Mollie Johnston | 2/13/2019 5:56:50 PM
Many insect pests have the potential to reduce the quality and yield of sweet potatoes.
Insects that damage the roots directly are the most troublesome and are referred to as soil insect pests. They can cause economic loss in relatively low numbers, and it is difficult to control them with insecticides because they live below the soil surface.
Insects that injure the foliage reduce the yield of the plants indirectly and are referred to as foliage feeding insects. These insects can cause economic loss, but usually only at very high numbers, and it is relatively easy to control them with insecticides because they are exposed on the plant.
The sweet potato root can be injured by several soil insects including: the sweet potato weevil, rootworms, wireworms, white grubs, whitefringed beetles and flea beetles. The sweet potato weevil larva is the only insect that tunnels throughout the root. Other soil insects feed on the surface of the developing root. The injury caused by rootworms and wireworms is similar and cannot be separated easily. These insects chew small, shallow circular or oblong holes through the skin of the root. Flea beetles chew shallow, narrow, winding tunnels along the surface of the root.
The injury caused by white grubs and whitefringed beetle larvae is similar to each other and different from the other soil insects. White grubs gouge broad, shallow to deep, irregular areas on the surface of the root. Whitefringe beetle larvae chew comparatively narrow and irregular ‘channels’ on the surface of the root.
Sweet Potato Weevil
The sweet potato weevil is a serious pest in the field and in storage. Even low numbers of larvae reduce sweet potato quality and marketable yield. The adult weevil feeds on all parts of the plant but reproduces only in the stems and roots. Sweet potato weevils are most numerous close to the soil surface. Eggs are laid in the roots or stems in holes (one per hole) eaten out by the females. Following egg lay, the holes are covered by a grayish mass that hardens to form protective caps over developing eggs. Depending on environmental conditions, incubation of eggs varies from four to 56 days. Larval development occurs inside the root and can range from 12 to 154 days. Pupation occurs within the sweet potato, and this stage lasts for five to 11 days. Newly emerged adults wait one to three days before leaving the root. Adults mate soon after emergence from the root but egg laying does not occur for four to seven days. The injury from the weevil is from adults and larvae. The adult feeds on the root, puncturing numerous small holes and laying eggs in these punctures. Larvae tunnel throughout the root. All stages of the weevil can be found in the root. Larvae also tunnel through the vine next to the soil. Larvae found in the vine usually come from eggs deposited in the plant production bed. Sweet potatoes infested with weevils are not only unsightly, but they taste bitter.
The adult of the sweet potato weevil is a snout beetle that looks antlike. It has a narrow head and thorax, long legs and a long body. The head and abdomen are metallic dark blue, and the thorax and legs are red. The long antennae are reddish-brown, with the last segment forming a thick club. The adult is about ¼ inch long. Eggs are white or pale yellow, broadly oval and are very tiny. The larvae have fat, legless, C-shaped bodies that are dirty white with a pale brown head.
Sweet potatoes and crowns left in the field after harvest may provide a haven for the weevil in winter, especially when sweet potatoes are covered by soil. Therefore sweet potatoes and intact crowns should not be left in the field after harvest. Devining at harvest helps to destroy crowns. When sweet potatoes are harvested under good conditions with a digger, all of the crop debris will be on top of the soil where it will decompose in winter. The field should not be plowed until late winter or spring because rots covered by soil provide a good over-wintering place for the weevil. Sweet potatoes should never be left in the field unharvested. When field graded at harvest, ingested sweet potatoes should not be left in the field.
Weevils may also survive the winter in stored seed. Eliminating weevils from planting material should begin with sanitation in and around the storage area as soon as it is emptied in the spring. Malathion can be sprayed in the storage areas to kill weevils. One small, infested sweet potato can produce many weevils.
At harvest, all the sweet potatoes to be stored should be dusted with 5% Imidan dust. Apply it with a recirculating duster for good coverage. Good insecticide coverage of all the sweet potatoes in a pallet box or James crate is necessary.
Foundation or certified seed free of weevils should be used. Place beds as far away as possible from last year’s sweet potato fields. This reduces the likelihood of any surviving weevils finding the beds. Ideally, beds should also be far away from the new fields (at least 1 mile). The separation makes it difficult for weevils surviving in the beds to find the new fields. After the last cutting of plants, destroy the beds.
