Bionomics of the Millet 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 (r2- 0.96, P < 0.001) and larval age (r2= 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 (λc) 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 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.