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Distribution and abundance of baculovirus populations in soil (click here for PDF reprints on baculovirus ecology)
 

The presence of baculovirus (Baculoviridae) in the soil can be quantified by mixing soil samples with insect diet (10% soil + 90% diet) and feeding the mixture to early instar larvae; a method developed by Richards & Christian (1999. J. Virol. Meth. 82, 63-75).

Larvae that consume viral occlusion bodies (OBs) in the soil become infected and die from virus disease several days later.

The OBs from each infected larvae can be amplified in third or fourth instar larvae to produce large quantities of virus for molecular characterization studies using PCR, RFLP or sequencing techniques (see Murillo et al. 2006a).

This technique has been applied to study the distribution and abundance of Heliothis/Helicoverpa nucleopolyhedroviruses in Australia (Richards & Christian, unpublished data) and Spodoptera exigua MNPV isolates in greenhouse soils in southern Spain (Murillo et al., 2006b).

Collections of soil from maize fields in southern Mexico, Belize and Guatemala were performed in collaboration with Andy Richards (CSIRO) and Peter Christian (NIBSC). These samples were subjected to bioassay by Guadalupe Melo Molina using neonate Spodoptera frugiperda larvae from a laboratory colony maintained in ECOSUR, Tapachula, Mexico (see map).

Distribution of Spodoptera frugiperda multiple nucleopolyhedrovirus (SfMNPV) in soils from maize fields in southern Mexico, Belize and Guatemala.  Of a total of 86 soil samples analyzed, 29% (N=25) proved positive (orange dots) for virus occlusion bodies (OBs). [Map designed by H. Weissenberger, ECOSUR, LAIGE ]

A subsample of isolates from soil samples was subjected to PCR-RFLP characterization using SfMNPV-specific primers and confirmed to be SfMNPV.

PCR products amplified using SfMNPV-specific primers were subjected to Sal I endonuclease digestion and electrophoresis.  All isolates were confirmed to be SfMNPV, although some polymorphism was evident in the last three isolates.  First and last lanes are molecular markers (photo: J. Jiménez).

Very similar results were obtained subsequently by Jaime Jiménez Fernández, who examined the local diversity of SfMNPV soil populations from maize fields around the town of Tapachula, Mexico.

OBs in the soil can be transported back onto plants by biotic and abiotic agents and these OBs are likely to be responsible for the initiation of infections in the host population in the subsequent growing season.

Clearly, the soil is an important environmental reservoir for SfMNPV and many other baculoviruses.

A detailed analysis of these results is currently being undertaken, including an examination of the influence of geology on the incidence of OB-positive soil samples.

Click here for a geological map of our sampling programme.

Click here for baculovirus ecology publications
 

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