Wireworm 101

HOW AND WHEN TO SCOUT; TIPS FOR MAKING BAIT BALLS

ONTARIO’S MAIN CROPS have long been plagued with wireworms, a field crop pest that can live in the soil for up to six years. Sporting three pairs of legs, wireworms are the larvae of various beetle species that feed on seeds and young plants or root systems of crops. Wireworms are a pest to many crops, and are a primary threat to corn, and a secondary threat to soybean and cereal crops.

Wireworms are typically spring pests, appearing as the soil warms, where they feed on newly planted seeds or plants. Once the soil gets too warm, they return to deeper, cooler soils for the summer, making them difficult to find during the summer and even into fall.

These hard-bodied larvae with distinctive flat heads are one of the major causes of plant stand losses in spring-seeded crops. “Insecticide seed treatments have traditionally been a grower’s insurance package against wireworms and other soil pests,” says Paul Hermans, certified crop advisor and agronomist with DuPont-Pioneer. “And once they are in the soil and causing damage, nothing can be done to prevent or recover from wireworm damage.”

New restrictions on access to neonicotinoid-insecticide seed treatments for corn and soybeans will now impact how Ontario growers manage wireworm pressure in fields.

SCOUTING AND PREVENTION
“Scouting is important,” says Hermans. “Growers will need to be more diligent on fields that aren’t treated with insecticide seed treatments, especially those more likely to be at risk to soil-borne pests.”

Where to find wireworms

•    Wireworms thrive in fields of grasses, pastures or hay and are attracted to fermentation or decay (i.e. manure and crop residue)

•    They prefer porous, well-drained loam

•    High risk fields include sandy soils, especially knolls and silty soils, corn and cereal crops and fields in rotation with grass (hay, sod, pasture)

•    Some species of wireworm can remain as larvae for up to six years in the soil

•    A slow crop emergence can attract wireworms because the soil is slower to warm up, keeping the seeds or plants at an ideal feeding stage

As a larvae-staged pest, wireworms are unable to move more than a few inches, making them opportunistic because they can only feed on what is present in their surrounding area.

What wireworms eat

•    Seed — preventing  germination and causing missing plants and uneven stands

•    Young plants and root systems — resulting in uneven development, stunted plants, or dead plants

•    Whorl or growing point above the seeds — may stunt or kill plants or cause abnormal development

“Corn seedlings will be affected up to the two-leaf stage, but if wireworms don’t start feeding until the four to five leaf stage, the more established root system makes it harder to take the plants down,” says Hermans who also reminds growers that wireworm damage at almost any stage of plant development will cause yield loss. “If you’re looking for wireworms you need to watch for pest signs in early plant development.”

DEALING WITH DAMAGE
“Scouting wireworms and determining the need for treatment is difficult,” says Hermans, “especially since growers have come to rely on insecticides to prevent pest damage.” Scouting fields for pests and crop damage and having a plan to deal with the pests is critical.

Wireworm management practices to consider

•    Avoid high risk fields and fields recently planted with sod or hay

•    Scout the entire field — large numbers of wireworms can be clustered throughout the field

•    Look closely — wireworm damage is mostly below ground soil; don’t confuse wireworms with other pests causing similar problems

•    Scout when wireworms are active — early spring before they move deeper as soils warm up

•    Count affected plant stands to determine pest pressure and potential yield loss

•    Keep records of affected fields and areas for future crop rotations and pest management considerations (Please note: plant stand losses over 15% for corn and 30% for soybeans need to be assessed and documented according to the new neonicotinoid regulations in order to access neonic-treated seed for these fields in 2017).

Wireworm prevention must be addressed at planting. Rescue treatments after emergence are ineffective. Hermans says that once infestation signs are apparent, growers need to make a management decision to take the yield loss or replant.

“Replanting is an economic and agronomic decision — the yield increase from the replanted crop should exceed the cost of replanting, and there are a number of factors that are part of that equation,” says Hermans. l

KEY CHARACTERISTICS OF WIREWORM DAMAGE
•    Missing plant or seed
•    Stunted or misshapen seedling
•    Holes across leaves as whorl unrolls
•    For positive pest identification, wireworm must be found

WIREWORM BAIT TRAP TIPS
•    Scout with bait traps in spring and fall, keeping in mind that wireworms like warm — not very warm — soil.
•    Set traps randomly throughout a field, consider 5-6 per field, depending on field size. Again, if scouting for regulatory assessment, five scouting locations are allowed, up to 100 acres. More land than that and you have to add scouting locations.
•    Make a trap by mixing oatmeal with honey, making a softball-sized ball. Place in a mesh bag (such as an onion bag) to hold it together and make it easier to find.
•    Any starch that will ferment in the ground, like corn or potatoes, can also be used
•    Bury the bait in a hole approximately one foot deep and mound with dirt to prevent rain collection.
•    Monitor traps regularly, digging up the bait ball to check for wireworms.
•    Wireworms can be tricky to find. Be sure to check the soil around the trap and cut into the centre of the ball.
•    Keep detailed records of traps and results (for your own records and for regulatory purposes).

HOW TO MAKE BAIT BALLS
One of the most effective wireworm scouting methods is setting bait traps. New regulations in Ontario list baiting wireworms as an approved way to prove pest levels. In order to meet the thresholds necessary to unlock access to neonic-treated corn and soybean seed, you must have an average of one wireworm per scouting location across a total of no more than 100 acres.
“With such a narrow window to find wireworms, bait traps are helpful to monitor pressure,” says Hermans. Wireworms are attracted to the CO2 that’s given off as material decays or as seeds sprout. That’s why fields with manure, crop residue, or previously planted in hay could have a higher wireworm pressure, explains Hermans. Bait traps can be used to proactively scout for wireworms in the spring and fall.

Looking for more information?
Guide to Early Season Field Crop Pests — Grain Farmers of Ontario www.gfo.ca/Production/ Production-Resources.
Scouting and Integrated Pest Management Resources — Ontario Ministry of Agriculture, Food and Rural Affairs: http://www.omafra.gov.on.ca/english/crops/ field/news/croptalk/2015/ct-0915a3.htm. •