Down to the wire! Winning the war on wireworms

The article below was written by Dr. Bob Vernon, AAFC-Agassiz, B.C., and has been reprinted in Hort Matters permission.

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 you aren't familiar with wireworms, it's time to learn more about these damaging pests, and determine if they are a threat in your 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, you will not generally see any above ground symptoms unless damage to the seed pieces is severe. Plants will then become discolored and may wilt.

Wireworm lifecycle

Wireworms have an interesting, and troubling, lifecycle. The larvae, the most damaging lifestage, are able to live in soil for several years (3-5) depending on the species. Here is a breakdown of their lifecycle:

• 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 3-5 years depending on the species
  
• Wireworms burrow deeper into the soil (up to a meter) when it is hot and dry (mid summer), 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 (C02) 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 3-5 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

Wireworm- the larval stage of click beetles - are glossy brown, slightly darker on each end and reach up to 3 cm in length

Click beetle - this is the adult stage of the wireworm

Wireworm lifecycle

Finding and baiting wireworms in your field

Wireworms are attracted to C02, whatever the source. Bait balls are a simple, effective way to check for wireworms in potato fields because they give off C02. Burying 1 cup of wheat flour or oatmeal in narrow 4-6 inch deep holes in fields will attract them. Mark the spot with a flag and check back in about 4-5 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 you find should be put in a small container such as a camera film canister with soil to be identified (see "Tracking the pest" below), 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 - You can help!

Dr. Bob Vernon, entomologist with Agriculture and Agri-Food Canada (AAFC) is behind a nation-wide 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.

You can help! By using the baiting approach described above, or if you notice wireworm damage in your crops, collect the wireworms you 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 collect as many as you can.

Please 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 to include a brief description of where the sample was collected (nearest town or address), what crop the wireworms were found in, your name and phone number. Once identified, you will be contacted with the results.

If you have any questions about this wireworm tracking initiative, please contact Dr. Bob Vernon at 1 604-796-1708 (bob.vernon@agr.gc.ca), Bayer CropScience at 1 888-283-6847, or the Pest Management Regulatory Agency at 1-800-267-6315.