RA 00110.pdf - OAR@ICRISAT

Acigona larvae devour the leaf whorl by penetrating
the main veins, then tunnel through the stems above
the node level (Fig. 2) and feed on the stem pith.
Subsequent dessication of the central leaves results
in deadheart formation. The plant may develop axillary
tillers but they are nonproductive. Sesamia larvae
enter the stem directly, and are responsible for
peduncle damage and plant lodging.
Acigona can complete 2-3 generations during the
cropping season. The adults emerge about 1 month
after the first rains. A source of infestation is larvae
that pass the dry season in diapause in stalks and
stubble left in the field after harvest, and panels
made from plant stems (Gahukar 1983a). Seasonal
incidence of both larval and adult stem borers
depends on several factors, such as the stage of crop
development, and climatic conditions. Severe damage
has been observed on landraces in Mali (Doumbia
et al. 1984), Burkina Faso (Bonzi 1977), Senegal
(NDoye 1977), and Niger (Maiga 1984, ICRISAT
1984). Stem borers are generally active in Aug-Sep;
however, Sesamia larvae have also been observed in
November (Doumbia et al. 1984) because they have
no diapause and can multiply on secondary plant
hosts throughout the year.
Head Pests
Pest damage caused between flowering and harvest
has a direct effect on yield. Several types of insect
pests have been reported at this stage.
Five species of the millet midge: Geromyia penniseti,
Contarinia sorghicola, Lasioptera sp., LestodipJosis
sp., and Stenodiplosis sp., are present in
West Africa (Coutin and Harris 1968). G. penniseti
Figure 2. Acigona ignefusalis damage on millet
stems.
is the most abundant and is common throughout the
Sahel. In M l i , secondary plant hosts of G. Penniseti
identified by Doumbia et al. (1984) include Echinochloa
stagnina, E. colonum, Pennisetum pedicellatum
P. asperifolium, and Setaria pallidefusca.
Damage is caused by larvae feeding on the ovary,
leading to grain abortion. Glumes of the infested
florets retain their flat shape. Incidence varies greatly;
it is high on late varieties when grown together with
varieties of different duration in the same field of
within the same region. Under normal conditions
the brood emerges in September. At the end of the
cropping season, the larvae enter dispause and then
quiescence in the spikelets, attached to the infested
grain. During the cropping season the life cycle is
completed in 2 weeks, allowing 4-5 generations per
season.
Deeming (1979) has reported the presence of a fly,
Dicraeus pennisetivora, that attacks maturing grain
in Burkina Faso, Senegal, and Nigeria. An early
attack leads to total ovary dessication and a late
attack causes lesions on the grain.
Spike worms have become major pests since the
1972-74 drought. This complex includes species of
the Raghuva, Masalia, and Adisura genera (Vercambre
1978, Laporte 1977, NDoye 1979b). R. albipunctella
is the most destructive species in Senegal
(NDoye 1979b, Bhatnagar 1984); it is also widespread
in all the Sahelian countries. The young larvae
perforate the glumes and devour the floret core.
They can be detected by the small whitish granular
excreta. Mature larvae cut through the peduncles in
a characteristic spiral, inhibiting grain formation or
causing grain shattering (Fig. 3). The biology of the
pest has been studied by Vercambre (1978) and
Guevremont (1982, 1983). The extent of damage
depends on synchrony of adult buildup with early
heading (NDoye 1979c), density of larval population,
and plant response to pests and damage. Peak
activity normally occurs in August and September
(NDoye 1979a; Guevremont 1982, 1983). At the end
of the cropping season, the mature larvae descend to
pupate in the soil where the pupae enter diapause
and remain inactive. Adults emerge 1 month after
the first rains. There is only one generation per year
(Fig. 4).
In clayey soils the pupae lie close to the soil surface
(5-15 cm deep), in sandy soils they are buried deeper
(15-30 cm).
Other lepidopterous pests, e.g., Heliothis armigera,
Eublemma gayneri, Pyroderces spp., and Celama
spp. also feed on developing grain, often cutting
them into small pieces. These pests are sporadic and
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