Disentangling effects of vector birth rate, mortality rate, and abundance on spread of a plant pathogen

Authors: Sisterson, Mark; Stenger, Drake

Submitted to: Journal of Economic Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/1/2015
Publication Date: 4/1/2016
Citation: Sisterson, M.S., Stenger, D.C. 2016. Disentangling effects of vector birth rate, mortality rate, and abundance on spread of a plant pathogen. Journal of Economic Entomology. 109:487-501.

Interpretive Summary: Insect-transmitted plant pathogens cause economic loss in crops worldwide. Long-term sustainable approaches to managing spread of insect-transmitted plant pathogens are needed. Abundance of insect vectors is determined by birth and death rates. Understanding factors affecting vector abundance is important as rates of pathogen spread increase with vector abundance. Despite this observation, theoretical models on spread of insect-transmitted plant pathogens routinely assume that vector population size is fixed; thereby ignoring parameters that determine vector abundance. To demonstrate the importance of explicitly including vector birth and death rates in models, results from models with fixed vector population size were compared to results from models with vector population growth. In models with vector population growth, magnitude of vector birth rate determined time required for vector populations to reach large size, thereby determining when pathogen spread occurred quickly. Similarly, mortality required to suppress a vector population could not be properly assessed unless vector birth rate and death rate were incorporated into the model. Currently, reproductive biology and mortality schedules of insect vectors are poorly studied. Results from this modeling study, indicate that assumptions regarding vector birth and death rates are important and warrant greater empirical study.

Technical Abstract: For insect-transmitted plant pathogens, rates of pathogen spread are a function of vector abundance. While vector abundance is recognized to be important, parameters that govern vector population size receive little attention. For example, epidemiological models often fix vector population size by assuming that deaths are offset by births. While such mathematical simplifications are often justified, deemphasizing parameters that govern vector population size is problematic as the reproductive biology and mortality schedules of vectors of plant pathogens receive little empirical attention. Here, the importance of explicitly including parameters for vector birth and death rates was evaluated by comparing results from models with fixed vector population size to models with logistic vector population growth. In fixed vector population size models, increasing vector mortality decreased the percentage of inoculative vectors, but had no effect on vector population size as deaths were offset by births. In models with logistic vector population growth, increasing vector mortality decreased the percentage of inoculative vectors and decreased vector population size. Consequently, vector mortality had a greater effect on pathogen spread in models with logistic vector population growth than in models with fixed vector population size. Further, in models with logistic vector population growth, magnitude of the vector birth rate determined the time required for vector populations to reach large size, thereby determining when pathogen spread occurred quickly. Assumptions regarding timing of vector mortality within a time step also affected model outcome. A greater emphasis of vector entomologists on studying reproductive biology and mortality schedules of insect species that transmit plant pathogens will facilitate identification of conditions associated with rapid growth of vector populations and could lead to development of novel control strategies. Similarly, models with more realistic vector population dynamics are needed to improve understanding of the role of vector population dynamics on pathogen spread.