Toxicity testing of Metarhizium conidia and toxins against aquatic invertebrates

Abstract

Entomopathogenic fungi offer much potential for the control of arthropod pests. Very little is known about the impact of inoculum drift and runoff on aquatic non-target invertebrates. In general, there are no clear guidelines on risk assessment of these biological control agents and their secreted compounds. In the current study, Metarhizium brunneum conidia and crude metabolite extracts were tested against the larvae of three mosquito species (Anopheles stephensi, Culex quinquefasciatus, and Aedes aegypti) and the aquatic, non-target arthropods Artemia salina and Daphnia pulex. Both A. salina and D. pulex are recognised ecotoxicology indicator species. Extracts were prepared from filtrates of three M. brunneum strains, grown in three different culture media, using conventional solvent and new nanofiltration technologies. Metarhizium conidia and extracts affected the survival of all the aquatic invertebrates with An. stephensi also exhibiting abnormal development 96 h after exposure to the extracts. Ingestion of M. brunneum conidia induced apoptosis in aquatic non-targets as reflected by elevated caspase activity. Mortality appeared to be due to a combination of stress and secreted fungal metabolites independent of culture media. Due to their extreme sensitivity, An. stephensi and D. pulex are highly recommended as ecotox indicator species when conducting risk assessment of inoculum and metabolites of entomopathogenic fungi in aquatic environments.