Key to Potato Pests
Irish potatoes are attacked by most of the insects which infest closely related solanaceous plants like tomato, eggplant, and pepper. However, since potatoes are grown for their edible tubers, they must receive greater protection from soil-inhabiting pests. Wireworms, tuberworms, white grubs, and vegetable weevils are pests for which growers should watch.
A. Pests that feed externally on the upper plant
- Chewing pests that make holes in leaves
- Blister beetles – Several species of slender, elongate beetles up to 19 mm long; have prominent heads; bodies variously colored but usually black, black with yellow margins, or black and yellow striped (Figure 1); stringy black excrement on heavily infested plants; foliage ragged; plants sometimes stunted
- Colorado potato beetle – Yellowish-brown, oval, convex beetle up to 14 mm long with 5 black longitudinal stripes on each wing cover and several black spots on pronotum (area behind the head) (Figure 2); feeds on leaves and terminal growth
- Flea beetles – Various species of tiny, darkly colored beetles 2.5 to 4.5 mm long; solid-color body or dark with pale yellow stripe on each wing cover (Figure 3); tiny round holes in foliage
- Sap-sucking pests that cause discoloration, deformation, or abscission
- Aphids – Soft-bodied, pear-shaped insects with a pair of dark cornicles and a cauda protruding from the abdomen; may be winged or wingless - wingless forms most common; feed in colonies; cause discoloration or mottling of the foliage; often transmit virus diseases; excrete honeydew on which sooty mold grows
- Green peach aphid – Pale yellow to green, wingless adult up to 2.4 mm long; winged adult with dark dorsal blotch on yellowish-green abdomen; cornicles over twice as long as cauda and slightly swollen toward tip; yellow-green nymphs with 3 dark lines on abdomen (Figure 4)
- Potato aphid – Adult and nymph both solid pink, green and pink mottled, or light green with dark stripe; adult up to 3.5 mm long; long slender cornicles about twice as long as cauda (Figure 5)
- Potato leafhopper – Spindle-shaped pest, up to 3 mm long; green body with yellowish to dark green spots (Figure 6); usually jumps instead of flies; extracts sap from underside of leaf causing leaf to crinkle and curl upward; also causes yellowing of leaf tips and margins (hopperburn)
- Aphids – Soft-bodied, pear-shaped insects with a pair of dark cornicles and a cauda protruding from the abdomen; may be winged or wingless - wingless forms most common; feed in colonies; cause discoloration or mottling of the foliage; often transmit virus diseases; excrete honeydew on which sooty mold grows
B. Pests that feed on underground plant parts, bore into stems, or mine in leaves and petioles
- European corn borer – Cream to light pink caterpillar with reddish-brown to black head; body up to 26 mm long with several rows of dark spots; has 3 pairs of legs near head and 5 pairs of prolegs (Figure 7); bores into stem; tangled frass and silk near entrance hole; stem may break, plant may yellow or wilt readily
- Potato tuberworm – Creamy white (young) or green to pink (mature) caterpillar up to 19 mm long with dark brown head; 3 pairs of legs near head and 5 pairs of prolegs (Figure 8); mines in older lower leaves causing grayish papery blotches; tunnels in exposed tubers or those close to soil surface, filling tunnels with excrement; entrance holes usually near the eyes and surrounded by a pink coloration
- Vegetable weevil and larva – Adult - dull grayish-brown weevil, about 6.4 mm long, with short, stout snout and light V-shaped mark on wing covers (Figure 9A); larva - pale green, legless, up to 10 mm long, with dark mottled head (Figure 9B); both feed at night making large open holes in surface of potatoes. Feeds primarily at night on underground plant parts, but sometimes consumes buds and foliage
- White grubs – Several species of creamy white grubs with a distinct brown head capsule, three pairs of legs near the head, and slightly enlarged abdomen; body C-shaped and up to 50 mm long (Figure 10); sever roots and stems of potatoes; leave large, shallow circular holes in tubers
- Wireworms -– Several species of slender, wire-like larvae with three pairs of short legs near the head and a pair of prolegs at the tip of the abdomen; large shallow cavities in sweet potatoes - evidence of early injury; deep ragged holes - later injury
- Melanotus communis – Yellowish-brown with darker head; body up to 25 mm long; last abdominal segment with scalloped edges (Figure 11A)
- Southern potato wireworm – Cream colored or yellowish-gray with reddish-orange head; body up to 17 mm long; oval notch in last abdominal segment (Figure 11B)
- Tobacco wireworm – White with brown head; body up to 19 mm long; V-shaped notch in last abdominal segment (Figure 11C)
Blister beetle, Epicautaspp., Meloidae, COLEOPTERA
DESCRIPTION (several species)
Adult – Blister beetles are slender insects 12 to 19 mm long. They have prominent heads and may be black with yellow margins or black and yellow striped.
