CLICK BEETLE (adult) and WIREWORM (larva)
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. An infested field will remain infested for three to six years and planting there should be avoided.
Description
Adults are slender beetles whose appearance varies greatly since many genera and species comprise the group called click beetles which produce wireworms.
Larvae from click beetles are called wireworms and these are the damaging form of the insect. During the early stages lasting one to two years, wireworms are very small and white. Mature larvae lasting two to six years are hard-shelled with dark transverse bands along the length of their body. The body color is a shiny yellow to rust. There are six (three pairs) slender legs toward the head region. At the head, there is a pair of pincer-like protrusions. Wireworms are 1/2 to an inch long at maturity and easily visible in traps and occasionally may be found in or hanging from a potato tuber.
Eggs are laid and pupa are formed deep in the ground; eggs are laid in grassy areas.
Life Cycle
Adults (click beetles) are relatively common in the summer and are often attracted to lights at night. Their presence does not necessarily indicate future problems. They overwinter in survive in the soil for several months, overwintering, and emerge in the spring. Once mated, click beetles seek egg-laying sites in grassy areas which may be pastures or other sod areas. For this reason, wireworms, the larvae, are most likely to be a problem in fields that have recently been broken out of sod or grass crops. Click beetles also may seek to deposit eggs in areas of cultivated fields where grassy weeds are a problem or in cereal crops. Eggs hatch in a few days to weeks, and the larva or wireworm emerges.
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 worms 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. They prefer soil temperature to be 50 to 60oF. After wireworm achieve full maturity during the summer, they will pupate in the soil, and the pupae will transform into click beetles after a few days.
Damage
Wireworms, the larvae of click beetles, damage potato by feeding primarily on tubers. The damage appears as straight, round holes with smooth walls. It is not clear whether the wireworm feeding is due to a search for shelter or due to thirst and not actually eating. Early in the season around planting, wireworms may drill into seed tubers or seed-pieces thereby weakening them possibly resulting in secondary infection especially by soft rot, in reduced stands and in weakened young plants. Wireworms also can feed on young sprouts with the same results. During the season, although not common, wireworms could damage roots and underground stem. For potato, the major damage is later in the season when tubers are maturing after bulking. Late-season damage will result when larvae feed on tubers causing feeding scars or feeding tunnels. The result of this feeding is reduced quality of the harvested tubers. Wireworm damage is most likely the result of feeding by larvae that are 2 or more years old. In most situations larvae of several ages can be found in an infested field. This does not affect seed production as seed vigor is not affected but the damage causes culling for the fresh market. In processing, potato chips will show gaps along the margin where the hole was located and likewise french fries will show the gap at their ends.
Soil Sampling
Wireworms would likely cause significant problems only in a proportion of a field. Even in those fields where they are present, wireworm damage may be spotty. Soil sampling and baiting are somewhat effective at determining if wireworms are present in fields to be planted to potato. Wet wheat flour or germinating wheat, barley or oat seeds can be used to attract wireworm larvae into buried bait stations. These pests seem to be attracted to fermenting grain seeds. Wireworms can be collected by sifting soil samples through window screen. Sampling and baiting must be carried out when soil temperatures are 50 to 60oF to ensure that wireworms are active near the surface of the soil. Threshold levels have been released based on the number of bait traps with wireworms. Economic risk assessment are based on 3% of harvested tubers having wireworm holes. When baited traps have two to four wireworms on the average, the level is considered very high and the application of a chemical at planting is often recommended. If the traps have more than four wireworms, the level is considered extreme and the recommendation is not to plant potato in that field. A rough estimate is that an average of one wireworm per trap is equivalent to 20,000 wireworms per acre.
Table. Risk of economic damage from wireworms: Economic damage is taken as 3% of tubers damaged at harvest.
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Bechinski, E.J., Sandvol, L.E., Carpenter, G.P. and Homan, H.W. 1994. Integrated Pest Management Guide to Wireworms in Potatoes. Univ Idaho Coop Ext Circ #760.
Management
Complications dealing with wireworm life cycles and biology make wireworms a difficult insect pest with which to deal.
Biological
Because of wireworms long life cycle, growers need to be aware of the cropping history of a field and also the severity of grassy weed problems in all of the fields areas. Fields with cereal cropping history, a history of grassy weed problems and newly cultivated soils need to be avoided to minimize wireworm problems. Crop rotations would have to exceed six years unless bait-traps indicate that wireworms are no longer present. Planting after an alfalfa field that has been in production for several years is excellent. Although not reported, personal observations suggest wireworm preferences for different varieties. For instance, wireworms have a distinct liking for Krantz and seem to have a moderate preference for Russet Norkotah. They do not have a particular taste for chipping potatoes nor for Shepody.
Chemical
If soil sampling or baiting indicate a significant presence of wireworms and planting in the field cannot be avoided, the options for wireworm control are to apply a pre-plant broadcast or planting-time application of an insecticide. Control of wireworms is difficult resulting in high use rates and incomplete control. 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. Where wireworm populations are very high and land cannot be rotated to a less sensitive crop than potatoes, soil fumigation is an option. Foliar treatments are not effective nor available.
Quick Review
Appearance:
* Adult - click beetles comprises many genera of insects
* Larvae - wireworms, shiny yellow to rust with dark bands, six frontal legs and pincer mouth parts, up to 1 inch long
Life Cycle:
* Wireworms survive in the ground for up to six years
* Adults live for up to a year
* Full life cycle is three to seven years in the same field
Damage:
* Wireworms eat holes in tubers and other underground organs
* Mostly tuber quality loss, some yield loss possible due to stand
Management:
* Biological - avoid field, plant after several years of alfalfa
* Chemical - soil-applied, long-residual product, soil fumigation