African Bollworm (Citrus)

Authors: T.G. Grout, S.D. Moore. 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. Buy the book.

African bollworm, helicoverpa armigra. Larva feeding on young fruit - W. Kirkman, CRI
African bollworm, helicoverpa armigra. Larva feeding on young fruit - W. Kirkman, CRI

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).