Egg – The yellow cylindrical eggs are 1.3 to 1.8 mm long.
Larva – Each of the seven larval instars differ in size, shape and color. They can be 2.5 to 13 mm long, slender to plump, and white to yellow or brown. All instars have three pairs of short ventral legs and 12 body segments, excluding the head.
Pupa – The white, 10-mm-long pupae darken gradually beginning with the eyes.
Distribution – Blister beetles are found throughout the continental United States and agricultural areas of Canada. Although fairly common in North Carolina, they are infrequently pests of importance.
Host Plants – Blister beetles have a wide host range. Important vegetable hosts include potato, tomato, melon, eggplant, sweet potato, bean, pea, cowpea, pumpkin, onion, spinach, beet, carrot, pepper, radish, corn, and cabbage.
Damage – Some species of blister beetles feed on flowers but most species are strictly foliage feeders. This latter group feeds gregariously, occasionally damaging foliage and stunting plant growth. Black stringy excrement often is found on heavily infested plants. Blister beetles also have been known to transmit the disease organism which causes southern bacterial wilt of potatoes. Larvae, on the other hand, are considered beneficial insects because they feed on grasshopper eggs.
Life History – Blister beetles have an unusual life cycle. They usually overwinter as sixth instar larvae 2.5 to 4 cm deep in the soil. In spring, resting larvae molt into active nonfeeding larvae which soon pupate. Adult blister beetles begin to emerge in June. Adults can be found well into September but are most abundant in July. During summer months, they congregate and feed voraciously on foliage or flowers (depending upon the particular species of beetle). Two to 3 weeks after mating, each female deposits up to six egg masses in the soil. These masses may contain 50 to 300 eggs apiece. Active larvae hatch from the eggs 11⁄2 to 3 weeks later and search for grasshopper egg cases. A few days after locating and feeding on the eggs, the active larvae molt and become fairly inactive. The grubs continue to feed on the eggs and molt until they are fat, almost legless fifth instars. These larvae create oval hibernating chambers in the soil, molt into sixth instars, and overwinter. Development usually continues the following spring but the larvae may remain inactive for as long as two years. Sometimes fifth instars molt directly into the pupal stage, by-passing the last two larval instars. As a general rule, however, blister beetles complete one generation each year.
Blister beetles often appear suddenly and may cause much damage before they are detected. Therefore, insecticides are generally applied as an emergency measure after beetles are found on a crop. Control failures usually are attributed to applying insecticides too sparingly, too late. Spot treatment is usually adequate. For recommended insecticides and rates, consult the current North Carolina Agricultural Chemicals Manual.
Colorado Potato Beetle
Colorado potato beetle, Leptinotarsa decemlineata (Say), Chrysomelidae, COLEOPTERA
Adult – This oval, convex beetle is yellowish-brown and about 9 to 14 mm long. It has five longitudinal black stripes on each wing cover and a variable number of black spots on the pronotum (area just behind the head).
