Fungal disease dynamics in insect societies: optimal killing rates and the ambivalent effect of high social interaction rates.
Entomopathogenic fungi are potent biocontrol agents that are widely used against insect pests, many of which are social insects. Nevertheless, theoretical investigations of their particular life history are scarce. We develop a model that takes into account the main distinguishing features between traditionally studied diseases and obligate killing pathogens, like the (biocontrol-relevant) insect-pathogenic fungi Metarhizium and Beauveria. First, obligate killing entomopathogenic fungi produce new infectious particles (conidiospores) only after host death and not yet on the living host. Second, the killing rates of entomopathogenic fungi depend strongly on the initial exposure dosage, thus we explicitly consider the pathogen load of individual hosts. Further, we make the model applicable not only to solitary host species, but also to group living species by incorporating social interactions between hosts, like the collective disease defences of insect societies. Our results identify the optimal killing rate for the pathogen that minimises its invasion threshold. Furthermore, we find that the rate of contact between hosts has an ambivalent effect: dense interaction networks between individuals are considered to facilitate disease outbreaks because of increased pathogen transmission. In social insects, this is compensated by their collective disease defences, i.e., social immunity. For the type of pathogens considered here, we show that even without social immunity, high contact rates between live individuals dilute the pathogen in the host colony and hence can reduce individual pathogen loads below disease-causing levels.
IST Austria (Institute of Science and Technology Austria), Am Campus 1, 3400 Klosterneuburg, Austria. Electronic address: sebastian.novak@ist.ac.at.
Sebastian
S
Novak
IST Austria (Institute of Science and Technology Austria), Am Campus 1, 3400 Klosterneuburg, Austria.
Sylvia
S
Cremer
D000818:Animals; D001522:Behavior, Animal; D003198:Computer Simulation; D007110:Immunity, Active; D032521:Immunity, Herd; D007113:Immunity, Innate; D019476:Insect Proteins / Q000378:metabolism; D007313:Insecta / Q000276:immunology / Q000382:microbiology*; D052981:Metarhizium / Q000472:pathogenicity*; D008954:Models, Biological; D012919:Social Behavior
D019476:Insect Proteins
Basic reproduction number; Biological control; Epidemiological model; Obligate killing entomopathogenic fungi; Social immunity