Authors: J. van der Berg, A. Erasmus, M. van Rooyen. Text extracted with permission from the editors from: Prinsloo, G.L. & Uys, V.M. (Eds) 2015. Insects of Cultivated Plants and Natural Pastures in Southern Africa. Entomological Society of Southern Africa.
Helicoverpa armigera
Other common names: American bollworm, cotton bollworm, Old World Bollworm, tomato caterpillar; Afrikabolwurm, Amerikaanse bolwurm, katoenbolwurm (A); lagarta-americana; lagarta-do-tomate (P)
Damage
Young larvae start feeding on young growth immediately after hatching. They feed on blossoms, young leaves and young fruit. Feeding on the fruit results in superficial, round feeding lesions, which causes downgrading to processing quality fruit. At harvest, three visually different forms of damage may be encountered. If the damage was caused during fruit growth by cell division, a callous is formed over the damage at harvest appearing as a round calloused swelling (illustrated). If the damage is caused soon after fruit growth by cell division, the lesions appear as indentations with callousing at the bottom of the indentation (illustrated). If the damage is caused shortly before the fruit starts maturing it appears as round holes in the fruit (illustrated).
African bollworm is a sporadic pest of apples. Although numbers are usually low, during some years high numbers are encountered. At present, the cause of these outbreaks is not known.
Occasionally some larvae may be found on young shoot tips later in the season, but feeding of these larvae is confined to foliage, so no economic damage ensues.
Life history
The eggs are laid singly on or near new growth. They hatch within 3-5 days and the larvae start feeding immediately. Because they are dependent on young growth, African bollworm is only a problem early in the season. By the end of November most of the damage to the fruit has been caused. They then disappear from apple orchards, presumably feeding on wild host plants in the vicinity of the orchards.
Natural enemies
No formal studies have been done on the natural enemies of African bollworm in local apple orchards. However, a small parasitic trichogrammatid wasp has frequently been found parasitizing the eggs. It appears too late in the season to be of any benefit in biological control.
Management
Because of the sporadic occurrence of African bollworm, a standardised monitoring system should be implemented. Orchards are divided into blocks of approximately 2 ha. Twenty-five evenly spaced trees in these blocks are marked. At weekly intervals, starting just prior to blossoming, five maturing spurs should be examined on each of the trees. Each spur should be classified as uninfested, infested with eggs (eggs present) or infested with larvae (larvae present or symptoms of larval damage). This should be continued until after petal fall. Just prior to and during blossom most of the eggs are laid. They take about 3-5 days to hatch. The young larvae are most sensitive to chemicals, but are very small and most difficult to see. When they are larger and more easily seen, they are no longer susceptible. Therefore, chemical treatments should be based on egg counts, particularly as the eggs only take about 3-5 days to hatch. At present, there are no fixed thresholds.
On apples, the eggs are initially white with vertically longitudinal ribs. As the embryo matures the eggs become darker and just prior to hatching they are almost black. As the larva matures the colour lightens from almost black to brown or green, with a conspicuous pale stripe on either side of the body.
Apple African Bollworm Helicoverpa armigera. Callousing, from larval during fruit growth by cell division. - K.L. Pringle Unv. Stellenbosch
Apple African Bollworm Helicoverpa armigera. Damage caused by larvae to maturing fruit. - K.L. Pringle Unv. Stellenbosch
Helicoverpa armigera
Other common names: American bollworm, cotton bollworm, Old World bollworm, tomato caterpillar; Afrikabolwurm, Amerikaanse bolwurm, katoenbolwurm (A); lagarta americana; lagarta-do-tomate (P).
Host plants
This pest has an extremely wide host range, including most vegetables, fruits and berries, many ornamental plants, as well as many wild host plants.
