Pathogens of tobacco and tomato hornworms

Abstract

The bacteria Serratia marcescens, Pseudomonas fluorescens, Pseudomonas aeruginosa, and Enterobacter aerogenes were the predominant pathogens in 7 of 11 samples of hornworms selected from populations with high mortality. Granulosis virus was the predominant pathogen in two samples; Bacillus thuringiensis caused most of the mortality in one sample taken from a population sprayed with a commercial preparation of this bacterium in the field; abnormal deposition of urate crystals in the fat body of mature larvae and prepupae appeared to cause mortality in one sample that was free of pathogenic microorganisms. More than one species of pathogenic bacteria usually occurred in diseased populations, and more than one species sometimes occurred in a single diseased larva. Bacterial pathogens were also isolated from samples attacked by virus or by B. thuringiensis and were sometimes found in individuals of presumably healthy populations when these were used to test the virulence of other pathogens. The voracious feeding of hornworm larvae provides good opportunity for them to ingest bacterial pathogens with their food, but bacteria seem to cause little harm unless the insects are also exposed to stress, such as infection with other pathogens, lack of food, or contamination of food by the feces. Such stress conditions probably occur rarely in the field, but do occur in the laboratory unless rearing is carefully conducted. Disease caused by nuclear-polyhedrosis and cytoplasmic-polyhedrosis viruses, protozoa, or fungi was not encountered.