Key to Pests of Carrots
Since carrots are root crops, soil-inhabiting pests such as wireworms and vegetable weevils have the most direct effect on produce quality. Armyworms, however, may cause indirect injury to the taproot by cutting stems and/or consuming foliage above ground. Few other insect problems are common in North Carolina.
A. Chewing insects that cut holes or entire leaves
- Caterpillars with three pairs of legs and five pairs of prolegs
- Armyworm – This pale green to yellowish or brownish-green, smooth-bodied caterpillar up to 35 mm long has three dark longitudinal stripes and a tan or greenish-brown head mottled with darker brown (Figure 1). It feeds primarily at night on foliage and succulent stems.
- Parsleyworm – This yellowish-green caterpillar up to 40 mm long has transverse black bands and deep yellow or orange spots. The head is greenish-yellow with black stripes (Figure 2). When disturbed, a pair of horn-like scent organs protrudes from behind the head. They feed during the day.
- Yellow woollybear – This caterpillar is white to yellow or brown or red and has dense white hairs covering its body (Figure 3). Young larvae feed in colonies on the underside of leaves; older larvae disperse and feed anywhere.
- Vegetable leafminer – These bright yellow maggots grow up to 3.0 mm long and make S-shaped leaf mines which are often enlarged at one end (Figure 4). Heavily infested leaves sometimes turn brown.
- Vegetable weevil and larva – Dull grayish-brown weevil with a short, stout snout and light V-shaped marks on wing covers, feeding primarily at night on buds and foliage (Figure 5A). Both weevils and grubs are about 6.4 mm long. The pale green legless larvae grow up to 10 mm long and have dark mottled heads (Figure 5B).
B. Insect with needle-like mouthparts that cause foliage to be yellowed or distorted
- Aster leafhopper – Yellowish-green aster leafhoppers are up to 5 mm long and have six black spots on front of head (Figure 6). Nymphs are sometimes light brown instead of yellow or green.
- Tarnished plant bug – Oval-shaped brown bugs are up to 6.4 mm long and have long legs, long antennae, and a white triangle between its "shoulders" (Figure 7A). Nymphs are yellowish-green to green with black spots on its back (Figure 7B).
C. Insects that feed on underground plant parts
- Vegetable weevil and larva – Dull grayish-brown weevil with a short, stout snout and a light V-shaped marks on wing covers, feeding primarily at night on buds and foliage (Figure 8A). Both weevils and grubs are about 6.4 mm long. The pale green legless larvae grow up to 10 mm long and have dark mottled heads (Figure 8B). Vegetable weevils feed at night, often attacking large taproots.
- Southern potato wireworm – These slender, wire-like, cylindrical larvae have three pairs of short legs and a pair of fleshy anal prolegs (Figure 9). The white, cream, or yellow-gray larvae have red-orange heads and grow to 17 mm long. The last abdominal segment has a closed oval notch. These wireworms form irregular holes in infested taproots.
Armyworm, Pseudaletia unipuncta (Haworth), Noctuidae, LEPIDOPTERA
Adult – The true armyworm moth has grayish-brown forewings, each with a white spot near the center, and grayish-white hind wings. The wingspan averages 38.5 mm.
Egg – The minute, greenish-white egg is globular in shape.
Larva – The young armyworm is pale green. The mature larva is basically yellowish or brownish-green with a tan or greenish-brown head mottled with darker brown. The smooth, practically hairless body is marked with three dark longitudinal stripes, one along each side and one down the back. A full-grown armyworm is 30 to 35 mm long.
Pupa – The reddish-brown 13-mm-long pupa darkens gradually until it is almost black.
Distribution – True armyworm occurs throughout the United States east of the Rocky Mountains. In North Carolina they are particularly abundant in the Piedmont and Coastal Plain regions. During daylight hours, larvae prefer to remain under litter on the ground.
Host Plants – Although true armyworms strongly prefer grasses and cereals, they have occasionally been reported to infest various vegetables, fruits, legumes, and weeds, especially when they are on the march. Some of their vegetable host plants include bean, beet, cabbage, carrot, cauliflower, corn, cucumber, lettuce, onion, pea, pepper, radish, and sweetpotato.
Damage – Preferring to feed at night, armyworms devour succulent foliage. By feeding on leaves and occasionally stems, they can severely damage seedling stands. Because they feed at night, armyworms may inflict much injury before they are detected. Having exhausted a current food supply, the worms migrate as an "army" to new host plants. Fields adjacent to or harboring lush grass are most commonly attacked.
Life History – True armyworms overwinter as partly grown larvae. Early in spring, larvae resume feeding at night, usually on grasses and small grains. First generation adults appear in May or June depending upon climatic conditions. Moths mate soon after emergence and feed on nectar for 7 to 10 days. Females then deposit up to 2,000 eggs in small clusters or rows on the leaf sheaths of grasses. About 6 to 20 days later, larvae emerge. After feeding for 3 or 4 weeks, they drop to the ground and pupate in earthen cells 5 to 7.5 cm deep within the soil. Moths emerge about 2 to 4 weeks later. True armyworms complete five or more generations per year in North Carolina.
