When wireworms are mentioned at a field day or cereal school, growers wonder if there will be any good news. From their perspective, good news is progress toward sustainable solutions. A new chemical control would be welcomed, too. Dr. Arash Rashed, an entomologist at University of Idaho’s Department of Entomology, Plant Pathology and Nematology, has become an expert on the wireworm species that damage Idaho cereal crops. When contacted for this article, Dr. Rashed remarked, “There is quite a bit of fundamental science being done to build a platform for an Integrated Pest Management approach to provide a sustainable solution. There are also some new advances on chemical control against wireworms, but chemicals are just one tool in the toolbox of integrated pest management (IPM)”.
Wireworms are one of the most difficult production problems facing Idaho producers. A wide range of crops, including wheat, barley, sugar beet, and potato, are affected by wireworms. When Dr. Rashed began working on wireworms, little was known about them except their basic life cycle as larvae of click beetles. Idaho wheat producers have partnered with Dr. Rashed to develop an IPM approach to wireworm control, providing partial financial support for wireworm research.
Wireworms are not unique to Idaho: they live in the soil of croplands throughout the United States and Canada. The larva feed on seedlings and roots of emerging crops, causing damage to the crop plants during the first few weeks of stand establishment and, to a lesser extent, throughout the growing season. Fields with enough wireworm damage to reduce profitability have patches of missing and/or stunted plants throughout a field of relatively healthy plants. Wireworms migrate up and down in the soil profile in response to temperature, plant volatiles, CO2 and/or soil moisture content. But they do not move very far laterally. The patches indicate a concentration of wireworms feeding heavily in that area. Adult click beetles lay their eggs in the soil in certain spots within fields, resulting in patches of wireworms rather than a uniform infestation. If thorough scouting is not in place, a wireworm-infested field may look deceptively healthy and productive. An estimate of wireworm numbers can only be obtained using solar bait traps or a shovel to dig and look for them in their underground habitat. It is difficult to develop a measure of the level of infestation that has negative economic impact justifying control measures.
Verifiable economic loss is a key data point in motivating agencies, industry, and regulators to facilitate research on, and implementation of, wireworm control measures. The Idaho State Department of Agriculture (ISDA) requires verifiable estimates of economic impact when applying for a Section 18 permit requesting emergency use of insecticides that are currently not registered for application on cereals. ISDA rejected the economic impact data presented by Idaho Grain Producers Association (IGPA) when they requested a section 18 permit for Fipronil in 2016.
Drs. Sanaz Shafiani and Patrick Hatzenbuehler are new UI-CALS researchers who have joined with Dr. Rashed to use aerial survey and bioinformatics as estimation tools to accurately determine economic losses to wireworms based on surface area exhibiting typical wireworm damage. Accurate estimates of damage can be calculated from trapped wireworm numbers compared with aerial measurements of affected field area. “Economic loss estimated from aerial survey data, validated by trap data, should overcome this barrier next time Idaho grain producers need a section 18 for wireworm control,” explained Stacey Satterlee, Executive Director of the Idaho Grain Producers Association. “This research will also provide more accurate information on threshold populations to determine when to initiate control measures in a particular field,” added Dr. Rashad.
Trapping wireworms across Idaho crop fields began in 2014. The goal was to identify predominant pest species in rainfed and irrigated fields and determine factors that affect fluctuations in populations. The most prevalent wireworm species in field traps has consistently been Limonius californicas, also known as sugar beet wireworm.
In the crop 2020 survey, sugar beet wireworm was collected in 95% of the traps in southcentral and southeastern Idaho and 70% of those in northern Idaho counties. L. infuscatus, common name western field wireworm, and Aeolus mellillus were collected frequently as well. Average numbers of wireworms collected per trap in infested fields remained relatively unchanged between 2019 and 2020, with a few exceptions. Numbers in Bonner, Kootenai, and Boundary counties decreased from 11 to 5 wireworms per trap. Latah County, where fewer wireworms are generally collected, saw its counts increase from 0.2 to 2 wireworms/trap. Wireworm numbers in Twin Falls county increased from 3.5 to 6.4 wireworms/trap in 2020. Across the state the number of Aeolus mellillus in traps was reduced compared to 2019. Dr. Rashed suggested this might be related to the increased rainfall in 2020 compared to 2019.
Understanding how wireworms choose what they eat is essential to an IPM control program. Belowground insects such as wireworms use CO2 and root volatiles to orient themselves toward the food source. Thus, planting a trap crop and/or intercropping with plants that direct wireworms away from the main wheat crop can be used as components of an IPM protocol controlling wireworms in the PNW. Through a series of experiments, Rashed and his team evaluated sugar beet wireworm preference for wheat versus pea and bean plants, which are common rotational crops with wheat in the region. Wireworms were consistently more attracted to these legumes than to wheat, corroborating a field study by Montana State University that showed, in small plots, planting legumes can divert wireworms away from nearby wheat.
Measuring volatile chemicals and CO2 concentration emitted from plant roots suggested that wireworms use both CO2 and various root volatiles to identify and select its host plant. Interestingly, however, it is not just the presence of volatiles that helps with host selection, the concentration of volatiles plays an important role as well. UI researchers found that if certain volatiles used by wireworms to select their host are present in higher than normal concentrations, it caused the wireworms to leave those plants alone. Identifying volatiles and concentrations that trigger pest response toward or away from the host plant is critical in developing plant genotypes that can be planted in wireworm infested fields. Understanding how wireworms use chemical receptors to navigate their environment has many uses in building IPM control measures for wireworms.
Pheromones are chemicals secreted by animals as alarm signals, to attract them to food trails, or to attract a mate. In a collaborative effort with colleagues in the USA and Canada, Dr. Rashad’s team used synthetic compounds based on pheromones to attract adult click beetles into traps. Pheromone traps can be used for monitoring click beetle populations and species. Trapping adult beetles reduces egg-laying by females, thus lowering infestations of larva, and reducing next generation adults. “Successful removal of adults will reduce egg-laying, which combined with controlling wireworms in the soil can result in more sustainable management of the problem,” explained Dr. Rashad.
In 2015, Dr. Rashad and a team of collaborators from University of Idaho’s Institute for Bioinformatics and Evolutionary Studies (IBEST) received an IBEST Technology Access Grant to sequence the mitochondrial genome of Limonius californicus (sugar beet wireworm). As a result of this collaboration, IBEST hosted a summit with researchers around the world to discuss what was currently known about this pest. As questions were posed, the most frequent answer from participants was, “good question, we don’t know”. Participants agreed that understanding how species are genetically and demographically related to each other would provide a foundation for interpreting all the applied work focused on wireworm management in crops. UI researchers took the lead in this effort. After 5 years of challenging work, results detailed in the paper "Wireworm (Coleoptera: Elateridae) genomic analysis reveals putative cryptic species, population structure, and adaptation to pest control" was accepted for publication in the scientific journal Communications Biology. This is a milestone for wireworm research benefiting all those working on wireworms around the globe. UI research teams are recognized as leaders in paving the way toward sustainable wireworm control.
Controlling wireworms is one of the most difficult problems facing cereal producers. New chemistry is an exciting possibility but how long will we be able to use chemical controls in light of environmental concerns, consumer driven demands for sustainable farming practices, and the rise in organic farming? Dr. Rashed’s integrated research answering both applied and more fundamental questions about wireworms has moved us toward a more sustainable approach to controlling this persistent pest.