Egg – The yellow or orange elongated eggs are deposited on end and grouped into rows (Figure KK). Each egg is about 1.8 mm long.
Larva – Red at first, this soft grub has a black head and black legs. As it matures, the larva turns yellowish-red or orange and develops two rows of black spots along each side of the body. It reaches a length of about 10 mm.
Pupa – Generally resembling the adult in shape, the pupa is approximately 13 mm long.
Distribution – The Colorado potato beetle can be found throughout most of North America.
Host Plants – Colorado potato beetles infest a wide variety of plants including tomato, potato, eggplant, pepper, tobacco, ground cherry, nightshade, and other solanaceous plants.
Damage – Adult beetles and larvae feed on leaves and terminal growth of their host plants. The loss of foliage hinders development of tubers or fruit thereby reducing yield. In cases of heavy infestation, entire plants may be killed. Colorado potato beetle damage often occurs in isolated spots throughout the field.
Life History – Colorado potato beetles overwinter as adults in the soil. After emerging in spring, beetles feed for a short period before mating and laying eggs. Females each deposit 300 to 500 eggs in clusters of 20 or more on the undersides of leaves. Four to 9 days later, larvae emerge and feed for the next 3 weeks. Once mature, larvae drop to the ground and pupate in the soil. Five to 10 days later, a new generation of beetles emerge. In North Carolina, at least two full generations and a partial third occur each year.
Many cultural enemies help keep Colorado potato beetle populations low. Birds feed upon adults and larvae while predatory bugs attack eggs and larvae. These predatory bugs may be gray, brown, or brightly colored and are often shield-shaped. Two kinds of gray and black tachinid flies also parasitize larvae.
Katahdin potatoes show some resistance to Colorado potato beetles. Early treatment of commercially grown potatoes with systemic insecticides normally control overwintering beetles and early hatching larvae. However, some insect activity may persist around the field. The application of a foliar insecticide is not recommended until the first eggs have hatched. As soon as damage is noticed, treatment should begin. Chemical control is directed toward the first generation since the buildup of subsequent generations may cause severe damage and defoliation. In some cases, spot treatments may be effective. For recommended insecticides and rates, consult the current North Carolina Agricultural Chemicals Manual.
Palestriped flea beetle, Systena blanda Melsheimer, Chrysomelidae, COLEOPTERA
Potato flea beetle, Epitrix cucumeris (Harris), Chrysomelidae, COLEOPTERA
Tobacco flea beetle, Epitrix hirtipennis (Melsheimer), Chrysomelidae, COLEOPTERA
Adult – The potato flea beetle is about 2.5 mm long and brownish-black to black in color. The equally small tobacco flea beetle is yellowish-brown with a dark band across the wings. Varying from 2.5 to 4.6 mm in length, the palestriped flea beetle has a pale yellow, brown, or black body, a reddish head, and one light-colored stripe along each wing cover.
Egg – The tiny elongate egg is white when first deposited.
Larva – The slender cylindrical grub has a whitish body, a brown head, and 3 pairs of tiny legs near its head. Potato and tobacco flea beetle larvae are 4 to 5 mm long when fully grown. The mature larva of the palestriped species is slightly longer than 6 mm.
Pupa – The white pupa roughly resembles the adult in size and shape. As it matures, it darkens gradually.
Distribution – The potato flea beetle occurs from Maine into the Carolinas and westward into Nebraska. Although the tobacco flea beetle is fairly generally distributed, it is primarily a problem in the South. The palestriped flea beetle occurs in most areas of this country, its northern limits lying in Utah, Colorado, Idaho, and New York.
Host Plants – Potato and tobacco flea beetles infest solanaceous plants such as tomato, potato, tobacco, pepper, horsenettle, etc. The palestriped flea beetle, however, is a more general feeder. Its hosts include potato, corn, eggplant, tomato, pea, bean, watermelon, pumpkin, sweet potato, peanut, oat, cotton, grape, pear, and strawberry.
