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

Authors: David, Aaron; Carmona, Andrea; Lake, Ellen

Submitted to: Biological Control
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/16/2021
Publication Date: 4/18/2021
Citation: David, A.S., Carmona Cortes, A., Lake, E.C. 2021. Bottom-up factors determine local, but not regional, distribution of a biological control agent against invasive Lygodium microphyllum. Biological Control. 159. https://doi.org/10.1016/j.biocontrol.2021.104632.
DOI: https://doi.org/10.1016/j.biocontrol.2021.104632

Interpretive Summary: 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’ effects that influence plant quality and, in turn, local agent establishment. Two types of bottom-up effects, fertilization and inundation, likely play a critical role for agent establishment on the invasive Lygodium microphyllum (Old World climbing fern) which invades a suite of habitat types throughout the state. We show through a series of experiments that the eriophyid mite Floracarus perrepae, introduced to help manage L. microphyllum, performs best on fertilized plants that are not inundated. Next, we conducted a regional study to validate our predictions in the field. While mite establishment throughout all of Florida was not related to fertilization or inundation, establishment within south Florida, where the mite has thrived, was generally higher in highly fertilized, non-inundated infestations as predicted. Our study demonstrates both the consequences and limitations of bottom-up effects to shape distributions of biological control agents in the field and has important implications for effective management of invasive plants.

Technical 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’ effects that influence plant quality and, in turn, local agent establishment. We investigated the roles of two bottom-up effects, 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 effects under controlled settings, and then validated these finding using a field survey of 19 infestations. Our shade house experiments demonstrated that fertilization generally increased F. perrepae galling and within-gall population density. Fertilizer addition increased N allocation to pinnae (i.e., leaflets) rather than rachises (i.e., stems) or rhizomes. Furthermore, we demonstrated that a pulse of nutrients did not induce galling of previously ungalled plants from a high susceptibility, suggesting that factors other than fertilization also determine mite establishment. Next, we investigated the interactive effects of fertilization and inundation by growing potted plants inundated with water and determined that the higher level of inundation strongly reduced mite abundance. Finally, in a regional study we found 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 galls were most prevalent, sites with galls had 33% higher leaf N than sites without galls. Moreover, nearly all inundated sites lacked galls. While bottom-up effects clearly influence local mite establishment, there are likely other factors (e.g., genetic factors underlying mite susceptibility) that are the primary control on mite establishment and could be particularly important at a regional scale. Our study demonstrates both the consequences and limitations of bottom-up effects to shape distributions of biological control agents in the field and has important implications for effective management of invasive plants.