Bionomics Of The M Illet Stem Borer Coniesta Ignefusalis (Hampson) (Lepidoptera: Pyralidae)

ABSTRACT

Study conducted from 1993 through 1996 at the ICRISAT Sahelian Centre, Niger,

on the millet stem borer Coniesta ignefusalis were designed to address larval instar

determination and life-fertility table construction, damage and yield loss assessment on

pearl millet, Pennisetum glaucum (L.) R. Brown, and oviposition preference and larval

development on two wild hosts, Andropogon gayanus Kunth and Pennisetum

pedicellatum (Trin.). Biological control studies investigated the suitability of C.

ignefusalis for the larval endoparasitoids Cotesia flavipes Cameron and Cotesia sesamiae

(Cameron).

The larval instars of C. ignefusalis reared on artificial diet in the laboratory were

determined based on head capsule widths and colour, body length and age of the larvae.

The frequency distribution of head capsule widths gave seven plus one intermediate instar

groups. The head capsule widths ranged from 0.2 to 2 mm, and body length from 1.5 to

32 mm. Head capsule colour changed gradually from yellow to brown. Larval age ranged

from 1 to 40 days. Mean head capsule widths were significantly different from one instar

to another and confirmed Dyar’s rule. Body length and age of larvae also differed from

one instar to another. The head capsule widths were positively correlated with body length

(x1- 0.96, P < 0.001) and larval age ( r= 0.91, P < 0.001). Life-fertility tables showed that

on average, 24.41 females were produced per female in a cohort generation time of 65.45

days. The estimated innate capacity of increase (rc) was 0.0133 and the finite rate of

increase (Ac) was 1.0135. C. ignefusalis had a type IV survivorship curve.

Even with natural infestation levels as high as 78.41%, late attack by the millet

stem borer caused bored stems to yield more than unbored ones. Artificial infestations

iv

with 5 and 10 larvae per plant at two weeks after plant emergence resulted in 50 to 70%

plants with dead hearts and 24 to 100% avoidable yield loss. With the same treatments at

four weeks after plant emergence, 7% yield increase and 16% yield loss, respectively were

recorded.

Andropogon gayanus and Pennisetum pedicellatum were non-preferred hosts for

oviposition by C. ignefusalis. Larvae also partially developed on these hosts without

pupae formation. This suggests that these wild hosts could be trap plants rather than

reservoirs for C. ignefusalis carry-over.

Cotesia flavipes and C. sesamiae were equally able to successfully parasitize C.

ignefusalis. The fourth instar was the most suitable host for both parasitoids. C. sesamiae

produced significantly more progeny than C. flavipes, but progeny development was faster

for C. flavipes than for C. sesamiae. Temperature had no effect on percent parasitism, but

for both parasitoid species, progeny developed faster at 26 and 30°C than at 18 and 22°C.