Post-emergence wireworm scouting
By Jeremy Boychyn (M.Sc)(P.Ag), Agronomy Research Extension Specialist
With the expert advice of Dr. Haley Catton (AAFC Lethbridge)
At this point, many producers and agronomists are familiar with the devastating effects of wireworm infestation.
For a description of wireworm biology, check out this article from the Prairie Pest Monitoring Network (PPMN). The PPMN website is a great resource for information on Western Canadian insect pests. You can sign up for weekly updates from the PPMN here: https://prairiepest.ca/subscribe/
Wireworm life cycles are typically two to five years long, depending on the species and environmental conditions. Wireworm eggs are laid by adult Click Beetles. Click Beetles emerge from the soil in the spring and lay their eggs in various locations within a field. Eggs hatch within a few weeks into very small neonates. The neonates themselves aren’t damaging to cereal crops. Once they survive their first winter, they are called resident wireworms and this larval form is the economically damaging life stage of the insect. Resident wireworms are in the soil before the crop is planted, waiting for a food source as the soil warms after winter. Resident wireworms live for several years before transforming into adults. Resident wireworms feed on seeds and growing seedlings, which reduces plant stands and leaves bare patches in fields. In extreme cases, the larvae can live in the soil for up to 11 years. However, they do not spread very far from where they hatched, meaning that damage will be patchy and depend largely on where the eggs were laid. This also means that locations within a field that contain wireworms will continue to see wireworm pressure until the population is no longer present. New wireworm patches arise from new eggs being laid. Small wireworm patches may grow due to populations maturing and expanding slightly or from new eggs being laid close to existing patches. This pattern depends on the species of wireworm as some species are limited to walking as adults, while others are able to fly and cover farther distances.
Figure 1: A small bare patch in a spring wheat field that scouting revealed to be pressure from wireworm
When wireworms first present themselves in a field, symptoms can be easily overlooked, or mistaken for other causes like frost or cutworm damage. However, as the population matures or more eggs are laid in that location, the pressure will increase year after year and become more obvious. For this reason, it is better to identify wireworm patches before they become large enough to cause significant economic impacts.
Scouting for wireworm can be done before seeding or after emergence. Pre-seeding scouting can be done with bait traps to detect wireworm presence. With that said, post-emergence scouting can be even more informative, as it shows you how much damage has occurred and where in the field that damage is located. Post-emergence scouting can be done after herbicide timing and before the beginning of stem elongation. This is the best time to spot small bare patches that could indicate wireworm presence (Figure 1) and will help determine if seed treatments are needed in the following years. If scouting occurs too late, the plants tiller and the wireworm may find refuge deeper in the soil to avoid the daytime heat, making them much more difficult to locate. Once you identify a spot that may have wireworms, it’s time to start digging.
Be sure to dig multiple holes within the same suspected patch (Figure 2). Your first goal is to find any seed that did not germinate and seedlings that did not emerge. If you find dead seeds or seedlings, look for feeding holes in the seeds or bottom portion of the roots. Also, look for seedlings where the central leaves are yellowing (dead heart). To confirm that the wireworm caused the damage, you will want to break apart soil clods and locate live wireworms. This will take time and patience.
Figure 2: Digging in multiple spots in the same wireworm suspected patch may be necessary to locate wireworms
If you do locate wireworms, note the field location with GPS. You can then use this information to decide if and how to implement control in future years. Unfortunately, nothing can be done to control wireworms after the crop is seeded. The best-known current management tool is to use a seed treatment.
There are currently no economic thresholds for seed treatment use to control wireworms. The decision to use seed treatment is based on perceived risk (number and size of patches), the historic appearance of damage within the field or farm, and the cost of treatment.
For future reference, AAFC has a field guide dedicated to Prairie wireworms coming out this summer (2021). The guide will have all the info you could want on wireworms in the Prairies, as well as great high-resolution photos. The guide will be available in French and English in both print and pdf versions at no cost.
Figure 3: Small wireworm located after digging through a suspected bare patch in the field. Note yellow wireworm in the center of the palm.
Figure 4: Small wireworm located after digging through a suspected bare patch in the field. Note yellow wireworm sticking out of a soil clod in the center of the image.