Prairie Grain Wireworm

Ctenicera aeripennis destructor

Insect Life Cycle

Host plants
The prairie grain wireworm is the most destructive wireworm pest of grain in Western Canada. It prefers annual and perennial grasses. It also attacks potatoes, sugar beets, corn, lettuce, sunflower, canola and onions.

Overwintering
Wireworms (the larval stage) live for several years in the soil and are quite resistant to adverse conditions. Those larvae that survive their first winter can go for at least two years without any food other than humus. The wireworm stage lasts anywhere from four to 11 years. They hibernate in the soil from 5 to 25 cm below ground level. Older larvae commonly feed to a depth of 15 cm in the topsoil. When fully grown, usually in July, the larvae pupate about 5 to 10 cm below the soil surface. Pupation lasts for less than a month; however, adults do not emerge until the following spring.

Spring appearance
Wireworm adults, called click beetles, emerge in April and early May from the soil in which they overwintered. They become active when the air temperature is above 10 degrees Celsius, mate and then seek egg laying sites. From late May through June individual females deposit 200 to 400 eggs in loose soil, or under lumps of soil. Depending on the moisture, temperature and firmness of the soil, eggs are laid anywhere from just below the soil surface to 15 cm deep. After three to seven weeks, the wireworms hatch and begin to feed on live roots or seeds of cereals or grasses. If no food is found within one to four weeks of hatching, the larvae die.

Number of generations
The generation time (4 to 11 years) varies with the quality and availability of food. Wireworms in all growth stages are likely to infest a field.

Natural enemies
Parasites - Parasitic nematodes attack larvae.
Pathogens - Larval mortality caused by bacterial diseases is especially high in soils with a high moisture content.

Damage Assessment

Economic importance
In Alberta, damage to wheat crops ranges from one to 50 per cent annually. Damage to other non-cereal crops varies from farm to farm. Over 80 species of wireworms occur in Alberta, but only the prairie grain wireworm is of major economic importance.

Damage description
The larvae feed on germinating seeds or young seedlings and shred the stems but seldom cut them off. The central leaves die but outer leaves often remain green for some time. Damaged plants soon wilt and die, resulting in thin stands. Poor seed and dry conditions can also cause thin stands; consequently, many wireworm infestations are passed off as due to poor seed or poor germination. Wireworms do the most damage in early spring when they are near the soil surface. During summer months, larvae move deeper into the soil where it is cool and moist. Wireworms do not ingest solid plant material, but chew tissues, regurgitate fluids containing enzymes, and then imbibe the juices and plant products made soluble by the enzymes.

Potato seed pieces are seldom damaged to a point where poor stands result. However, new tubers can be damaged severely. Tunnels made by the wireworm allow disease organisms to enter, and the damaged tubers are less marketable.

Damage is generally higher in silty, medium textured, well-drained soils and in soils cultivated for at least 12 years. Damage is less likely in heavy or very light soils. Crops grown in newly broken sod can suffer great losses for one to two years, then the damage decreases rapidly only to gradually increase in succeeding years if no wireworm control measures are applied.

Sampling and monitoring
Whole potatoes buried in marked locations in a field in the spring or from early to mid August will indicate whether wireworms are present. Bury the potatoes 10 to 15 cm deep then dig them up after a couple of weeks, and examine them for wireworm tunnels. Monitor your fields each year.

To sample for larvae, sieve the soil through a screen. Mark out areas 50 cm x 50 cm and sieve the soil to a depth of 15 cm. Repeat in different areas of the field to determine an average number of larvae per square metre.

Economic threshold
None have been established. Treat seed for two consecutive years after breaking sod to reduce the problem to a non-economical level.

Management Strategy

Effects of weather
Larval activity is governed by temperature and moisture conditions. Cool wet weather forces wireworms closer to the surface; dry hot weather forces them deeper into the soil. Cool weather restricts adult activity and lengthens the egg laying period. Eggs laid near the soil surface or in compact soil are subject to high mortality when moisture levels and temperature fluctuate rapidly. Mortality is from 92 to 98 per cent in eggs and young wireworms. Most wireworm mortality occurs during the first two weeks of larval life.

Cultural practices
Crop rotation - Crop rotation and other cultural practices usually prevent wireworms from becoming a major problem in sugar beet fields. Because sugar beets are normally grown in a four-year rotation in Alberta, crops less susceptible to wireworm attack can be grown on infested fields so that populations will not build up. Root and row crops such as potatoes, corn, onions or beans should not be grown in a rotation where wireworms have been a problem. Wireworms can also be present when sugar beets are grown on land previously uncultivated or planted to grass or pasture. Deep plowing in the fall and frequent cultivation in early summer are suggested when wireworms are known to be present in these fallow fields.

