RA 00110.pdf - OAR@ICRISAT

ance to the pest) because of their ability to produce a
large number of tillers.
The synchrony of head emergence with peak activity
of adults is determined by rainfall, cultivar duration,
planting date, and cultural practices. For
example, late varieties such as Sanio in Senegal;
N K K and C N M 127 in Mali; Sadore, Torini, Haini
Kirei, and their hybrids in Niger (Gahukar 1983b,
Doumbia et al. 1984, I C R I S A T 1984); or the very
early varieties such as Souna from Segou (SRCVO
1980), are not attacked by the spike worm (pseudoresistance).
There is significant correlation between
infestation rate and crop duration (r=-0.83) and time
of head emergence (r=-0.70 to -0.88) (ICRISAT
1981; Guevremont 1982, 1983).
Certain head characteristics could be related to R.
aJbipunctella damage, e.g., length and position of
bristles; and head length, compactness, and diameter.
Of all these characteristics, only compactness —
measured by the number of florets or grains—is
correlated to pest damage (r=0.81) (Guevremont
1983, Gahukar 1984). In the Malian Souna variety,
the combination of compactness and short involucre
and peduncle bristles probably make it unsuitable
for oviposition and larval development (Guevremont
1982).
Stem-borer tolerance is found in the varieties
I N M B 106, I N M B 218, and I N M B 155 from Niger
(ICRISAT 1984). In Zongo, a secretion in the galleries
where the larvae are lodged (NDoye 1977) may be
a resistance mechanism (antibiosis).
Blister beetle incidence was not related to head
chracteristics (position of florets within the glumes,
shape, length, direction, and rigidity of bristles) in
Mali (Doumbia et al. 1984).
Biological Control
Risbec (1950, 1960) has listed the natural enemies
(predators, parasites, pathogens) of most millet
pests. Recently about 20 auxiliary parasites of
Raghuva have been identified for different development
stages (Table 1) (Vercambre 1978; Gahukar
1981; Guevremont 1982, 1983; Bhatnagar 1983,
1984). Among these, Bracon hebetor (Braconidae),
Litomastix sp. (Encyrtidae), and Cardiochiles spp.
(Chalcidiae) appear to be of major importance with
parasitism of up to 48% for eggs, 95% for larvae, and
2% for pupae (Guevremont 1983, Bhatnagar 1984).
Their activity has a significant effect only at the end
of the cropping season, especially in dry years.
The biology of these three parasites, and their
subsequent use as biological control agents has been
studied.
B. hebetor is a very active larval ectoparasite with
a life cycle of 7-10 days. But its performance can be
inhibited by hyperparasites: Eurytoma sp. (Pteromalidae)
and particularly Pediobus sp. (Eulophidae)
which attacks up to 56% of the parasites in Niger
(Guevremont 1983). B. hebetor survives on Ephestia
sp., a pest of stored grain, during the dry season.
Studies are underway to determine the most effective
use of B. hebetor in pest control by transferring
it naturally to pearl millet fields in Aug-Sep.
Litomastix sp. is a polyembryonic egg parasite
that remains in dispause until February when it
emerges from prepupal host larvae in the soil. Up to
800 parasites have been collected from a single larva.
Although parasitism can potentially reach 80-90%
under favorable conditions, it has not yet exceeded
31% in the field (Vercambre 1978, Bhatnagar 1984).
Cardiochiles sp. parasitized only 8% of the larvae
during flowering (Bhatnagar 1984), but the number
of parasites is expected to increase with crop diversification.
This pest develops in Jul-Aug on Heliothis
armigera in association with Acarthospium hispidium,
and later appears on the same insect host as it
infests maize crops in Aug-Sep.
Larvae and pupae of Acigona are attacked by
several parasites (Table 2) (Risbec 1950, NDoye
1977, Gahukar 1981), including Syzeuctus sp. (Ichneumonidae).
This parasite is common in Senegal
and Nigeria where it is reported to have reduced the
larval population by up to 30% (Harris 1962, Bhatnagar
1984). Another parasite Gonozius procerae
attacks only 1-2% of the diapausing larvae (NDoye
1980).
Six parasites infest the larvae and pupae of the
midge Geromyia penniseti: Tetyrastichus diplosidis,
Platygaster sp., Aphanogmus sp., Eupelmus popa,
Eupelmus sp., Tetrastichus sp. The most important
is Tetrastichus sp. (Eulophidae), which represents
80% of the total pest population at the end of the
season (Coutin and Harris 1968).
Diptera parasites (Heliocobia sp.) were collected
from Mylabris holocericea adults (CILSS 1985).
The parasites are less active during the dry season
when their insect hosts are in diapause and climatic
conditions break the parasite-pest synchrony. Although
local natural enemies of certain pests contribute
greatly to reducing the insect host population,
they are not capable of controlling the pests.
The introduction of well-selected exotic species
would be more useful. These should have a good
host-location ability, specificity to the host, high
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