A defensive endosymbiont fails to protect aphids against the parasitoid community present in the field

Abstract

1. The value of protective mutualisms provided by some facultative endosymbionts has been well demonstrated in the laboratory, yet only recently has their effectiveness in the field been studied. ‘Candidatus Hamiltonella defensa’ is known to defend aphids from parasitoid wasps in laboratory trials. However, the efficacy of this defence varies among parasitoids, suggesting that protection will vary spatially and temporally depending on parasitoid community composition.2. This demonstrated specificity and a dearth of studies on Hamiltonella in the field prompted the authors to quantify parasitism rates of Hamiltonella‐infected and ‐uninfected Aphis craccivora Koch aphid colonies in a manipulative field study.3. It was found that A. craccivora in central Kentucky alfalfa were parasitised by Lysiphlebus testaceipes (Cresson) and Aphelinus sp. Surprisingly, Hamiltonella infection did not lower successful parasitism by the naturally occurring parasitoid wasps. Whether Hamiltonella was effective against L. testaceipes was subsequently tested in a controlled laboratory assay, and no effect on parasitism rate was found.4. This study emphasises the fact that defensive symbionts sometimes provide no tangible defensive benefits under field conditions, depending on parasitoid community composition. It is hypothesised that the protective mutualism may be beneficial in geographically localised areas. When the symbiosis is effective against a local parasitoid community, aphid clones may experience eruptive population growth and rapidly disperse across a large area, allowing spread to habitats with different parasitoid communities where the mutualism is an ineffective defence.

Diversity begets diversity: do parasites promote variation in protective symbionts?

“…by 3,4], although notable exceptions exist [e.g. 21,22]. This specificity even extends to variation within species.…”

Impacts of a novel defensive symbiosis on the nematode host microbiome

“…We further hypothesized that the protective effects of E. faecalis would remain in microbiome-colonised hosts, but be less effective overall. It has been previously shown that fitness constraints from multiple interactions in polymicrobial communities can dilute phenotypes normally observed in reduced systems [21]. However, the minimal shifts in microbiome composition we found suggests that novel, rapidly evolving symbionts can be maintained and remain effective without major disruption to the native microbiome.…”

A window of opportunity: Subdominant predators can use suboptimal prey

“…There are certainly other aphid examples in which individuals are protected from some enemies but vulnerable to others (e.g., Asplen et al., ; Cayetano & Vorburger, ; Michaud, ; Müller, Adriaanse, Belshaw, & Godfray, ). Whether an herbivore population will benefit from such narrow‐spectrum defenses ultimately depends on the composition of susceptible versus less susceptible enemies in the local community (Lenhart & White, ), and the degree to which compensatory attack by less susceptible enemies undercuts the defensive virtue of the selectively toxic trait for the herbivore (Letourneau, Jedlicka, Bothwell, & Moreno, ).…”