Question of the Week
Answer: Wireworms are the larvae of click beetles. In Vermont, the eastern field wireworm (Limonius agonus) occasionally develops in corn fields under very clean cultivation on sandy soils. The most troublesome wireworm in Vermont is the wheat wireworm. It attacks crops planted in succession to pasture or hay fields. There are many other species of wireworm, however, including the Pacific coast wireworm, Limonius canus, the sugarbeet wireworm, Limonius californicus, or the dryland wireworm, Ctenicera pruinina. The adults are slender and cylindrical in shape, yellow to brown in color, and average between ½" and 2" in length. The adult beetles are not pests but the larvae can cause severe damage to seeds and young root systems. The life cycle of the wireworm can last up to five years depending on the species. Most of this is spent in the larval stage with the greatest feeding damage occurring in the second and third years.
Adults mate in early summer and the eggs are laid singly or in clusters below the soil surface in grassy/weedy areas (1). The eggs hatch within a month and the larvae feed on plant matter, growing slowly and taking between three and five years to mature. The larvae are initially white with dark jaws, then turn dark yellow to brown as they mature. In late summer, the mature larvae move deeper into the soil and create small cell hollows where they pupate. They may emerge within a month or overwinter and emerge in spring. Soil temperatures are important in wireworm development and control. Larvae start to move upward in the spring when soil temperatures at the 6-inch depth reach 50°F. Later in the season, when temperatures reach 80°F and above, the larvae tend to move deeper than 6 inches, where most remain until the following spring (2). Because the female beetles fly very little, infestations do not spread rapidly from field to field.
Wireworms are difficult to control. There is no silver bullet, organically acceptable insecticide that will provide good control. Not only do wireworms have an extended larval stage, they also feed on many different crops. Oats, wheat, barley, and clover are especially susceptible. Horticultural favorites of wireworms are beans, brassicas, carrots, corn, cowpeas, lettuce, melons, onions, peas, potatoes, strawberries, and sweet potatoes. Wireworm populations tend to build up on grass and sod, in fields that have been in continuous cereal production, or that have not been in annual crop production for years (3). Adult females of the corn wireworm prefer to lay eggs in grassy undisturbed soil (4). Since rotations to grasses are key to sustainable soil fertility management in many systems, eliminating sod crops and small grains from a rotation scheme should not be done without thorough planning for alternatives. Baiting is a good way to confirm wireworm presence but does not give a reliable estimate of the density of the population.
Monitoring by Baiting (5)
The most direct way to detect wireworms in a field is by general observation during plowing or disking of a field, particularly where old alfalfa, clover, or pasture is being taken out. Wireworms can also be detected by baiting, using carrots, packets of untreated corn and/or wheat seed, or ground whole wheat flour, provided they are used when soil temperatures are 50°F 4 to 6 inches deep. Baiting does not give a good estimation of the density of the population. If baiting shows the presence of wireworms, take soil samples to estimate the wireworm density. Use a 6-inch post hole digger and a shaker/sifter to sample. Take samples in the spring when soil temperatures are 45°F or higher at the 6-inch level or in late summer at the 18-inch level. The following guide is used in some production areas.
Treatment Thresholds for Wireworms in California (5)
Acres in field Number of soil samples Treatment threshold (# of wireworms) 10 30 1 22 45 2 40 60 2 90 90 4 160 120 5
Summer fallow along with frequent tillage helps reduce populations, although frequent tillage has negative impacts on soil organic matter. Crop rotation to alfalfa or another wire worm-tolerant host may help to decrease populations in the long run. Rotation to non-host crops such as onions, lettuce, alfalfa, sunflowers, and buckwheat will reduce wireworm populations and is recommended. Most importantly, avoid planting highly susceptible crops like grains, grasses, pasture, carrots, and potatoes following sod (6). Such practices as shallow tillage, shallow seeding, seeding with a press drill, and clean summer fallow are all helpful in reducing future wireworm damage (7). Cultivation eliminates food sources, desiccates the larvae and pupae and exposes them to predation, but at the cost of increasing the rate of oxidation of organic matter in the soil. Larval populations are most dense close to the surface in early spring when temperatures begin to warm up, and again in early fall. Disking or tillage at these times is more effective than at other times, when larvae burrow deeper down, sometimes as much as two feet deep. Populations usually decrease following cultivation but may persist for 3-4 years if the land is weedy (1).
Treatment options are very limited. Use of parasitic nematodes is an option if the area to be treated is not too large. Ron Engeland, in Growing Great Garlic, The Definitive Guide for Organic Gardeners and Small Farmers recommends the use of beneficial nematodes (Steinernema feltiae) applied in a water spray at the base of the plant for wireworm control. For best results, he recommends that they be applied the year before garlic is planted (8). It may be worthwhile to partner with your local cooperative extension agent and experiment with treating small plots to establish cost/benefit data relative to using parasitic nematodes for wireworm control.
