Pathology of amber disease in the New Zealand grass grub Costelytra zealandica (Coleoptera: Scarabaeidae)
Amber disease of the New Zealand grass grub Costelytra zealandica (Coleoptera: Scarabaeidae) is a chronic infection of the larval gut caused by strains of the bacteria Serratia entomophila or S. proteamaculans. Following ingestion of pathogenic bacterial cells, larvae cease feeding after 2-5 days and become amber colored due to gut clearance. However, death does not occur until 1-3 months from initial infection. Light and electron microscopy demonstrated that bacteria colonize the gut, most specifically the foregut in the region of the cardiac valve. Bacteria adhere to the foregut intima and grow across the crop membrane, forming large clusters in folds of the crop. Adhesion appears to be mediated by fimbriae of the bacterial cells. As the disease progresses the larvae become shrunken due to a general degradation of the fat cells. The bacteria are not directly invasive, penetration of the hemocoel not occurring until the late stages of the disease when general septicemia is accompanied by death of the insect. Culturing of the hemolymph confirmed that invasion of the hemocoel was restricted to the final stages of disease. Enumeration experiments confirmed that the majority of the bacteria in the foregut were Serratia Spp.
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