Spatial distribution of and sampling plans for Mussidia nigrivenella (Lepidoptera : pyralidae) on cultivated and wild host plants in Benin

The spatial distribution of Mussidia nigrivenella Ragonot was studied in ma ize fields and on four major wild host plants in the Republic of Benin, Wes t Africa. Maize ears were sampled at han-est in 1994 and 1995, and fruits o f the wild host plants were collected monthly from January 1996 to December 1997 during the fruiting periods of the respective host species. The spati al distribution was analyzed using the variance-to-mean ratio (s(2)/m), an index of departure from Poisson distribution, Iwao's patchiness regression, and Taylor's power law. The variance-to-mean ratio and the index of depart ure from Poisson gave similar results of nonrandom distribution of M. nigri venella populations on the different host plants out of Gardenia sokotensis Hutch (Rubiaceae). On this specific host plant, the variance-to-mean ratio was not different from 1, whereas there was no overlap of the observed pop ulation and the Poisson distribution. Iwao`s patchiness regression was inap propriate for our data as shown by the nonhomogeneity of variance, whereas Taylor's power law fitted the data well. Based on Taylor`s power law, M. ni grivenella was aggregated on all host plants out of G. sokotensis. The degr ee of aggregation of M. nigrivenella was positively correlated with the fru it size of the host plants. The optimal number of secondary sampling units needed to estimate M. nigrivenella densities on the respective host plants in Berlin. varied from four fruits on C. sokotensis to 10 pods on P. biglob osa. These values were used to calculate the time expenditure needed to sam ple maize fields or wild host plant trees to achieve a predefined precision level of 25%.