Doctor of Philosophy (PhD)
Management of plant invasions using biological control has the potential to generate spatial patterns which reflect geographic or genetic variation in invader or control agents. Despite its rarity in practice, investigations into the biogeography of interacting species (i.e., plant invader and control agent) in the context of biological control can lend insights into species distribution-abundance patterns and provide predictions for spatial variation in control success. I explored spatial variability in biological control agent-plant interactions using two wetland weed study systems with large geographic distributions: flowering rush (Butomus umbellatus L.) and alligatorweed (Alternanthera philoxeroides (Mart.) Griseb). Through literature and database review, I found that geographic variability in biological control success is relatively common, and abiotic factors are more often implicated than biotic factors. For flowering rush, I explored whether genetic and geographic variation in susceptibility to fungal pathogens could cause variation in plant performance and biological control. I found that patterns of disease varied between flowering rush cytotypes in field (higher disease rates in diploid plants) and laboratory (higher disease impacts in triploid plants) studies and were spatially variable along a latitudinal gradient for triploid plants only. I hypothesized that variation in alligatorweed biological control among sites and seasons in Louisiana was due to variation in plant quality (foliar nitrogen; FN). Over four years, I found that FN varied among sites and seasonally, with peak FN in spring and fall. Foliar nitrogen decreased the duration of larval development across a range of temperatures and slightly reduced dispersal at moderate conspecific densities. Finally, I explored the role of weather on biological control of alligatorweed across an environmental (climate) gradient in Louisiana. Biological control agent mean and maximum density decreased with latitude, population variability increased, and host (alligatorweed) density increased with latitude, likely due to low agent abundance at higher latitudes. Agent phenology and variability were influenced by weather variables and better explained alligatorweed density than weather or beetle densities alone. By combining literature review on geographic variation in biological control success with complementary studies in the above systems, my work is an important addition to the invasion biology and biological control literature and lends insights into how a biogeographical approach can be applied to study biological control of plant invasions and make predictions about the success of future agents.
Harms, Nathan, "Biogeography of Biological Control: Spatial Variation in Agent-Host Interactions" (2020). LSU Doctoral Dissertations. 5206.