Rate of increase of Autographa californica nuclear polyhedrosis virus in Trichoplusia ni larvae determined by DNA:DNA hybridization

In Vivo Production of Agrotis ipsilon Nucleopolyhedrovirus for Quantity and Quality

The black cutworm, Agrotis ipsilon (Hüfnagel) (Lepidoptera: Noctuidae), is a pest causing damage to a variety of plants including turf and row crops. A recently discovered baculovirus has the potential to be developed as a microbial-based biological pesticide to provide targeted control of this insect pest. In an effort to develop this baculovirus as a biological pesticide, experiments were conducted to determine parameters necessary to maximize in vivo production using cutworm larvae. Treatment combinations including three larval diets, larval age at infection (6- to 10-d old), and dosage of virus exposure (1 × 105 to 1 × 108 occlusion bodies [OBs]/ml) were evaluated. Production quantity and quality were measured as number of OBs produced and insecticidal activity of the virus, respectively. Generally speaking, insect diets that maximized larval growth resulted in a greater quantity of virus OBs. Less virus was produced when younger (small) larvae were exposed to higher dosages of virus resulting in rapid mortality and when older (large) larvae were exposed to low dosages of virus resulting in low levels of infection. Virus quality was measured as insecticidal activity (low LC50 representing high activity) and was highest for larger larvae exposed to minimal virus concentrations needed to initiate infections. When considering both quantity and quality measurements, maximum production was achieved for 8- to 9-d-old larvae fed a general purpose lepidoptera diet. These results will support the development of this baculovirus as an additional tool for the integrated control of the black cutworm.

Characterization of Adoxophyes honmai single-nucleocapsid nucleopolyhedrovirus: morphology, structure, and effects on larvae

A nucleopolyhedrovirus (NPV) was isolated from a diseased larva of the smaller tea tortrix, Adoxophyes honmai, collected from a tea field in Tsukuba, Ibaraki, Japan. Electron microscopic observations confirmed that A. honmai NPV (AdhoNPV) was a single-nucleocapsid type virus. The genome size of AdhoNPV was estimated to be 111.6 +/- 0.9kb (mean +/- SE) by restriction endonuclease analysis. AdhoNPV was also infectious to two other Adoxophyes species, the summer fruit tortrix Adoxophyes orana and Adoxophyes dubia. The LD50 values for neonatal, second, third, fourth, and fifth (final) instar larvae of A. honmai were determined as 61, 107, 688, 1,961, and 4,085 occlusion bodies/insect, respectively. Most of the infected larvae died 5-9 days after molting to the final instar, regardless of the timing of inoculation. However, when neonates were exposed to extremely high doses of AdhoNPV (greater than 100 x LD90), larval development was prevented and most of the larvae died in the first instar.

Effects of incubation temperature on the dose-survival time relationship of Trichoplusia ni larvae infected with Autographa californica nucleopolyhedrovirus

The interaction between virus multiplication and host development was studied by determining the survival time of Trichoplusia ni larvae inoculated with a wide range of doses of Autographa californica nucleopolyhedrovirus and incubated at five different temperatures spanning the biologically relevant range. The results support earlier findings that the course of baculovirus infection follows the mathematical description of the birth-death model. Both in vivo rate of virus increase and larval growth rate increased linearly with increasing temperature from 14 to 29 degrees C; developmental zeros for virus replication and larval growth were estimated from these data to be 10.2 and 10.4 degrees C, respectively. The data were used to generate a description of the combined effects of dose and temperature on median survival time at doses greater than the LD(50). Implications of the lag times before onset of viral replication and between cessation of replication and larval death with respect to model-based estimation of the critical population level (i.e., amount of virus in the host just prior to death) are discussed.

The impact of host developmental status on baculovirus replication

The yield of progeny occlusion bodies from a baculovirus infection is a critical parameter governing viral population dynamics in the field. Previous evidence has suggested that the ability of the virus to block host development may be an important factor in determining yield of progeny virus. Here, we explore the relationship between yield, dose, and host developmental status at the time of infection during Autographa californica nucleopolyhedrovirus infection of final instar Heliothis virescens. The data indicate that occlusion body yield is strongly inversely dependent on the time elapsed after ecdysis before the insect is infected. The later in the instar the infective dose is received, the lower the efficiency with which the virus can block host development. The occlusion body yield from insects whose development is completely arrested is more than fourfold greater than the yield from insects that have initiated prepupal development. Dose is also an important factor, with high-dose infections more likely to lead to developmental arrest. Thus, the infection parameters that give rise to optimal progeny virus yield are infection at high dose early in the instar. Analysis of ecdysteroid titers demonstrated that only low levels of ecdysteroids are detectable in insects whose development is completely arrested. In contrast, in insects whose development was only partially arrested, extremely high ecdysteroid titers were frequently observed. These data support the hypothesis that the function of the baculovirus ecdysteroid UDP-glucosyltransferase gene is to delay or block host development, with the benefit of increasing the yield of progeny virus. Copyright 1998 Academic Press.

