Legumes (snap bean and cowpea)
Legumes: Cowpea aphid, Aphis craccivora Koch, Hemiptera: Aphididae
Biology: Snap bean rarely experiences high levels of aphids in Arkansas. Cowpea, however, commonly experiences high levels of cowpea aphid that may require management. The cowpea aphid is minute (2 mm long) and dark purple to black in color (Photos 105 and 106). Clear membranous wings may be present but wingless forms are more common. Aphids occur in colonies that contain different size nymphs and adults. As the newly born nymphs increase in size, molting occurs and the white exoskeleton is left on the leaf surface. Development of nymphs is rapid and many generations occur each season. Aphids feed by inserting their stylet or beak into the plant tissue and removing plant sap. Large amounts of sap are removed and the partially digested contents are excreted onto the plant surface in the form of a clear sticky honeydew. A dark grey mold may later form on the honeydew. Heavily infested plants develop yellow curled foliage, wilt, develop distorted pods and may be killed. Aphids also transmit viral diseases but aphid control is not effective in viral disease management.
Management: Early planted cowpea seldom harbors large aphid populations throughout the field. In late planted cowpea, aphid populations may be very high, but when this population buildup occurs on maturing cowpea, aphid management will likely have little economic benefit. Numerous beneficial organisms affect aphids including naturally occurring insect pathogens, parasites and predators and insecticide use may reduce their effectiveness. Excessive aphid populations on actively growing cowpea may be managed with foliar insecticides but the benefits may be very limited.
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Legume: Bean leaf beetle, Cerotoma trifurcata (Forster), Coleoptera: Chrysomelidae
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Photo 107 |
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Photo 108 |
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Photo 109 |
Biology: Adult beetles are up to 6 mm long (Photos 107 and 108) and emerge from overwintering sites in plant debris near field edges, mate and deposit up to a few hundred eggs near plants but in the soil. Larvae develop within the soil by feeding on plant roots but damage from feeding by the larvae is usually minimal. Damage on bean and pea occurs from adult feeding and when beetle populations are low to moderate, this foliar feeding has little impact on yield. On seedling cowpea and bean, adult feeding can be substantial (Photo 109). Much defoliation can quickly occur and plant growth may be retarded with high numbers of bean leaf beetles. Adults may also feed on the outer skin of bean pods but this is generally minimal. Multiple generations occur each year.
Management: Low numbers of adult bean leaf beetles may be tolerated with a minimal effect on yield. If beetle numbers increase beyond acceptable levels, use of insecticides may be warranted. Traditional insecticides include bifenthrin, zeta-cypermethrin, carbaryl, or acephate. Systemic insecticides such as thiamethoxam, that are applied for other insects will likely provide effective control of bean leaf beetles.
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Legume: Corn earworm, Helicoverpa zea (Boddie), Lepidoptera: Noctuidae
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Photo 110 |
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Photo 111 |
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Photo 112 |
Biology: The corn earworm is generally the most important insect pest of snap bean in Arkansas. In spring planted snap bean, populations are generally low and some commercial fields never develop sufficient populations for management. In fall planted snap bean, however, populations are generally sufficient that management is required. Although yield is not affected, quality of pods is reduced and larvae may be found within the pods at harvest. Cowpea is also susceptible to corn earworm but because cowpea is harvested after drying, the likelihood of getting corn earworm larvae in harvested peas is low. Adult earworms are large bodied moths and are covered in scales that often dislodge and give the appearance of being dusty (Photo 110). Moths generally have green eyes. Shortly after emergence, moths mate and begin to lay eggs on foliage and stems. Eggs are laid individually. Eggs hatch within two to six days depending on temperature and emerging larvae feed on foliage, blooms or fruit (Photo 111). Corn earworms usually do not develop completely on snap bean foliage and require blooms or pods for full development. The color and stripe pattern of larvae vary greatly and at maturity larvae may approach 5 cm in length (Photo 112). At maturity, larvae migrate to the soil where they pupate. Multiple generations of caterpillars occur each season in Arkansas and generations overlap. Caterpillars feed on numerous host plants both wild and cultivated.
