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Preparing for wireworms in potatoes during 2009 season


April 30, 2009
By Dr. Bob Vernon

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Wireworms, which are the larval stage of click beetles, have become major pests of many horticultural crops across Canada.

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Wireworms are the larval stage of click beetles and have become major pests of many horticultural crops across Canada, including potatoes. Favoured crops are cereals and other grassy hosts and forages, where wireworms can build up to millions per acre in a short period of time. (Contributed photo)


Wireworms, which are the larval stage of click beetles, have become major pests of many horticultural crops across Canada.

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There are several pest species of wireworms, and one or more of these species can be found in a field causing damage. Favoured crops are cereals and other grassy hosts and forages, where wireworms can build up to millions per acre in a short period of time. If a field is in pasture or has been planted to a cereal crop, emerging click beetles from the field or from surrounding grassy headlands in the spring will lay eggs by the hundreds. Later in the spring and summer, the new, or ‘neonate’ wireworms, begin to feed and grow.

Wireworms will generally grow about five millimetres per year, and can generally cause crop damage in their third growing season. Depending on the species, the wireworm stage can last from three to five years, culminating in a brief pupation period and adult click beetles late in the summer. The new click beetles stay buried in the ground until the next spring, and emerge to mate and lay more eggs.

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Dr. Bob Vernon surveys a field of potatoes. The Agriculture and Agri-Food Canada researcher based at Agassiz, B.C., is currently involved in a wireworm survey across Canada. (Contributed photo) 

The life cycle of wireworms
To control wireworms, we must first understand how they behave in the soil. Typically, wireworms are mostly dormant and deeper in soil in winter months when temperatures are low and food is scarce. In spring, as soil temperatures increase to five to 10oC, wireworms become active and move to the surface in search for food. Wireworms are attracted by CO2 , which is produced by germinating seeds or existing plants (i.e. pasture) in the spring. In the case of cereal crops (wheat, barley, etc) or forage crops (corn), wireworms are attracted to the germinating seeds and can wound or kill the seedlings, resulting in reduced stand and yield.

In crops such as potato, wireworms are attracted to potato seed pieces during sprouting and will feed on the seed piece or on the developing roots and shoots. During the hot and dry months of summer, wireworms will often move deep into the soil until cooler temperatures and higher moisture occur later in August and September. Wireworms will then feed on the maturing crops at that time. In the case of established cereal and forage crops, wireworm feeding activity on roots causes little yield damage in late summer, however, feeding on crops such as potato and carrot can cause significant cosmetic injury by harvest.

The importance of field management
How you manage and prepare your fields prior to the growing season can have a marked effect on wireworm behavior and the amount of damage to your crops. You are at greatest risk of wireworm damage when you take a field out of pasture and into a crop situation. Wireworm populations will likely be extremely high in the pasture, and remain in the soil when the pasture is removed. When you plant other crops in the spring, wireworms will be compelled to feed on those crops (cereals, forages, vegetables, strawberries, etc) and can cause extensive damage for several years.

Work at Agriculture and Agri-Food Canada (AAFC) in Agassiz, B.C., suggests that how and when you remove pasture has an effect on wireworm damage and on how well insecticides will work. For example, if you remove the pasture by plowing in the early spring, many wireworms will be attracted to the CO2 produced by the rotting vegetation about 30 centimetres (one foot) below the surface. If you then plant potatoes with an in-furrow insecticide (i.e. Thimet 15G), only part of the wireworm population will move to the treated rows and be killed. Later in the season, after the plowed pasture has rotted away, surviving wireworms will then be attracted to the developing daughter tubers at a time when the insecticide has lost its effectiveness.

If you fall plow, there will be no rotting pasture at the time of planting in the spring, and the majority of wireworms will be attracted to the insecticide-treated potato rows and be killed. The same principle applies to planting other crops following pasture. Certain crop rotations also favor wireworm buildup. If your rotation includes a cereal crop, these crops will be selected by click beetles for egg laying. From these eggs, wireworms will be large enough in a couple of years to cause major damage to your crops. Be prepared for this eventuality.

Control options
After 1950, several insecticides were registered for wireworm control in Canada. Among the most effective were the organochlorine insecticides (i.e. DDT, aldrin, heptachlor and lindane). Some of these insecticides (heptachlor) controlled wireworms in the soil for up to 13 years with a single application. Lindane was used for decades by cereal and forage crop growers as a seed treatment (Vitavax Dual, Agrox DL Plus), and wireworms were controlled in fields with one treatment every three to four years. Work by AAFC has shown that lindane killed not only existing wireworms in the soil, but persisted long enough in the growing season to kill the new neonate wireworms produced that year.

Repopulation of fields by wireworms large enough to cause damage would take at least another three to four years, at which time the seed treatment would be used again. All of the effective organochlorines, however, including lindane (2004) have now been de-registered in Canada and elsewhere.

At the present time, there are no insecticidal seed treatments for cereal or forage crops that will kill both existing and neonate populations of wireworms, although some products will prevent early season wireworm damage. Other insecticides, including various carbamates (e.g. Furadan), organophosphates (e.g. Thimet) and pyrethroids (e.g. Force) have been registered for wireworm control on various crops in North America, but many of these insecticides are no longer registered or will soon be de-registered in Canada.

The next article in this series will focus on what the current insecticide options are for controlling wireworms and what we might expect in the future.

AAFC is conducting a wireworm survey across Canada, and you can help. If you have a wireworm problem, contact Dr. Bob Vernon at AAFC, Agassiz, B.C. at (604) 796-1708.


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