Transmission of Pandora neoaphidis in the presence of co-occurring arthropods

Unhealthy herds and the predator–spreader: Understanding when predation increases disease incidence and prevalence

Disease ecologists now recognize the limitation behind examining host–parasite interactions in isolation: community members—especially predators—dramatically affect host–parasite dynamics. Although the initial paradigm was that predation should reduce disease in prey populations (“healthy herds hypothesis”), researchers have realized that predators sometimes increase disease in their prey. These “predator–spreaders” are now recognized as critical to disease dynamics, but empirical research on the topic remains fragmented. In a narrow sense, a “predator–spreader” would be defined as a predator that mechanically spreads parasites via feeding. However, predators affect their prey and, subsequently, disease transmission in many other ways such as altering prey population structure, behavior, and physiology. We review the existing evidence for these mechanisms and provide heuristics that incorporate features of the host, predator, parasite, and environment to understand whether or not a predator is likely to be a predator–spreader. We also provide guidance for targeted study of each mechanism and quantifying the effects of predators on parasitism in a way that yields more general insights into the factors that promote predator spreading. We aim to offer a better understanding of this important and underappreciated interaction and a path toward being able to predict how changes in predation will influence parasite dynamics.

The effect of synthetic female sex pheromone on the transmission of the fungus Metarhizium brunneum by male Agriotes obscurus click beetles

Autodissemination techniques can potentially be used to distribute insecticides, including microbial insecticides, to cryptic pests. This approach is reliant on the target insect either passing the pathogen passively to other insects or the pathogen cycling within the population after the initial host dies. Here we examine, in small scale experiments, whether male Agriotes obscurus click beetles passively transmit the spores of the fungus Metarhizium brunneum directly, or indirectly via the environment, and whether this is influenced by exposure to synthetic female pheromone. We found that the beetles did not avoid M. brunneum spores and that this behaviour was not affected by pheromone. Exposure to pheromone increased beetle movement and uptake of spores, but this did not result in an increase in infected beetles under our conditions. Beetles were able to transfer spores at high levels via environmental contamination. However, contamination of the environment declined rapidly after exposure to the spores. The results are discussed in the context of developing an autodissemination strategy for click beetles.

Symbiont-mediated protection against fungal pathogens in pea aphids: a role for pathogen specificity?

Here we show that a bacterial endosymbiont, Regiella insecticola, protects pea aphids (Acyrthosiphon pisum) from the aphid-specific fungal entomopathogen Zoophthora occidentalis but not from the generalist insect fungal pathogen Beauveria bassiana. This finding highlights the complex influence of fungi on the dynamics of this economically important agricultural pest.

Intraguild interactions between the entomopathogenic fungus Pandora neoaphidis and an aphid predator and parasitoid at the population scale

The interactions that occur between the entomopathogenic fungus Pandora neoaphidis and a predator (Coccinella septempunctata) and a parasitoid (Aphidius ervi) were assessed in microcosm and polytunnel experiments. Transmission of P. neoaphidis to the pea aphid, Acyrthosiphon pisum, was enhanced in the presence of both C. septempunctata and A. ervi in microcosm experiments done under fixed abiotic conditions. In contrast, the reproductive success of A. ervi was reduced in the presence of P. neoaphidis. Despite the increased fungal transmission in the presence of C. septempunctata, there was no additional decrease in the aphid population indicating that P. neoaphidis is functionally redundant in the presence of the coccinellid. In polytunnel experiments the reproductive success of A. ervi was not affected by P. neoaphidis. These results do not support those of the microcosm and may be due to the more natural abiotic conditions in the polytunnel reducing the competitive advantage of the fungus. Microcosms therefore provide an arena in which the interactions between fungal pathogens and other aphid-natural enemies can be assessed however, further assessments at increased spatial scales under more natural abiotic conditions are also required to accurately determine the outcome of these interactions.