking sod to reduce wireworm problems to a non-economical level, but this should be done only if the risk of infestation is high.
If wireworms are found in an emerging crop, there is no easy solution as there are no foliar insecticides currently registered in Canada for wireworm control. The damage which occurs is often noticed too late for reseeding of the damaged patch, and the yield loss doesn’t justify the cost of reseeding. Infestations this year are a signal to draw up a plan of action to deal with the wireworm problem next year.
Including a legume like alfalfa or a pulse crop like peas in a crop rotation with cereals and oilseeds is an effective way to gradually reduce wireworm populations on the Prairies. These crops can better withstand wireworm damage, and are not preferred crops for click beetles laying eggs.
A three-year study conducted by AAFC research scientist Christine Norohna at Charlottetown, P.E.I. showed that growing brown mustard or buckwheat for two consecutive years as part of a three-year rotation significantly reduced wireworm damage in potatoes grown in year three. One-acre plots of brown mustard, buckwheat, barley and alfalfa were planted in three different fields in three different areas of P.E.I. Each plot grew two consecutive years of that crop and then a potato crop in the third year.
Results for the barley and alfalfa crops were similar, as were results for the brown mustard and buckwheat. The potatoes following the brown mustard and buckwheat had significantly less holes per damaged tuber than those following barley, and also had far greater amounts of undamaged tubers. †