Eye on Potatoes: Down to the wire! - Fruit and Vegetable Magazine

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	Eye on Potatoes: Down to the wire! Down to the wire!
Written by Eye on Potatoes

Winning the war on wireworms

Wireworm – the larval stage of click beetles – are glossy brown, slightly darker on each end, and reach up to three centimetres in length.

Wireworms are on the rise in Canadian crops such as potato, sugar beet, carrot, cole crops forages and cereal grains. Wireworms are the larvae of slender beetles known as click beetles. There are approximately 30 pest species of wireworm in Canada – with many found in potato fields. Before pesticides, potato growing was abandoned in some areas of Canada due to wireworm damage. There are indications that wireworm numbers are increasing, and damage is growing in many crops. If growers aren’t familiar with wireworms, it’s time they learn more about these damaging pests, and determine if they are a threat in their area.

Tuber damage
Wireworms do their damage when they feed on potato seed pieces and daughter tubers, burrowing shallow holes and opening the way for secondary diseases, including
Rhizoctonia and blackleg. The greatest damage occurs when they tunnel into daughter tubers destined for the processing and table markets, leaving them unmarketable. While wireworms are doing their damage under the surface, growers will not generally see any above ground symptoms unless damage to the seed pieces is severe. Plants will then become discoloured and may wilt.

Wireworm life cycle
Wireworms have an interesting, and troubling, life cycle. The larvae – the most damaging life stage – are able to live in soil for several years (three to five) depending on the species. Here is a breakdown of their life cycle:

Click beetles (adult wireworms) enter fields, preferably those with pasture, cereals and certain weeds, between April and June to lay eggs (about 200 per female).

Eggs hatch into wireworm larvae in about three weeks and live and feed on plant roots and germinating seeds in the soil for three to five years, depending on the species.

Wireworms burrow deeper into the soil (up to a metre) when it is hot and dry (midsummer), or when it is cold (winter), or when there is nothing to eat.

In potato fields in the spring, wireworms move towards the soil surface, following carbon dioxide (CO₂) trails produced by potato seed pieces after planting.

In late August, wireworms return to the surface to feed on daughter tubers and damage from wireworms can double every three weeks until the crop is harvested.

After three to five years, wireworm larvae metamorphosize into click beetles (adult wireworm), which overwinter in the soil and emerge in spring to lay eggs and continue the cycle.

Finding and baiting wireworms in the field
Wireworms are attracted to CO₂, whatever the source. Bait balls are a simple, effective way to check for wireworms in potato fields because they give off CO₂. Burying one cup of wheat flour or oatmeal in narrow four to six-inch-deep holes in fields will attract them. Growers should mark the spot with a flag and check back in about four to five days – no later. About 20 evenly spaced baits per acre should suffice.

This technique will indicate wireworm presence but is NOT an indication of population threshold. Any wireworms growers find should be put in a small container – such as a camera film canister – with soil to be identified, because some wireworm species may not be adequately controlled with certain insecticides.

Important information on control
Wireworm populations are high in fields that have had a recent history of pasture and rotations with forages and cereal grains. If growing potatoes in high risk fields, the
effectiveness of insecticides will be reduced if green manure is present in soil at planting. This is because green manures produce CO₂, which will attract and hold wireworms away from the treated areas. Later in the season, wireworms will then attack daughter tubers. Ideally, a well-fallowed field prepared well in advance of potato planting would ensure that wireworms would visit the seed furrows and come in contact with the insecticides
applied.

Tracking the pest – Growers can help!
Dr. Bob Vernon, an entomologist with Agriculture and Agri-Food Canada (AAFC), is behind a nationwide wireworm tracking survey. Since some insecticides do not control or suppress all wireworm species, it is important to know which type of wireworms are present in the major growing areas of Canada so that the right control option(s) are chosen to get the job done.

Growers can help! By using the baiting approach described above, or if producers notice wireworm damage in their crops (especially potato), they are urged to collect the wireworms they find, along with some of the field soil, and put them in a hard plastic container. There may be more than one species present, so they should collect as many as they can. They can mail the sample(s) to Dr. Vernon at:

Agriculture and Agri-Food Canada
6947 #7 Hwy, P.O. Box 1000
Agassiz, B.C. V0M 1A0

It is important they include a brief description of where the sample was collected (nearest town or address), what crop the wireworms were found in, their name and phone number. Once the wireworms are identified, the grower will be contacted with the results.

If there are any questions about this wireworm tracking initiative, producers are urged to contact Bayer CropScience at 1-888-283-6847, Dr. Bob Vernon at 1 604-796-1708 or the Pest Management Regulatory Agency at 1-800-267-6315.

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