Management: In commercial snap bean fields produced for canning, the food processors have zero tolerance for corn earworm larvae in pods. In fields produced during the spring/early summer in Arkansas, larvae may not attack all fields. Thus, sampling for corn earworm prior to insecticide application is important. Currently, the method of choice is to make 300 sweeps with a standard sweep net in each 50 acres of snap bean. Sampling needs to be continued at approximately 4-day intervals from bloom until harvest. If a single larvae is detected, insecticide application will likely be required. In fall produced snap bean, corn earworm larvae are much more common and will likely require management. Several insecticides are currently labeled and effective against corn earworm larvae on snap bean. In areas of cotton production, effectiveness may be reduced due to the level of exposure to insecticides applied to cotton. In snap bean produced in small gardens and for fresh markets, alternative management is effective. Numerous beneficial organisms help to suppress corn earworm populations including insect diseases, predatory and parasitic insects such as Trichogramma wasps. Efforts at conserving these beneficials is of utmost importance. To accomplish this, plants should be scouted for caterpillars. The use of “hard insecticides” should be delayed for as long as possible. Low numbers of caterpillars may be tolerated with a minimal effect on yield. If numbers of caterpillars increase beyond acceptable levels, use of insecticides is warranted. Spinosad likely offers the best alternative to the “hard insecticide”. Also, after exposure to “soft insecticides”, caterpillars will generally stop feeding but death may not occur for several days. Thus determining the effectiveness of the “soft insecticide” should be delayed for at least one week. If high levels of caterpillar populations persist despite use of “soft insecticides”, use of tradition insecticides may be warranted. Use of corn as a trap crop may also reduce the presence of caterpillars on beans and peas.
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Legume: Cowpea curculio, Chalcodermus aeneus (Boheman), Coleoptera: Curculionidae
Biology: Cowpea acreage continues to increase in and adjacent to Arkansas. One of the principal reasons for this increase is the ability to produce cowpea in the absence of cowpea curculio. In the southeastern U.S. the cowpea curculio requires commercial producers of cowpea to apply insecticides numerous times. Currently, the cowpea curculio is not significantly affecting cowpea production in Arkansas. However, because this situation may change a brief discussion of cowpea curculio biology and management is summarized. The cowpea curculio overwinters as adults in plant debris near previously infested legume fields. Adult beetles are about 6 mm long and have the characteristic snout-shaped mouth parts (Photo 113). After mating, female beetles chew a shallow pit in cowpea and bean pods. An egg is inserted and the hole sealed. Upon hatching, larvae feed internally on developing seed within the pod. Cowpea contaminated with infested pods is usually rejected by commercial processors.
Management: Currently cowpea curculio management is not necessary in Arkansas produced cowpea. If this insect becomes more damaging, insecticide applications will likely be required from the time plants bloom until shortly before harvest.
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Legumes: European Corn Borer, Ostrinia nubilalis (Hubner), Lepidoptera: Pyralidae
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Photo 114 |
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Photo 115 |
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Photo 116 |
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Photo 117 |
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Photo 118 |
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Photo 119 |
Biology: In more northern states, the European corn borer is a major pest of snap bean because larvae feeding in pods are difficult to detect during processing and are difficult to manage. Fortunately, in Arkansas, European corn borer larvae are rarely detected in snap bean pods. Spring planted snap bean has been rarely attacked. This may be due to the availability of alternate hosts and the period of snap bean blooming not coinciding with corn borer egg laying. Fall snap bean and cowpea sustain some damage, however. Cowpea plants can usually be detected with European corn borer larvae in stems in late summer and fall. By searching for piles of frass on the ground (Photo 114) and then searching for damage on cowpea stems (Photo 115), larvae within the stem can be located. Damaged plants are usually low in number, impact on yield has been minimal and control efforts on cowpea have not been justified. On fall snap bean, particularly in southern Missouri, occasional damage, as indicated by a part of the plant wilting (Photo 116), can be detected. Damage, however, has generally been confined to plant stems. European corn borer adults are small, about 3/4 in. in length, and fragile moths (Photo 117). Wings are yellow and brown and exhibit a zigzag pattern. Males are slightly smaller and darker than females. In Arkansas, European corn borer adults may be confused with adults of the garden webworm, Achyra rantalis (Guenee), a much more common but slightly smaller moth. Mature European corn borer larvae overwinter in corn stubble and other host plant material in Arkansas. In April and May, European corn borer adults begin to emerge from overwintering sites and mate. European corn borer eggs are laid on foliage in a mass of up to about 25 cream colored eggs in an overlapping fish scale arrangement. Upon hatching, European corn borer larvae are about 1 mm in length. Larvae initially feed on foliage but soon enter the stem and may enter bean and pea pods (Photo 118). The tubercles of the European corn borer larvae are pronounced and the upper integument is darker than the lower integument (Photo 119). European corn borers can be found throughout the Arkansas River Valley, north central, northeastern, and eastern Arkansas. None have been found in southwestern or southeastern Arkansas during recent surveys. The greatest concern with European corn borer larvae is with larvae which go undetected in snap bean pods harvested for canning.
