Early identification and effective control practices can significantly reduce the impact of pests on crops.
Farmers should not only familiarise themselves with pests that pose threats to their crops, but also learn more about their developmental stages and the best methods to control them.
Dr. Annemie Erasmus, a research entomologist from the South African Agricultural Research Council’s Grain Crops Institute (ARC-GCI) offers tips for opposing some of the most common pests found on maize crops:
- the maize stem borer
- the Chilo-borer
- the Pink stem borer
- the common cutworm
- the African bollworm
- the False bollworm
African bollworm (Helicoverpa armigera) attacks a wide range of agricultural crops. Bollworm moths fly at night and lay their yellow-white eggs on maize ears and leaves. These eggs hatch after three to five days and larvae moult five to six times, during which time they change colour. A characteristic pale white band runs along the sides.
Larvae initially feed on the silks and later penetrate the tips of maize ears. When ears are still young and bollworm infestations are severe, the silk can be damaged to such an extent that poor pollination occurs. If leaves of young ears are damaged during rainy periods, water can enter the ears and lead to fungal growth that will cause kernels to become discoloured. However, in most cases only the tips of ears are damaged and yield losses are slight.
- Bollworm eggs and larvae may be attacked by various parasitic flies, wasps and other natural enemies.
- Larval populations are sometimes suppressed by bacteria and a polyhedrosis virus.
- Cultivation during winter can kill pupae in the soil, but cannot be recommended as a general measure, as bollworm outbreaks are sporadic.
- If chemical control is necessary, insecticides should only be applied during the larval stage.
- Chemical control of bollworm has to be planned properly since unnecessary and long-term application may lead to the development of insecticide resistance, Erasmus cautions.
False bollworm (Acantho leucania loreyi) is not as abundant as African bollworm. Moths lay their eggs in batches of up to 100 eggs between the leaf sheath and the stem. Eggs hatch about 5 days later.
Larvae have the characteristic white bands along the sides, similar to the African bollworm, but on this species it is pale in colour. Leaves are skeletonised by young larvae, and later older larvae become gregarious and feed voraciously, eating the entire leaf.
Damage to maize ears is similar to that caused by the African bollworm. Larvae may be present on younger maize whorls and maize ears, as well as tillers.
- Because this pest is not that serious on maize and not that abundant, research is still ongoing to identify the most effective control measures, says Erasmus.
SA is home to a wide variety of cutworms. Common cutworm poses the greatest risk to maize seedlings. Cutworm moths usually lay their eggs during autumn and winter on the leaves of weeds and volunteer plants.
Larvae hatch a week after the eggs were laid and feed on the leaves of weeds and winter crops. After the second moulting, the larvae burrow into the soil and only come out at night to feed.
During August and September, the larvae develop into pupae and live in pupal cells in the soil. The first-generation moths for the new season emerge from these pupae about two weeks later.
Cutworms sever seedlings at the soil surface with a neat round hole in the subterranean part of the seedling stem. They move from seedling to seedling, which can result in a great number of seedlings being damaged in a single night.
- The most effective way to control cutworms is to remove weeds before planting.
- Where practically possible, all fields should be cultivated 35 days before planting and kept free of weeds and volunteer plants until planting.
- Chemical control can also be used preventatively or correctively.
MAIZE STEM BORER
Stem borer (Busseola fusca) reduces crop yield on average by 10%. Total losses in SA have been estimated at 5% to 75% and sometimes even higher.
2 to 3 peaks in moth flights generally occur per season. Maize plants are usually attacked by either the first or second generation, depending on the planting dates. Female moths usually lay egg batches behind the leaf sheath.
The newly hatched larvae are dark brown, but become lighter as larvae mature. Soon after hatching, larvae migrate to the maize whorls where they start feeding. Damage looks like small “windows” after the whorl leaves unfold. As the larvae grow, the holes in the leaves become bigger, until rows of “shot hole” damage appear.
Further damage causes the growth point to die, resulting in wilting of the central whorl leaves, commonly called “dead heart”. Larvae remain in plant whorls for a long period before they bore into the stem.
The duration of the larval cycle is about six weeks after which larvae start to pupate. Larvae spend winter in stubble which is the main source of infestation the following season.
Chilo-borer (Chilo partellus) has a short life cycle resulting in large-scale overlapping of generations during the season. They can have up to 5 generations per season. Moths lay eggs either on the upper or lower surfaces of leaves.
Eggs are disc-shaped, yellow and arranged imbricate. The larvae are cream-coloured and have 4 rows of spots on their backs and one on each side. Larvae migrate to the maize plant whorls where they feed for 10 to 14 days.
They then start entering the stem. Damage caused by Chilo is similar to that of the maize stem borer. About 30% of the larvae remain behind the sheath, which makes it difficult to control them with chemicals. Larvae become quiescent in winter and take shelter in stems or behind leaf sheaths in winter.
PINK STEM BORER
Pink stem borer (Sesamia calamistis) is mainly found in the coastal regions of the country, but is increasingly being encountered in interior provinces. Moths lay their eggs in batches between the leaf sheath and the stem of the maize plant.
Larvae that hatch under leaf sheaths penetrate the stem directly. “Dead heart” symptoms are the first observed and larvae don’t damage whorl leaves. Generations overlap to such an extent that specific generations cannot be distinguished. Larvae develop throughout the year and there is no rest stage.
STEM BORER CONTROL
Genetically modified maize can be used to control all three of the stem borer species. The Chilo-borer and the Pink stem borer are controlled effectively, but according to Erasmus, there have been reports of the survival of the maize stem borer. Non- BT maize has to be planted with BT maize to prevent the development of insect resistance.
Chemical control is an important tool for stem borer control in maize. The economic threshold for stem borer control in maize is when 10% of the plants in a field exhibit visible symptoms of whorl damage. Since a large part of the life cycle of stem borers is spent in plant whorls they are relatively easy to reach with insecticides. Whorl applications are the most effective method of control.
Plant residues in fields after harvest are the most important source of stem-borer infestations during a subsequent season, since stubble provides winter sites for larvae. Removing and destroying stubble before planting is an effective way to kill a large proportion of stem borer larvae.
Plant residues in fields after harvest are the most important source of stem borer infestations during a subsequent season, since stubble provides winter sites for larvae. Removing and destroying stubble before planting is an effective way to kill a large proportion of stem borer larvae.
- This article was written by Rene Bosman and first appeared in Farming SA.