Parasites, various diseases, insect predators, and birds usually keep armyworms under control except after cold, wet springs. When practical, cultural methods, such as disking large areas, can help reduce future armyworm populations by exposing the pupae to natural enemies and hot weather. However, since armyworm moths are strong fliers, most areas will be subject to constant reinfestation.
Because armyworms feed exposed, are active during their larval stages, and are susceptible to several insecticides, they are easily controlled chemically when buildup occurs. For specific control information, consult the current North Carolina Agricultural Chemicals Manual.
Aster leafshopper, Macrosteles fascifrons (Stal), Cicadellidae, HEMIPTERA
Adult – Usually 3.5 to 4 mm long, this light, smokey green to yellowish-green leafhopper may be as short as 2 mm or as long as 5 mm. Sometimes called the six-spotted leafhopper, this species has six black spots arranged in pairs on the front of the head.
Egg – No description.
Nymph – The nymph ranges from 0.6 to 3 mm long. It has the same head markings as the adult but varies in color from yellow or light brown to a pale greenish-gray.
Distribution – During the growing season aster leafhoppers can be found from Mexico to Alaska. Habitats as varied as grasslands, swamps, and dry prairies can all support populations of these leafhoppers. Unless they are migrating northward in spring, the leafhoppers usually do not move more than 100 m (about 200 feet) over a 4-week period.
Host Plants – Aster leafhoppers attack a wide range of vegetables, fruits, herbs, grasses, and weeds. Some common vegetable and herb hosts include lettuce, celery, carrot, parsnip, parsley, dill, onion, shallot, pepper, tomato, cucumber, and sweet corn.
Damage – Nymphs extract plant sap from the underside of leaves and cause a general yellowing of plant foliage. Adults of this species, however, also damage plants by transmitting diseases like aster yellows to carrot, lettuce, and aster. Aster leafhoppers are the only known vector of this disease in the eastern United States. Infected plants yellow, become stunted, branch excessively, and develop short internodes. Young plants are most affected by this disease.
Life History – The biology of this leafhopper has been studied primarily in northern states or in the laboratory but not in North Carolina. Here, it is believed to overwinter on perennial weeds or fall-planted small grains, probably both as eggs and adults. In spring, adults migrate to other herbaceous host plants. They pick up the aster yellows virus by feeding on infected plants but cannot transmit it until after an incubation period of 10 to 18 days. Viruses are usually transmitted by adult leafhoppers because nymphs molt fairly often and mature into adults in about 18 days (under lab conditions). Thus, the virus often does not have enough time to incubate in nymphs. Adults, excluding overwintering forms, have been reported to live an average of 42 days (on host plants in laboratory). Life cycles as short as 20 days occur in summer in Michigan. Many generations per year are possible in North Carolina.
Fortunately, the leafhoppers and the yellow disease they transmit are rarely much of a problem in North Carolina. Should large populations develop, consult N.C. Cooperative Extension for current control measures.
Parsleyworm, Papilio polyxenes asterius Stoll, Papilionidae, LEPIDOPTERA
Adult – The black swallowtail butterfly varies in size and coloration, the female being slightly larger and more somber than the male. Both sexes are velvety black and have hind wings with tails and peacock-like eyes (Figure S). Some individuals, especially the males, have bands of yellow spots across the wings. The wingspan may be as much as 76 mm.
Egg – Spherical and about 1 mm in diameter, the egg gradually changes from pale yellow to reddish-brown.
Larva – When fully grown, the parsleyworm is about 40 mm long. Its yellowish-green body has transverse black bands and deep yellow or orange spots on these black areas (Figure S). Behind the yellowish-green, black-striped head is a pair of yellow or red scent organs which protrude like horns when the caterpillar is disturbed. Early instar larvae resemble mature larvae in coloration but may have spines.
Pupa – Known as a chrysalis, the sharply angled pupa is dull gray and mottled with black and brown. It is about 32 mm long.
Distribution – This insect occurs from southern Canada into Florida and westward to the Rocky Mountains. It has also been reported in Arizona, New Mexico, and Mexico.
Host Plants – Parsleyworms feed primarily on celery, parsley, carrot, parsnip, dill, and other closely related herbs and weeds.
Damage – Though foliage feeders, parsleyworms are not usually serious pests. They usually eat leaves, but occasionally attack blossoms and underdeveloped seeds. Often noticed, even in low numbers because of their striking and sometimes formidable appearance, these caterpillars do little damage to plants and do not sting or attack man.
Life History – Parsleyworms overwinter as pupae, or chrysalides, attached to stems or fallen leaves. Black swallowtail butterflies emerge in April or early May. Soon afterwards, eggs are deposited on plant foliage. Four to 9 days later, eggs hatch and the young larvae begin feeding on leaves. The period of larval development varies with geographic location but rarely lasts longer than 4 weeks. Mature larvae become chrysalides and are attached to host plants by silken threads about their middles. Butterflies emerge from these non-overwintering pupae in 9 to 15 days. The number of annual generations varies from two in northern states to three in St. Louis to four in the Gulf States. There are probably three generations per year in North Carolina.
