Wireworms and WDS complex

[Home] [Insect and mite pests ] [ Diseases ] [ Nutrient disorders ] [ Nematodes ] [Glossary ]

 

Scientific name: Several species of click-beetle larvae (family Elateridae).

 

Common name: Wireworms; click-beetles

Scientific name: Several species of the family Elateridae.

Common name: Cucumber beetles; rootworms

Scientific name: Diabrotica spp., family Chrysomelidae

Common Name: Flea beetles

Scientific Name: Systena spp.; Chaetocnema confinis (Crotch), of the family Chrysomelidae, subfamily Alticinae

 

Taxonomy

 

Class

Insecta

Order

Coleoptera

Family

Elateridae, Chrysomelidae

 

Economic importance

 

Soil-dwelling beetle larvae can cause considerable damage to storage root marketability in some areas.  As wireworms, rootworms and flea beetle larvae cause similar damage, the problem is sometimes referred to as the "WDS complex", where the contribution of particular species to the damage is not specified.  Secondary infection of the wounds may cause rot of some storage roots, but more often the scars heal.  The roots are still edible, but may not be marketable.

Geographical distribution

Damage by these soil insects was only reported from sub-tropical to temperate production areas.  No reports were found from tropical countries: they may be present but of less concern.

Wireworms are widespread, with different species and genera present in various countries.  In USA, various species of the genus Conoderus have been reported, including C. scissus Schaeffer, C. rudis (Brown), C. vespertinus (F.), C. amplicollis (Gyllenhal) and C. falli Lane, as well as the corn wireworm, Melanotus communis (Gyll.), which is more common in cooler areas.  Species of the genus Agrypnus are widely distributed, and in Australia, Hapatesus hirtus Candeze was recorded.

Cucumber beetles (rootworms) are native of North America.  All references found to cucumber beetle and flea beetle damage to sweetpotato were from USA, and it is not known if they are a problem elsewhere.

Damage

Storage root surface is damaged with round to irregular crater-like holes or shallow channels, usually less than 5 mm deep but sometimes over 1 cm.  The holes are larger than those produced by sweetpotato weevils, and are often referred to as "shot-holes" by farmers.  Older scars may appear healed, and may be deepened by the expansion of the root.  Recent scars have ragged edges and may contain frass.

Initially the larvae make a small hole in the skin of the sweetpotato, then eat an enlarged cavity just under the skin.  In the case of flea beetle larvae, they may make short winding tunnels below the skin. The skin over the cavity dies, leaving ragged edges on recent scars and an open cavity on healed scars.

Wireworm scars are usually randomly scattered over the root, while those of cucumber beetle are usually in groups.  Wireworm damage is reported to be greatest late in the season producing many fresh scars at harvest (but this may only apply to Conoderus spp with a single-year lifecycle), while cucumber and flea beetles often attack at early stages of storage root development, resulting in mainly healed-hole injury.  Cucumber beetle scars are most commonly round, while flea beetles produce some elongated or irregular channels as a result of tunnelling.

Flea beetle larvae feed primarily on the fibrous roots, but when populations are high, they can cause significant damage to storage roots. 

The larvae of all species live in the soil and don't remain inside storage roots for prolonged periods.  Thus they may not be easily detected during harvest.

Adult click beetles do not feed on the crop.  Cucumber beetle adults feed on leaves producing irregular-shaped holes.  Sweetpotato flea beeltes produce narrow brown tracks or grooves, while the Systena flea beetles characteristically make small circular shot-holes on the leaves.

Morphology

Wireworms

The larvae are slender, 1 to 4 cm long, with a hard, often shiny and distinctly segmented cylindrical to flattened body.  They can vary in colour from cream to yellowish to red-brown, usually with darker brown heads and tails.  Their three pairs of short legs on segments behind the head are barely visible from the dorsal view.  They have bristly hairs at the sides of body segments, and a flattened tail segment that is typically forked or notched in the centre and serrated around the margin. 

