Key to Sweetpotato Pests
The sweetpotato flea beetle is the most common pest of sweetpotatoes in North Carolina. Soil-inhabiting pests lower grades or make potatoes unmarketable and are most economically important. Though the sweetpotato weevil is the most damaging pest of this crop nationwide, it has only recently been found in North Carolina and is a potential problem here.
Many pests attack sweetpotato foliage. In the plant bed, this injury can be threatening. In the field, however, there is little evidence that foliar pests do enough damage to warrant treatment.
A. Pests that feed on aboveground plant parts
- Caterpillars with three pairs of legs and five pairs of prolegs
- Southern armyworm – Gray or nearly black larva up to 36 mm long with greenish or pinkish tint; lightly colored longitudinal stripes and paired triangular spots down back; pale yellow head capsule with bright reddish-brown markings (Figure 1); feeds on leaves, tender stems, and tips of branches; congregates round bases of plants during hot portion of the day
- Sweetpotato hornworm – First instar: white with black anal horn; later instars: green or brown with black angled marks down each side and black anal horn; body up to 90 mm long; head green or brown with black stripes (Figure 2); defoliates plants; often hides near base of plant under large leaves
- Yellowstriped armyworm – Pale gray to black caterpillar up to 45 mm long with yellow-orange stripes along each side and paired triangular spots on the back of most segments (Figure 3A); head capsule brown with black markings and a white inverted V (Figure 3B); feeds much like southern armyworm
- Potato leafhopper – Spinkle-shaped pest up to 3 mm long; green body with yellowish to dark green spots (Figure 4); usually jumps instead of flies; extracts sap from underside of leaf causing yellowing of leaf tips and margins; one of several leafhopper species attacking sweetpotato
- Small fruit or vinegar fly – Small yellowish fly about 3 mm long with red eyes (Figure 5); hovers around overripe or decaying produce; often found with small creamy maggots in the cracks of sweetpotatoes
- Sweetpotato flea beetle – Black oval beetle about 1.6 mm long with a bronze tinge, reddish-yellow legs, and deeply ridged wing covers (Figure 6); leaves narrow channels or grooves in upper surface of leaves; injured areas turn brown and die
- Sweetpotato weevil adult and larva – Ant-like snout beetles are about 6 mm long with dark blue wing covers and red-orange legs and thorax; fat, legless, dirty white larvae are about 9 mm long with pale brown head; beetle makes small holes over surface of sweetpotatoes particularly at stem end; larva tunnels inside filling tunnels with frass and causing sweetpotatoes to turn bitter. Feed on foliage, but primarily on underground plant parts
- Tortoise beetle adult and larva – Oblong-oval, basically gold-colored beetle, up to 8 mm long, with various black or red markings on its flattened, shell-like body (Figure 7A); larva with dull yellow, brown, or green body up to 12 mm long with black head, legs, spots, and spines; long spines on larval abdomen hold excrement (Figure 7B); adult and larva chew leaves leaving them riddled with holes
- Spider mites – Tiny pale or reddish spider-like arthropods feed on the bottom of leaves (Figure 8); heavily infested plants become yellowish, bronzed or burned in appearance
B. Pests that feed on belowground plant parts
- Sweetpotato flea beetle larva – Slender, white, cylindrical larva, up to 5 mm long, with 3 pairs of legs near head (Figure 9); etches shallow, winding tunnels on surface of sweetpotato roots and sweetpotatoes; tunnels darken, split, and leave scars
- Sweetpotato weevil adult and larva – Ant-like snout beetles are about 6 mm long with dark blue wing covers and red-orange legs and thorax (Figure 10A); fat, legless, dirty white larvae are about 9 mm long with pale brown head (Figure 10B); beetle makes small holes over surface of sweetpotatoes particularly at stem end; larva tunnels inside filling tunnels with frass and causing sweetpotatoes to turn bitter
- White grub (spring rose beetle) – Dirty white grub up to 25 mm long with brown head and 3 pairs of legs near head (Figure 11); leaves large, shallow feeding scars on sweetpotatoes
- Wireworms – Several species of slender, wire-like larvae with 3 pairs of short legs near the head and a pair of prolegs at the tip of the abdomen; large shallow cavities in sweetpotatoes - evidence of early injury; deep ragged holes - later injury (Figure SS)
- Melanotus communis – Yellowish-brown with darker head; body up to 25 mm long; last abdominal segment with scalloped edges (Figure 12A)
- Southern potato wireworm – Cream colored or yellowish-gray with reddish-orange head; body up to 17 mm long; closed oval notch in last abdominal segment (Figure 12B)
- Tobacco wireworm – White with brown head; body up to 19 mm long; V-shaped notch in last abdominal segment (Figure 12C)
- Whitefringed beetle larvae – These yellowish-white legless, 12-segmented grubs, up to 13 mm in length, have small, pale heads. They gouge on roots, reducing marketable sweetpotatoes
Small Fruit Flies
Small fruit flies, Drosophilus spp., particularly Drosophila melanogaster Meigan, Drosophilidae, DIPTERA
Adult – About 3 mm long, these flies have red eyes and yellowish bodies with dark bands. Though commonly referred to as fruit flies, they are more correctly termed small fruit flies, vinegar flies, or pomace flies.