Damge – Flea beetles attack the foliage leaving small round holes. Most serious early in the growing season, this injury eventually kills infested leaves. In addition, potato flea beetles may transmit early blight. As a general rule, flea beetles are much less of a problem on potato than on other solanaceous crops.
Life History – Flea beetles overwinter as adults among debris in or near fields of host plants. They resume activity in spring and feed on weedy hosts until crop hosts are available. Eggs, deposited in soil near the bases of host plants, may require a week or more to hatch. Grubs feed on or in roots, tubers, and lower stems for 3 to 4 weeks before pupating. After a pupal period of 7 to 10 days, a new generation of beetles emerges. The palestriped flea beetle completes only one generation each year. Potato and tobacco flea beetles produce three to four annual generations in North Carolina.
Cultural methods are primary sources of defense against flea beetle infestations. First, it is important to keep fields free of weeds. Destruction of plant residues, especially piles of cull potatoes and trash where beetles hibernate, prevents the buildup of high populations. Late planting favors growth of the host plant over establishment of flea beetles. Lastly, covering beds of seedlings with a gauze-like material prevents beetle entry.
A number of insecticides (granular and foliar) are available to control adult flea beetles. On potatoes, an in-furrow insecticide application at planting can prevent flea beetle damage early in the season. For control throughout the season on all vegetable crops, spray plants when adults appear and repeat as needed. For recommended insecticides and rates, consult the current North Carolina Agricultural Chemicals Manual.
No common name, Melanotus communis Gyllenhal, Elateridae, COLEOPTERA
Adult – This wireworm adult is a hard, slick, reddish-brown to black click beetle about 13 mm long.
Egg – The white, glistening egg is oval to spherical in shape and 0.33 mm long.
Larva – A wireworm larva has 3 pairs of short legs near its head. M. Communis has a pale yellow to reddish-brown body, a brown, flattened head, and a scalloped last abdominal segment. When fully grown, this wireworm ranges from 21 to 25 mm in length.
Pupa – The white, soft-bodied pupa has no protective covering and is approximately the same size and shape as the adult.
Distribution – This wireworm species can be found throughout the United States, but is particularly common in the mid-western and southeastern states. Damage tends to be prevalent in fields which follow sod or no-till corn.
Host Plants – The wireworm, M. Communis, feeds on the roots of many grasses, including corn and many small grain crops. It may also attack the roots, seeds, and tubers of many flower and vegetable crops, especially potatoes and sweet potatoes. This species has been known to infest tobacco.
Damage – This wireworm creates holes in potato and sweet potato roots similar to those caused by the tobacco and southern potato wireworms. However, due to the size of this species, the holes are noticeably larger and deeper than those of other species. Damaged roots and tubers are downgraded and discarded. M. Communis is the most damaging wireworm in the more northern sweet potato-growing areas.
Life History – This wireworm species has a 6-year life cycle. In June of the first year, adults deposit eggs singly among the roots of grasses. First instar larvae emerge in July and begin feeding on roots. Larvae continue to develop throughout the summer and overwinter in the ground as second instars during the first year. Most of these immatures remain in the larval stage for 5 years although life cycles as short as 3 years have been reported. In late July or August of the 6th year, mature larvae construct oval cells 15 to 30 cm deep in the soil and pupate. M. communis adults emerge about 18 days later and feed on pollen before hibernating in protected areas. They become active and deposit eggs the following May or June.
Control consists of avoiding land previously in sod or out of production. The preplant use of fumigants for nematode control 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 this wireworm species.
Potato leafhopper, Empoasca fabae (Harris), Cicadellidae, HEMIPTERA
Adult – Because many species of leafhoppers look alike, entomologists studying these insects must rely heavily on examination of internal genital structures, as well as external characters, to distinguish various species. About 3 mm long and generally spindle-shaped, the potato leafhopper has a green body with very small yellowish, pale, or dark green spots. Though it can fly, the leafhopper usually jumps when disturbed.
