Charles C. Doane

Primary pathogens and their role in the development of an epizootic in the gypsy moth.

Abstract The development of an epizootic was studied in a dense population of larvae of the gypsy moth, Porthetria dispar. The two pathogens involved were a nuclear-polyhedrosis virus and a variant of Streptococcus faecalis. It was known that there was approximately 10% acute infection from trans-ovum transmission of the nuclear-polyhedrosis virus. The origin of the initial inoculum of the variant of S. faecalis is unknown but it became common in the second, and later instars. The behavior of the larvae increased the relative density of the population and enhanced the rate of larva-to-larva spread of the pathogens. Larvae in the first four instars fed most heavily in the tops of trees and dead larvae accumulated on the upper surfaces of the leaves. These cadavers disintegrated and adhered firmly to the leaves, forming an abundant source of inoculum for feeding larvae. This occurred early enough to account for the later, massive increase in disease. Results from counts in the field and collections of larvae reared in the laboratory indicated that there was an increasing rate of infection and mortality that reached a climax when larvae were in the last instars. The threatened defoliation did not occur and the population declined sharply. Although the nuclear-polyhedrosis virus appeared to be most important, up to 50% of the larvae in some collections were killed by the variant of S. faecalis. Observations indicated that the epizootic was density-dependent and that the rapid spread of pathogens in the susceptible population was enhanced by the behavior of the larvae during the early instars.

Trans-ovum transmission of a nuclear-polyhedrosis virus in the gypsy moth and the inducement of virus susceptibility

Abstract Eggs of the gypsy moth, Porthetria dispar, were collected from six sites differing in topography from wet to dry, in density of the gypsy-moth population, and in age or duration of the outbreak. Eggs collected from one site on March 23, 1968, responded differently than eggs of any of the other collections after as little as 7 days at 0–2°C. About 80% of the larvae hatching from these eggs died of nuclear polyhedrosis in the first instar. These virus-susceptible larvae made it possible to demonstrate, by surface disinfection, that trans-ovum transmission of the nuclear-polyhedrosis virus (NPHV) takes place via the surface of the egg. The hatching larva becomes infected by ingesting part of the egg shell. (An improved method of surface disinfection with 0.1% sodium hypochlorite is described. This reduced the incidence of polyhedrosis in the susceptible larvae from 80% to 0.1%.) Larvae from eggs collected at hatching time from the same site as the susceptible group of March 23 had less than 10% polyhedrosis. This was also true for larvae from eggs collected at the other sites both at hatching time and in the previous months of December and January. Larvae from eggs collected in the winter months and refrigerated at 0–2°C for up to 7 months suffered less than 10% polyhedrosis. Thus, long periods of refrigeration did not induce susceptibility to the NPHV. The data suggest that initial mortality of first-instar larvae in the field, exclusive of larva-to-larva transmission typically would be less than 10%. By comparison with the 80% polyhedrosis observed in the virus-susceptible larvae it appears that there is normally a high incidence of transmission of the virus and that most larvae become infected as they hatch. Since less than 10% die in early instars there appears to be some natural resistance to acute infection. Apparently by March 23 eggs in the field undergo some change as the weather begins to moderate. After this the unhatched larvae cannot tolerate prolonged chilling at 0–2°C without becoming susceptible to acute infection from NPHV.

Bioassay of nuclear-polyhedrosis virus against larval instars of the gypsy moth

Abstract The relative susceptibility of three larval instars of the gypsy moth, Porthetria dispar , to a nuclear-polyhedrosis virus was studied by bioassay in the laboratory. The LC 50 values for first-, second-, and third-instar larvae were 0.23, 2.3, and 2.5 polyhedral inclusion bodies per mm 2 of food surface (PIBs/mm 2 ). LT 50 values of larvae fed on serial-dilution concentrations of 69,879 to 0.06 PIBs/mm 2 were established. From measurements of the amount of feeding on surfaces treated with 69 PIBs/mm 2 , a concentration producing approximately 100% mortality, the average doses were 2,233, 11,690, and 25,197 PIBs, respectively. Ingestion of PIBs was initially benign and, with heavy doses, depended on their concentration on the food. The effects of low doses on molting, pupation, and adult emergence were observed. If the larvae survived two molts following exposure to the virus no heavy mortality occurred later. The lowest concentration preventing a second molt in the three instars was 69 PIBs/mm 2 .

Characteristics of motile strains of Streptococcus faecalis pathogenic to larvae of the gypsy moth

Abstract A bacterial disease of larvae of the gypsy moth, Porthetria dispar , is caused by infection of the gut with a streptococcus. The morphological, biochemical, and serological features of the organism were consistent with those of Streptococcus faecalis . The bacterium is ingested with food, multiplies in the digestive tract, and causes diarrhea. The diarrheic discharge contains a high concentration of infectious streptococci. The gut, especially the foregut, becomes distended with liquid from the hemocoel. Streptococci are found in especially high concentration in the midgut. The infected larva does not feed and, as liquid is lost continually, body length decreases markedly and death occurs in 3 to 15 days. The cadavers usually have a characteristic mummified or desiccated appearance. Serological studies with antisera prepared by immunizing rabbits with several isolates indicate that at least three distinct serotypes were isolated from infected larvae. They appear to be serologically different from noninsect S. faecalis strains tested. It is suggested that this antigenic difference may form the basis of pathogenicity of the species. A pilot study revealed that an immunofluorescence technique may be an extremely valuable tool in identifying these organisms in insects and in determining their distribution in nature.

Field application of a Streptococcus causing brachyosis in larvae of Porthetria dispar

Abstract A motile strain of Streptococcus faecalis originally isolated from a naturally infected larva and tested for pathogenicity under laboratory conditions was tested in the field. Mistblower sprays prepared from broth cultures induced brachyosis in gypsy moth larvae feeding in apple trees. One application was made against larvae in late second, early third instar, and a second application was tested against larvae in the fourth instar. The test demonstrated the ability of the bacterium to produce disease and prevent defoliation even in an area with a heavy population.