Adults are typically small (less than 1 cm) elongated oval-shaped beetles with distinctly segmented antennae, pitted bodies and may have various markings on the elytra (wing cover).  They are referred to as "click-beetles" because of their ability to "click" themselves upright when placed on their backs.

Cucumber beetles

Eggs are oval, orange-yellow colour and found in clusters of 25-50 on the underside of host leaves.

The larva is a soft-bodied cylindrical grub, about 12-20 mm long when mature.  It is cream to yellowish with a dark head and six brown legs.

The adult beetles are around 5-6 mm long with domed, smooth, shiny elytra carrying distinctive markings.  The 12 spotted cucumber beetle has a yellow to greenish body with black spots on the elytra, and black head and legs.  The banded cucumber beetle is greenish with a red head and broad yellow bands across its elytra.

Flea beetles

The flea beetle species that feed on sweetpotatoes (both foliage and roots) are the pale-striped flea beetle (Systena blanda), the elongated flea beetle (Systena elongata), and the sweetpotato flea beetle (Chaetocnema confinis). They are small beetles around 2 mm long, and have strong rear-pointing back legs with enlarged femurs, with which they jump like a flea when disturbed.

The creamy coloured eggs are laid in the soil.  The larvae are white, soft-bodied and grow to around 9 mm long.  They have brown heads and a fleshy pointed tail.


Biology and ecology

Wireworms

Most species of wireworm feed on decaying plant material.  Relatively few species feed on living plants (feeder roots as well as storage roots and tubers).  These may be pests in a range of crops, particularly affecting seedling establishment of cereal and vegetable crops.

Larvae are most abundant in the upper layers of the soil, with younger instars concentrated in the upper 10 cm and older instars often found somewhat deeper, to around 30 cm.  They are highly mobile in the soil, and don't stay at one feeding site for prolonged periods.

Species differ widely in the length of their lifecycle (1 to 7 years) and the number of larval instars (around 4 to 15).  For multiple-year species, larval instars of varying age can be found all year.  The final larval instar is longer-lived than the earlier ones, and is the most damaging.  Pupae form in earthen cells in the soil near the food source.  Pupation may take one to three weeks, but emergence may be delayed if soil is dry.  Adults generally hide under leaf litter or soil clods during the day.  They do not feed on the crop.

Cucumber beetles

Larvae of both the banded cucumber beetle (Diabrotica balteata) and the spotted cucumber beetle (Diabrotica undecimpunctata howardii Barber) feed on the storage roots of sweetpotatoes.

Like click-beetles, adults lay eggs near the base of plants, often under fallen leaves.   The larval stage lasts from eight to 30 days, and pupae are found in earth cells just below the soil surface.  There are two to three generations per year.

Flea beetles

Eggs are laid in the soil near the base of host plants.  Larvae feed for about three weeks.  The whole life cycle takes around 30 days in warm weather, and there are typically several generations per year.

Host Range

Wireworms have a very broad host range, including many weeds grasses, cereal and vegetable crops.

Cucumber beetles prefer cucumber, melons, pumpkin and squash but may feed on a wide range of crops, including corn, peanuts, potato and wild grasses.

Particular species of flea beetle tend to specialize on one or a few host plants, but preferences differ with each species.

Detection and inspection

Wireworms may be quite difficult to find in soil samples and in damaged roots.  The most reliable method for detecting them and determining population density is through the use of baits, such as buried pieces of storage root or germinated grains left in the ground for 1 to 3 weeks, the optimum time depending on species. 

Adult click-beetles are only active at night and shelter under plant debris or soil clods during the day.  They have been collected by inspecting under clods in bare fields before planting, or using black light traps placed above the crop.  Adult flea beetles and cucumber beetles are active during the day, and can be collected using sweep nets.

Management

Cultivars vary in their susceptibility to attack by soil insects, and use of resistant cultivars is the most reliable method of damage control.  Some cultivars show parallel resistance to weevils, but this is not always linked.