Since parsleyworms are so highly visible and pose so small a threat, handpicking these caterpillars from plants is the best method of controlling them. In commercial production, consult the North Carolina Agricultural Chemicals Manual for control recommendations.
Tarnished Plant Bug
Tarnished plant bug, Lygus lineolaris (Palisot de Beauvois), Miridae, HEMIPTERA
Adult – Like most lygus bugs, the tarnished plant bug is oval in shape and has a characteristic white triangle between the "shoulders." Its legs and antennae are relatively long. The adult may be one of several shades of brown and is approximately 6.4 mm long.
Egg – The tiny elongate egg is slightly curved.
Nymph – The wingless nymph is yellow-green to green in color with several black spots on its back. Length varies from 1.5 mm to slightly less than adult size. The fourth instar nymph has wing pads.
Distribution – Tarnished plant bugs are generally distributed because warm, dry climates are most conducive to the buildup of lygus bug infestations, but tarnished plant bugs pose a limited threat in North Carolina. In states further south and southwest, economic injury occurs annually.
Host Plants – Tarrnished plant bugs infest more than 50 economic plants including many field, forage, fruit, and vegetable crops. Also, weeds such as butterweed, fleabane, goldenrod, aster, vetch, dock, and dogfennel commonly harbor these insects in southern states.
Damage – Shiny circular spots of excrement on various plant parts indicate the presence of tarnished plant bugs. These bugs pierce buds and terminal growth with their needle-like mouthparts and extract plant juices. New growth may be yellowed and distorted, causing plants to appear unthrifty.
Life History – In North Carolina, adults hibernate in plant debris and resume activity in spring. At this time females insert eggs into succulent host plant tissue with their sword-like ovipositor. Eggs hatch 11⁄2 to 3 weeks later. The nymphs develop through five instars over a three-week period as they feed on plant sap. Mature nymphs molt and emerge as adults. The first few generations develop on preferred hosts such as small grains, alfalfa, wild grasses, vetch, dock, and fleabane. As hay is cut or as other plants dry out, tarnished plant bugs migrate in large numbers to succulent hosts such as cotton or vegetable crops. During summer, the life cycle (from egg hatch to adult emergence) is completed in 4 weeks. As many as 5 annual generations are possible.
Vegetable crops planted as far as possible from the forage and cotton crops preferred by this pest are not likely to be heavily damaged. Weed control and destruction of crop residue help eliminate sources of food and shelter for the tarnished plant bug. Chemical control should not be necessary; however, in the case of heavy infestations, consult N.C. Cooperative Extension for recommendations.
Yellow woollybear, Diacrisia virginica (Fabricius), Arctiidae, LEPIDOPTERA
Adult – With a wingspan of nearly 40 mm, the moth is nearly pure white except for the abdomen and a few black spots on each wing.
Egg – The white to golden-yellow spherical eggs occur in clusters of 50 to 60 and are usually covered with hairs from the body of the moth. Individual eggs are about 0.6 mm in diameter.
Larva – Densely clothed with long and short hairs, the woolly caterpillar may be pale yellow, brownish-yellow, red, or white. It usually blackens in color near the head. A fully grown caterpillar may be as long as 50 mm.
Pupa – Approximately 16 mm long, the reddish-brown pupa is enveloped by a thin, fragile silken cocoon.
Distribution – The yellow woollybear is common throughout North America.
Host Plants – Yellow woollybear caterpillars feed on a wide range of garden, field, and ornamental crops as well as weeds. Some vegetable hosts include asparagus, bean, beet, cabbage, carrot, cauliflower, celery, corn, eggplant, onion, parsnip, pea, potato, pumpkin, radish, rhubarb, salsify, squash, sweetpotato, and turnip.
Damage – All stages of the foliage-feeding caterpillar consume flowers, leaves, tender stems, and fruit buds, primarily in summer and autumn. Plants may be skeletonized by heavy infestations.
Life History – This insect overwinters in the pupal stage. Cocoons are often found in large numbers beneath a single shelter (old board, tree bark, etc.). In spring, moths emerge and mate. Females soon deposit clusters of eggs on leaves. After about 7 days, the eggs hatch.
Young larvae feed in colonies on the underside of leaves. As they mature, caterpillars disperse and feed on more exposed sites. After feeding for about 4 weeks, woollybears are fully grown and begin to seek sheltered places in which to pupate. Here, caterpillars spin cocoons and spend the next 1 to 2 weeks transforming into moths. The life cycle is then repeated. Several generations are completed each year.
Several natural enemies limit yellow woollybear populations. Eggs are parasitized by Trichogramma wasps and the caterpillars are susceptible to diseases caused by Bacillus thuringiensis and granulosis virus. This insect usually does not become a problem on crops which are being sprayed for other pests. Should an infestation develop, consult N.C. Cooperative Extension for current recommendations.
Publication date: Feb. 10, 2003
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