Management: Spring planting dates and early summer harvest of snap bean in Arkansas likely limit opportunity of corn borer larvae to enter pods. Thus, management on spring planted snap bean has not been required. A similar situation occurs with cowpea. Although corn borer is somewhat common in cowpea, it is usually found only in stems. This renders it very difficult to manage and current economics of cowpea production prevent intensive corn borer management. In fall snap bean, European corn borer numbers have been greater but are still usually too low to require management. If management needs change in the future, several tools are available. First, adult European corn borers can be sampled with light and pheromone traps. Based on Foster and Flood (1995) if 10 or more moths are caught in light traps on each of three consecutive nights and snap bean is in the “bud” stage, insecticide treatment may be required. Also, during dry periods, searching grassy areas surrounding the field that hold moisture, e.g., under center pivot irrigators, for European corn borer adults may provide a good indication of population. The time of egg hatch until larvae enter a plant structure may be as little as 3 days. Thus timing of insecticide application is critical. Stalk destruction, particularly in field corn, can reduce overwintering corn borer populations. From the insect management view, the most effective method of stalk destruction is to mow and disk stalks shortly after harvest in late summer or early fall. This leaves the overwintering larvae on or near the soil surface where the effects of low temperatures and rainfall have their greatest effect on producing larvae mortality. Natural enemies including beneficial insects, rodents and birds also contribute to larvae mortality in stalks left on soil surfaces. In late winter, prior to corn borer pupation, the stalk residue should be turned under as much soil as possible. This makes it more difficult for the emerging moths to make their way out of the soil.
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Legumes: Vegetable leafminer, Liriomyza sativae Blanchard, Diptera: Agromyzidae
Biology: Adult vegetable leafminers are minute flies and are only about 1.5 mm long. After mating, eggs are inserted into the underside of the leaf. Host plants include most vegetables and numerous weeds. Eggs hatch in about 1 week and emerging larvae feed by tunneling within the leaf. Larvae feed from 4 to 12 days and as they feed the mine is enlarged resulting in a serpentine like appearance (Photos 120 and 121). When miners are plentiful, foliar loss can be significant and plant nutrition may be reduced resulting in fewer and lower quality fruit. Larvae generally pupate within the larger end of the mine but may fall to the ground where pupation occurs. Adults emerge after about 1.5 weeks and begin the process again. Numerous generations occur each year.
Management: Low numbers of leafminers may be tolerated with a minimal effect on yield. In home gardens, infested leaves can be removed and destroyed. Insecticides directed at other pests may result in a great increase of leafminers which were originally suppressed by beneficial insects. Delaying use of these insecticides may be sufficient for leafminer management.
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Legumes: Lesser cornstalk borer, Elasmopalpus lignosellus (Zeller), Lepidoptera: Pyralidae
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Photo 122 |
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Photo 123 |
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Photo 124 |
Biology: Cowpea and fall produced snap bean often have individual plants that wilt and die (Photo 122). Seedlings are particularly susceptible and this occurrence is generally associated with very dry conditions and light sandy soils. Upon splitting the plant stem or digging in the ground around the stem, turquoise colored lesser cornstalk borer caterpillars can often be detected. Adults are small (25 mm wingspan) fragile moths that lay eggs on cowpea, snap bean or soybean stems or on the nearby soil surface. Other host plants include peanut, field corn and several weeds. Upon hatching, larvae penetrate seedling plants and feed within the stem causing the plant to quickly wilt and die. At maturity, larvae are about 20 mm in length and are turquoise in color on the abdomen (Photo 123). A key identification characteristic of larvae is a rapid wiggling when prodded. Larvae are capable of killing several adjacent seedlings within a row. With larger plants, larvae feed on the exterior of the stem often within a silken web. While larger plants may survive, yield is reduced. Pupation occurs within the stem or within the web on the outside of the stem (Photo 123).
Management: Plant loss from lesser cornstalk borer is generally associated with hot dry conditions and sandy soil. Irrigation is likely the best method for reducing borer impact. Although numerous beneficial organisms affect lesser cornstalk borers, their impact on population development appears to be low. Low numbers of caterpillars may be tolerated with a minimal effect on yield. Sanitation, including removing plant debris from adjacent areas and destruction of crops immediately following the last harvest, may have some effectiveness in reducing damage in subsequent plantings.
