The bacterium Yersinia entomophaga was isolated from larvae of the New Zealand grass grub, Costelytra zealandica (Coleoptera: Scarabaeidae), found in soil. Following ingestion of a lethal dose of bacteria, larvae of C. zealandica reduced feeding activity and movement. After approximately 4h infected larvae convulsed and regurgitated dark digestive fluid and expelled frass pellets leaving the midgut empty and the larva amber in appearance. In the initial stages of infection, ingested bacteria were mostly contained within the peritrophic membrane and expelled with the gut fluid or transferred into the hind gut. While few Y. entomophaga were associated with the midgut epithelial cells, by 24h cells were swelling and bursting with vesicles being expelled into the midgut lumen. By 48h, bacteria had entered the haemocoel and the midgut cells had further deteriorated. After 72h, the cellular remnants were totally detached from the basal membrane the infected insects were filled with bacteria and moribund or dead with septicaemia. Mortality was directly proportional to dose and time after infection. By applying a range of doses, the LD50 was determined as 2.9×10(4)Y. entomophaga per C. zealandica larva, with an LT50 of 2.94days for doses of>1×10(5) per larva. Ingestion of low doses of bacteria did not inhibit feeding activity but led more slowly to death. By time of death, Y. entomophaga had multiplied, approximately 500 fold, in the cadavers of the infected larvae.
Keywords: Costelytra zealandica; Entomopathogen; Pathology; Scarab; Yersinia entomophaga.
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