Insect Note #24 (Revised)

Dated 8/93
Placed on the Web 4/95 by the Department of Entomology, NCSU
Revised: 03/05

Wireworms on Sweetpotatoes

Although wireworms are present throughout NC, comparatively few fields suffer economic damage. Wireworm abundance is greatly dependent upon such factors as soil type, rotation, drought, previous crop, insecticide program, and other cultural practices. An assessment of the need to treat must be based on the above factors, knowledge of the life history of wireworms and previous experience. One can also monitor adult click beetle flights with a black light insect trap or use soil baits of corn or wheat to detect larvae present. Bait may be placed 6 inches in the soil at 6 or so locations in the field in early Spring and checked for insect species and numbers 3 weeks later.

Tobacco Wireworm

Biology - The eggs, averaging 240 per female, are laid singly on or slightly beneath the soil surface in the summer. Larvae hatch and feed on roots of corn, tobacco, potatoes and other plants. The winter is passed in the larval stage. Pupation in June occurs in the soil. Adults emerge during early summer with the greatest activity from late June through July. There is only one generation per year. The typical life cycle requires about 348 days in North Carolina as follows: egg, 10 days; larva, 315 days; pupa, 10 days, and pre-oviposition period, 13 days.

Description

Egg - The newly laid egg is spherical, white and about 0.5 mm in diameter.

Larva - The newly hatched larva is approximately 1.5 mm long. Except for the head capsule, which is tinged iron brown, the larva is white. Its last abdominal section terminates in a V- shaped notch.

Pupa - The brown pupa is slightly larger than the adult; it occurs in the soil near the food source.

Host Plants

Damage

Distribution

Control

1. No insect parasites or predators of this wireworm have been discovered.

2. Three disease-causing agents - a fungus, a protozoan and a parasitic nematode - have been isolated, but their usefulness in the control of this wireworm has not been determined.

3. Avoid fields planted to a winter crop, not plowed during the fall and winter and those not recently in row crops.

4. To control wireworms on sweetpotatoes, apply a granular insecticide broadcast over the foliage when the roots begin to form and adult beetles are abundant (usually late July) or apply a liquid or granular insecticide preplant and incorporate. A problem has been this wireworm's development of resistance to chlorinated hydrocarbons and organophosphate insecticides.

The Southern Potato Wireworm

Biology - While the biology of this insect pest has not been studied in North Carolina, adults are found in fields throughout the year in South Carolina. There are two generations annually. Adults from overwintering larvae begin to appear in large numbers during May, reaching their peak abundance in June. Each first- generation female lays an average of 36 eggs. They hatch into the "short cycle" brood which requires 42 to 109 days to mature. Adults of this "short-cycle" brood are abundant in late August and throughout September. They mate and lay eggs of the "long cycle" brood and the overwintering generation requires 239 to 318 days for the egg to reach adulthood.

Description

Egg - The spherical egg is smooth and translucent white with an average diameter of 0.5 mm.

Larva - The newly hatched larva is white, later becoming cream colored or yellowish-gray with a reddish-orange head. The fullgrown larva may be as long as 17 mm. The last abdominal segment of this larva, unlike that of the tobacco wireworm, terminates in a closed oval notch rather than a V-shaped notch.

Pupa - Slightly larger than the adult, the pupa is white when first formed, soon changing to a creamy yellow.

Host Plants

Damage

Distribution

Control

1. No insect parasites or predators of this wireworm have been discovered.

2. Three disease-causing agents -- a fungus, a protozoan, and a parasitic nematode -- have been isolated, but their usefulness in the control of this wireworm has not been determined.

3. Avoid fields planted to a winter crop, not plowed during the fall and winter, and those not recently in row crops.

4. To control wireworms on sweetpotatoes, apply a granular insecticide broadcast over the foliage when the roots begin to form and adult beetles are abundant (usually late July) or apply a liquid or granular insecticide preplant and incorporate. A problem has been this wireworm's development of resistance to chlorinated hydrocarbons and organophosphate insecticides.

The Corn Wireworm

The corn wireworm, a large wireworm, is likely the most damaging species in the more northern sweetpotato growing areas. The injury, often extensive, is similar to that caused by other wireworms. However, the holes are large and deeper.

Not a great amount of information is known about the corn wireworm. This wireworm does spend more than one year as destructive larvae in the soil. Adult click beetles prefer to lay eggs in grassy undisturbed soil. Hence, land put out of production for several years or land previously in sod harbor this species.

Control consists of avoid land previously in sod or out of production. The preplant use of fumigants for nematodes will also provide some control of this soil inhabitant. Traditional applications of a granular insecticide over the row in late July are thought to be of little value against the corn wireworm.

Wireworm Control

Lorsban and Capture gives suppression of sweetpotato flea beetles and white grubs.

Foliar sprays of endosulfan (Thiodan, Phaser) 3EC at 1 qt/acre or Phosmet (Imidan) 70WSB at 1.33lb per acre or Phosmet (Imidan) 2.5EC at 3.25pt. per acre when used for cucumber beetles and tortoise beetles will give some control of adult click beetles present in the field. Use Bifenthrin (Capture) 2EC at 3.2oz as a sidedress incorporated in the soil and to a lesser extent as foliar sprays will give some control of wireworms, sweetpotato flea beetle, and other beetles and worms. Endosulfan can be used 3 times per year and there is a 1 day time limitation between last spray and harvest.

Wireworm Management

1. Keep records of fields where damage has been severe and avoid those fields.

2. Fields at risk: avoid fields planted to a winter cover crop, not plowed during the fall and winter and those not recently in row crops.

3. Crop rotation with tobacco is effective.

4. Control weeds in and around fields, as adult beetles feed on weed seeds.

5. Place soil baits of corn and/or wheat or oatmeal for corn wireworm in 6" holes in fields in March and check 3 weeks later. Record number and species of insects collected.

6. Monitor black light insect traps in June, July and August for adult wireworm (click beetles). Record number of species collected by date.

7. Use a soil fumigant for nematodes.

8. Use a tolerant variety. Jewel has some resistance, while Resisto and Regal have more resistance. Beauregard, Pope, Hernandez, Cordner have low resistance or none.

9. Preplant and mid-season side dress insecticide applications give best control. Foliar sprays late give little control of corn wireworm.

10. Mid-season applications of granular insecticides should be made based on adult activity, not enlargement and when one can pass through the field for last cultivation. Applications made the last week of July just prior to a significant rain or irrigation will give the best results.

11. Foliage sprays during July and August help control adult beetles.