Effects of insect-vector preference for healthy or infected plants on pathogen spread: insights from a model.

Insects vector many plant pathogens and often have higher or lower densities on infected plants than on healthy plants. Two hypotheses may explain this observation: insects may preferentially orient toward and select one plant type (referred to as orientation preference) or insects may reside on infected plants for longer or shorter periods than on healthy plants (referred to as feeding preference). The effects of feeding preference and orientation preference were compared alone and in combination using a spatially explicit model. With feeding preference for healthy or infected plants, the qualitative relationship between the percentage of plants infected and the rate of pathogen spread was not affected. However, feeding preference for healthy plants increased rates of pathogen spread, whereas feeding preference for infected plants decreased rates of pathogen spread. Unlike feeding preference, orientation preference for healthy and infected plants produced qualitatively different relationships between the percentage of plants infected and the rate of pathogen spread. With orientation preference for healthy plants, the pathogen spread slowly when few plants were infected, but quickly once most plants were infected. In contrast, with orientation preference for infected plants, the pathogen spread quickly when few plants were infected, but slowly once most plants were infected. In sensitivity analyses, we found that assumptions about the latent period (time between infection and when insects can recognize a plant as being infected) and persistence (length of time an insect remains inoculative) altered the aforementioned effects in some cases. The results illustrate that feeding and orientation preference affect pathogen spread differently, highlighting the importance of elucidating the mechanisms that control vector preference for healthy versus infected plants.