Ivan Drahun and Dr. Bryan Cassone, Brandon University – Spring 2020 Pulse Beat
What are wireworms and why is it important to scout your fields for them?
Wireworms are the soil-living larvae of click beetles. They are small (1/2″ or so) and worm-like, with three pairs of legs on their underside. Wireworms are pests of several important crops grown in Manitoba, including wheat, soybean, corn and potatoes. Although the adult beetles pose no harm, the larvae are voracious feeders and can also cause plant injury by burrowing into tissues. Most of the damage occurs to seeds and seedlings, and can result in cosmetic damage, stunted plants, and even plant death.
Heavy infestations can cause significant yield losses or reductions in crop value. Historically, wireworms were successfully managed by the pesticide lindane, but since its ban in 2004, wireworm numbers have exploded. While nothing on the current market is as effective as lindane, there are other seed treatments available and cultural management practices that can help to reduce damage. The larvae are unusually long-lived (two to six years), which means they can be present in your fields for several growing seasons and this makes their control even more important. Over 90% of southern Manitoba fields we surveyed over a three-year period had wireworms, but infestations were rare. We found at least seven pest species, with Hyponoidus bicolor being the predominant species.
What are some clues that you have wireworm infestations?
The presence of click beetles in crop fields or the surrounding vegetation may serve as a sign of nearby wireworm infestations. In heavily infested fields, it is common to find the larvae near or even above the soil surface. Patchy crop growth is also a good indicator of wireworm damage but should be investigated through scouting before preventative measures are undertaken, as it could also be due to other factors.
Our data also suggests that moisture within the soil could reduce wireworm abundance. This means that in years of high precipitation, wireworms are less likely to be problematic and that flooding could serve as a natural management option. It should also be noted that wireworm infestations are often localized. So heavy infestation in one area does not necessarily mean the entire crop field is infested to the same degree.
When should you scout for wireworms?
Most experts recommend scouting for wireworms in the spring, prior to planting and once soil temperatures rise above freezing (around 5°C). Wireworms are attracted to carbon dioxide (CO2) produced by germinating seeds, which means finding them after seeding can be more challenging due to the competing CO2 sources. Post-harvest is another reliable time to scout for wireworms, as long as it is before the freezing temperatures start to occur (as cold temperatures cause wireworms to burrow deeper into the soil).
Since we have found that in some fields the pest species composition changes throughout the growing season, we recommend testing your fields at two or three different times. Should patchiness become apparent in your crop throughout the growing season, scouting within the patchy area will still indicate whether wireworms are present.
How to scout your fields for wireworms
The good news is that scouting for wireworms in your fields is not too difficult or expensive. All of the needed items can be purchased at your local gardening store. There are two different methods available — soil coring and bait trapping.
Soil coring consists of random soil sampling and inspection to estimate wireworm presence. Since the distribution of wireworms in a field tends to be patchy and unpredictable, large numbers of samples are needed to obtain an accurate measure. This process is labour-intensive and often not sensitive enough to detect problems.
Bait trapping is the more accurate and preferred method for most fields. The traps are designed to create an optimal environment for wireworms. They should be installed in areas suspect of infestations or along the peripheries of fields, about five metres apart. As wireworm infestations can be localized within a field, we recommend trapping in more than one area of a field. For our scouting, we trapped in four different field sections with four traps per section. The more traps, the more accurate your estimates will be.
How many wireworms per trap is a sign of infestation?
The University of Nebraska estimates that an average of one wireworm per trap is equal to 20,000 wireworms per acre. While we are currently working to determine the economic thresholds for wireworms in Manitoba, studies in the Midwest USA indicate fields with an average of one to two wireworms per trap have a 50% to 74% chance of economic damage. Fields with more than four wireworms per trap have a greater than 90% chance of economic damage. Before spending money on treatment options, it is a good idea to first seek advice from an expert.
Do you want further information? Please contact Dr. Bryan Cassone, Associate Professor of Biology – cassoneb@brandonu.ca
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