Stressors and rearing diseases of Trichoplusia ni: Evidence of vertical transmission of NPV and CPV

Inability to consistently rear healthy Trichoplusia ni led to a study of it s rearing diseases. Four diseases were designated after preliminary researc h which included electron microscopy: cytoplasmic polyhedrosis (due to cyto plasmic polyhedrosis virus, or CPV), nuclear polyhedrosis (due to nucleopol yhedrovirus, or NPV), "neonate death" syndrome (mortality in first or secon d instars), and "late-instar" syndrome (death in late instars accompanied b y bacterial decomposition). Infectious agents were not detected by electron microscopy in insects with the latter two diseases. Prevalence of CPV and NPV, but not the neonate-death or late-instar diseases, in progeny was sign ificantly associated with pairs of mating adults. In conjunction with egg-s urface decontamination, this indicated that both viruses may be transmitted transovarially. Pupae killed by CPV had virions in the cytoplasm of infect ed cells, but polyhedra were empty, not occluding virions. None of the dise ases had a consistent pattern of prevalence associated with the date on whi ch eggs were laid after oviposition began. Prevalence rates of cytoplasmic polyhedrosis, nuclear polyhedrosis, and the late-instar disease were signif icantly greater at 95-100% relative humidity (RH) than at RH levels of 75% or below. These same three diseases killed significantly more insects in cr owded rearing conditions (four or five larvae per cup with 10.2-cm(2) diet surface) than in uncrowded conditions tone to three larvae per cup), As a r esult of these experiments, healthy I: ni have been reared for 10 generatio ns by use of a modified Pasteur method and rearing cups containing no more than two larvae. (C) 1999 Academic Press.