Lack of crop growth may be due to an insect, not a seeder miss

When you’re checking for pests, this insect’s wily ways could give you the slip

For Scott Hartley, an insect pest management specialist at the Saskatchewan Ministry of Agriculture, it’s an all-too-familiar story. A producer notices areas in the field where growth is slow, plants are dying back or missing altogether. The damage is often put down to a seeder miss or poor germination, poor nutrition or environmental conditions, or even cutworm damage.

Alterations to correct the problem are made the following year — which ultimately fail because the diagnosis is wrong. The problem is an unwanted pest with an elusive nature, making it one of the most difficult insects on the Prairies to identify and control: the wireworm.

“Wireworm is hard to monitor because the larvae remain in the soil,” says Hartley. “They may not be noticed until you’ve had them for a couple of years because you thought the problem was due to some other issue.”

Often, once the damage is caught and wireworms are discovered, it’s too late in the year to reseed.

Hartley says producers seeking a leg up on wireworm should be scouting for damage early in the spring. “Knowing what’s going on in your fields is No. 1,” he says.

Wireworms, which are the larval form of click beetles, are attracted to the soil surface in spring when carbon dioxide is released from germinating seeds. They feed on seeds, roots and seedlings, and shred stems and plant tissue below ground causing a plant’s central leaves to wilt and die, eventually killing it entirely.

In the field, stands may look thin with plants dying back or crop rows missing altogether. Other signs of infestation include hollowed out seeds, dead seedlings and dying plants still attached to their roots (and not severed at the soil surface as with cutworm damage).

It is often thought that wireworms prefer cereals, but there are more than one species and they will feed on most crops including pulses, canola and potatoes.

“Be aware that just because you’re not growing cereals, doesn’t mean you won’t have a wireworm problem,” says Hartley.

Understanding the wireworm’s life cycle will also give producers an edge, he says. Adult click beetles overwinter in the soil and emerge in the spring, entering cereal — and other economic — crops, pasture, or grasses to lay their eggs. The eggs typically hatch within three to seven weeks (by mid-summer) producing neonate larvae. Once neonates survive for one year they are known as resident larvae, which typically live for another two to four years, resulting in many wireworm larval stages in the soil at any one time.

Wireworms move freely up and down in the soil as temperature and moisture conditions change. They like the cooler, moister conditions spring offers, moving within inches of the soil’s surface and retreating deeper when temperatures rise and the soil dries out as summer advances. They will move closer to the surface again to eat once a canopy develops, but plants are well established at this stage and are not affected by feeding.

Pupation takes place during the last year of the wireworm’s life cycle, usually in July, and lasts for about a month. The adult click beetle emerges from the pupa and overwinters in the soil and emerges in the spring to begin the cycle all over again.

Soil sieving or using bait balls and whole potatoes to attract wireworms can help producers establish and monitor populations in a field, but will not give an accurate picture of population size as many factors influence when and where they are located in the soil profile and their responsiveness to bait.

Using bait

The key to using bait, says Hartley, and a better estimate of wireworm numbers, is more samples. “With more samples, you’re increasing your knowledge of the field,” he says. Place bait in the field once the ground is warm enough for heightened wireworm activity (between 10 C and 20 C) and clearly mark the location of each. Check bait balls or potatoes within a week, before the bait gets too mushy to find or the wireworms have time to move away deeper into the soil, says Hartley.

Sieve soil during germination when wireworms are attracted to the surface. Entomologists question the use of an economic threshold for wireworm. In the past, an action threshold of 32 wireworms per square metre was recommended, however, due to the insect’s complex nature, even low numbers may be cause for concern, depending on the crop. Hartley is cautious about using an economic threshold, but says it can still serve as a rough guide when soil sampling.

“If you’ve already found 20 wireworms in a square foot, I’d say you’re well over the economic threshold. At least it gives you a gauge,” he says.

A producer who has already seeded and discovers a wireworm problem may have to sit tight and wait for the following spring to use a seed treatment with an insecticide component and cultural control methods, when viable.

However, when it comes to wireworm management, options continue to be limited for cereal and pulse crops.

“Leading up to spring, the key is you’ve got to make sure you’ve got your seed treated because there is no alternative later on in the season. You can’t use a foliar spray for this. You can’t do anything else. It is a seed treatment issue and that is your management option,” says Hartley. “Ultimately, seed treatment is still the answer.”

Seed treatments containing neonicotinoids, which are the only option for chemical control in cereal and pulse crops, are available in products such as Cruiser Maxx, Cruiser 5FS, Alias 240 SC, Raxil Pro Shield, Raxil WW and Stress Shield 600. In the past, seed treatments could only be applied by commercial seed treaters, but now some products, such as Cruiser 5FS and Stress Shield 600, may be applied on-farm.

Wireworms move up and down in the soil, but there is little lateral movement, so damage can be patchy, with low-lying, moister areas at greater risk. Hartley points out producers can save money by using treated seed only in areas of the field known to be populated or at risk for wireworm damage.

Protective measures

Neonicotinoid seed treatments protect the crop when it is at its most vulnerable — during the early stages of growth. Wireworms feed on treated seed and become lethargic and enter a period of suspended animation during the seedling growth stage, allowing the plant stand to develop to the point where it is no longer vulnerable to wireworm feeding. The comatose wireworms recover by mid-summer.

However, treated seed only causes low mortality in resident wireworm populations and does not affect newly-hatched neonates. Therefore, seed treatments must be applied on a continuous basis each spring, meanwhile wireworm populations continue to rise.

“Even if there isn’t high mortality, you’re still protecting your crop for the year,” says Hartley.

The seed treatments on the market today protect the crop and get it off to a good start, but Hartley says there’s hope for future products that also offer high mortality.

“We are still trying to find more control options. There are gaps in our options for wireworm. There are options the United States has that we’re looking at trying to get here. It is ongoing… to try and find new viable options that get better mortality.”

In the meantime, cultural control methods can reduce damage, says Hartley. Shallow seeding, increasing seeding rates, altering seeding dates while avoiding very early or late seeding, crop rotation, and shallow cultivation with soil packing (to impede wireworm movement close to the seed row) are some methods that could curb populations. Watch for high numbers in no-till fields, where cooler, moister conditions are favourable.

Agronomic practices that establish quick and even germination and healthy crop establishment also mitigate damage. “Get that crop up and growing,” says Hartley. “The quicker you get it up into a more robust plant, the less vulnerable it is.”

Although none of these methods is 100 per cent effective, says Hartley, wireworms taking on a field where producers have combined seed treatments, cultural control methods and good agronomic practices may find they’ve bitten off more than they can chew.

About the author

Editor

Kari Belanger

Kari Belanger has been a writer and editor since graduating from the University of Calgary with a B.Sc. in Biology and a BA in English Literature in 1996. For more than twenty years, she has worked in many different industries and media, including newspapers and trade publications. For the past decade she has worked exclusively in the agriculture industry, leading a number of publications as editor. Kari has a particular passion for grower-focused publications and a deep respect for Canadian farmers and the work they do. Her keen interest in agronomy and love of writing have led to her long-term commitment to support, strengthen and participate in the industry.

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