Millet

It is usually an occasional pest, but when outbreaks occur damage to millet can be devastating. The caterpillars eat the above-ground parts of the plants leaving only the base of the stem.

Stemborers are the most important insect pests of maize in sub-Saharan Africa. Yield losses vary between 10-70%. Several species have been reported. The importance of a species varies between regions, within a country or even the same eco-region of neighbouring countries. At least four species attack maize in eastern and southern Africa, with yield losses reported to vary from 20 to 40%, depending on agroecological conditions, crop cultivars, agronomic practices and intensity of infestation.

The most important are the African maize stalkborer (Busseola fusca) and the spotted stemborer (Chilo partellus) (see also below).

The pink stalkborer (Sesamia calamistis) and the sugarcane stalkborer (Eldana saccharina) are of minor importance in maize.

Early warning signs: Young plants have pinholes in straight lines across the newest leaves. This is the time to treat - before the caterpillars move into the stem.

Lesions on foliage are elliptical or diamond-shaped, approximately 3 x 2 mm. Lesion centres are grey and water-soaked when fresh but turn brown upon drying. Lesions are often surrounded by a chlorotic halo, which will turn necrotic giving the appearance of concentric rings. The disease is favoured by hot, humid conditions.

Symptoms often vary as a result of systemic infection. Leaf symptoms begin as chlorosis at the base and successively higher leaves show progressively greater chlorosis. On the lower leaf surface of infected leaves greyish white fungal growth may be observed. Severely infected plants are generally stunted and do not produce panicles. Green ear symptoms result from transformation of floral parts into leafy structures. The disease is prevalent during rainy seasons.

Cream to pink sticky "honeydew" droplets ooze out of infected florets on panicles. Within 10 to 15 days, the droplets dry and harden, and dark brown to black sclerotia (fungal fruiting bodies) develop in place of seeds on the panicle. Sclerotia are larger than seed and irregularly shaped, and generally get mixed with the grain during threshing. Conditions favouring the disease are relative humidity greater than 80%, and temperatures between 20 to 30 degC.

The sclerotia falling on the soil or planted with the seed germinate when the plants are flowering. They produce spores that are wind-borne to the flowers, where they invade the young kernels and replace the kernels with fungal growth. The fungal growth bears millions of tiny spores in a sticky, sweet, honeydew mass. These spores are carried by insects or splashed by rain to infect other kernels.

Several species of grasshoppers attack millets. Short-horned grasshoppers include Zonocerus spp, Oedaleus senegalensis, Kraussaria angulifera, Hieroglyphus daganensis, Diabolocantatops axillaris among others. The long horned edible grasshopper (Homorocoryphus niditulus) is a pest in East Africa.

Grasshoppers defoliate and eat the panicles. They are not of economic importance when present in low numbers. However, invasion by a swarm of grasshoppers may result in serious grain losses.

Immature, green fungal bodies (sori) larger than the seed develop on panicles during grain fill. A single fungal body (sorus) develops per floret. As grain matures, sori change in colour from green to dark brown. Sori are filled with dark spores. Infection takes place at temperature range of between 21 and 31°C, and at relative humidity greater than 80%. The disease is spread by wind-borne spores and rain.

Mealybugs are small, flat, soft bodied insects covered with a distinctive segmentation. Their body is covered with a white woolly secretion. They suck sap from tender leaves, petioles and fruits. Seriously attacked leaves turn yellow and eventually dry. This can lead to shedding of leaves, inflorescences, and young fruit. Mealybugs excrete honeydew on which sooty mould developed. Heavy coating with honeydew blackens the leaves, branches and fruit. This reduces photosynthesis, can cause leaf drop and affect the market value of the fruit.

A wide range of natural enemies attacks mealybugs. The most important are ladybird beetles, hover flies, lacewings, and parasitic wasps. These natural enemies usually control mealybugs. However, mealybugs can cause economic damage to mango when natural enemies are disturbed (for instance by ants feeding on honeydew produced by mealybugs or other insects) or killed by broad-spectrum pesticides, or when mealybugs are introduced to new areas, where there are no efficient natural enemies.