Egg – The tiny white elongate eggs are only about 0.5 mm long and have 2 slender filaments near the head end. Though individual eggs are too small to be easily noticed, clusters of eggs often resemble white mold on the surface of produce.
Larva – The cream colored maggots develop through three instars. They are about 5 mm long when fully grown.
Pupa – Yellowish-white at first, the 3-mm-long pupae soon turn brown.
Distribution – Cosmopolitan in occurrence, small fruit flies are most likely to attain large populations around piles of overripe produce or in sweetpotato storage houses.
Feeding Habits – Drosophila flies consume yeast and bacteria associated with the initial decay of plant materials. Sap flows, mushrooms, and overripe produce are all very attractive to these flies.
Damage – Unlike real fruit flies, Drosophila flies do not break the skin of sound fruits and vegetables. They breed only in cracked or decaying overripe produce. As a result, these flies and their maggots are most likely to develop large populations in cull piles, storage houses, or processing plants.
Life History – Small fruit flies sometimes overwinter as larvae or pupae in sheltered locations with an abundance of dry fermented plant material. However, they have been known to breed throughout the winter in sweetpotato storage houses and in root cellars as far north as New Jersey. Egg laying, though, is much reduced at temperatures below 13°C (55°F) or above 38°C (90°F).
Eggs are deposited in cracked produce and incubate about 24 hours before hatching. When temperatures average 25°C (77°F), larvae feed and develop to maturity in about 4 days. Pupation then occurs within the shrunken skin of the last larval instar. About 5 days later, adult flies emerge. Within 2 days, females begin ovipositing at the rate of about 25 eggs per day. This process continues for several weeks, each female eventually depositing an average of 500 eggs.
The length of a complete life cycle (adult to adult) varies with temperature. At 20°C (68F) about 15 days elapse, but at 29°C (85°F), a life cycle is completed in only 8 days. Generations may be produced all year if temperature permits and fermenting produce is available.
Small fruit flies are subject to many natural enemies. Adults are parasitized by protozoa, fungi, nematodes, and mites and preyed upon by spiders and certain species of flies. Maggots are parasitized by certain wasps and preyed upon by staphylinid and nitidulid beetle larvae.
Infestations can be prevented by the destruction of piles of culled produce. Storage houses should be well screened and sealed to minimize fly entrance as much as possible. Still, chemical control in storage areas may be necessary. For up-to-date chemical recommendations, consult the current North Carolina Agricultural Chemicals Manual.
Southern armyworm, Spodoptera eridania (Cramer), Noctuidae, LEPIDOPTERA
Adult – The southern armyworm moth has a wingspan of 30 to 36 mm. The forewings may vary in color from pale yellowish to dark brown. A darker streak extends from the center of each forewing almost out to the wing tip. The hind wings are white with brown veins and margins.
Egg – The circular greenish egg is about 0.5 mm wide and 0.4 mm high. Viewed under magnification, the egg can be seen to have a ridged surface.
Larva – The fully grown caterpillar is gray or nearly black with whitish stripes tinged orange or pink and is about 36 mm long. The background body color sometimes has a green or pink tint. Viewed from above, the larva has a pair of black triangular spots on each body segment, except the segment near the head bearing the first pair of legs. The larva has 3 pairs of true legs and 5 pairs of prolegs. The head capsule is pale yellow with bright reddish-brown markings. The southern armyworm does not have a white inverted "V" on its head capsule.