Damage
Larvae feed on young growth, including blossoms, young leaves and young fruit, immediately after hatching. On raspberries and blackberries, damage is found on closed buds, blossoms, fruitlets and growing tips. Younger larvae feed in developing leaf buds; the larvae tunnel into the tightly folded young leaves, leaving perforations along the veins when the leaves unfold. Entire portions of leaves can be eaten away by older larvae. First instar larvae eat through the drupelets in and can then feed, often unnoticed, inside the fruit. Early colouring of the damaged drupelets gives away their presence. Older instars also feed on blossoms, young leaves and immature or mature berries. Blossoms and immature fruit can be almost totally devoured. Malformed berries result from partial feeding. Deep circular holes are eaten into older fruit. Berries tend to fall apart when more drupelets are eaten. On blueberries, the larvae feed on both leaves and fruit; on the latter, deep circular holes are eaten into the immature and mature fruit, as illustrated. Infestation by micro-organisms can occur in the damaged area, which can lead to extensive rotting.
African bollworm is a sporadic pest. The severity of the damage varies between crops and regions, and between seasons. The causes of outbreaks are not properly understood. In the Western Cape, African bollworm has reached pest status on raspberries in the Hemel and Aarde Valley near Hermanus and on Porterville Mountain. On blueberry, serious damage by this pest has been recorded in the Vyeboom area and on Portervill Mountain.
Management
Management is best achieved by implementing a monitoring system based on the inspection of shoot tips and drupelets of selected bushes for eggs, larvae, or larval damage symptoms. Young larvae are the most susceptible to chemicals, but when they are large enough to easily be seen with the naked eye, they have passed their sensitive stage. The necessity for and timing of chemical treatments should therefore be based on egg counts.
Helicoverpa armigera
Other common names: American bollworm, cotton bollworm, Old World bollworm, tomato caterpillar; Afrikabolwurm, Amerikaanse bolwurm, katoenbolwurm (A); lagarta-americana, lagarta-do-tomate (P)
Damage
African bollworm is highly polyphagous. Larvae start feeding on blossoms, young leaves and young fruit immediately after hatching. They eat circular holes into the fruit, the size depending on the age of the larva. Infestation by micro-organisms can occur in the damaged area, which can lead to extensive internal rotting.
African bollworm is a sporadic pest; the severity of the damage varies between crops and regions and between seasons. In epidemic years, extensive damage and resultant crop loss can occur.
Management
Management is best achieved by implementing a monitoring system based on the inspection of shoot tips of selected trees for eggs, larvae or larval damage symptoms. Young larvae are the most sensitive to chemicals, but they become increasingly more difficult to control as they get older. The necessity for and timing of chemical treatments should therefore be based on egg counts.
The eggs are pearly white and ribbed, about half the size of a pinhead, and are laid singly; they turn almost black just prior to hatching. Larval colour varies considerably, making them difficult to describe. Young larvae are generally yellowish to almost blackish, usually darker than older larvae, their backs with numerous black bumps with short hairs, lending them a spotted appearance. Older larvae sometimes retain these spots and often have dark stripes along the sides of their bodies. All instars are characterized by a longitudinal white or beige stripe that extends along each side of the body. They can grow to 30-40 mm in length.
Helicoverpa armigera
Other common names: American bollworm, cotton bollworm, Old World bollworm, tomato caterpillar; Afrikabolwurm, Amerikaanse bolwurm, katoenbolwurm (A); lagarta-americana, lagarta-do-tomato (P)
Origin and distribution
African bollworm is widespread in Asia, Africa, Oceania and southern Europe and is probably not indigenous to southern Africa.
Host plants
African bollworm is highly polyphagous and has been recorded from a wide range of indigenous and cultivated plants in southern Africa, including a variety of field crops, vegetables and deciduous and subtropical fruit.
Damage
On citrus all damage is caused by the feeding activity of the larvae. Fruitlets are the primary targets (illustrated). They can be destroyed before all the blossom petals have fallen. Heavily damaged fruitlets can drop. However, infestation has to be extremely severe to result in any crop reduction. After fruit set, the appearance of damage can range from shallow marks to deep holes in the peel. On mature fruit, this early damage leaves unsightly blemishes of uneven shape and depth which makes fruit unacceptable for export. On navel oranges, 86% of bollworm-induced damage occurs on the navel end of fruit, which can result in unacceptable enlargement or malformation of the navel end. The edges of young leaves may also be eaten, but this damage is of secondary importance.
