INFECTIVITY STUDIES AND HISTOPATHOLOGY OF A MOSQUITO NUCLEAR POLYHEDROSIS VIRUS
Abstract
The experimental host range of the nuclear polyhedrosis virus (NPV) of Aedes sollicitans was expanded following the screening of 29 mosquito species. A total of 9 mosquito species representing the genera Aedes, Wyeomyia, Toxorhynchites and Eretmapodites were found to be susceptible to viral infection (the later three genera have not previously been reported susceptible). It was determined that increasing age of A. epactius larvae caused a significant reduction in the susceptibility of this species to infection. A bioassay was conducted utilizing first instar A. epactius larvae and a geometric progression of viral doses. The relationship between log of viral dose and percent infection was exponential. However, the maximum percent infection observed only approached 50%. A comparison of larval midgut pH values was made utilizing three mosquito species (A. epactius, A. atropalpus, and A. scutellaris of varying viral susceptibilities. Comparisons were made using second, third and fourth instar larvae. No significant difference in midgut pH was observed between the three species for any given instar. However, a significant difference in the length of the high midgut pH (9.0 to 10.0) region between the different instars tested was observed, with high pH extending further posteriorly in the midgut in older larvae. Study of the infection sequence revealed that virions were released from polyhedra in the midgut of A. epactius larvae but electron microscopic examination could not determine how virions reached the midgut cells and effected penetration. The first signs of progeny virus appeared in midgut cells 12 hours after the inoculation period. Mature nucleocapsids apparently acquired envelopes through a process of de novo envelope morphogenesis. Singly enveloped virions were occluded; polyhedra were rough surfaced, irregular in shape and averaged approximately 0.5 (mu) by 1.5 (mu) in size. Large smooth surfaced, spindle shaped inclusions were not observed in this host. NPV infection was restricted to midgut and gastric caecae cell nuclei. NPV infection was confirmed in W. smithii, T. Brevipalpus, E. quinquevittatus and A. annandalei by electron microscopy. Infection was again restricted to the midgut and gastric caecae nuclei in these hosts. Unusual tubular structures were observed in some infected nuclei of W. smithii. Large spindle shaped inclusions devoid of virions were observed in some infected nuclei of T. brevipalpus. The experimental host range and the age and viral dose effects upon NPV infection are discussed in relation to the potential of the mosquito NPV as a biological control agent.
Degree
Ph.D.
Subject Area
Entomology
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