Search

Amber disease in the New Zealand grass grub (Costelytra zealandica) is caused by some strains of Serratia entomophilaor Serratia proteamaculans(Enterobacteriaceae). When treated with pathogenic isolates, larvae ceased feeding within 48 h, developed an amber coloration after 72 h, and entered a long chronic phase without feeding. An acute dose of 2–4 × 104pathogenic bacteria was sufficient to produce disease in 50% of treated larvae. Time to death was directly related to temperature. At 15°C, infected larvae remained in a chronic, nonfeeding state for more than 4 months prior to death. Nonpathogenic isolates, lacking the disease-causing plasmid (pADAP), had no effect on either feeding or disease. Twenty-four hours after ingestion, bacteria were found predominantly in the hindgut and growth occurred primarily within the fermentation chamber and in the head section of the larvae. Nonpathogenic strains did not multiply in treated larvae. Treatment of diseased larvae with antibiotic eliminated Serratiacells from the insects but did not result in restoration of feeding or the dark gut characteristic of the healthy larva.

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 entomophilaor 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 Serratiaspp.