The next step in weevil control is to spray the plant beds. This will control the weevils that survived the winter in the previous year’s fields. This spraying will also take care of an occasional infested seed that did not get enough Imidan dust to kill weevils. Spray the beds weekly with Imidan, Sevin, Thiodan or Penncap M. Start spraying as soon as the plastic mulch is pulled off, and continue weekly applications until the last cutting of plants. A bar placed on the spray boom ahead of the nozzles will bend large plants over and let the spray hit the base of the plants. Use enough water to wet the plant stems and soil surface. The reason for spraying the base of the plants and the soil surface is that the female weevils stay on the soil surface of just above or below it, and almost all the eggs are placed within an inch of the soil surface. Cut the plants at least an inch above the soil. This leaves nearly every weevil egg in the plant bed. Also, it helps reduce the spread of numerous diseases and root-knot nematodes.
Finally, monitor activity in the field. Place three pheromone traps about equal distance apart per 40-acre field. If a single trap catches as many as four weevils in a week, begin weekly foliar spray applications (Imidan, Penncap M, Sevin, Thiodan) until weevil numbers drop to below the threshold level. Good coverage is essential for control. Use at least 15 gallons of water per acre with the insecticide.
Individual practices recommended to produce weevil-free plants will not be completely effective if used alone. But if all the practices are used together, they assure that the plants taken to the field will be as weevil free as possible.
The banded cucumber beetle is the most common cucumber beetle attacking sweet potatoes in Louisiana, but the spotted cucumber beetle will also feed on sweet potatoes. Both species feed on several different host plants including vegetable and field crops, as well as weeds. The larvae, or rootworms, eat small holes through the skin of roots and form irregular cavities under the skin. Feeding holes are in groups and may enlarge as roots develop. Roots injured during early development will have many unsightly healed holes at harvest. Injury that occurs closer to harvest will not be healed. Feeding by adults on sweet potato foliage produces irregular holes, but the adult beetles do not consume enough foliage to injure the plants. Adults lay eggs in the soil where larvae emerge in one to two weeks, depending on temperature. Larvae of these two species are very difficult to tell apart. The larval stage may last eight to 30 days, depending on food supply and temperature. Pupae are found in cells just below the soil surface, and adults emerge in about one week. There are several generations per year.
Cucumber beetles are oblong-oval, have beaded antennae and are about ¼ inch long. The adult banded cucumber beetle is marked with alternating green and yellow bands, and the spotted cucumber beetle has a bright yellowish-green body with 12 black spots and a black head. Cucumber beetle eggs are orange-yellow and oval and found in clusters in the soil at the base of plants. Cucumber beetle larvae, called rootworms, have a yellow-white, somewhat wrinkled body with three pairs of brownish legs near the head and a single pair of prolegs near the tip of the abdomen. Fully grown cucumber beetle larvae are 1/4 to 1/3 inch long.
The banded cucumber beetle is the most common rootworm that injures sweet potato roots in Central Louisiana. In North Louisiana, the spotted cucumber beetle predominates. Because this species has several generations per year, a wide host range and is very mobile as adults, it is one of the most difficult soil insect pests to control. Repeated cultivation of the fields in the spring will help remove alternate food sources and expose the larvae to birds and other predators. A preplant soil insecticide (Mocap, Lorsban) will kill rootworms in the soil and provide residual control for four to six weeks after application. Adult beetles moving into the field will lay eggs, and resulting rootworms will injure the roots once these soil insecticides break down. At this point, if adults exceed threshold levels, resort to foliar applications to prevent rootworm injury. Depending on environmental conditions, the insecticide used and the coverage obtained with the spray application, a foliar-applied insecticide may kill beetles for anywhere from one day to one week. Adult beetles can be monitored with sweep nets. Two banded cucumber beetles per 100 sweeps indicate the need to spray on a seven-day interval. With higher numbers of the beetle, reduce the spray interval to five days. These beetles are highly mobile. They may move into a field in large numbers and make management difficult.
Several species of white grubs are pests of sweet potatoes, but the most prevalent species is Phyllophaga ephilida. Larvae are often abundant in pasture, sod or weedy fields and can injure roots severely when sweet potatoes are planted in fields that follow pasture. Larvae gouge out broad, shallow areas on the root, reducing the marketability of the crop. Adults are active at night and feed on the leaves of deciduous trees. They emerge in late May through mid July. Females oviposit on bare soil or soil planted to crops. Larvae over-winter in the soil. The length of the life cycle varies; some species complete a generation in one year, and others require two to three years to complete a generation.
Adults, known as May beetles or June beetles, are shiny reddish-brown to black and are 1/2 to 1 inch long. Eggs are oval to spherical and dull pearly white when first deposited, turning dark just before larval emergence. The C-shaped grub has a distinct brown head; a shiny, smooth body; and three pairs of legs just behind the head.
