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Abstract
Pea aphids, Acyrthosiphon pisum, have the potential to increase reproduction as a defence against pathogens, though how frequently this occurs or how infection with live pathogens influences this response is not well understood. Here we determine the minimum infective dose of an environmentally common bacterium and possible aphid pathogen, Pseudomonas syringae, to determine the likelihood of pathogenic effects to pea aphids. Additionally, we used P. syringae infection to investigate how live pathogens may alter reproductive rates. We found that oral bacterial exposure decreased subsequent survival of aphids in a dose-dependent manner and we estimate that ingestion of less than 10 bacterial cells is sufficient to increase aphid mortality. Pathogen dose was positively related to aphid reproduction. Aphids exposed to low bacterial doses showed decreased, although statistically indistinguishable, fecundity compared to controls. Aphids exposed to high doses reproduced significantly more than low dose treatments and also more, but not significantly so, than controls. These results are consistent with previous studies suggesting that pea aphids may use fecundity compensation as a response to pathogens. Consequently, even low levels of exposure to a common plant-associated bacterium may therefore have significant effects on pea aphid survival and reproduction.
Highlights
The plant-associated bacterium Pseudomonas syringae (Gammaproteobacteria) is a common plant pathogen and is frequently found epiphytically on leaf surfaces without causing disease symptoms [1,2]
Previous work has relied on high bacterial doses and artificial diet, making it unclear if the minimum infective dose of P. syringae in insects is within a naturally occurring range for bacterial population sizes, or if insects feeding on healthy plants are better able to defend against, or compensate for, the pathogen
Feeding behaviour or limitations in the size of the digestive tract may constrain the number of bacteria that can be ingested by many hemipteran insects [5,6] and bacterial population sizes may fluctuate dramatically in nature [1], making it unclear how frequently insects are infected by pathogenic strains of plant-associated bacteria
Summary
The plant-associated bacterium Pseudomonas syringae (Gammaproteobacteria) is a common plant pathogen and is frequently found epiphytically on leaf surfaces without causing disease symptoms [1,2]. Suspended in artificial diet, leads to bacterial growth in the insect and up to 100% mortality within a 2 few days These studies showed that insects could become infected while feeding on plants with epiphytic P. syringae populations, how commonly insects become infected in nature is unknown. Previous work has relied on high bacterial doses and artificial diet, making it unclear if the minimum infective dose of P. syringae in insects is within a naturally occurring range for bacterial population sizes, or if insects feeding on healthy plants are better able to defend against, or compensate for, the pathogen. In order to assess the potential for P. syringae to infect and kill insects we use pathogen exposure via artificial diet to control pathogen dose and determine if the minimum infective dose of P. syringae in pea aphids is within naturally occurring bacterial population size ranges. We investigated how pea aphids respond to infection, with regards to fecundity responses, by orally exposing aphids to varying doses of P. syringae and tracking their survival, development time, and reproduction rates on healthy plants
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