Pupa – The darkly colored pupa is about 18 mm long and 5 mm wide.
Distribution – Florida, California, New Mexico, and central South America are year-round homes for the southern armyworm. Each year moths migrate northward as far as Tennessee and Virginia. In North Carolina, this armyworm is only an occasional problem.
Host Plants – This armyworm is a general feeder with a wide host range. Weeds like spiny amaranth and pokeweed are preferred food plants. Some vegetable crop hosts include beet, cabbage, carrot, celery, collards, corn, cowpea, eggplant, okra, pepper, potato, rhubarb, sweetpotato, and tomato.
Damage – Though southern armyworms feed primarily on leaves, they have been known to consume tender stems and tips of branches. These caterpillars feed freely during the daytime but often are not observed because they tend to congregate around the bases of plants. Here they gnaw on stems or feed on potato tubers or sweetpotatoes near the soil surface. During the morning and evening, or on cloudy days, southern armyworms are likely to be found on foliage.
Life History – Southern armyworms overwinter either as larvae or pupae in Florida. Egg-laying moths probably arrive in North Carolina in July. Each female deposits hundreds of eggs in masses on foliage. These masses are fuzzy in appearance since they are covered with scales from the bodies of moths. Eggs hatch in 4 to 6 days. For approximately 17 days, larvae feed and develop through 6 instars. At the end of this time, larvae drop by means of silken threads to the soil surface, enter the soil, and pupate. Nine to 13 days later a new generation of moths emerge. About 5 weeks elapse from egg stage to adult emergence during the summer. As many as 5 generations occur each year in Florida, but only 2 or 3 are likely to be completed in North Carolina.
The variety NC Porto Rico 198 has been found to have some resistance to the southern armyworm. Populations of this armyworm species rarely reach high enough numbers to warrant chemical control in North Carolina.
Spring Rose Beetle
Spring rose beetle, Strigoderma arboricola Fabricius, Scarabaeidae, COLEOPTERA
Adult – This slightly hairy beetle, sometimes referred to as the spring rose beetle, is basically greenish-black with a greenish-purple iridescence. The wing covers, however, are dull brownish-yellow in color. The beetle averages 10 to 12 mm long.
Egg – At first oval in shape, each white egg gradually enlarges, becoming more globose. From an initial size of 2 by 1.2 mm, the egg often increases in size to 2.5 by 2 mm.
Larva – The dirty white grub has a brown head and 3 pairs of forelegs. About 4 mm long when newly emerged, it reaches a maximum length of about 25 mm and resembles a common white grub in shape.
Pupa – The pupa is white when first formed but gradually darkens as it matures. It is approximately the same size and shape as the adult beetle.
Distribution - This beetle occurs from Canada southward through Kansas and North Carolina and is native to North America.
Host Plants - Adult beetles have been taken from many flowers, including those of clover, rose, blackberry, timothy, wild parsnip, dogfennel, and plantain. Larvae have been reported infesting roots of peanut, strawberry, sweetpotato, and certain pasture grasses.
Damage - These grubs feed on most underground plant parts. In certain cases they have been known to strip the taproot bare. Sweetpotatoes injured by these grubs have large but shallow feeding scars over their surface.
Life History - These insects overwinter as larvae in soil. In spring, grubs hollow out elongate, slightly curved earthen cells about 30 mm long. Within these cells, they spend approximately 6 days as inactive prepupae and 13 days as pupae. In Virginia, adult beetles usually emerge between May 15 and June 10. Further north, they often do not appear before the end of June. Several days after mating, females deposit eggs singly in soil (4 to 5 eggs/female based on lab studies). Eggs hatch an average of 17 days after deposition. By the time larvae begin feeding, at least one month has elapsed since adult emergence. Only one generation is completed each year.
White grub infestations are typically associated with fields formerly in sod or pasture. Such a relationship has not been documented for Strigoderma arboricola grubs, but it may exist nonetheless. Chemically, these grubs are controlled by granular insecticides incorporated into the soil before planting. For recommended insecticides and rates, consult the current North Carolina Agricultural Chemicals Manual.