Mature Valencia oranges can also be attacked occasionally, but this only happens in spring when trees begin to blossom and fruit has not yet been harvested. Bollworm is only a pest in spring and occurs sporadically in all production areas. It is an important pest as it can cause significant damage and crop loss within a few days.
Management
From blossoming until petal fall, scouting for eggs and larvae must be conducted at least weekly. Extensive sweep surveys should be conducted to identify localised areas that may be threatened by heavy infestations. Commercial pheromone traps are also available for monitoring this pest, although treatment thresholds have not yet been determined.
An increase in egg presence on blossoms will provide an indication of the extent of the larval attack to come. A treatment should be applied when more than 20% of blossom clusters are infested with larvae or mature eggs. Enlarged navel end in navel oranges can be exacerbated by bollworm attack, in which case, a threshold of 11% of clusters infested should be used. Only once around 40% or more clusters are infested, will a reduction in crop load result.
A few organophosphates and a carbamate are registered for control of bollworm on citrus. However, it is preferable to use IPM compatible treatments such as nucleopolyhedrovirus, Bacillus thuringiensis, spinosad or spinetoram (the latter two are derivatives of a soil-borne actinomycete bacterium).
Helicoverpa armigera
Other common names: American bollworm, cotton bollworm, Old World bollworm, tomato caterpillar; Afrikabolwurm, Amerikaanse bolwurm, katoenbolwurm (A); lagarta-americana; lagarta-do-tomate (P)
Locally, Helicoverpa armigera (Heliothis armigera) has in the past often been referred to as the American bollworm because it was thought to be identical to Helicoverpa zea (Boddy), the corn earworm from the Americas. The two species are, however, distinct and, to prevent any confusion, the use of the name American bollworm is discouraged.
Origin and distribution
The African bollworm, which is probably native to Africa and found throughout the continent, was originally described from Europe and is also widespread in Asia and Australia.
Host plants
The African bollworm is undoubtedly the most polyphagous agricultural pest in southern Africa, attacking almost all field crops in addition to a wide range of deciduous fruit, vegetables and ornamental flowers. It is also found on a large variety of indigenous plants, many of which are listed by Kroon (1999). It also occurs commonly on weed species found in and around cotton fields, including several malvaceous plants, in particular Abutilon austro-africanum, Abultilon guineense, Acanthospermum hispidum, Cienfuegosia hildebrandtii, Corchorus trilocularis, Hibiscus vitifolius, Fusticia flava and Pavonia burchellii.
Damage
Young larvae feed on the small buds (illustrated), known as pin-head squares, making holes on the outer bracteoles of the squares before entering the bud to feed. Although it is mainly the squares that are damaged, the flowers and bolls are also attacked, but to a lesser extent. A high percentage of squares may drop to the ground, so-called shedding of squares, which affects yield negatively. Larger larvae often damage open flowers (illustrated) and fourth, fifth and sixth instars damage larger bolls (illustrated). Larvae also feed on the leaves and stems of cotton plants.
Life history
The African bollworm is mainly a middle season pest on cotton, and it occurs in the fruit formation phase and peak phase of cotton, from 12-14 weeks after planting. The fruit formation stage starts at six weeks after planting when the first squares start to form, and it is at this stage when the African bollworm starts to lay eggs, mainly on young squares, but also on the upper surface of cotton leaves. The female moth starts to mate and lay eggs about four days after emerging from the pupa and lays an average of about 730 eggs and at most 1600, during about 2-3 weeks or more of her lifespan. The eggs, usually laid 2-3 per leaf or square hatch after 2-4 days in late spring or summer after which the young larvae enter the squares to feed, later attacking the flowers and bolls. There are five or six larval instars and the entire larval stage last about 2-3 weeks, depending on temperature.
The fully grown larva burrows into the soil where it pupates at a depth of up to about 180 mm beneath the surface. In midsummer the pupal stage may be as short as 15 days, which may give rise to two or more moth flights during February and March. However, the pupal stage usually becomes longer with the onset of cool weather in late summer and most pupae will overwinter in a state of diapause, emerging as adults in spring during September and October. The adult moths are strong flyers and are most active from sunset until dark.
