What an absolute disaster! Wireworms can be so damaging that even the most experienced grower can be throwing their hands up in defeat. However, they are manageable, and below is what you need to know to control wireworms.
Wireworms are one of my “Covid Pest Series.” These are pests that have become a growing concern because of habits we have formed during the Pandemic. Wireworms are an instant classic. Lawn and grasses provide the perfect cover for them: dense roots protect them from predation. And during this pandemic an enormous number of people have turned grassy areas into flower and veggie gardens, not knowing what lies beneath. So, let’s now control them together:
First, put the chemistry away. Some harsh chemicals worked well against them but are banned because they killed everything else in the soil. There is no chemical available (registered) that will get the affect you need. Even if it does manage to kill the wireworms, it’ll kill the things that would eat them normally, and you’re back to spraying again when no predators show up.
Second, understand the bug. It’s not a worm. The “worm-like” stage is the larva of a click-beetle. Click Beetles are active in Spring and summer feeding above ground on plant tissue and laying their eggs in the soil. The larva emerge and live for three to five years feeding on roots, tubers, and young, tender plants. In their final soil-borne year they pupate and emerge as new adults in the spring. The adult beetles are long oval shaped with blunt rounded heads. Not all click beetles and wireworms are plant pests. Many just eat dead plant material. There are other beetles that look similar, too, like some boring beetles. It is usually easy to diagnose wireworm damage by pulling a young, damaged plant or tuber and finding the wireworms on them, in them or around them by digging around in the soil. They are a noticeable orange-cream colour with darker head and six legs towards to head side of the body. They are very firm, hence the name. One grower even showed me that if they “snap” in half they are a wireworm!
Common Click Beetle scientific names: Agriotes sputator; Agriotes lineatus; Agriotes obscurus . You won’t typically see these beetles as they travel and feed at night. It is believed these are European species that established in North America as early as the mid 1800s.
Understanding the beetle and its lifecycle can help understand how to control them: Adults are free to roam from property to property, so other than trapping adults, little can be done to prevent them from arriving. There are pheromones produced by Agriculture Canada and other government agencies and some have been combined with fungi-based bio pesticides to provide a knock-down of pest pressure by attracting and infecting adults. But another obvious life stage to attack is the eggs and small larva within the soil and thus cutting off their life cycle.
Metarhizium and Beauvaria
These are biopesticides that are simply naturally-occurring parasitic fungi. Most wireworms are already infected with low levels of one or more of these, so buying and applying larger amounts is needed. This stuff works. However, it also affects non-target organisms. Fortunately the non-target pests are usually other soil dwelling pests, so some of you may want to go this route. I’d recommend this for a disastrous infestation on a crop with a large profitability, since their costs can be quite high. As far as I can tell, no Metarhizium is registered for use against Wireworms in Canada, but it can be used in soil to protect against other pests. Beauvaria have several strains that are commercially available in North America and Europe. Here’s one example:
Stratiolaelaps scimitus
I could have put this first, because it is the best solution, but I started writing this with no order in mind, and I think I’ll keep it that way. Stratiolaelaps scimitus is a predatory mite that lives in soil and feeds on first instar wireworm larva. They hunt by movement, so the only eggs that are consumed are ones that begin to hatch and start to wiggle. And as the mites are small, the larger wireworms are resistant to predatory mites. However, why Stratiolaelaps is the best is it can be applied at any time and will live permanently in the soil. This is essential as Click beetle adults are regularly laying eggs and those eggs are regularly hatching. So you will get predation, but because of the 3-5 year life cycle of click beetles, you’ll really begin to see dramatic reductions only after that time. And if you’re thinking “I can’t wait that long,” well keep this in mind: 1. You can use this with all the other options; 2. it is extremely inexpensive; 3. It’s non-target pests include: overwintering spider mites, pupating thrips (80% reduction in thrips), fungus gnats, black vine root weevil, springtails, symphylans, flea beetle larva, and nematodes. 4. They do not attack any other known Wireworm predator (spiders, rove beetles, birds..etc.) 5. You don’t have to reapply every year. So, you really can’t go wrong! You can buy small amounts of it off of Amazon under the name “Grub Grenade,” but it’s likely you have a garden bed or more that needs protection, so you can find a list of distributors here:
Dalotia coriaria