White grubs have a longer life cycle than cucumber beetles. As with cucumber beetles and wireworms, white grubs may be in the field before planting. Repeated cultivation of the fields in the spring will help remove alternate food sources and expose grubs and wireworms to birds and other predators. White grubs may be found by turning over (shovel or plow) the soil and checking for grubs in the loose soil.
Because of the long life cycle of white grubs, their presence in the previous season increases the likelihood of injurious numbers in the current season. White grubs are often found in large numbers in pastures and grassy areas. Planting sweet potatoes into these areas will also increase the likelihood of these pests. A preplant soil insecticide (Mocap, Lorsban) will provide some protection for these pests in the same way it will control rootworms and wireworms. Adult white grubs (May beetles, June beetles) are generally active in the spring and early summer. During this time these adults may lay eggs in sweet potato fields. These new larvae will not be killed once the soil insecticide has broken down. June beetles tend to be active at night and will not be captured with a sweep net during the day. Foliar applications of insecticides to control cucumber beetles will also control click beetles and June beetles.
Whitefringed beetles can be a serious problem for growers. Whitefringed beetles have been reported on more than 385 plant species, including peanuts, corn, sugarcane, cotton, cowpea, beans, cole crops, alfalfa and sweet potatoes. Larval injury to sweet potato roots resembles that caused by white grubs. Larvae gouge out narrow, shallow to deep channels on the surface of the root. Whitefringed beetles over-winter mainly as mature larvae in the soil at a depth of about 9 to 15 inches but can over-winter in the egg stage. Pupae are white and are found in the soil at a depth of 3 to 6 inches from late May to the end of July. Adult emergence begins in May and continues until the middle of August. Newly emerged adults feed on the undersides of leaves or at the base of the stems near the soil surface. Adults prefer to feed on the older plant foliage, but consume little. Females are parthenogenetic (reproduce without males) and initiate oviposition on the lower plant parts 10 to 12 days after emergence from the pupal stage. Females can live for two to five months. Eggs are deposited near the soil surface on objects such as plant stems and debris lying on or protruding from the soil, but they can also be deposited in the soil at depths of 1 to 4 inches. The larvae emerge from eggs in two weeks to three months, depending on temperature. The larvae live underground and feed on the root system of plants. Since this insect cannot fly, its only means of movement are by walking and by being carried on plants being transported to new locations.
Adult beetles are black and covered with dark-gray and grayish-brown scales. They have two longitudinal stripes and a marginal band of white hairs. They are about 1/2 inch long. The white, oval eggs become pale yellow before larval emergence. Larvae are slightly curved, yellowish-white, legless and have light-brown heads.
Whitefringed beetle larvae are very difficult to control because the larvae often remain deep in the soil where even preplant incorporated soil insecticides may not reach them. The adults will feed above ground and can be controlled with foliar applications of insecticides (Penncap or Imidan). These beetles can be sampled with a vacuum sampling device, they can be searched for by hand or their presence can be detected by the presence of notched leaves. Adults are fairly long lived and may be present in the field from May through September. Whitefringed beetles cannot fly, so they move very little from year to year. The presence of an infestation the previous year then indicates close attention is warranted in the current year. Crop rotation can be used effectively to reduce infestation levels. The planting of small grains on heavily infested portions of a farm will reduce the numbers of beetles, because small grains are poor food for adults and whitefringed beetles feeding on them produce few eggs.
Several species of wireworms seriously injure sweet potatoes. Adults, known as click beetles, do not feed on the crop but oviposit on the soil near the crop, weeds or other vegetation. Wireworms have a very broad host range and will feed on weeds in the fields. Larvae produce small, round shallow feeding holes on the root surface. The original holes are usually less than 1/4 inch deep, but may be considerably deepened by later growth of the sweet potato root. The life cycle varies from species with two generations per year to species with life cycles lasting two to three years.
Adults are reddish-brown to black and elongate. The eggs are white, oval to spherical and very small. A wireworm larva is cylindrical with three pairs of short legs near its head. They have a pale yellow to reddish-brown body and a brown, flattened head. When fully grown, wireworms range from 1/2 to 1 inch in length.
