Plant vigor metrics determine dynamics of interactions between the cereal leaf beetle and its natural enemy

The cereal leaf beetle, Oulema melanopus (L.) (Coleoptera: Chrysomelidae), is an emerging alien pest of cereal crops in western Canada. Since its discovery, the beetle has expanded its geographic range significantly with localized population hot-spots. The principal natural enemy of the beetle, Tetrastichus julis (Hymenoptera: Eulophidae), tracked the beetle naturally or has been relocated so that it is now, present in several infested areas. Currently, the beetle is in its early establishment phase and understanding the factors determining the spatio-temporal distribution dynamics of the pest and its natural enemy can help to design effective management strategies. We investigated the role of, and interactions among, host-plant nutrients, soil available nutrients, and plant vigor metrics on the distribution dynamics of O. melanopus and T. julis on a spatio-temporal scale. Spatial associations among the pest, the parasitoid, host-plant and soil available nutrients (N, P, K, S), and the plant vigor metrics were analyzed using Spatial Analysis by Distance Indices (SADIE) software. We found a strong spatial association between the beetle and the parasitoid indicating density dependence of the parasitoid on its host. Host-plant nutrients varied in their availability across grid plots and were influenced by soil nutrient availability. Plant vigor expressed in terms of basal stem diameter, number of leaves, and plant height indicated a high degree of spatial association with beetle activity, and thus were the major determinants of distribution dynamics of the beetle.