Armyworm
The armyworm exists in two distinct phases:
1.The solitary passive phase, where the larvae have green and brown stripes on the back
2.The active phase, where the larvae are velvety black on the back surface, with several yellow lines running the length of the body, broad white lateral bands and a pale green lower surface. It is the latter phase that is predominant in an outbreak, although some larvae may be between the two phases in appearance.
Crop damage
The African Armyworm is notorious for its sudden appearance in outbreaks of varying intensity every few years in Zimbabwe and the region. They feed on the leaves of cereal plants and can seriously strip the leaves so that only stems and mid-ribs remain. They can wipe out young cereal crops.
Management/control
•Warnings of imminent outbreaks are usually broadcast in the media. If such warnings are received, or even as a regular precautionary measure, scout the crop for the appearance of the pest in December/January.
•Chemical control is possible with carbaryl, endosulfan, malathion and trichlorfon.
•Digging a shallow trench ahead of marching caterpillars will slow them down and concentrate them in an area where they can be physically destroyed.
African Bollworm
They vary in colour, but are often dark, and may not be immediately noticeable, especially in sorghum, where they have a habit of feeding well within the heads. Small whitish frass pellets are a good indication of their presence.
Crop damage
Bollworms infest both whorls of leaves and heads. Young larvae feed on tender, folded leaves in the whorl. When heads emerge, they feed on developing florets and hollow out developing seeds.
The most damage is from adult larvae which feed on grain at the milky stage. Larval excrement or frass is common in infested heads and on the tops of upper leaves and the ground under plants. Under certain conditions, seeds of infested heads may develop mold fungus.
Management/control
•Spray the crop with endosulfan
•Use sorghum cultivars with loose or open panicles. Larvae on open panicles are exposed to predaceous insects and birds
Grasshoppers
Small grains are attacked by many species of grasshoppers. Among these are short horned and long horned grasshoppers.
Crop damage
The grasshoppers eat and defoliate the panicle. They feed on the leaves of the plant. They are not a threat when present in small numbers. However, large numbers can wipe out the whole field.
Management/control
•Dig or cultivate the field before planting to kill the eggs and expose them to predators.
•Grasshoppers lay eggs on bare ground, so minimum inter row spacing should be used to give the ground enough cover.
•Spray with carbaryl.
Spotted and pink stem borers
They infest the crop any time from the second week of emergence to maturity. They can easily be seen inside the stem of infested plants.
Crop damage
The appearance of small, elongated holes in young whorl leaves is the first sign of stalk borer infestation. The holes join to give the leaves a ragged appearance. The borers then bore the stem, where they eat the central portion, killing off the growing point to give dead heart symptoms. The larvae may also attack heads, resulting in the breakage of the head before maturity.
Management/Control
•Deep ploughing to bury the residues early after harvesting
•Uprooting of crop stubbles and chopping of stems prevents borer carry over to the next season
•Tolerant varieties can be grown if available
•Carbaryl or carbofuran may be applied on the plant whorls about four weeks after crop emergence
Harvesting
The best stage to harvest the crop is when it has reached physiological maturity. The crop is physiologically mature when leaves turn yellowish and begin to dry naturally, and a black spot appears at the bottom of the grain. The longer the crop is kept in the field after physiological maturity, the greater the losses due to birds, insects, rodents, mold and bad weather.
Basin method
•Remove heads (and take them for drying), leaving stalks standing in the field
•Cut stalks at the base at a point that will not allow rationing of the crop (very close to the root)
•Spread the cut stalks between rows in the field
Ripper-tine method
•Remove heads (and take them for drying), leaving stalks standing in the field
•Cut stalks at the base at a point that will not allow rationing of the crop (very close to the root)
•Spread the cut stalks between rows in the field
Conventional method
•Remove heads (and take them for drying), leaving stalks standing in the field
•Cut stalks at the base at a point that will not allow rationing of the crop (very close to the root)
•Spread the cut stalks between rows in the field, or collect and put them in a safe place so that they can be fed to livestock later in winter
Field management in dry season
Basin method
•Remove weeds that are still in the field
•Prepare basins in the same positions as last season and start all over again
Ripper-tine method
•Remove weeds that are still in the field
•Prepare the rip lines in the same positions as last season and start all over again
Conventional method
•Using the conventional plough, turn the soil, burying all crop residues and weeds
•Prepare planting furrows and start all over again
Post-harvest management
Drying: Sorghum, pearl millet and finger millet can reach physiological maturity when the grain still contains high moisture. Therefore, the harvested heads should be dried before threshing, by exposure to the sun.
Threshing: The heads are ready for threshing when the grain easily falls off the head with minimal impact. At this stage threshing can be done by pounding or beating the heads with sticks. Threshing must be done on a clean hard surface free from soil particles and stones. Motor or electric driven stationary threshers, which thresh and clean the grain at one can be used.
Winnowing: After threshing winnowing is done using natural wind to remove larger particles and trash from the grain. The final grain must be clean and free of any foreign matter and glumes.
Grain treatment: Before storage, the grain is treated to avoid damage by storage pests. Traditional methods of treatment can be used, as well as registered grain storage chemicals. A range of grain treatment chemicals are available on the market. Choice should be based on price, time between grain treatment and consumption, efficacy and spectrum of storage pests controlled.
Storage: Clean, treated grain can either be stored in bags, or direct in the granary. Grain stores best at a moisture content of 10-12%. The use of improved grain storage structures is encouraged. Where grain is stored in bags the bags should be placed on wooden pallets (or their equivalent) and off the wall to prevent absorption of moisture from the floor and walls.
We thank the Zimbabwe government and the Food and Agricultural Organisation of the United Nations for the use of this guide.
