A COMPARATIVE STUDY OF THE COMPOSITION, ABUNDANCE AND LARVAL PARASITISM OF INSECT PESTS IN FRESHLY HARVESTED COBS FROM IRRIGATED AND RAINFED MAIZE IN SAMARU, ZARIA, NIGERIA

Abstract

Cob infestation, insect species composition, lepidopterous larval pest density and larval parasitism rates were assessed in freshly harvested cobs from irrigated and rainfed maize in Samaru, Zaria, Nigeria. Maize cob sampling was conducted at three commercial sales points at weekly intervals over eight months. Infested cobs were collected and kept in the laboratory for observations till adult insect emergence. Data were analyzed using Shannon Diversity Index (H′) and Kruskal-Wallis test (H). Mean percentage cob infestation was significantly higher in cobs from rainfed (40%) than irrigated maize (32%). On the contrary, mean percentage number of damaged grains was significantly higher in cobs from irrigated (74%) than rainfed (16%) maize. Four species (Spodoptera frugiperda (J. E. Smith), Ephestia rectivittella Ragonot, Pococera africalis Hampson and Agromyza verdensis Spencer) and two species (S. frugiperda and E. rectivittella) were respectively recorded as dominant insect pests in cobs from rainfed and irrigated maize. Insect diversity was significantly higher in cobs from rainfed (4.2) than irrigated maize (1.1). Lepidopterous larval density per cob was significantly higher in cobs from rainfed (3.9) than irrigated maize (1.2). Nonetheless, the mean abundance of adult moths (S. frugiperda (2.5/5.1), E. rectivittella (1.0/3.2) and Eldana saccharina Walker (1.0/1.5) did not vary per species in cobs from both rainfed and irrigated maize. Three parasitoid species including Gronotoma sp., an unidentified diptera and Eupelmidae were recovered in cobs from rainfed maize. The mean abundance of adult parasitoids and larval parasitism rates were similar in cobs from both rainfed and irrigated maize. In conclusion, maize cob infestation is moderate to high across seasons. Spodoptera frugiperda and E. rectivittella are the dominant pest species attacking cobs from both irrigated and rainfed maize. Larval parasitism rates are too low and insufficient to manage lepidopterous larval pests in these maize cobs.