Natural enemies
Numerous indigenous parasitoids, mainly tachinid flies and parasitic wasps of various families, are known to attack Helicoverpa armigera in Africa, many of which have been documented by Van den Berg et al. (1988). Common among the wasps known from southern Africa are the egg parasitoids Trichogrammatoidea lutea Girault (Trichogrammatidae) and Telenomus ullyetti Nixon (Scelionidae), and the larval parasitoid Chelonus curvimaculants Cameron (Braconidae). Tachinid flies include the larval parasitoids Paradrino halli (Curran) and Drino laxa (Curran). During the 1970s and the 1980s several parasitoids were introduced into South Africa from various other countries for the control of African bollworm on cotton and other crops. These included the trichogrammatid egg parasitoids Trichogramma chilonis Ishii, T. perkensi Girault, T. semifumatum (Perkins), T. ostriniae Pang & Chen, T. pretiosum Riley and Trichogrammatoidea brasiliensis (Ashmead) in addition to the larval parasitoid Cotesia kazak (Telenga) (Braconidae). With the exception of T. pretiosum none of the species became established. Recorded predators include a wide range of insects such as ground and ladybird beetles, ants, earwigs, anthocorid and mirid bugs and lacewings, in addition to spiders and predatory mites.
Management
Although the complex of indigenous parasitoids and predators contribute to the suppression of bollworm populations in the field, it is unable to keep bollworm numbers under economic threshold. With the introduction of transgenic Bt-cotton varieties to South Africa, chemical control of the bollworm complex, comprising the African bollworm, red bollworm and spiny bollworm, has decreased. Only in rare instances where bollworm thresholds are reached for Bt-cotton varieties, is it still necessary to spray, in which case contact insecticides should preferably be used, followed by a systemic insecticide if necessary
Bt-cotton varieties, expressing inherent bollworm resistance, are planted to control the bollworm complex. These varieties may require chemical intervention when bollworm infestations are extreme. To combat the possible development of resistance to the Bt proteins, an area of non-Bt-cotton, called refuge, is planted. The purpose of the refuge is to harbour an adequate population of individuals not exposed to the Bt-protein and therefore not resistant to it. These non-resistant individuals will pair with potentially resistant individuals emerging from adjacent Bt fields. The refuge cotton planted should either be five or 20% or the total area; in the latter case only, is chemical intervention indicated and then only once the threshold has been reached.
The threshold for conventional (refuge) cotton is reached when five or more larvae of the bollworm complex per 24 plants are observed. For Bt varieties, the threshold is reached when one or more larvae of the complex is observed on five or more plants out of the 24 plants scouted.
Small, first instar larvae are often found inside squares, which should be opened during scouting to observe and identify the different instars of all the species in the bollworm complex. During scouting it is important to distinguish between first instar larvae that have not yet fed and larvae that have fed, as fed larvae, having ingested the Bt-endotoxin, should soon succumb. Distinct morphological differences are evident in fed and unfed larvae. Unfed larvae have a short, narrow pronotal shield and a relatively short body compared to a fed larvae, where the pronotal shield is long and wide and the body appears longer (K. Hake, pers. comm.). Unfed and moribund larvae should be disregarded when determining whether the economic threshold has been reached, as moribund larvae, although contributing to the larval count, do not warrant chemical intervention.
Guidelines for the use of pesticides for the integrated pest management of cotton pests in South Africa is provided by Basson (1987) who cautions against the use of synthetic pyrethroids, which are non-selective, often giving rise to secondary outbreaks of pests such as mites, cotton stainers and aphids. In any event, pyrethroids should never be used in the pyrethroid free window, between, 8-12 weeks after plant emergence.
Egg: Shiny, yellowish-white at first, becoming dark brown before the hatch, almost spherical, about 0.5 mm in diameter, the surface with distinct longitudinal ridges; laid singly, usually on the upper side of leaves, leaf stems or young squares or flowers.
Larva: There are 5-6 larval instars on cotton. Newly hatched first instar larvae (neonates) are about 1.5 mm in length and grow to 3 mm; fully grown larvae attain a length of 28-40 mm, depending on temperature. On cotton, larvae are often divided into three categories to facilitate scouting, and they are generally referred to as small larvae, or larval stage one and two (L1-L2, varying in size from 1.5-5.0 mm), medium larvae (L3, 5-10 mm) and large larvae (L4-L5, >10 mm). Larval colour varies considerably (illustrated), making it difficult to describe. Young larvae are generally yellowish to almost blackish, usually darker than older larvae. Older larvae often have dark stripes along the sides of their bodies and are characterized by a longitudinal white or beige stripe that extends along each side of the body.
