Lack of avoidance of the fungal entomopathogen, Metarhizium brunneum, by male Agriotes obscurus beetles.

Fungal entomopathogens can greatly reduce the fitness of their hosts, and it is therefore expected that susceptible insects will be selected to avoid exposure to pathogens. Metarhizium brunneum is a fungal pathogen that can infect Agriotes obscurus, which in its larval form is a destructive agricultural pest and is repelled by the presence of M. brunneum conidia. Due to the subterranean nature of larval A. obscurus, recent research has focused on targeting adult A. obscurus with M. brunneum. No-choice and choice behavioural assays were conducted to determine if male adult A. obscurus avoid M. brunneum mycosed cadavers, or conidia applied to either food or soil. To further investigate the response of A. obscurus beetles to conspecific cadavers, the movement and behaviour of beetles placed at the centre of a semi-circular arrangement of mycosed or control cadavers was examined using motion tracking software. We found little evidence to suggest that A. obscurus male beetles avoid M. brunneum conidia or mycosed conspecific cadavers or alter their behaviour in their presence.

Modification of reproductive schedule in response to pathogen exposure in a wild insect: Support for the terminal investment hypothesis.

Trade-offs in the time and energy allocated to different functions, such as reproductive activities, can be driven by alterations in condition which reduce resources, often in response to extrinsic factors such as pathogens or parasites. When individuals are challenged by a pathogen, they may either reduce reproduction as a cost of increasing defence mechanisms or, alternatively, modify reproductive activities so as to increase fecundity thereby minimizing the fitness costs of earlier death, a behaviour consistent with the terminal investment hypothesis (TIH). The TIH predicts that individuals with decreased likelihood of future reproduction will maximize current reproductive effort, which may include shifts in reproductive timing. We examined how wild, adult female click beetles (Agriotes obscurus) responded after exposure to the fungal pathogen Metarhizium brunneum. Field-collected beetles exposed to a high concentration of M. brunneum died earlier and in greater numbers than those exposed to a low concentration. Using a multivariate approach, we examined the impact of pathogen challenge on lifespan and a suite of reproductive traits. Stepdown regression analysis showed that only female lifespan differed among the fungal treatments. Fungal-induced reductions in lifespan drove changes in the reproductive schedule, characterized by a decrease in preoviposition period. Moving the start of egg laying forward allowed the females to offset the costs of a shortened lifespan. These changes suggest that there is a threshold for terminal investment, which is dependent on strength of the survival threat. From an applied perspective, our findings imply that exposing adult click beetles to M. brunneum to reduce their population density might not succeed and is an approach that needs further investigation.

Metarhizium brunneum - An enzootic wireworm disease and evidence for its suppression by bacterial symbionts.

Wireworms (Coleoptera: Elateridae) are serious agricultural pests, with soil-dwelling larvae attacking subterranean tissues of crop plants and their fruit when in contact with the soil surface. Researchers collect wireworms for laboratory experiments to study their behaviour and test pest control agents but frequently lose them to Metarhizium Petch (Ascomycota: Hypocreales: Clavicipitaceae) infection. We found latent M. brunneum infection in 13-100% of live, asymptomatic Agriotes obscurus and A. lineatus wireworms acquired from agricultural fields and in wireworms maintained indoors, indicating its enzootic presence. M. brunneum DNA in the wireworms maintained indoors sometimes exceeded 250pg/ug total DNA (0.025% of whole-sample DNA mass). Expressed as copies of M. brunneum DNA/g, unadulterated soil levels of M. brunneum ranged from 4037 in agricultural field soil to 721,538 in soil harbouring a wireworm collection indoors, with the prevalence of latently-infected live wireworm specimens being directly related to soil levels. M. brunneum levels in live wireworms, when regressed against relative levels of 394 bacteria species in the microbiome, were proportionally related to only four: Pantoea agglomerans, Pandoraea pnomenusa, Nocardia pseudovaccinii, and Mycobacterium frederiksbergense. All four of these bacteria have previously been reported to express antimicrobial mechanisms. Consistent with occurrences of disease immunity reported for other pathogen-insect pairs, symbiotic bacteria may be suppressing M. brunneum-induced wireworm mortality. This would help explain why wireworms commonly succumb to infection after being brought into sterilized conditions, as well as the sometimes limited efficacy of M. brunneum when using it as a pest control agent in the field.