I’m throwing this in the mix with a bit of hesitancy. It works, but…. It’s not proven in the field. Yes, they eat them; Yes, you can get a knockdown of wireworms; but no, I can provide no evidence that they stay in high-enough numbers to be responsible for true control. The good news is they can be used with the above stratiolaelaps and they will take out lots of other, bigger pests that stratiolaelaps does not. But until true field-studies have been conducted, I’d only do this if you’re a believer (rightfully so) that the more predators out there the better. Dalotia is a new-kid-on-the-block and used religiously in a massive number of applications, but the only true, peer-reviewed studies of it in the field were to control fungus gnats and shore flies in green houses. They were found to control flea-beetles in farms, however the farms doing the trial found it was better to apply chemicals since they couldn’t control new flea beetles from flying in from their neighbouring properties. Anyways, it’s likely worth a try. You can also buy dalotia from one of these distributors:
Nematodes
Nematodes are certainly good for a knock-down. And the best part is that people who are used to applying a chemical find this an easy step towards biocontrol since you just mix it and pour it. However, what isn’t often discussed is that these nematodes don’t live very long. If they don’t make contact with their prey immediately, there is no effect. So environmental conditions are a limiting factor. You can only use them (without wasting money) for a couple weeks of the year when outdoor and soil temperatures are right and soil moisture is just right and not likely to change within the time frame needed for them to infect. But, they are a living thing, so once in the soil and working, you can get a bit of cycling. And while stratiolaelaps eat nematodes, they are unlikely to eat these ones since most of the time they’ll be inside their host. Instead, Stratiolaelaps eat nematodes that are killing your plants. Nematodes are also easy because they are easy to find in garden centres.
Crop rotation.
This isn’t applicable for everyone, but it is an important strategy for control. The click beetles lay their eggs near specific plants they know will feed their young. There are lots of host plants, but obviously most of our information comes from commercial crops like potatoes. But, if you have tons of wireworms in your soil and you plant a crop of something that they don’t like they will still eat it because they have no other option. So crop rotation is a long-process and for the first couple of years it makes more sense to plant a sacrificial crop. Consider something like buckwheat (not a host) but with rows of potatoes that will be dug-up and composted. The potatoes will attract the larva and adults away from the rest of the field for you to deal with (and create a concentrated place to apply one of the controls above).
If you happen to be reading this ahead of establishing a garden or crop, then I applaud you for being proactive and I will suggest these following two things: But if you have a garden or crop already you can still apply these in principle:
Barriers:
If you want a garden or crop where there is currently grass, create a barrier. Digging up the turf is ideal, but either way, put a dense layer or two of cardboard to prevent the immediate migration of larva up into your soil. Regardless, give the new soil stratiolaelaps and you’ll thank yourself later. The great thing about cardboard is you can usually get it free, water can flow through it and it eventually breaks down. Yes, there are chemicals in the manufacture of cardboard, but it will be of little consequence. There are certainly other barriers available, but remember, no barrier is perfect and you will always keep out predators disproportionately compared to pests, so be careful cutting your garden-off from natural soil.
Black tarp:
Ahead of planting, use heavy black plastic (and rocks to hold it down) across the growing area. Predators, which are generally more mobile will leave the super-heated area, but many pests including wireworm larva will die. Those that don’t die will make their way to the edges and be eaten by birds and other predators. It’s a quick way of preparing your soil. The draw back is you have a blank slate – which sounds great, but not when growing. The problem with barren soil is that the first thing to come back will be the pest, since you are supplying their food. Only after they have established will they be sought out as a food source for the predators. Furthermore, naturally-occurring fungi which contribute to pest control and plant health are also gone. But you also have warmer soil to plant heat-loving plants like cucurbits.
You can tell this is a complicated pest to manage and really all of these types of control may need to be considered depending on where you are. The main issue with this pest is its duration. Many trials have been conducted but only to switch gears or give-up before the five years needed to see it through. Furthermore, things like nematodes and stratiolaelaps are difficult to see, and therefore some will not trust they are working. So the most important bit of advice I can give is to use multiple methods and have patience. Growing organic has it’s benefits, but it also has it’s challenges.
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