Click beetles and their larvae, commonly known as wireworms, have recently become important pests of cultivated plants, especially root crops. Due to changes in land management (conservation agriculture, monocultures, improper crop rotation, promotion of no-till systems, heavy weed infestation), they have found favorable conditions for development and colonization of fields. Adult insects have no economic significance as pests and live freely in agricultural areas. During the spring flight period, adults feed on plant pollen. Potato tubers are only at risk from larvae, which undergo a long developmental cycle in the soil and cause significant quality losses. Wireworms (larvae) are polyphagous, feeding on underground plant parts, damaging potato tubers, beets, germinating cereals, maize, grasses, onions, root vegetables, and ornamental plants.

Elateridae are specific species present in Europe, as well as North and South America, Africa, Asia, and Australia, except in very cold regions.

Bionomics

• Eggs – elongated-oval, milky white to cream-colored, laid in the soil (at 1-2 cm depth) singly or in groups of 2–10. Eggs fail to develop and die if soil moisture is insufficient.
• Larvae (pest stage) – called wireworms due to their resemblance to copper wire; newly hatched larvae are colorless and transparent, 1.2–2.5 mm long. Larvae are oligopodial with a worm-like, 14-segmented body, a well-developed strong head, chewing mouthparts, and three pairs of legs (photo 2). The body is cylindrical along its entire length. The cuticle of older larvae is heavily sclerotized, light yellow to yellow-brown. Largest larvae reach 17–20 mm (only A. sputator are smaller – 15–20 mm). Larvae pass through multiple stages differing in body length. Larval development in soil: A. lineatus and A. obscurus – 4 years, A. sputator – 3–4 years, with 8 molts. The morphology of the IX segment (apparently the last) is taxonomically important; it is the longest segment, conical in shape (Agriotini tribe, including Agriotes sp.). They overwinter in deeper soil layers.
• Pupa – free type, 20 mm long, white. Body smooth and soft. Eyes, jaw tips, feet, and wings darken during final maturation.
• Adults – possess a jumping apparatus on the pronotum, enabling the inverted beetle to jump back to its feet. Body length: 7–9 mm (Agriotes obscurus), 6–8 mm (Agriotes sputator), 7–11 mm (Agriotes lineatus) (photo 1). Elytra brown (striped in A. lineatus), pronotum dark, glabrous. Insects are secretive, walk actively on soil, rarely fly, active in evenings and during spring flight. Young adults overwinter in soil after pupation. This stage occurs once every 4–5 years (one generation). Female fecundity: 100–200 eggs.

Development cycle

Overwintering stages are adult beetles in winter cocoons (pupated in late summer) and larvae at various stages in soil up to 60 cm deep. First adult emergence occurs from late April to early May (during potato planting) when soil reaches +7°C. Mass emergence occurs in May, followed by reproduction and egg-laying in soil. Females lay eggs from late May, 2–10 per spot, at 1–2 cm depth in the moistest areas of the field. Embryonic development lasts 2–4 weeks. Larvae hatch in June. Larval development lasts 3–4(5) years (depending on species), with 8 molts (most voracious L4–L8 responsible for major yield losses). Mature larvae in the final year feed until June–July, then pupate in deeper soil layers at 30–60 cm depth (pupation lasts 1 month). Young adults emerge at the end of summer and remain in soil cocoons until the next spring.

Occurrence and harmfulness

Wireworms occur abundantly under favorable soil and environmental conditions, mainly in poorly managed farmland with limited plowing, weed-infested, no-till systems, fields after monocultures or adapted grasslands, fallows, and set-aside land (photo 4). These insects are phylogenetically associated with densely vegetated grasslands. Due to lack of available plant protection products and the specific life conditions of larvae, they are currently very difficult to control. Wireworms are among the most dangerous potato pests. Young larvae (thin – L1) feed on plant residues and humus, while larger larvae after 2 years of development (L4–L8) damage all underground parts (tubers, stolons, roots). Heavy infestations (e.g., on long-term fallows) can completely destroy tubers (2 mm diameter tunnels). Direct yield losses can reach 30–50% (on heavily infested fields), usually not exceeding 10–15%. Damaged tubers have numerous external holes (~2 mm diameter) (photos 5A, 5B), with shallow or deep feeding tunnels inside filled with excrement (photos 6A, 6B, 6C). Wireworms are often found inside tubers. Damaged tubers are secondarily infected by soil fungi or bacteria, causing dry or wet rot, making them unsuitable for processing or consumption.

Economic threshold

Chemical control is indicated when the economic threshold is exceeded. Previously, 11 large larvae per 1 m² was used as the threshold; in specialized potato crops (for fries, chips, high-quality packaged tubers), the threshold is 6 large larvae per 1 m².

Monitoring wireworm occurrence can be done by field observation and soil analysis (32 pits diagonally across the field) before planting. Bait traps (soil-based) can also be used, 10–20 per hectare along field diagonals (photo 7).

Control methods

Protection is difficult due to the limited number of registered products. Currently, the only method is treating tubers with a registered insecticide-fungicide coating Prestige Forte 370 FS during planting, which acts contact and systemically from tuber placement in soil, eliminating wireworms feeding on seed tubers. Later, it protects against Colorado potato beetle and aphids as the insecticide circulates systemically in young plants. The coating effect lasts 10–12 weeks; larvae not exposed to it can damage young tubers until harvest, often causing damage shortly before extended mechanical harvesting. Other organophosphate soil treatments are currently not allowed.

Agrotechnical methods are partially effective, mainly reducing larval numbers (proper crop rotation, cover crops such as mustard or oilseed rape, non-host plants, timely agrotechnical operations, deep plowing, loosening). Agrotechnical control can be applied during winter plowing to expose larvae (birds or low temperature). Overwintering larvae are controlled during the 3–4 year cycle in spring by treating tubers with coating. Larvae are eliminated as they approach the treated seed tuber, around which a protective insecticide layer forms. Optimal timing is during potato planting (larvae awaken seeking food after winter): April–May.

Biological agents (based on living organisms, mainly entomopathogenic nematodes from the Heterorhabditidae family) support control during the growing season. Biological treatments should coincide with larval soil activity, requiring careful field monitoring. High soil moisture or irrigation is necessary for nematode effectiveness. Treatments are applied with a field sprayer on cloudy days. This method is expensive and mainly reserved for small areas.