One simple thing you can do to improve wireworm control over the long term is to send a sample of any wireworms you find to Dr. Bob Vernon. Put a few of the wireworms in a small plastic container, like an empty pill vial, and add some soil. Send the sealed container along with information on the location where you collected them, the crop, the date, and your name and contact information, to:
Dr. Bob Vernon Agriculture and Agri-Food Canada,
Pacific Agri-Food Research Centre
6947 Highway 7, PO Box 1000
Agassiz, B. C. V0M 1A0
Wireworms are out of sight but they shouldn’t be out of mind,” says Dr. Bob Vernon, an entomologist with Agriculture and Agri-Food Canada (AAFC) based at Agassiz, B. C. “Wireworm problems are increasing around the world because of the ban on most of the chemicals used to kill wireworm populations. Now we’re just suppressing damage, and the populations are growing. Growers need to be vigilant.”
Wireworms are the voracious larval stage of click beetles. The beetles lay their eggs in the spring and a few weeks later the larvae hatch. Most species have a long larval stage that can last four or five years. After a short pupal stage during the final summer of their life, adults are produced and remain in the soil until the following spring.
Wireworms munch on the underground parts of many crop types, damaging or killing the roots and below-ground stems. The larvae grow bigger with each moult and can eventually be over an inch long. The larval populations move up and down in the soil in response to temperature and moisture conditions and to meet their life cycle needs. In cool, moist conditions, they tend to rise up into the top few inches of soil for feeding.
Vernon and his research team are evaluating wireworm control options. An important aspect of that effort is a nation-wide survey to identify wireworm species in agricultural areas (see below).
Species identification is crucial because Vernon’s studies show that response to insecticides varies with the wireworm species. As well, the survey will help identify which are the major pest species in key growing areas, so researchers can focus on those.
“Wireworms are different than other pest species, such as Colorado potato beetle, where you’ve got just one species to deal with. With wireworms we have anywhere from 25 to 30 pest species across Canada that could cause farmers problems,” he explains. The species vary from region to region across the country, and there can even be more than one species in a single field.
In Alberta, the most common types collected in the survey so far are Ctenicera destructor (prairie grain wireworm), various species of Athous, and Limonius californicus (sugar beet wireworm), which dominates in irrigated areas.
Vernon says, “Although it varies from species to species, wireworms will attack all kinds of different crops. One of the biggest problems we have in Canada is in potatoes. On the prairies, quite often potatoes are rotated with cereals and other crops. Cereal crops are probably wireworms’ favourite food. You’ll get a lot of egg laying in cereals and forages. So if you have cereal crops or pasture for long periods of time, the wireworm populations of certain species can get very high.”
This population buildup means the risk of wireworm damage is greatest when growers rotate into some other crop after several years of cereals or forages. The hungry wireworms will feed on almost anything planted in that field and will cause significant damage for several years.
WHAT TO SCOUT FOR
A key sign of wireworm damage in cereals, corn and forages is a patchy thinning of the crop stand. “If some areas of your crop are thinner than others, then you might go into your crop rows with a spoon and dig around the base of plants that look like they have died back. In cereals, corn and so on, you’ll see browning leaves and eventually the plant just withers and turns brown. That’s a dead giveaway of wireworm damage. If you dig in about a two-inch radius around that plant, you’ll probably pop up a wireworm or two,” says Vernon.
In these crops, yield reduction by wireworms occurs soon after planting. “For about the first three weeks after emergence, you get this wilting and dying. About a month after planting, the plant is usually big enough to withstand wireworms,” he notes.
In potatoes, wireworm damage is also easy to diagnose but it can be season-long. “Wireworms feed on the developing daughter tubers earlier in the season which causes the potato to grow around the hole they made, resulting in tubers you can’t peel or chip. And they put holes directly into fully formed potatoes at the end of the season,” says Vernon.
You can scout for wireworms using some type of bait; wireworms are attracted by carbon dioxide, which is emitted by growing plants and during the breakdown of decaying plant matter, so lots of things will work as bait. For example, Alberta Agriculture and Rural Development recommends burying whole potatoes about four to six inches deep at marked locations randomly across a field in either early spring or early to mid-August. After a couple of weeks, dig up the baits and look for wireworms and tunnels in the tubers.