Sweetpotato Flea Beetle
Sweetpotato flea beetle, Chaetocnema confinis Crotch, Chrysomelidae, COLEOPTERA
Adult – The tiny oval beetle is black with a bronze tinge and about 1.6 mm long. It has reddish-yellow legs and deeply ridged wing covers.
Egg – Each white, oblong-oval egg is about 0.2 mm long.
Larva – The slender, white, cylindrical larva has three pairs of legs near its head. It is about 4.8 mm long when fully grown. This larva has no dark spot or fleshy tubercle on its tail-end like cucumber beetle or palestriped flea beetle larvae.
Pupae – The pupa is white at first but gradually darkens and is approximately the same size and shape as the adult.
Distribution – The sweetpotato flea beetle occurs in practically all areas of this country where sweetpotatoes are grown.
Host Plants – Sweetpotato, corn, small grains, bindweed, raspberry, and sugar beet are the main food plants of this pest.
Damage – Adult flea beetles feed on foliage leaving narrow channels or grooves in the upper surfaces of leaves. These injured areas turn brown and die. Larvae live underground and feed on roots. Shallow winding tunnels etched into root surfaces indicate an infestation of flea beetle larvae. These tunnels eventually darken and split open leaving shallow scars. This type of damage usually is restricted to fibrous roots, but, during heavy infestations, larvae may injure the fleshy marketable portion of roots in the same manner as fibrous roots (Figure OO).
Life History – Sweetpotato flea beetles overwinter as adults under logs and leaves, along fence rows, and at the edges of wooded areas. They resume activity in spring and begin to deposit eggs in soil near host plants. A few days later eggs hatch. Newly emerged grubs feed for about 3 weeks before pupating in the soil. During summer, the entire life cycle is often completed in 30 days. Several generations per year are possible. From June onward, however, most eggs are deposited near bindweed, and flea beetle populations on sweetpotato decline.
Cultural practices are instrumental in preventing flea beetle infestations. Controlling weeds along fence rows and plowing under crop debris destroy overwintering and egg-laying sites. However, the use of resistant varieties such as Jewel or Centennial is the most effective means of preventing sweetpotato flea beetle injury.
In fields with a history of flea beetle infestation, chemical control may be justified. Preplant, soil-applied insecticides are available for this purpose. For recommended chemicals and rates, consult the current North Carolina Agricultural Chemicals Manual.
Sweetpotato hornworm, Agrius cingulatus (Fabricius), Sphingidae, LEPIDOPTERA
Adult – This grayish, heavy-bodied moth has a wingspan of 105 to 110 mm. The hind wings and abdomen bear bright pink bands.
Egg – Nearly spherical and about 1 mm in diameter, the translucent egg has a slightly greenish tint.
Larva – The first instar of the sweetpotato hornworm has a white body and a black anal horn. Later instars are basically green or brown with prominent, slanted black markings on each side of the body and a black anal horn. The head is also green or brown with 3 dark stripes on each side. A fifth instar hornworm may be 90 mm or more in length.
Pupa – The reddish-brown pupa is about 15 mm wide and 64 mm long. The large tongue case has a pitcher-handle-like appearance.
Distribution – Sweetpotato hornworms are common in tropical America and the southern United States. Moths stray northward as far as Nova Scotia, but the larvae are too scarce to be pests that far north.
Host Plants – Sweetpotato and morning glory are the primary food plants of this hornworm although jimsonweed has also been reported as a host.
Damage – These large worms consume much foliage leaving only bare stems and petioles on plants. Sweetpotato hornworms have been reported to display armyworm-like habits in Florida; however, their movement in large groups has not been observed here in North Carolina. Larvae often hide under large leaves at the base of plants.
Life History – The biology of this pest is not well documented. Its life history is probably very similar to that of tomato and tobacco hornworms (see Pests of Tomato). Moths appear in early June, again in August and September, and once more in early fall. There are probably 21⁄2 generations per year.
In small gardens, hornworms can be controlled simply by picking them off plants. Chemical control, however, may be necessary in commercial production. For recommendations, consult the current North Carolina Agricultural Chemicals Manual.