Pupa: Smooth, shiny brown, about 15-20 mm long, with two short spines at the posterior tip of the body. Found in the soil, often directly under the plant.
Adult: Colour varies considerably, the forewings dull young, pale brown (illustrated) to reddish-brown, often with an irregular brownish cross-band and a dark mark in the middle of each wing; hind wings paler, greyish-white, the hind margins broadly marked with dark brown. Moths are about 15-20 mm long when at rest with a wingspan of 35-40 mm.
Helicoverpa armigera
Other common names: American bollworm, cotton bollworm, Old World bollworm, tomato caterpillar; Afrikabolwurm, Amerikaanse bolwurm, katoenbolwurm (A); lagarta-americana; lagarta-do-tomate (P)
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African bollworm, Helicoverpa armigera.. Larva on a maize ear. - A. Erasmus, ARC
African bollworm, Helicoverpa armigera. Typical larval damage to a maize leaf. - T. Joffe
Damage
Bollworm larvae attack both the leaves and ears of maize plants. Damage to leaves of maize is not common but is sometimes visible when larvae eat larger holes through the whorl leaf roll. When leaves grow out, the holes are visible in rows across the leaves, as illustrated. The most common damage on maize is to the silks of young ears. This damage may be of such an extent that pollination cannot occur. Larvae (illustrated) damage the tips of pollinated ears by feeding on the top part of leaves and kernels, exposing ears and making them vulnerable to secondary attack by pests and diseases. This exposure may cause ear rot due to rainwater that flows int the ear. Holes eaten by larvae into the sides of the ears also result in ear rot when ears remain wet. Faeces that are characteristic of the presence of African bollworm larvae are always found near feeding sites.
The extent of the damage depends on the stage of development of the crops when bollworm eggs are laid. If bollworms feed only on the tips of ears after pollination, damage is not severe. When larvae damage silks to such an extent that pollination cannot occur, it can contribute to direct yield loss.
Life history
Moths are nocturnal and lay eggs singly near or on maize ears. One female can lay more than 1000 eggs during her lifespan. Prior to mating and laying eggs, moths feed on nectar and other sources of sugar in the environment. Eggs hatch within 3-5 days. Larvae moult 5-6 times, during which they change colour. The larval stage lasts for approximately one month. When fully grown, they migrate from where they fed on the maize ear and burrow 60-100 mm deep into the soil where they pupate. The pupal stage lasts for about two weeks but, in cooler environments such as the Highveld region of South Africa, may last longer during winter when pupae enter a dormant stage. This explains a peak in moth flights that usually occurs during August to October in areas with cold winters. In areas with mild winters and long warm summers where a succession of crops is grown under irrigation, bollworm generations succeed one another at short intervals. When pupae do not go through a dormant stage, the insect takes about 50 days to complete its life cycle.
Natural enemies
Bollworm eggs and larvae are attacked by various parasitic flies, wasps and other natural enemies. In addition, larval populations are sometimes suppressed by bacteria and a polyhedrosis virus. Bollworms infected with the virus are inactive and do not feed, usually have a puffy appearance and lie on the upper surface of leaves or hang upside down from plants. The efficacy of the virus increases when the temperature and humidity are high, resulting in total eradication of severe bollworm infestation within days. However, natural enemies are not always able to suppress outbreaks of this pest.
Management
Although cultivation during winter can kill pupae in the soil this practice is not recommended as a general measure, as bollworm outbreaks are sporadic and only occur in isolated areas. Insecticide application in fields where large numbers of bollworm eggs are observed is not justified, since up to 90% of these eggs do not hatch or are killed by natural enemies. If it is necessary to control bollworm, insecticides should only be applied during the larval stage. Chemical control should be planned properly since unnecessary and long term application of insecticides may lead to development of insecticide resistant populations of bollworm.