Wireworms are attracted to carrots. These may be used to catch the insects if a relatively small area is planted, or for spot infestations. Plant nearly full grown carrots every 3 feet in the garden. Every 2 to 3 days pull up the carrots, remove and kill the wireworms and replace the carrot. Pieces of potato may also be effective (9).
A team of scientists in Canada have experimented with using a fungus, Metarhizium anisopliae to control wireworm biologically (10). They concluded that the fungus has potential for control of wireworms in a field situation, but unfortunately, there are no commercial products now registered for use on wireworm that contain the particular strain of this fungus most effective against wireworms. Contact information for the researchers is:
Pacific Agri-Food Research Centre
Agassiz, BC, Canada V0M 1A0
ph. 604-796-2221 ext. 215
Dr. Mark Goettel
Lethbridge Research Centre
Box 3000 Lethbridge, AB, Canada T1J 4B1
Dr. Bob Vernon
Pacific Agri-Food Research Centre
Agassiz, BC, Canada V0M 1A0
ph. 604-796-2221 ext. 212
Sue Blodgett of Montana State University is also researching the efficacy, formulation, and soil persistence of Metarhizium strain F52 in small grain and potato field plots. Her contact information is:
Montana State University
Department of Entomology
410 Leon Johnson Hall
Bozeman, Montana 59717-3020
A research team in Wales, UK examined using fodder rape and mustard as green manures for control of wireworms on potatoes (11). There was a slight trend toward lower wireworm (and slug) populations in the black mustard (Brassica nigra) treatments, which consisted of growing the mustards for six weeks, then rototilling them into the soil prior to planting potatoes. It should be noted that this experiment was done on non-organically managed land that had previously been in pasture.
As noted above, there are limited options when it comes to organic management of wireworm infestations. Many of the options available are not optimum because they can damage the soil quality (multiple shallow cultivations) or farm finances (keeping the field fallow and weed free for 2-3 years). It’s possible that combining trapping (using carrots) with use of predator nematodes around the bait could provide some control of wireworm damage, but this would be an experiment you would have to undertake on your farm with no guarantee of success. Row covers might provide some control by limiting access to the plants by egg-laying females, but that is assuming that the wireworm adults do not emerge under the row cover, mate and oviposit on the carrots. Another option is a more long-term, experimental consideration. Wireworm adults, known as click beetles, are active at night and have few predators as a result. However, bats are insectivores and increasing bat populations may help reduce the number of adults and subsequent number of wireworms in the soil. As I said, this is a long-term strategy, since there is a "wireworm bank" of larvae in the soil that may take up to 3 years to mature to adult beetles. At the very least, the bats will help manage a host of other night-flying insect pests, such as cutworm and armyworm moths. You may wish to access ATTRA’s Farmscaping to Enhance Biological Control, which has information about increasing bat habitat.
1) McKinlay, R.G. (ed.) 1992. Vegetable Crop Pests. CRC Press, Boca Raton, FL. p. 33.
2) Jensen, A., and B. Stoltz. 2002. PNW Insect Management Handbook. Wireworms Supplement. http://pnwpest.org/pnw/insects?27IPMW11.dat
3) Sandvol, Larry E. et al. Wireworms in Potatoes. College of Agriculture, University of Idaho. 2 p.
4) Sorensen, K.A. 1995. Wireworms on Sweetpotatoes. Insect Notes #24. Department of Entomology, NCSU. www.ces.ncsu.edu/depts/ent/notes/Vegetables/veg24.html
5) Godfrey, L.D. 2000. Entomology, UC Davis. UC IPM guidelines for potato. www.ipm.ucdavis.edu/PMG/r607300111.html
6) Metcalf, R.L. and R.A. Metcalf. 1993. Destructive and Useful Insects, 5th ed. McGraw-Hill, New York, NY.
7) Glogoza, P. 2001. Wireworm Management for North Dakota Field Crops. NDSU Extension Service. http://www.ext.nodak.edu/extpubs/plantsci/pests/e188-1.htm
8) Engeland, Ron L. Growing Great Garlic. 1991. Filaree Productions, 182 Conconully Hwy, Okanogan WA 98840.
9) Spring, A., and Eric Day. Department of Entomology. Virginia Tech., Blacksburg, VA. http://everest.ento.vt.edu/~idlab/vegpests/vegfs/wireworms.html
10) Kabaluk, T., M. Goettel, B. Vernon, and C. Noronha. 2001. Evaluation of Metarhizium anisopliae as a Biological Control for Wireworms. Pacific Agri-Food Research Centre and Lethbridge Research Centre. www.organicagcentre.ca/ResearchDatabase/res_biol_ctrl_wireworms.html
11) Frost, D., A Clarke, B M McLean. 2002. Wireworm control using fodder rape and mustard – evaluating the use of brassica green manures for the control of wireworm (Agriotes spp.) in organic crops. ADAS Pwllpeiran, Cwmystwyth, Aberystwyth, Ceredigion, SY23 4AB, June. www.organic.aber.ac.uk/library/Wireworm%20control%20and%20brassica%20green%20manures.htm
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