African bollworm (Apple)

Authors: K.L. Pringle, B.N. Barnes, T.L. Blomefield. 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.

K.L. Pringle Unv. Stellenbosch - Apple African Bollworm Helicoverpa armigera
Apple African Bollworm Helicoverpa armigera. Indentation by larval feeding - 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)

Identification

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. 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
Apple African Bollworm Helicoverpa armigera. Damage caused by larvae to maturing fruit. - K.L. Pringle Unv. Stellenbosch

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.