The latter is the case of two serious mealybug pests on mangoes in Africa: Rastrococcus invadens in West and Central Africa and Rastrococcus iceryoides in East Africa. These mealybugs, of Asian origin, were introduced into Africa, where they developed into serious pests since the natural enemies present were not able to control them. They cause shedding of leaves, inflorescences and young fruits. In addition, sooty moulds growing on honeydews excreted by the insects render the fruits unmarketable and the trees unsuitable for shading. They cause direct damage to fruits leading to 40 to 80% losses depending on locality, variety and season. Rastrococcus invadens was brought under control in West and Central Africa by two parasitic wasps (Gyranusoidea tebygi and Anagyrus mangicola) introduced from India (Neuenschwander, 2003).

Rastrococcus iceryoides is a major pest of mango in East Africa, mainly Tanzania and coastal Kenya. Although several natural enemies are known to attack this mealybug in its aboriginal home of southern Asia (Tandon and Lal, 1978; CABI, 2000), none have been introduced so far into Africa (ICIPE).

Insecticides do not generally provide adequate control of mealybugs owing to their wax coating.

It is the most important insect pest of pearl millet in the Sahel. Moths deposit their eggs on the heads of millet, preferring half-emerged and fully-emerged flowering heads. The caterpillars mine into the seeds of the millet head, damaging the millet panicle (i.e. the flower head, where the grain is formed). It has been reported to cause complete crop loss. Pupation takes place in the soil.

Sorghum shoot fly, (Atherigona soccata), is a particularly nasty pest of sorghum in Asia, Africa, and the Mediterranean area. Females lay single cigar-shaped eggs on the undersides of leaves at the 1- to 7-leaf stage. The eggs hatch after only a day or two of incubation, and the larvae cut the growing point of the leaf, resulting in wilting and drying. These leaves, known as 'deadhearts', are easily plucked. When a "dead heart" is plucked, it releases an obnoxious odor.

Adult resemble small houseflies. They are about 0.5 cm long. The shoot fly has been reported as attacking pearl millet.

Damage occurs 1-4 weeks after seedling emergence. The damaged plants produce side tillers, which may also be attacked. The shoot fly's entire life cycle is completed in 17-21 days. Infestations are especially high when sorghum planting is staggered due to erratic rainfall. Temperatures above 35°C  and below 18°C  reduce shoot fly survival, as does continuous rainfall.

Several species of stemborers attack millet including the millet stemborer (Coniesta ignefusalis), the maize stalkborer (Busseola fusca), the spotted stalkborer (Chilo partellus), and the pink stalkborer (Sesamia calamitis).

Stemborer caterpillars bore into stems of millets disrupting the flow of nutrient from the roots to the upper parts of plants. Attack on young millet plants causes damage known as "dead hearts". In older plants the top part of the stem dies as a result of tunnelling by the borers.

The millet stemborer (Coniesta ignefusalis)

It is the dominant stemborer of millet in the Sahelian zone of Africa, and also attacks sorghum, maize, and wild grasses. Major damage has been reported in West Africa. It has also been found causing considerable damage to millet in Western Eritrea, being considered as the major pest of millets in Eritrea (B. Le Ru, icipe, personal communication). The moths have golden brown forewings. They are active throughout the night and during the day rest on the lower surface of leaves or along stems. Caterpillars are cream-coloured with black spots along the body. However in the dry season, when caterpillars enter in diapause (a resting period) they change colour to pale yellow or uniform cream white. They stay in this resting period from 6 to 7 months, but occasionally for more than a year.

Moths lay eggs between the leaf sheet and the stem in batches of 20 to 50 eggs. Caterpillars tunnel in the leaf sheets and in the underlying stem. They normally pupate within the stem. Small plants on which eggs are laid may be thoroughly riddle with caterpillars and soon collapse, but in larger plants external symptoms show two to three weeks after stems have been infested.

Economic damage results from early plant death ( "dead-heart") stem tunnelling, disruption of nutrient flow, steam breakage, poor or no grain formation and empty heads. Crop losses have been estimated at $91 million a year.

For more information on the African maize stalk borer click here. For more information on the Spotted stalkborer click here.

Grains of pearl millet are attacked by major pests such as the lesser grain borer and the khapra beetle. For this reason, the popular concept that millets are hardly susceptible to damage by storage insect pests is erroneous, except for the very small-grained millets such as tef and fonio. The lesser grain borer and the kapra beetle are relatively well adapted to extremely dry conditions and will cause serious damage to millet. Other secondary storage pests do not thrive in semi-arid climates where millets are grown, where stored grain is typically very dry.

Other non-insect pests such as rats and birds may destroy a considerable part of the harvest.