Egg – About 1 mm long, the white egg is elongate in shape.
Nymph – Several nymphal stages exist, each of which is wingless and smaller than the adult. Though paler, the nymph is colored similarly to the adult.
Distribution – During summer, potato leafhoppers are found from the Atlantic coast to the Rocky Mountains. They are absent throughout most of the winter months which they spend in the Gulf states. Leafhoppers migrate into North Carolina in early summer and become established on a wide range of plants throughout the growing season.
Host Plants – Potato leafhoppers feed on more than 200 cultivated and wild plants. In addition to fruit trees and forage crops, vegetables such as bean, potato, eggplant, and rhubarb are also subject to infestation.
Damage – Potato leafhopper nymphs and adults feed on the undersides of leaves. By extracting sap, they cause stunting of plants, curling of leaf margins, and crinkling of the upper surfaces of leaves. While feeding, leafhoppers also inject a toxic substance into plants which, in most vegetable hosts, causes a condition known as "hopper-burn." This disease is characterized by a yellowing of the tissue at tips and margins of leaves which increases until the leaves die. Symptoms of leafhopper damage are sometimes confused with drought stress.
Life History – Potato leafhoppers winter in the Gulf States and migrate northward each year on spring winds. They arrive in North Carolina by midsummer. Three to 10 days after mating, females use their sharp ovipositors to thrust eggs into the main veins or petioles of leaves. Each female leafhopper lives a month or more and produces an average of two to three eggs daily. Eggs hatch in about 10 days, and the nymphs mature in about 2 weeks. Nymphs usually develop on the leaves where they hatched from the eggs. They molt five times before becoming adults. Mating occurs approximately 48 hours after maturation, and the life cycle is repeated. Three or four generations are produced each year in North Carolina.
The potato variety Delus is highly resistant to potato leafhoppers. The use of systemic insecticides at planting or contact insecticides as necessary throughout the season will adequately control these pests. For recommended insecticides and rates, consult the current North Carolina Agricultural Chemicals Manual.
Potato tuberworm, Phthorimaea operculella (Zeller), Gelechiidae, LEPIDOPTERA
Adult – The small, slender moth has narrow gray forewings with dark brown spots. The hind wings are yellowish-brown. Both sets of wings are fringed - the forewings near the base and the hind wings along the lower margin. The moth's body is about 8 mm long and its wingspan about 13 mm. The female is slightly larger than the male.
Egg – The oval egg is about 0.5 mm long. Pearly white when first deposited, it gradually turns yellow and finally brown.
Larva – The larva, upon hatching, is creamy white with a dark brown head and prothorax (area immediately behind the head). The larva varies from green to pink as it matures and just before pupation takes on a purplish cast. The larva is 13 to 19 mm long when fully grown.
Pupa – White with green blotches at first, the spindle-shaped pupa soon turns brown. It is about 8 mm long and enclosed in a flimsy, white silken cocoon.
Distribution – The potato tuberworm is a cosmopolitan pest, occurring in most areas where potatoes or other solanaceous plants are grown. It occurs in at least 25 states from the Atlantic to the Pacific coasts.
Host Plants – The tuberworm generally attacks potato foliage and tubers, but will also feed on tobacco, tomato, eggplant, pepper, jimsonweed, nightshade, and horsenettle.
Damage – Potato tuberworm larvae act as miners of leaves, petioles, and stems of the above crops and as borers in potato tubers and, occasionally, tomato fruits. Tuberworms feed and tunnel between upper and lower surfaces of leaves causing grayish papery blotches which become brownish and very brittle. Such injury is usually concentrated on older, lower leaves.
The caterpillars enter tubers near the surface of the ground by moving through cracks in the soil. Tubers covered with at least 5 cm of soil are not subject to infestation. Tubers exposed at harvest often are infested soon afterwards by larvae moving from the foliage. Therefore, infestations are likely to increase in areas where cull potatoes are allowed to remain in the field following the harvesting of the spring crop.
