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Biology and Ecology of Baculoviruses (click here for publications in PDF format)

Understanding the ecology of insect pathogenic baculoviruses requires a basic appreciation of their biology.

Baculoviruses (comprising nucleopolyhedroviruses and granuloviruses) have two virion forms.

The occlusion derived virions infect insect midgut cells following ingestion of contaminated foliage by susceptible insect larvae.

These virions fuse with the cell membrane releasing nucleocapsids into the cell.

The nucleocapsids migrate the nucleus and begin replication or pass straight through the midgut cell to infect the cells of other tissues.

Initial replication results in the production of virions that bud through the basal cell membrane.

These budded virions disperse throughout the insect.

Later in infection, virions are enveloped (individually or in groups) and are occluded into large (~1-3 µm) occlusion bodies (OBs) designed for insect-to-insect transmission.

Multiple enveloping appears to be a strategy for overcoming host cell responses to viral infection, but this has important evolutionary consequences because each cell can be infected by multiple virus genotypes.

Shortly before death, the infected insects become pale and flaccid and often climb to the apical points of the plant where they die.

The body ruptures releasing millions of OBs that contaminate foliage for transmission to other larvae.

Once ingested, the OBs dissolve in the highly alkaline midgut of phytophagous insects, liberating the occlusion derived virions for the next cycle of infection.
 

Rod-shaped nucleocapsids are enveloped to form virions in the cell nucleus and occluded by a protein matrix.
 

Virus occlusion bodies (seen here as brilliant dots) can be quantified using a hemocytometer (x400)

The relatively large size of viral occlusion bodies (1-3 µm) means that they can be visualized and quantified by counting under a phase-contrast microscope.

For example, a diluted suspension of OBs placed on a the grid of a Neubauer-type hemocytomer can be counted to estimate the number of OBs present in the original suspension of virus.

Several counts are performed on each suspension and the average value is calculated to minimize the variation that arises from dilution effects or handling of small volumes of OB suspension.

Different concentrations of OB suspension can then be prepared and used to inoculate insects in order to determine the insecticidal activity of different virus isolates.

The dose of OBs required to infect and kill 50% of the larvae of a particular developmental stage (instar) is known as the LD50 (or LC50 in the case of an OB concentration).

Predatory invertebrates eat virus-infected prey and subsequently disperse OBs, over distances of many meters, in their feces (photo R. Lasa).
 

Because most of these viruses only infect a few closely related species of insects, particularly Lepidoptera, they may interact passively with other insects to achieve dispersal and/or transmission to new hosts.

For example, the OBs do not dissolve in the acid guts of predatory insects.

As a result, predators that consume virus infected hosts may disperse the OBs during several days and over considerable distances as they defecate the remains of their infected victim.

Similarly, parasitoid wasps that have stung an infected insect can act as vectors introducing the virus to susceptible hosts during subsequent acts of oviposition.

The ecology of these viruses and the relationship between ecology and pest control have been reviewed in detail (Williams 2018).
 

A predatory bug (Nesidiocoris) about to feed on a virus-infected larva. Bugs can transmit the virus to other larvae because their mouthparts and body become contaminated with virus particles (photo R. Lasa).

Virus isolates from insects or from the soil can be compared by their genetic restriction profile using enzymes that cut the virus DNA into fragments of different lengths.

The soil represents a major virus reservoir in the environment (reviewed in Williams 2023).

OBs can persist in acid or neutral soils for months or years before being transported back onto leaf surfaces by rainsplash, air currents, or by the movement of soil surface dwelling arthropods.

Work by Murillo et al. (2006) indicates that certain genotypes present in baculovirus populations may be better adapted to survival in soils than others.

Certain virus genotypes may also be better adapted to survive in soils of different acidity or alkalinity.

The principal factors that limit OB persistence in the environment are solar UV radiation and exposure to alkaline conditions such as occur in calcium rich soils and on the leaf surfaces of certain plants (e.g. cotton).

OBs can be isolated from soil samples by mixing the soil with insect diet and feeding the mixture to susceptible larvae, a technique developed by Richards & Christian (1999).

Click here to see how we are using this technique for the study of Spodoptera nucleopolyhedroviruses in agricultural soils.
 

Virus occlusion bodies are highly resistant and can survive long periods in the environment (scanning electron microscope image).

Baculovirus populations are genetically heterogeneous and individual isolates often comprise a mixture of different genotypes, including defective variants that are incapable of achieving transmission or on their own.

The interactions between genotypes can have surprising consequences for the phenotype of the mixture and the probabilities of transmission of each of the constituent genotypes.

For example, work by López-Ferber et al. (2003) has demonstrated increases in pathogenicity of mixtures containing complete and defective genotypes.

Repeated steps of insect-to-insect transmission of such mixtures rapidly results in an equilibrium in which the proportion of defective genotypes is precisely the proportion seen in the wild population.

When two genotypic variants (or two distinct nucleopolyhedroviruses) replicate in the same cell, the nucleocapsids and ODV envelope proteins are shared among the variants to produce virus particles with a mixed-variant pseudotype that may differ in its pathogenic characteristics compared to each of the component variants (Williams et al., 2022).

This work underlines the importance of genotypic diversity on the transmissibility and stability of baculovirus populations.
 

Schematic of co-occluded genotypic variants. Variants (shown in green or blue colours) that replicate in the same cell become enveloped individually or in groups to form mixed-variant virions (ODVs). Nucleocapsids have a rod-like appearance in longitudinal section and are circular in transverse section.

Click here for publications on baculovirus ecology
 

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