Wireworms have a longer life cycle than cucumber beetles and as adults are less mobile. As with cucumber beetles and white grubs, wireworms may be present in the field as larvae before planting. Repeated cultivation of the fields in the spring will help remove alternate food sources and expose wireworms to birds and other predators. Because of the long life cycle of wireworms, the presence of these insects in the previous season increases the likelihood of injurious numbers in the current season. Wireworms can be sampled in the spring using corn or small potatoes as baits before planting. These baits should be buried in the soil about 3 to 4 inches deep and left for seven to 10 days. The samples may then be dug and checked by washing through a sieve. A preplant soil insecticide (Lorsban) will provide some protection from wireworms and a layby application will extend the residual in the soil. Adult wireworms (click beetles) are generally active in the spring and summer. During this time click beetles may lay eggs in sweet potato fields. These new wireworms will not be killed once the soil insecticide has broken down. Click beetles tend to be active at night and will not be captured with a sweep net during the day. Foliar applications of insecticides to control cucumber beetles will control click beetles.
The sweet potato flea beetle feeds on several plants including sweet potato, morning glory and bindweed. The adults chew shallow narrow channels or grooves in the upper surface of leaves. These injured areas will turn brown and die. Larvae feed on roots, leaving shallow, narrow winding tunnels. Such tunnels will split open, leaving shallow, dark scars. Usually, flea beetle larvae feed on fibrous roots but, when high numbers of beetles are present, they will feed on marketable roots. The over-wintering stage of the flea beetle is the adult. Over-wintering occurs in protected places such as under logs or leaves, along fencerows and at the edges of wooded areas. Eggs are laid in the soil near plants in the spring when adults emerge from over-wintering sites. Larvae emerge from eggs in a few days, feed for about three weeks and pupate in the soil. The life cycle is completed in 30 days in the summer, and there are several generations a year. Other species of flea beetles such as the pale-striped flea beetle may be found in sweet potato fields.
The adult is oval, black with a bronze tinge and very small. The wing covers have deep ridges and the legs are reddish-yellow. The first segment of the hind pair of legs is greatly enlarged and allows flea beetles to jump like fleas. The eggs are very small, oblong-oval and white. Larvae have three pairs of true legs near the head but do not have prolegs. The body is cylindrical and slender, reaching a length of 1/5 inch.
Flea beetles are an occasional problem for growers. It has several generations per year and can pose a problem throughout the growing season. Because flea beetle larvae feed primarily on the fibrous roots, high numbers must be present to cause significant root injury. Soil insecticides and foliar-applied insecticides will provide protection from flea beetles. Adult flea beetles are active during the day and can be captured in sweep nets. A threshold of five beetles per 100 sweeps is recommended before applying insecticides.
Many insects feed on the foliage. As a general rule, defoliation must exceed 30 percent before yields are reduced. Defoliation late in the season will have less of an impact on yield than defoliation in the earlier stages. The natural enemies of these insects usually keep them from reaching levels that may cause yield loss. Foliar insecticide applications for the soil insect pests will control the defoliating insects. Microbial insecticides may be used to control caterpillars.
Sweet potato hornworm moths are present from June to September. They are active at night. Eggs are laid singly on the lower surface of leaves. Larvae feed on the foliage for about three weeks and then they burrow into the soil where they pupate. The pupal period lasts three weeks in the summer, then adults emerge to start a new generation. There are two to three generations per year, with the pupa being the over-wintering stage. This insect will also feed on morning glory.
The adult moth is heavy-bodied and gray with a wingspan of about 4 inches. There are bright pink spots on the hind wings and abdomen. The spherical egg is translucent with a greenish tint. Larvae are green to brown with slanted black lines on both sides of the body and a black anal horn. The head is green or brown with three stripes on each side. Mature caterpillars reach a length of 3 ½ inches. The pupa is reddish-brown and is found in the soil.
Several species of armyworms feed on sweet potato foliage, including the southern armyworm, yellow-striped armyworm and beet armyworm. These armyworms have a broad host range, feeding on many plants including several vegetable and field crops as well as weeds. Larvae feed on leaves, tender stems and branch tips. They are daytime feeders but are sometimes difficult to find because they move to the base of the plant. Armyworm larvae are often found on foliage during the morning, evening or during the day if it is cloudy. Larvae will chew on exposed sweet potato roots. Eggs are placed in masses on the foliage by moths. Larvae, or caterpillars, emerge from eggs in four to six days and spend three to four weeks as larvae feeding on the plant. At the end of larval development, larvae drop to the ground, where they burrow into the soil and pupate. Adult moths emerge to start a new generation. The life cycle from oviposition to adult emergence requires about five weeks in the summer. There are from three to five generations per year, depending on the species.
