Spatial Pattern of Saissetia oleae (Homoptera: Coccidae) in Greece

Abstract

The spatial pattern and the spatial density dependence of mortality of Saissetia oleae (Olivier) were studied under field conditions. The results indicated that the distribution of S. oleae was highly aggregated and the degree of aggregation changed considerably during the generation, as indicated by the values of the exponent k of negative binomial distribution and the I δ index of aggregation. Aggregation increased during the ovipositional period in summer and during winter, whereas it decreased during autumn and spring. The increased aggregation during summer was caused by the combined effect of crawler behavior as they searched for settlement, and of mortality, which is negatively related to population density. During autumn, and possibly during spring when the main action of natural enemies occurred, the reduced aggregation was caused by the direct density dependence of mortality. This relationship was determined to a certain extent by the action of the internal parasitoid Metaphycus helvolus (Compere). The increased aggregation during winter was attributed to an inverse density-dependent relationship between mortality and population density and also to larval movement from leaves to branches. A strong relationship between the mean and variance of population density (Taylor power law) was established with 2 types of sampling units (leaf, branch). The corresponding 2 values of the coefficient b were remarkably similar. However, the exponent k of negative binomial and the Morisita index of aggregation ( I δ) were found best to describe changes in the spatial pattern during the generation compared with the Taylor relationship.