Life cycle and description
- Adult stage: Adult moths generally have dark brown, patterned foreqings and white to gray-brown hind wings. In wet, cold conditions adult moths develop darker coloring. Adult moths are nocturnal and have a wing span of 25 to 40 mm. Females are generally slightly larger than males. Females usually mate on the night of emergence and will oviposit on the subsequent 3 to 4 nights. Females lay eggs in batches of 100 to 200. Each female can lay up to 1000 eggs in a lifetime. Adults have an area of dispersal of up to 1.6 km and occasionally further.
- Egg stage: Eggs are light yellow in color, spherical, and approximately 1 mm in diameter. Eggs are characterized by approximately 70 radial ridges on the upper surface. Eggs are laid in a single column on the stem under leaf sheaths. Eggs hatch in about 10 days.
- Larval stage: Young larvae are generally dark brown, purple, or black in color. They migrate to the whorl to feed. There are usually 6 larval instars although 8 are possible in unfavorable conditions. Instars burrow into the stem and feed on the central stem tissue. Typically only one larva is found per stem, as larvae are cannibalistic. Later, instars are creamy-white with a gray or pink tint. Mature larvae can be up to 40 mm in length and have small black spots along the length of the body. Larvae mature in about 35 days and pupate in the stem. Prior to pupating, larvae cut a small hold in the stem which enables the adult moth to emerge.
- Pupal stage: Pupae are generally 25 mm in length and shiny yellow brown to dark brown in color. Males are usually smaller than females. Pupae are found in galleries within the stem and have a pair of plain spines located on the terminal cremaster. The pupae stage lasts from 2 to 3 weeks.
- Usually two to three generations occur during the growing season.
- Often, eggs from the second or third generation are laid around the ears and emerging larvae, then cause extensive damage to young kernels before burrowing into the stalks.
- In favorable conditions, the life cycle can take 7 or 8 weeks.
Stem borers can not be identified from the larvae along; therefore, it is necessary to assess adults either by rearing them from larvae or using moth traps to catch adults at the start of the season.
Problems with similar symptoms
The African pink borer and the African sugarcane borer also bore into maize stems and ears. The African pink borer is very similar in appearance to the African maize stem borer.
Why and where it occurs
- Busseola fusca is indigenous to Africa and is considered the most important insect pest of maize at altitudes of 500m above sea level in sub-Saharan Africa.
- In maize crops, female moths prefer to oviposit on plants less than two months old and on the youngest unfolded leaf. The female also prefers plants with thicker stems.
Maize, sorghum, millet, sugarcane, and various thick-stemmed grasses such as sudan and napier grass.
This species occurs throughout sub-Saharan Africa in altitudes ranging from sea level to above 2000 m.
- Mechanism of damage: Damage is caused by the destruction of the growing point in the whorl (dead heart), loss of photosynthetic leaf area due to foliar feeding, lodging due to burrowing in the stem, and extensive damage to young kernels due to feeding of larvae from the second and third generations.
- When damage is important: Damage is critical when the growing points of young plants are completely damaged, when there is extensive lodging due to stem boring, and when second and third generation larvae feed directly on the cob.
- Economic importance: Busseola fusca is considered the most important pest of maize in sub-Saharan Africa. Yield loss as high as 40% has been attributed to stem borers.
- Light and pheromone traps can be used to assess the incidence of adult moths.
- Plants should be inspected for signs of larval feeding in the whorl, dead heart, and stem boring.
- Plants should also be inspected for presence of eggs under leaf sheaths or on cobs.
- Export of ornamental grasses or stalks should be monitored for the presence of eggs and diapausing larvae to prevent introduction of stem borers to other regions.
- As larvae survive in the diapausing state in overwintering crop debris, exposing crop residue to direct sunlight or using crop residue for livestock feed or compost can reduce the incidence of diapausing larvae.
- Pheremone and light traps can be used to trap adult moths.
- Modifying sowing periods to avoid periods of heavy infestation can reduce crop damage.
- Intercropping with legumes or cassava can reduce the incidence of stem borer infestation on maize.
- Under severe infestation, chemical control can provide an effective means of pest management. However, chemical applicaion is only effective if pest scouting and monitoring has been successful prior to crop damage. Furthermore, as stem borers burrow into the stem, they are often protected from insecticide application.
- There are a number of natural enemies of Busseola fusca including various Hymenoptera parasitoids of the larvae, pupae, and eggs.
- Effective host resistance has yet to be integrated into most locally adapted varieties.
- Under the push-pull system, maize is intercropped with plants that release odors that repel ('push') adult moths out of the maize field to trap crops outside the field that 'pull' adult moths. Typically, napier and sudan grass are used as trap crops. They are important fodder crops and produce a gummy substance that traps moths and prevents over 80% of larvae from reaching adulthoood.
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Contributors: Gabrielle Turner, David Bergvison, and Biswanath Das