It can be tricky to get reliable results from baiting. Vernon explains, “For example, if you plough a field and then put baits in, you might not catch a lot of wireworms because they are preoccupied with feeding on the ploughed-under plant material.” He recommends growers create areas in the field where there is no competing plant material and place baits in those areas.
Researchers are attempting to develop economic thresholds for applying insecticides based on the number of wireworms or wireworm tunnels in baits. However it’s difficult to find something that works consistently, with about 30 different pest species that all behave differently, that occur in patches in a field, and that move up and down in the soil, sometimes going several feet deep.
LIMITED CONTROL OPTIONS
Wireworms are a growing concern because lindane, an organochlorine insecticide, has been banned from North America. Vernon explains, “Growers generally used lindane on their wheat seed once every three to four years, and that kept wireworm populations down. The wireworms would feed on the seed and contact or ingest the lindane and eventually die. Also the lindane was persistent enough in the year of use that any new eggs laid in that field in that year and hatched into little wireworms would also be killed. The following year there would be very few wireworms in that field and it would take a few years for the populations to again build up.”
Some of the products currently registered that are known to suppress wireworm damage include: Cruiser (thiamethoxam) for wheat and barley; Cruiser, Poncho (clothianidin) and Titan (clothianidin) for corn (and damage suppression in potatoes); and Thimet (phorate) for potatoes. (Thimet has had its registration extended several times; it is scheduled to be phased out by 2012.) For many other crops such as canola, oat and rye, there are no registered products.
Vernon’s wireworm research team (involving three AAFC scientists across Canada) is testing some of the newer insecticides, such as neonicotinoids and pyrethroids, on various wireworm species. So far nothing works as well as the old organochlorines (lindane) or the organophosphates (Thimet) in terms of reducing wireworm populations.
Neonicotinoids include chemicals like thiamethoxam and clothianidin. Vernon says, “When wireworms contact the neonicotinoids they become intoxicated immediately, and this intoxication can last for a long time. When they are intoxicated, they don’t feed. They’ll writhe back and forth, or they’ll go into a coma. Depending on the neonicotinoid and the wireworm, these comas can last for several months, after which the wireworms recover fully. At higher doses, they will die, but the rates being used on various crops around the world in practice appear to be below a toxic level.
“In cereal or corn crops, the wireworms are knocked out long enough for the crop to become established, after which the wireworms may recover fully. Along with the wireworm survival, you’ve also got new egg laying in those crops by the click beetle stage of the wireworm.”
In potatoes, Vernon is finding that the ability of neonicotinoids to prevent damage for the whole season seems to vary with the wireworm species. In 2010, he is planning to conduct a trial with Limonius californicus, which is likely affecting potatoes in Alberta.
Pyrethroids include chemicals like tefluthrin. “The pyrethroids are also being used at levels that do not kill wireworms, and their effectiveness against wireworms appears to be through repulsion. Around either the seed or in the case of potatoes in the potato hill area, pyrethroids likely form a force field that keeps the wireworms from getting in,” explains Vernon. That repulsion effect can last for months. “The pyrethroids also work wonderfully for reducing damage in certain field crops like cereals and forages.”
AAFC researchers are also assessing various cultural control options. Based on crop rotation studies by other researchers, Vernon says, “I expect rotations with cereal crops are likely to give rise to high wireworm populations. With certain crops in the mustard family (being evaluated through work underway by Dr. Christine Noronha, AAFC, Charlottetown, P. E. I.), there might be less egg laying. Crops like alfalfa are reported to have reduced populations because they dry the soil out.”
Shallow cultivation in the early spring may help limit wireworm population growth by harming the eggs or small larvae. Although some people believe wireworms may be more of an issue in minimum and no-till systems, Vernon notes there hasn’t been any research done specifically to confirm this.
Unfortunately Vernon hasn’t found a silver bullet to control wireworms so far. He advises, “In cereals or forages, growers need to be vigilant in looking for wireworm damage, so if they have thinning stands, they need to know the cause. Then in subsequent years they could consider using seed treatments to at least maintain crop health.
“In potatoes, only Thimet is still available to growers for fairly generic control of wireworms (both reducing damage and wireworms), and AAFC is trying to develop new options before Thimet is abolished in 2012. If growers have a cereal/potato rotation, they can anticipate having wireworm population buildups, and should be prepared to take steps to control them with the control options available.”
Please make it Carolyn King is a freelance agriculture journalist based at Kingston, Ont.