The in vivo production of Spodoptera littoralis nuclear polyhedrosis virus

The in vivo production of the nucleopolyhedrovirus (NPV) of the Egyptian cotton leafworm Spodoptera littoralis was studied experimentally. Larvae (7 days old) of 30-50 mg were experimentally infected with a range of NPV doses then harvested alive at various times after dosing to determine the effect of dose and incubation time on NPV productivity. Maximum NPV production achieved after 7 days incubation was 1.86 x 10(9) polyhedral inclusion bodies (PIBs) per larvae using an inoculum of 1 x 10(4) PIBs. Adjusting the inoculum dose had limited impact on NPV productivity but the correct selection of harvesting time was crucial in maximising the yield, both to achieve peak NPV production in individual larvae and to avoid losses from the death and disintegration of larvae if harvesting was delayed too long.

A polymerase chain reaction for the detection of nucleopolyhedroviruses in infected insects: the fate of the Spodoptera littoralis virus in Locusta migratoria

The Spodoptera littoralis nucleopolyhedrovirus (SlNPV) is a potential pest control agent of Spodoptera spp. As part of our studies to establish the use of this virus, a polymerase chain reaction (PCR)-based method was developed for the detection of viral DNA in infected insects. PCR amplification of the polyhedrin sequences enabled the detection of low levels of viral DNA directly from viral occlusion bodies or from total larval DNA. The use of different sets of synthetic DNA primers allowed us to differentiate between SlNPV and the Autographa californica NPV (AcNPV) and to identify a new AcNPV variant isolated from a cotton pest, Pectinophora gossypiella NPV. The PCR method was also used to test for the possible infection of Locusta migratoria larvae by SlNPV, reported by Bensimon et al., 1987. The progress of SlNPV infection in L. migratoria larvae was monitored by PCR for 2 weeks. The reaction revealed decreasing amounts of viral DNA in infected larvae. During this time, no signs of disease were observed in the infected locusts.

In vivo production, stabilization, and infectivity of baculovirus preoccluded virions

Wild-type and polyhedrin-negative isolates of Autographa californica nuclear polyhedrosis virus were replicated in fifth-instar Trichoplusia ni larvae. Insect tissues infected with wild-type virus contained two types of virions that are highly infectious when ingested, those occluded in polyhedra and preoccluded virions. Tissue infected with the polyhedrin-negative virus contained only preoccluded virions. The relative potencies of the two types of infected tissue were determined by dose-mortality bioassays by using the neonate droplet feeding procedure. On a fresh weight basis, preparations of tissues infected with the polyhedrin-negative virus were approximately four times more potent than equivalent preparations of tissue infected with wild-type virus. Approximately half of the observed potency of the wild-type-virus preparations was due to polyhedra, and the remaining activity was due to preoccluded virions present in the tissue. The potency of the polyhedrin-negative preparations was not reduced significantly by lyophilization. The polyhedrin-negative isolate produced about 60% more infectious virus per unit of larval weight than did the wild-type isolate. The ability to produce large amounts of high-potency viral preparations in larvae and the convenience of being able to lyophilize the preparations for long-term storage shows promise for the use of preoccluded virus preparations as biopesticides.

Non-linear transmission rates and the dynamics of infectious disease

This study considers how non-linearities in the transmission of microparasitic infections affect the population dynamics of host-parasite systems in which the disease is potentially lethal to the host. Non-linearities can either lead to a locally stable or unstable host-parasite equilibrium point, depending on the respective contributions of healthy and infected hosts to the functional form of the transmission rate. Analysis of the non-linear transmission model results in a revealing pair of local stability criteria. Specifically, stability requires sufficient total levels of intrinsic growth of the host population and total levels of density-dependent transmission. The most stable systems occur when increases in the density of healthy hosts result in increases in transmission efficiency, and increases in the number of infected hosts result in small decreases in transmission efficiency. These appear to be very reasonable relationships for directly transmitted microparasites.