Occurrence of a Disease Caused by a Rickettsia-Like Organism in a Larval Population of the Cabbage Looper, Trichoplusia ni1 , in Southern California

Abstract

A study of natural mortality in larvae of the cabbage looper, Trichoplusia ni (Hübner), feeding on experimental broccoli plantings in Riverside, Calif., revealed that ca. 10% of the larvae collected from July through October 1979 had a disease caused by a rickettsia-like organism. The disease occurred most commonly in 2nd- and 3rd- instars, where the rates of incidence were 12.8 and 15.2%, respectively. All larvae that developed patent infections died from the disease. The disease was most easily recognized in larvae which were reared individually in the laboratory on artificial diet. Disease signs included an opaque yellow-white discoloration on the body, incomplete shedding of the molted cuticle, reduced feeding activity, and a retarded growth rate with concomitantly increased larval longevity. At the histological level, the hemolymph of diseased larvae was milky white and contained numerous refractile vesicles that were diagnostic for the disease. Electron microscopy demonstrated slightly curved bacilliform particles, 160 by 420 nm, developing within these vesicles. Vesicles and particles also were found in the cytoplasm of epidermal, tracheal matrix, and fat body cells. The disease was transmitted to healthy T. ni larvae by injecting them with diluted hemolymph from diseased larvae. About 50% of the treated larvae developed typical signs of disease and subsequently died.