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Legumes: Spotted cucumber beetle, Diabrotica undecimpunctata howardii Berber, Coleoptera: Chrysomelidae
Biology: Adult beetles (Photo 125) are up to 7mm long and emerge from pupa within the soil, mate and deposit up to a few hundred eggs near plants but in the soil. Larvae develop within the soil by feeding on plant roots but damage from feeding by the larvae is usually minimal. Damage on bean and pea occurs from adult feeding and when beetle populations are low to moderate, this foliar feeding has little impact on yield. Adults may also feed on the outer skin of bean pods but this is generally minimal. Multiple generations occur each year.
Management Low numbers of adult Chrysomelids may be tolerated with a minimal effect on yield. If beetle numbers increase beyond acceptable levels, use of insecticides may be warranted. Traditional insecticides include methomyl. Systemic insecticides such as imidacloprid, that are applied for other insects, will likely provide effective control of Chrysomelid beetles.
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Legumes: Thrips including flower thrips, Frankliniella tritici (Fitch), tobacco thrips, F. fusca (Hinds) and soybean thrips, Sericothrips variabilis (Beach), Thysanoptera: Thripidae
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Photo 126 |
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Photo 127 |
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Photo 128 |
Biology: Thrips are minute insects found on the leaves and in blooms (Photo 126) of both snap bean and cowpea. Adults are tan to dark brown and about 1mm. long. Two pairs of wings are present and long hairs occur on the edges of the wings (Photo 127). Immature thrips are smaller and lighter in color. Adults and immature stages “rasp” the plant surface with their mouthparts and feed on the exuding plant sap. After mating, females insert eggs into the plant tissue. Larvae emerge and feed by scraping the plant surface and ingesting plant sap. Maturity is reached in less than one week and pupation occurs in soil. Adults emerge in about four days and the cycle is repeated. Multiple generations occur annually in Arkansas. On snap bean, thrips rarely cause significant injury although high populations can cause leaves to curl. Because of the rapid growth of snap bean this damage has little effect on snap bean yield. On cowpea, however, thrips generally constitute the major insect pest in Arkansas. As soon as cowpea plants emerge, thrips attack the new foliage. The result is leaf distortion and yellowing (Photo128). Although seedling injury appears substantial, studies have shown the cowpea tolerates high numbers of seedling thrips with little effect on yield. The greatest impact of thrips is on blooming cowpea. Flower thrips feed within the bloom and often significantly reduce cowpea yield.
Management: On commercial snap bean, thrips rarely reduce yield and toleration is generally the best policy. This is also the case with thrips on seedling cowpea. Although damage on cowpea seedlings may appear severe, plant appearance greatly improves in about two weeks. Flower thrips can be successfully managed on cowpea with systemic insecticides, including acephate and imidacloprid. In periods of drought, a single application of acephate just prior to cowpea bloom has often resulted in substantial yield increases. The future registration of acephate for use on cowpea and other vegetables is currently unclear due to label changes. Thus, prior to its use, registration should be confirmed.
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Legume: Minor pests
Several additional insects feed on snap bean and cowpea in Arkansas. They generally cause little damage and management is usually not required. They are listed here primarily for purposes of identification.
Seedcorn maggot, Delia platura (Meigen), Diptera: Anthomyiidae
The seed corn maggot has occasionally been reported damaging snap bean grown in Arkansas. Problems have always been related to planting into soils with large amounts of decaying plant matter. Particularly, recently converted alfalfa and pasture fields with decaying plants are attractive to the adult seedcorn maggot that deposits eggs near the decaying plants. Larvae later move to snap bean seed and seedlings that may be killed. Additional information can be found in the sweet corn section of this book.
Cabbage looper, Tricoplusia ni (Hubner), Lepidoptera: Noctuidae
Velvetbean caterpillar, Anticarsa gemmatalis (Hubner), Lepidoptera: Pyralidae
Both insects are lepidopterous caterpillars which feed solely on the foliage of legumes including snap bean and cowpea. Little damage occurs and management is not warranted. In snap bean, insecticides applied for corn earworm are effective against both cabbage looper and velvetbean caterpillar.
Leafhoppers, Empoasca sp., Hemiptera: Cicadellidae
Biology: Adult leafhoppers are minute (<3mm in length) insects that use their piercing mouthparts to extract sap from leaves. They also inject digestive juices to assist feeding. The most noticeable form of damage is a yellowing of the leaf edges which is called “hopper burn”. Adult leafhoppers insert eggs into the petiole or stem tissue and hatching occurs in about 1 week. Emerging nymphs are about 1mm long, without wings and generally feed on the bottoms of foliage. Five nymphal stages precede the adult. Low to moderate numbers of leafhoppers have little effect on yield.
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