Bottom-up factors determine local, but not regional, distribution of a biological control agent against invasive Lygodium microphyllum

Abstract

The efficacy of biological control as a tool to manage invasive plants relies on widespread agent establishment, yet achieving such establishment remains a central challenge to the field. One potential explanation for patchy distributions of agents is the role of ‘bottom-up’ factors that influence plant quality and, in turn, local agent establishment. We investigated the roles of two bottom-up factors, soil fertilization and inundation, on establishment of the leaf galling mite Floracarus perrepae (Acariformes; Eriophyidae), an agent introduced to control the noxious vine Lygodium microphyllum (Lygodiaceae; Old World climbing fern) found in both upland and inundated habitats in Florida, USA. We conducted a series of shade house experiments to quantify bottom-up factors under controlled settings, and then validated these finding using a regional survey of 19 L. microphyllum infestations. Our shade house experiments demonstrated positive effects of fertilizer and negative effects of soil inundation on F. perrepae populations. Our regional survey showed no difference in leaf percent N between sites with or without F. perrepae galls. However, when we restricted our analysis to the area of the state where the mite had established, sites with galls had 33% higher leaf N than sites without galls, and nearly all inundated sites lacked galls. Bottom-up factors determine local mite establishment, but regionally, other factors (e.g., genetic factors underlying plant susceptibility) are likely at play. Our study demonstrates both the consequences and limitations of bottom-up factors to shape distributions of biological control agents in the field and has important implications for effective management of invasive plants.