Population Ecology of the Legume Pod Borer Maruca testulalis Geyer (Lepidoptera: Pyralidae) in Relation to its Natural Enemies on Cowpea in Western Kenya

Abstract

Studies were conducted at the International Centre of Insect Physiology and Ecology, Mbita Point Field Station (MPFS) fana and the Lambwe Valley fanaer's field during 1983 - 1985 , to detenaine the natural population changes of Maruca testulalis Geyer (Lepidoptera; Pyralidae). The role of natural enemies, host plants and cliaetic factors on natural population, development and survival of M· testulalis was investigated. Further investigations on the biology of Tetrastichus sesaaiae (Chalcididae, Eulophidae) a gregarious pupal endcr-perasitoid as well as bioassays on Nosema sp., a protozoan pathogen of the pest were conducted in the MPFS laboratory. It was found that M· testulalis colonised the crop at least 15 days after plant emergence when the first adult moths were recorded in the pheromone traps. The egg and larval populations .started increasing steadily at the flower initiation stage reaching a peak between 42 and 54 DAPE at both sites. Only one generation of M· testulalis occurred on each crop of cowpea. From the partial ecological life tables it was found t hat total real aortality for the generations at MPFS and Lambwe ranged between 51.7 - 98.~ and 96.4 - 97.7~ respectively indicating very low survival in this species. Several natural enemies were found associated with M· testulalis, including 7 parasitoids, five of which attacked pupae and 2 on larvae. No egg parasitoid was found. The major pathogenic microorganisms found on dead M· testulalis larvae and pupae were Nosema sp. and Bacillus sp. Observable parasitism played negligible role in causing mortality on M· testulalis being only 0.02 - 0.09 and 0.04 - 0.0~ at MPFS and Lambwe respectively. Pathogens especially Nosema sp. and Bacillus sp. contributed significantly to mortality of M· testulalis at both sites. Disappearance designated as other losses not due to either parasitoids or pathogens also accounted for a large proportion of mortality. Analysis using key factor and correlation methods revealed that disappearance at the egg stage (k0 ) was the key-factor causing population change at MPFS, while in Lambwe, although, more observations are necessary for this analysis, the factor kJ, representing disease at 3rd instar larval stage, was identified as the key factor. Temperature and rainfall also affected seasonal abundance of M· testulalis under field conditions. Under laboratory conditions, temperature was found to be an important factor affecting development and survival of the pest. Biological studies on !· sesamiae showed that the development period range from 14 - 18 days. Major factors identified to influence the biology and efficiency of the parasitoid include quality of food, age of host pupae and different host species. Results from bioassay for pathogens showed that Nosema sp. was the 11<>st virulent naturally occurring pathogen on M· testulalis .