Sweetpotato weevil, Cylas formicarius elegantulus (Summers), Curculionidae, COLEOPTERA
Adult – This ant-like snout beetle is about 6 mm long. The head and wing covers are metallic dark blue and the thorax and legs, bright red-orange.
Egg – Each white or pale yellow egg is inserted into a shallow hole in the vine. Broadly oval and about 0.6 mm long, the egg is slightly narrower at the attached end. The dark head of the larva becomes visible inside the egg, just before hatching.
Larva – The fat, legless, slightly crescent-shaped larva has a dirty white to gray body and a pale brown head. When fully grown it is about 9 mm long.
Pupa – When newly formed, the pupa is the same color as the larva and about 5 mm long. Before transformation to the adult, the eyes, wing pads, and legs turn dark brown and the rest of the body is pale yellow. The last abdominal segment has two outward and backward curved tubercles.
Distribution – Sweetpotato weevils are a serious problem in some coastal areas from North Carolina to Texas. Discovered in eastern North Carolina in 1967, sweetpotato weevils are now largely under control in this area. Rarely, weevils are found as far north as New Jersey.
Host Plants – Sweetpotato and related wild plants such as morning glory are the only hosts of sweetpotato weevils.
Damage – These weevils and their larvae are the most destructive sweetpotato pests. Infestations may reduce plant growth during the first month after planting, but other damage often is not evident until harvest. Larvae and adults feed on foliage but they prefer to attack stems and sweetpotatoes underground. Small holes scattered over the surface of infested sweetpotatoes, particularly at the stem end, are the beetles' egg-laying and / or feeding punctures which cause sweetpotatoes to turn bitter. Such sweetpotatoes are unfit either for human consumption or stock feed.
Life History – Beetles become active in the field as soon as host plants are available. They first feed on leaves and stems. As plant stalks enlarge and become woody, adult females prepare to deposit eggs. They make holes in stems and fleshy roots near the soil surface. Eggs are placed in these holes and covered with a jelly-like secretion. Each female deposits an average of 120 eggs.
Larvae hatch less than a week after eggs are laid. They burrow deep into stems and fleshy roots for about 2 to 3 weeks. At the end of this period, third instar larvae return to the plant surface nearest the soil line to pupate. Pupae transform into adults in about a week, but another 4 days often elapse before the new beetles emerge from their pupal cells. Adults live about 2.5 to 3 months in summer and up to 8 months in winter.
Sweetpotato weevils continue to feed and breed throughout winter in stored sweetpotatoes. Development and activity, however, are much slower at temperatures below 15°C (60F). As many as 6 to 8 generations may be produced each year.
Cultural practices such as crop rotation, use of weevil-free planting stock, and destruction of volunteer plants and crop residue are primary elements of weevil control. Planting sweetpotatoes in the same fields year after year leads to increased weevil populations. If slips for planting cannot be obtained from a weevil-free area, each sweetpotato chosen for seed should be examined carefully and destroyed if infested. Also, use of deep-rooted varieties such as Porto Rico over shallow-rooted varieties like Gold Rush is advisable.
Postharvest insecticide treatments can be applied to prevent development of weevils in storage. Treated sweetpotatoes, however, will need to be washed thoroughly once they are removed from storage. For recommended insecticides and rates, consult the current North Carolina Agricultural Chemicals Manual.
Argus tortoise beetle, Chelymorpha cassidea (Fabricius), Chrysomelidae, COLEOPTERA
Blacklegged tortoise beetle, Jonthonata nigripes (Olivier), Chrysomelidae, COLEOPTERA
Golden tortoise beetle, Metriona bicolor(Fabricius), Chrysomelidae, COLEOPTERA
Mottled tortoise beetle, Deloyala guttata (Olivier), Chrysomelidae, COLEOPTERA
Striped tortoise beetle, Agrioconota bivittata (Say), Chrysomelidae, COLEOPTERA
Adult – These oblong-oval beetles are basically gold in color with various black and / or red markings depending upon species. Slightly flattened and squared at the shoulders, tortoise beetles' bodies are somewhat shell-like in appearance. Body margins extend in a roof-like manner over much of the head and legs. Most species are 5 to 8 mm long.
Egg – Tortoise beetle eggs usually occur in masses. Each individual egg is stalked, the long stalk being attached on the plant surface by gelatinous substance. The beige or white eggs of some species have a reddish tubercle on the upper end. Eggs are about 1.6 mm long.