Larvae tunnel through potatoes, filling tunnels with excrement and webbing on which disease-causing fungi grow. Such potatoes are unsightly and of little food value. Larvae usually enter tubers near eyes, covering the small entrance holes with webs and excrement. Infestations are more evident a few days later when a pink coloration develops in the flesh around entrance holes. More excrement also is present. Larvae cut galleries 2.5 to 7.5 cm (1 to 3 in) long in the tubers, either just beneath the skin or deep in the flesh.
Life History – Potato tuberworms may overwinter as larvae or pupae in the soil or in potatoes that are not subjected to freezing temperatures. Cull dumps, storage houses, and cellars are all suitable hibernation sites. The weak-flying moths emerge in spring and dart from plant to plant when disturbed. They are most active at dusk and dawn. Each female deposits, singly, 60 to 200 eggs in 4 days or less. Eggs are usually deposited on rough surfaces such as potato tuber eyes or the hairy leaf undersurfaces. Hatch occurs 3 to 6 days later, depending on temperature. Larvae feed and mature in 7 to 10 days under ideal summer conditions, but take longer at cooler temperatures. When fully grown, larvae leave their host and pupate in the soil near the bases of plants, in leaf remains, or in some other suitably sheltered site. A new generation of moths emerges in 6 to 9 days. Five or 6 generations occur each year.
Preventive measures usually are effective in controlling the potato tuberworm. Some important control practices include: 1) keeping potatoes well hilled so there are always at least 5 cm of soil over the tubers, 2) cultivating or irrigating to prevent deep cracks in soil, 3) planting fall potatoes as far as possible from the location of the spring crop, 4) destroying cull potatoes, perhaps by feeding them to livestock, 5) eradicating volunteer plants early in spring, 6) harvesting potatoes soon after maturity and removing them from the field immediately after digging, 7) storing tubers, when possible, at temperatures below 10°C (50°F), 8) screening storage area from egg-laying moths, and 9) fumigating or steam cleaning sacks before they are reused. Practices to avoid include planting infested seed pieces, covering newly dug potatoes with vines, and leaving piles of potatoes out overnight when egg-laying moths are active.
Tuberworms are rarely a problem in fields where rigid pesticide schedules are followed. Once an infestation develops in a field, chemical treatment should begin at once and be repeated until the pest is controlled. For recommended pesticides and rates, consult the current North Carolina Agricultural Chemicals Manual.
Southern Mole Cricket
Southern mole cricket, Scapteriscus acletus Rehn & Hebard, Gryllotalpidae, ORTHOPTERA
Adult – The beady-eyed adults are about 32 mm long, brown, and covered with fine short hairs. They have short front wings, long and membranous hind wings which fold under the forewings, and short, broad, shovel-like front legs for digging.
Egg – Eggs are oval and about 32 mm long.
Nymph – Nymphs are similar in appearance to adults but are smaller and wingless.
Distribution – The southern mole cricket occurs from North Carolina to Texas. In North Carolina it is more prevalent in the Coastal Plain.
Host Plants – Nymphs and adults tunnel in the soil and feed on decomposing organic matter and roots. By tunneling, mole crickets injure tobacco seedlings, garden vegetables, peanuts, strawberries, and grasses. Severe damage may be done to the roots and tubers of potatoes, turnips, and sweet potatoes.
Damage – The southern mole cricket is one of several species of mole crickets which injury young plants by tunneling in the soil and feeding on roots. Seedlings may be uprooted by the tunneling activity of mole crickets. Heavily tunneled soil dries out quickly causing further stress to plants.
Life History – The southern mole cricket overwinters as a nymph or adult, migrating downward in the soil during cold weather. In spring, eggs are laid in the soil in cells constructed by the females. About 35 eggs are placed in each cell. Hatching occurs in 10 to 40 days depending on temperature. Nymphs develop through 6 or 7 molts and may become adults by winter or may overwinter as immatures. One generation occurs per year.