Controlling weeds along borders of the field and plowing under crop debris destroys overwintering and egg laying sites.  Repeated cultivation of the fields before planting will help remove alternate food sources and expose the larvae to birds and other predators.

Waterlogging of soil will reduce the insect population.

Soil-applied insecticides are usually effective in reducing damage, but do not eliminate the pest.

Experimental control methods, using parasitic nematodes, have been successful in controlling damage.

 

References

 

Chalfant, R. B., Bondari, K., Sumner, H. R., Hall, M. R. (1993). "Reduction of Wireworm (Coleoptera, Elateridae) Damage in Sweet-Potato with Insecticides Applied by Chemigation." Journal of Economic Entomology 86(1): 123-130.

Chalfant, R. B., Hall, M. R., Johnson, A. W., Seal, D. R., Bondari, K. (1992). "Effects of Application Methods, Timing, and Rates of Insecticides and Nematicides on Yield and Control of Wireworms (Coleoptera, Elateridae) and Nematodes (Tylenchida, Heteroderidae) That Affect Sweet-Potato." Journal of Economic Entomology 85(3): 878-887.

Fronk, W. D. and L. E. Peterson (1956). "Wireworm Control in Iowa Sweet Potato Fields." Journal of Economic Entomology 49(4): 479-481.

Horne, P.A and Horne, J.A (1991). The Life-History and Control of Hapatesus hirtus Candeze (Coleoptera: Elateridae) in Victoria. Australian Journal of Agricultural Research 42, 827-34.

R.P Griffin and J. Williamson, Clemson Extension Home & Garden Information Center. Clemson University. http://hgic.clemson.edu/factsheets/HGIC2215.htm

 

E. R. Morgan, L. Gianessi and M. Marcelli, Crop Profile for Sweetpotatoes in Mississippi.  Mississippi State University Pesticide Impact Assessment Program and National Center for Food and Agricultural Policy. http://pestdata.ncsu.edu/cropprofiles/docs/mssweetpotatoes.html

Schalk, J. M., Bohac, J. R., Dukes, P. D., Martin, W. R. (1993). "Potential of Nonchemical Control Strategies for Reduction of Soil Insect Damage in Sweet-Potato." Journal of the American Society for Horticultural Science 118(5): 605-608.

Seal, D. R. and R. B. Chalfant (1994). "Bionomics of Conoderus-Rudis (Coleoptera, Elateridae) - Newly Reported Pest of Sweet-Potato." Journal of Economic Entomology 87(3): 802-809.

Seal, D. R., Chalfant, R. B., Hall, M. R. (1992). "Effectiveness of Different Seed Baits and Baiting Methods for Wireworms (Coleoptera, Elateridae) in Sweet-Potato." Environmental Entomology 21(5): 957-963.

Seal, D. R., Chalfant, R. B., Hall, M. R. (1992). "Effects of Cultural-Practices and Rotational Crops on Abundance of Wireworms (Coleoptera, Elateridae) Affecting Sweet-Potato in Georgia." Environmental Entomology 21(5): 969-974.

Sorensen, K.A. 1993. Cucumber beetles.  Vegetable Insect Note 25, North Carolina Cooperative Extension Service, North Carolina Stat University. http://www.ces.ncsu.edu/depts/ent/notes/Vegetables/

 

Contributed by: Jane O'Sullivan

 

Taxonomy

Economic importance

Geographical distribution

Damage

Morphology

Biology and ecology

Host Range

Detection and inspection

Management

References

Wireworm damage is often described as "shot-holes" (E. Coleman).

 

A wireworm (Hapatesus hirtus) found in the soil at harvest (E. Coleman).

 

A wireworm (Conderus sp.) (Texas T&M University).

 

An adult click-beetle (Conderus rudis) (D.R. Seal and R.B. Chalfant)

 

12 spotted cucumber beetle (Diabrotica undecimpunctata howardii) larva, pupa and adult (K.A. Sorensen, NCSU)

 

Banded cucumber beetle (Clemson University / USDA).

 

A sweetpotato flea beetle (University of Georgia).