Shallow cultivation - The long life and underground habit of wireworms make them hard to control. The most vulnerable period for the wireworm is from the egg to early larval stage. Only two to eight per cent of eggs and young wireworms survive. Newly hatched wireworms must feed within four weeks. Thus, cultivate fallow land in early spring to starve hatchlings. Shallow cultivation in early spring can expose eggs and injure larvae. Thorough cultivation of summer fallow during the latter half of July can destroy pupae as well as larvae. Use a rod weeder and disturb only the upper soil layer.

Summer fallow - Summer fallow for wireworm control is not recommended, because it has almost no effect on mature larvae, which can survive for two years on soil humus alone. In fact, wireworm damage is more severe after fallow. If early spring cultivation is used to starve and injure young larvae and eggs, plant a resistant crop.

If summer fallow must be part of the rotation, starve newly hatched wireworms by destroying all green growth during June and July. Work summer fallow as shallow as possible for weed control. Seed shallow, pack the seedbed to induce quick germination, and avoid very early or very late seeding.

Shallow cultivation or seeding combined with soil packing -Pack behind the seed drill to reduce damage to grain in wireworm infested land. Wireworms are very poor travellers. Some remain in the larval stage for nine or 10 years during which time they travel only a few yards. Firming the soil further impedes wireworms. Use a press drill for best results. If a press drill is not available, use a packer hitched behind the seeder in such a way that all wheels of the packer "follow" the drills of the seeder. Thus, the seed row is packed firmly, making wireworm movement so difficult that most of the worms will seek their food in the looser soil between the seed rows. If the packers do not follow the seed rows they will tend to leave them loose while firming the intervening strips. This may encourage the worms to follow the seed rows and cause heavy damage.

Farmers should restrict tillage operations to the upper 5 to 8 cm of the soil to maintain a compact soil layer beneath the tilled layer.

Adult click beetles are also affected by a compact sub-layer because they are forced to lay their eggs close to the surface, where the eggs can easily dry out or be discovered by predators.

Seeding practices - Avoid very early or very late seeding. Use methods that speed germination and early growth of the crop to help reduce the impact of wireworm damage. Cultivate before spring seeding to give the crop a competitive advantage with weeds and to prepare a good seedbed. On land that was in summer fallow the year before, a rod weeder will produce a very compact seedbed. On stubble, use a one-way disc; then seed at the proper time. If the soil is moist, seed 10 to 14 days from when drilling was first possible so that the soil is warm and seeds will grow quickly. If the soil is dry, delay seeding until it rains. As much as 90 to 95 per cent of a crop has been destroyed when seeded into a dry soil compared with 5 to 10 per cent loss when seeded into moist soil. Moisture also helps young seedlings recover from wireworm damage.

Shallow seeding - Seed preferably at a depth of 2 to 5 cm. This speeds the early growth of plants (as long as moisture is present). Use a drill press or standard drill with press attachment. If not available, use a standard double disc drill. Avoid the use of single-disc drills, and hoe-drills; but if you must use them, use press attachments or follow with a packer.

Increase the seeding rate in fields infested with wireworms, especially for wheat. Use as much as an extra bushel per acre, or for a patchy infestation, drill twice. Use healthy seed. A light topping of rotted manure or an application of phosphate fertilizers will help reduce wireworm damage. The phosphorous in the manure probably encourages root development and early maturity. Manure can be applied late in the fallow year or early the next spring, and should be incorporated into the soil.

Do not plant susceptible crops on the same land in two successive years. A crop rotation with resistant varieties and legumes is useful.

Buckwheat and flax are not usually damaged by wireworms. If recommended seeding practices are followed, oats and barley can be planted except when infestations are severe. Fall rye and winter wheat are more resistant owing to early vigorous growth in spring. However, a wet autumn preceded by a dry summer and fall has resulted in damage to rye. Plant legumes with a light nurse crop or in a mixture with a grass. Seedlings of sweetclover and alfalfa can be seriously damaged by wireworms. The seedlings usually escape damage in a mixture. Mature plants are not damaged by wireworms.

Take special precautions after breaking sod. If a crop on new land is destroyed, reseed immediately with a resistant crop. This is preferable to leaving the land fallow, since a recurrence of the problem would then be likely.

Biological control
Parasites - A few nematode parasites control wireworms that pupate below ground. At present these nematodes are not available for commercial use.

Pathogens - Wireworms are susceptible to bacterial and fungal diseases, and mortality is higher in moist soils.

Predators - Click beetles and their larvae are prey to both birds and small rodents. The adults are preyed upon when they lay eggs, the larvae are eaten in spring when they are near the surface or when exposed by cultivation. Birds pull them from the soil, and moles and shrews dig for them.