Mole crickets can be controlled by applying treatments before planting. For specific chemical recommendations, consult the current North Carolina Agricultural Chemicals Manual.
Southern Potato Wireworm
Southern potato wireworm, Conoderus falli Lane, Elateridae, COLEOPTERA
Adult – The adult, 6 mm to 8.5 mm long, is a brownish, oblong click beetle. Its legs are light tan.
Egg – The spherical egg is smooth and translucent white with an average diameter of 0.5 mm.
Larva – The newly hatched larva is white and later becomes cream colored or yellowish-gray with a reddish-orange head. The fully grown larva is about 17 mm long. 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 but soon changes to a creamy yellow.
Distribution – The southern potato wireworm was apparently introduced into the United States. from South America. In this country, it has been reported on coasts from North Carolina to Louisiana. Within North Carolina, it occurs mainly in the southeastern counties of the Coastal Plain.
Host Plants – The southern potato wireworm appears to prefer potato tubers. Newly transplanted tobacco seedlings, roots of sweet potatoes, carrots, corn seedlings, and stems of tomato transplants are also frequently attacked. Less frequently damaged hosts are melons, the roots of beets, and the fruits of strawberries, cantaloupes, watermelons, and tomatoes that touch the soul surface.
Damage – Wireworms chew ragged holes on the roots. Oftentimes a single root may have 10 or more small holes. Early feeding appears as shallow but large cavities. Late or most recent feeding appears as ragged, deep holes. This wireworm usually attacks sweet potatoes late in the season.
Life History – While the biology of this insect pest has not been studied in North Carolina, in South Carolina adults are found in fields throughout the year. 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 in September. They mate and lay eggs of the "long-cycle" brood, or the overwintering generation, which requires 239 to 318 days for the eggs to reach adulthood.
No insect parasites or predators of this wireworm have been discovered. 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.
As a cultural control, susceptible crops should not be planted in fields that were planted with a winter crop, those not plowed during the fall and winter, or those not recently in row crops. No resistance to this pest has been found in Irish potatoes. However, the sweet potato varieties Nugget and All Gold do possess some resistance.
Insecticides for the control of wireworms can be applied in furrow at planting, broadcast and incorporated into the soil, or broadcast later over the top of sweet potato foliage. A problem has been this wireworm's development of resistance to chlorinated hydrocarbons and organophosphate insecticides. Therefore, for specific information or insecticides and rates, consult the current North Carolina Agricultural Chemicals Manual.
Tobacco wireworm, Conoderus vespertinus (Fabricius), Elateridae, COLEOPTERA
Adult – The adult, called a click beetle, is reddish-brown with yellow markings, oblong, and about 8.5 mm in length, though the size varies considerably.
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 and grows to a length of 14 to 19 mm. Except for the head, which is tinged iron brown, the larva is white. Its last abdominal segment 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.
Distribution – The tobacco wireworm is common in the southeastern states. In North Carolina it occurs throughout most of the Coastal Plain. It is much more prevalent in areas where tobacco, cotton, or corn are the main crops than in areas planted chiefly with truck crops.
Host Plants – The tobacco wireworm apparently prefers tobacco, but it feeds on a variety of other plants including cotton, corn, potatoes, sweet potatoes, and various truck crops.
Damage – Damage occurs as ragged holes in the roots. Oftentimes a single root may have 10 or more small holes. Early feeding appears as shallow but large cavities. Late or most recent feeding appears as ragged, deep holes. Damaged roots are downgraded or discarded.
Life History – Eggs, averaging 240 per female, are laid singly on, or slightly beneath, the soil surface in summer. Larvae hatch and feed on roots of corn, tobacco, potato and other plants. Winter is passed in the larval stage. Pupation in June occurs in the soil. Adults emerge during early summer with 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: egg, 10 days; larva, 315 days; pupa, 10 days; and preoviposition period, 13 days.
