Presence of Spodoptera frugiperda Multiple Nucleopolyhedrovirus (SfMNPV) Occlusion Bodies in Maize Field Soils of Mesoamerica

Abstract

The occlusion bodies (OBs) of lepidopteran nucleopolyhedroviruses can persist in soil for extended periods before being transported back on to the foliage for transmission to the host insect. A sensitive insect bioassay technique was used to detect OBs of Spodoptera frugiperda multiple nucleopolyhedrovirus (SfMNPV) in 186 soil samples collected from maize fields in the southern Mexican states of Chiapas, Tabasco, Campeche, Yucatán, and Quintana Roo, as well Belize and Guatemala. Overall, 35 (18.8%) samples proved positive for SfMNPV OBs. The frequency of OB-positive samples varied significantly among Mexican states and countries (p < 0.05). Between 1.7 and 4.4% of S. frugiperda larvae that consumed OB-positive samples died from polyhedrosis disease. Restriction endonuclease analysis using PstI and HindIII confirmed that the soil-derived isolates were strains of SfMNPV and that genetic diversity was evident among the isolates. The prevalence of OB-positive soil samples did not differ with altitude or extension (area) of the maize field, but it was significantly higher in fields with the presence of living maize plants compared to those containing dead plants or crop residues (p < 0.05). Georeferenced soil samples were used to identify soil types on digitized soil maps. Lithosol and Luvisol soils had a higher than average prevalence of OB-positive samples (42−45% positive) (p = 0.006), as did Andosol, Gleysol, and Vertisol soils (33−60% OB-positive), although the sample sizes were small (<5 samples) for the latter three soils. In contrast, Cambisol soils had a lower than average prevalence of OB-positive samples (5% positive). Bioassays on Acrisol, Fluvisol, Phaeozem, and Rendzina soils resulted in intermediate levels of OB-positive samples. We conclude that certain soil types may favor OB persistence and virus-mediated biological pest control. The soil is also likely to provide a valuable source of genetic diversity for the design of virus-based insecticides against this pest.

Keywords: Alphabaculovirus; Baculoviridae; fall armyworm; genetic diversity; insect bioassay; soil type; virus persistence.

Figure 1

Distribution of soils samples that proved positive (orange points) or negative (black points) for SfMNPV OBs in the soil-diet bioassay. Samples were distributed across fives states of southern Mexico, Belize, and Guatemala. Intensive sampling was performed in the Soconusco region in the south of Chiapas State, Mexico (shown enclosed in a black rectangle).

Figure 2

Restriction endonuclease analysis of a selection of soil-derived isolates of SfMNPV (lanes A-H) compared to the SfMNPV-NIC reference isolate (Nic). Genomic DNA was digested with (A) PstI or (B) HindIII and subjected to electrophoresis in 0.6% and 0.8% agarose, respectively. The isolates used for this analysis were SfMNPV-NIC (labeled Nic) sample #121 (labeled A), #36 (B), #37 (C), #19 (D), #152 (E), #92 (F), #177 (G), and #50 (H), following the sample numbers given in Supplemental File S1. Restriction fragments and sub-molar bands that differed from those of SfMNPV-NIC are labeled with an asterisk. Molecular markers were NEB 1 Kb ladder (M1) and NEB λ mono-cut DNA (M2).

Figure 3

Prevalence of OB-positive and -negative samples taken from maize fields with (A) living green crops and (B) dry crops or post-harvest crop residues. Percentage values shown in gray rectangles are based on the indicated sample sizes (N).

Figure 4

Soil type composite maps for (A) southern Mexico, Belize, and Guatemala and (B) the Soconusco region of Chiapas State, Mexico. Georeferenced soil samples were plotted as OB-positive samples (open black circles) and OB-negative samples (black circles with a cross). High resolution versions of these maps are available as Supplemental Material (Figures S1 and S2).

Figure 5

Prevalence of samples from different types of soil that were positive for SfMNPV OBs. Values above columns indicate the number of samples tested (N).