Larva – The spined larvae may be basically dull yellow, brown, or green depending upon the particular species. They all have black heads, prothoracic shields (area behind head), legs, spots, spine-like setae, and anal forks. The anal forks are long spines near the tip of the abdomen which hold large masses of excrement. Fully grown larvae are 10 to 12 mm long.
Pupa – Pupae are oblong-oval in shape like adult beetles but have spines along the abdomen like larvae. They are approximately the same size as adult beetles.
Distribution – Argus and mottled tortoise beetles occur in all arable sections of the United States and Canada. The golden and blacklegged species are most common from the Rocky Mountains eastward.
Host Plants – Most tortoise beetles feed on sweetpotato and closely related plants such as morning glory and bindweed. Argus tortoise beetles also infest cabbage, corn, raspberry, strawberry, milkweed, and plantain. Golden tortoise beetles have been found on eggplant.
Damage – Both larvae and adults feed on leaves causing them to be riddled with holes (Figure RR). This type of damage is most threatening to seedlings or newly set plants.
Life History – Tortoise beetles overwinter as adults under bark, in leaf litter, or in other dry, protected places. In spring, beetles emerge and feed on weed hosts until sweetpotato plants are available. Female adults deposit clusters of 15 to 30 eggs on the undersides of leaves. Larvae emerge 7 to 10 days later. After feeding for 21⁄2 to 3 weeks, larvae transform into pupae. About a week later, a new generation of beetles emerges. Several generations may occur each year in southern states.
Tortoise beetles and other leaf-feeding insects do not affect sweetpotato production if growing conditions are satisfactory. Cultural practices such as adequate fertilization, good weed control, and well-timed planting effectively deter excessive tortoise beetle injury. Generally, chemical control is not necessary.
Yellowstriped armyworm, Spodoptera ornithogalli (Guenee), Noctuidae, LEPIDOPTERA
Adult – The yellowstriped armyworm moth has dark forewings with white and brown markings and white hind wings. The wingspan ranges from 32 to 38 mm.
Egg – Approximately 0.5 mm and 0.4 mm in diameter, the ribbed, greenish egg gradually becomes pale pink or brown before hatching. The egg mass is covered with scales from the moth's body.
Larva – This smooth-skinned, pale gray to jet black caterpillar has a yellowish-orange stripe along each side and a pair of black triangular spots on the top of most segments. Its head capsule is brown with black markings and a white inverted "V." The sixth larval instar may be as long as 45 mm.
Pupa – The brownish pupa is about 18 mm long and 5.5 mm wide.
Distribution – The yellowstriped armyworm occurs from New York southward into Mexico, westward to the Rocky Mountains, and in some areas of California and the West Indies. In this country, however, it is most common and most injurious in the southern states. In North Carolina, this caterpillar is observed annually in field and vegetable crops.
Host Plants – The yellowstriped armyworm is a general feeder. Some of its hosts include alfalfa, asparagus, bean, beet, cabbage, clover, corn, cotton, cucumber, grape, grass, jimsonweed, morning glory, onion, pea, peach, peanut, sweetpotato, tobacco, tomato, turnip, wheat, watermelon, and wild onion.
Damage – This foliage-feeding caterpillar is sporadically injurious to young crop stands. Defoliation at this stage can be a problem.
Life History -– Yellowstriped armyworms overwinter as pupae in the soil. In southern states, moth emergence begins in early April and continues into May. After mating, females deposit egg masses on foliage, trees, or buildings. Approximately 6 days later, the eggs hatch and the larvae begin feeding. Although these caterpillars emerge by late spring in the South, they may not appear before July in northern and midwestern states. Larvae feed during the day on tender foliage over a 3-week period. Mature 6th instars burrow into the soil and change into pupae. Two weeks later, the second generation of moths emerges. In southern states, including North Carolina, 3 to 4 generations occur each year.
Yellowstriped armyworms seldom require control in North Carolina. Since large larvae are difficult to control with insecticides, early detection is important in maintaining populations below economic injury levels. For chemical control recommendations, see the current North Carolina Agricultural Chemicals Manual.
Publication date: Feb. 10, 2003
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