Manipulating floral resources dispersion for hoverflies (Diptera: Syrphidae) in a California lettuce agro-ecosystem

Abstract

Conservation biological control as a low-input method of pest management is unlikely to be universally adopted until easily deployable protocols are developed and targeted at farmers. However, critical questions remain concerning maximizing the efficacy of these pest management methods. For example, managing the currant–lettuce aphid on Californian organic lettuce farms currently relies on the presence of hoverfly species with aphidophagous larvae. Although floral resources are planted by the grower to provide food for adult hoverflies, little is known about how much and where floral resources should be provided. In this study, such a grower-initiated field design that is not based on published research is tested. The strips of sweet alyssum planted by the grower at 48m intervals within organic romaine lettuce fields were covered with agricultural frost cover to effectively double the distance between floral strips. Data collection, including egg, larvae and aphid counts, visual censuses of adult flies as well as pan trapping, were conducted before, during and after the application of the covers to establish the effect of changing the dispersion of floral resources. While the period of the row-cover manipulation was too short to establish an effect on egg, aphid and larval numbers, visual counts of adult hoverflies suggested that they reacted rapidly to the change in the local environment when the covers were applied and when they were removed. However, there was evidence to suggest that despite apparent aggregation in visual counts, hoverflies uniformly dispersed throughout the crop, regardless of the presence of covers, suggesting that floral resource provision could be reduced in this system.