Because armyworms feed above ground on the plant foliage, they are fairly easy to control; however, the beet armyworm is resistant to pyrethroid insecticides and may have some tolerance to organophosphates as well (Penncap M, Imidan). Spintor and Confirm are effective against the beet armyworm. Whereas Spintor will control all species of Lepidoptera on the foliage, however, Confirm will provide only fair control of loopers.
Several species of tortoise beetles feed on sweet potato foliage. Both adults and larvae can be found on the leaves.
Adults over-winter under bark or in leaf litter. The adults become active in the spring and feed on weed hosts until sweet potatoes are available. Eggs are laid on the underside of leaves, and larvae feed for two and a half to three weeks before pupating. Adults emerge from pupae about a week later. There are several generations a year.
Tortoise beetles are brightly colored, usually gold with black and/or red markings, depending on the species. These beetles are oblong-oval and slightly flattened. The front wings are wide, and the first hardened plate of the thorax covers the head. The bodies of tortoise beetles have a hell-like appearance. They are called tortoise beetles because they resemble tortoises. The head and legs are covered in a roof-like manner by the margins of the body. They vary from 1/5 to1/3 inch long. The body of the larvae may be yellow, brown or green, and the head is black
Two species of loopers may occur on sweet potato foliage, the soybean looper and the cabbage looper. Loopers can be distinguished form armyworms by the ‘looping’ motion when they crawl: they arch their back up while moving in a forward direction because of a reduced number of legs on the abdomen. The soybean looper has shown resistance to pyrethroids but will be controlled by Spintor or the organophosphates (Imidan, Penncap M), the carbamates (Sevin) or the Bts. The cabbage looper is susceptible to all the insecticides.
The sweet potato whitefly is the most common species of whitefly on sweet potato. Adults are white insects (up to 3/32 inch) that resemble tiny moths. They are often found on the undersides of leaves and are often associated with honeydew and sooty molds. They flutter when disturbed.
Direct damage is caused by the removal of sap, and indirect damage as a disease vector. Chlorotic spots sometimes appear at the feeding sites on leaves, and heavy infestations cause leaf wilting. The excretion of honeydew and the subsequent development of sooty mold fungi also may reduce photosynthesis and other physiological functions of the plant.
Admire is the most effective insecticide for controlling whiteflies.
The melon aphid and green peach aphid are the most common species of aphid on sweet potatoes. The adults and nymphs of these soft-bodied, pear-shaped insects are usually pale to dark green but sometimes yellow. Though winged forms develop periodically, most adults are wingless and are about 3/32 inch long. All forms have a pair of tailpipe-like appendages known as cornicles.
Congregating on lower leaf surfaces and terminal buds, aphids pierce plants with their needle-like mouths and extract sap. New growth becomes stunted and leaves curl downward and pucker. Wilting and discoloration follow. Heavily infested plants turn brown and die from the top down. Honeydew secreted by aphids makes plants sticky and enhances development of black sooty mold on plant foliage.
Aphids tend to spread rapidly from field to field, transmitting a number of viral diseases. Melon aphid is a primary vector of sweet potato feathery mottle virus.
Admire is the most effective insecticide for controlling aphids.
The most common species of thrips on sweet potatoes are the eastern flower thrips and the onion thrips. Thrips are spindle-shaped and 1/16 inch or less in length. Adult thrips have pairs of long, narrow wings fringed with long hairs. The flower thrips is yellowish brown to amber with an orange thorax. The onion thrips is dark brown.
Thrips rasp the tender parts of center leaves and/or terminal buds with their sharp mouthparts and feed on escaping juices. Leaves develop silvery blotches or curl upward. This symptom should not be confused with aphid injury, which causes leaves to curl downward. Injury to plants is always more severe under hot, dry conditions.
The most common species of cutworm in sweet potatoes is the granulate cutworm. This cutworm remains close to the soil surface and feeds on plant stems and leaves by cutting them off at the ground level. Cutworms have a very wide host range and may feed upon a variety of weeds and grasses. There are multiple generations per year. The adult stage is a brown and black moth that is active at night. It does not feed on the sweet potato plant, but will lay its eggs in the field at the base of plants. The feeding by the larvae (worm stage) can be troublesome under dry conditions late in the season where there is a lot of soil cracking. Under such conditions the granulate cutworm may crawl into the soil cracks and feed directly on the developing root, chewing a circular hole that may be ¼ to ½ inch deep.
Control of cutworms can be achieved with many insecticides including Sevin. Once cutworms crawl into soil cracks, control will be difficult. The use of a high rate of gallonage and high pressure will help penetrate the foliage to get the insecticide to the soil surface level where the cutworms occur.
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