Crop rotation is an effective management tool for control and should be practiced where possible. Fields planted to a winter cover crop, those not plowed during fall and winter, and those not recently in row crops are not suitable sites to plant wireworm-susceptible crops. Low soil moisture, high summer temperatures, disease, predation and cannibalism are helpful in reducing wireworm populations.
Insecticides for the control of wireworms can be applied in furrow at planting, broadcast, and incorporated into the soil or broadcast later over the top of sweet potato foliage. For specific information on insecticides and rates, consult the current North Carolina Agricultural Chemicals Manual.
White grubs, Phyllophaga spp., Scarabaeidae, COLEOPTERA
Adult – Known as May beetles, the shiny reddish-brown to black adults are 19 to 26 mm long.
Egg – A dull pearly white when first deposited, the oval to spherical egg turns dark just before hatching. It may be 1.5 to 3 mm in diameter. Small masses of 15 to 20 eggs occur in cells in the soil.
Larva – The young grub is creamy white and about 5 mm long. The grub is about 26 mm long, and the mature grub about 30 mm long. The C-shaped grub has a distinct brown head; a shiny, smooth body; and three pairs of legs just behind the head. Two rows of hairs on the underside of the last segment distinguish May beetle grubs from similar grubs.
Pupa – Approximately the same size as the adult, the pupa may be creamy white, pale yellow, or dark brown.
Distribution - More than 200 species of white grubs are found throughout North America. During the summer, these beetles are often seen flying around lights at night. Populations of grubs tend to be highest in older plantings of sod or soils high in decomposing organic matter. In North Carolina, they occur in all counties but are most numerous from the Piedmont to the coast.
Host Plants - White grubs attack the roots of many cultivated crops as well as pasture, field grasses, and nursery plants. Corn, sorghum, soybean, strawberry, potato, barley, oat, wheat, rye, bean, turnip, and tobacco are a few of their common hosts. Adult beetles are strongly attracted to fragrant flowers and ripe fruits.
Damage - White grubs are among the most destructive soil insects in North America. In addition to severing roots and stems of potatoes, white grubs feed on tubers, leaving large shallow circular holes in them. The infested plants often do not show symptoms on above-ground plant parts. As a result, considerable damage may be done before the grub problem is discovered. In heavy infestations, the soil may become soft and fluffy due to grub movement.
Life History - In spring, overwintering adults emerge from the ground at dusk, feed on the leaves of trees, and mate during the night. At dawn, they return to the ground where females lay 15 to 20 pearly white eggs in cells several centimeters below the surface. Eggs hatch in 3 to 4 weeks. Newly hatched grubs feed on plant roots throughout the summer and complete 1⁄3 of their development before fall. These grubs burrow below the frostline (to a depth of 1.5 meters) and hibernate.
The following spring, May beetle grubs return near the soil surface and resume feeding. They continue to feed and grow throughout the growing season and overwinter again the second year. The grubs become fully grown by late spring of the third year. At this time, they dig cells in the soil and pupate. Pupae become adults by late summer but the beetles do not leave the ground. New May beetles overwinter in their earthen cells and emerge the following spring to feed and mate. White grubs complete one generation every 3 years.
Several cultural practices help reduce white grub populations. Late summer or early fall plowing may expose larvae, pupae, or even adults to predaceous birds. Crop rotation, however, is the most effective control method. Corn and potatoes should be rotated with resistant or less susceptible crops like clovers or other legumes. These crops should never follow grasses in a rotation, especially in years following a heavy beetle flight.
Chemical control is rarely necessary for white grubs in vegetable crops. Should a serious infestation develop, consult the current North Carolina Agricultural Chemicals Manual.
Publication date: Feb. 10, 2003
Other Publications in Insect and Related Pests of Vegetables
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