Wireworms in potatoes need new control
Wireworms are the larvae of many species in several genera called click beetles. Wireworms cause the damage not the adult click beetles. Since every geographic location has its own set of species, to know which wireworms are causing damage to potato, it is necessary to trap them and identify those in the potato field, according to Extension specialists from Virginia to California.
A wireworm-infested field will remain infested for three to six years and planting has to be avoided if wireworm levels reach too high a level in the soil. Wireworms are large enough and infest in high enough levels that a potato grower is likely to observe them in fields while plowing or discing, according to the University of California Pest Management Guidelines fact sheet.
Bait trap procedures are recommended by nearly every Extension service where potatoes are grown. “A rough estimate is that an average of one wireworm per trap is equivalent to 20,000 wireworms per acre,” according to the University of Nebraska-Lincoln “Potato Education Guide.” But at this level of infestation, an advanced insecticide, meaning one with more than one mode of action, is often recommended.
It isn’t until four wireworms per trap that a field is not recommended for potato planting, the “Potato Education Guide” suggests. But it should also be noted that wireworms are attracted to high moisture areas in general; therefore, densities of the pest are often higher in low-lying wetter portions of fields. Wireworms will likely cause significant problems only in a proportion of a field, according to the Extension specialists. Even in those fields where they are present, wireworm damage may be spotty. Also, as most potato growers know, high wireworm densities will tend to occur over many years after a field has been transferred from sod or pasture or previously planted with grass cover crops.
Historically preventative soil insecticides, either preplant as a broadcast or at planting treatments, have been lost due to environmental concerns or efficacy problems. Potato growers have limited options for wireworm control. This is why MANA agrochemical company recently registered a dual mode of action insecticide, Skyraider, with bifenthrin and imidacloprid active ingredients.
Adults click beetles overwinter in the soil and emerge in the spring. Once mated, click beetles seek egg-laying sites in grassy areas, which include pastures, sod areas, cereal crops or even grassy weed spots in cultivated fields. Eggs hatch in a few days to weeks, and the larva or wireworm emerges.
Concern should have been heightened this year with grassy weeds growing rapidly in fields because of the unusual weather patterns in many areas of the country.
Wireworms emerge in the soil and may survive for two to six years. In the winter, they survive about two feet deep, and as the temperature warms, they move up through the ground to the top two to three inches of soil. Wireworms move up and down in the soil during the season depending on temperature, according to the Nebraska guide. They prefer soil temperature to be 50 to 60 degrees F. After wireworms achieve full maturity during the summer, they will pupate in the soil, and the pupae will transform into click beetles after a few days.
During the growing season, wireworms have been difficult to control requiring high use rates of insecticides and incomplete control. The Nebraska Extension reports, “This is the result of the difficulty in trying to move the insecticide down into the soil in a high enough concentration to obtain wireworm control and for an insecticide to last long enough to protect mature tubers near harvest.”