13.1.2 Entomopathogenic Viruses in Biocontrol
13.1.2.1 Spodoptera exigua
The sugar beet armyworm, Spodoptera exigua (Lep.: Noctuidae), is a major pest of
many crops (Dai et al. 2000). This pest is native to Asia but has been recorded in
different regions throughout the world, such as Australia, Europe, and North America (Pathak and Khan 1975). Its larvae feed on the foliage and the upper portion of
beet roots and can completely defoliate small plants, frequently causing economic
damage. Due to the problem of the pest resistance to insecticides, the adverse effects
of insecticides on the environment (Asi et al. 2013), negative effects of insecticides
on wildlife and natural enemies (Ordóñez-García et al. 2015), biological control as a
pest management strategy is desirable.
Various populations of S. exigua are infected by different species/strains of
baculoviruses (Virto et al. 2014). Among baculoviruses, S. exigua multiple
nucleopolyhedrovirus (SeMNPV) was isolated from S. exigua larvae (Jakubowsaka
et al. 2005; Virto et al. 2013). The SeMNPV is a specific pathogen of S. exigua and
has the potential to be used as a biocontrol agent as it produces epizooty in larval
populations (Virto et al. 2014).
Manzari et al. (1998), surveyed pathogenicity of Mamestra brassicae nuclear
polyhedrosis virus on S. exigua. This virus had a high pathogenicity for Spodoptera
exigua larvae. In laboratory conditions, a concentration of 12.58 Occlusion bodies
(OBs) per mm
2 of artificial food led to 50% mortality of 2nd instar larval of S. exigua
after 6 days. The mortality percentage for 1st, 3rd and 5th larval instars were
100, 69 Æ 6, and 11 Æ 5%, respectively. In surface contamination of eggs with
1 Â 10
5 OB per cm
3 concentration, all larvae died 3 days after hatching (Manzari
et al. 1998). Also, Kamali and Pourmirza (2000) surveyed pathogenicity of a NPV
strain on 1–5 instars larval of S. exigua in laboratory condition. The median lethal
does (LD 50 ) in 1–5 instars larvae were 14.46, 154.55, 1576.14, 2917.39, and
5902.88 OB per mm, and the mortality value was 93.3, 93.3, 80, 92.9, and 73.3%,
respectively. These results indicated that there was a correlation between larval
weight and LD 50 . Thus, for virus spraying, the larval weight must be considered.
Rabie et al. (2008) performed an experiment about efficacy of MbNPV against
different larval stages of S. exigua and found that older larvae were more susceptible
than younger larvae.
During 2014–2015, a survey was done to identify the natural enemies of sugar
beet armyworm, S. exigua larva. Different larval stages were collected from beet
fields in Mashhad and Chenaran (Northeast of Iran) and kept in laboratory conditions. Finally, several species of parasitoids and pathogens including virus, nematode and fungus were identified. For virus identification, two genes, lef8 and
Polyhedrin were amplified, sequenced and analyzed. The reconstructed phylogenetic trees inferred from two loci confirmed that two isolates of SeMNPV were
natural enemies of S. exiguae. The identified entomopathogenic virus isolates were
very important because they had high incidence and virulence (Darsouei and Karimi
13 Insect Pathogenic Viruses, Microsporidians and Endosymbionts
507
13.1.2.1 Spodoptera exigua
The sugar beet armyworm, Spodoptera exigua (Lep.: Noctuidae), is a major pest of
many crops (Dai et al. 2000). This pest is native to Asia but has been recorded in
different regions throughout the world, such as Australia, Europe, and North America (Pathak and Khan 1975). Its larvae feed on the foliage and the upper portion of
beet roots and can completely defoliate small plants, frequently causing economic
damage. Due to the problem of the pest resistance to insecticides, the adverse effects
of insecticides on the environment (Asi et al. 2013), negative effects of insecticides
on wildlife and natural enemies (Ordóñez-García et al. 2015), biological control as a
pest management strategy is desirable.
Various populations of S. exigua are infected by different species/strains of
baculoviruses (Virto et al. 2014). Among baculoviruses, S. exigua multiple
nucleopolyhedrovirus (SeMNPV) was isolated from S. exigua larvae (Jakubowsaka
et al. 2005; Virto et al. 2013). The SeMNPV is a specific pathogen of S. exigua and
has the potential to be used as a biocontrol agent as it produces epizooty in larval
populations (Virto et al. 2014).
Manzari et al. (1998), surveyed pathogenicity of Mamestra brassicae nuclear
polyhedrosis virus on S. exigua. This virus had a high pathogenicity for Spodoptera
exigua larvae. In laboratory conditions, a concentration of 12.58 Occlusion bodies
(OBs) per mm
2 of artificial food led to 50% mortality of 2nd instar larval of S. exigua
after 6 days. The mortality percentage for 1st, 3rd and 5th larval instars were
100, 69 Æ 6, and 11 Æ 5%, respectively. In surface contamination of eggs with
1 Â 10
5 OB per cm
3 concentration, all larvae died 3 days after hatching (Manzari
et al. 1998). Also, Kamali and Pourmirza (2000) surveyed pathogenicity of a NPV
strain on 1–5 instars larval of S. exigua in laboratory condition. The median lethal
does (LD 50 ) in 1–5 instars larvae were 14.46, 154.55, 1576.14, 2917.39, and
5902.88 OB per mm, and the mortality value was 93.3, 93.3, 80, 92.9, and 73.3%,
respectively. These results indicated that there was a correlation between larval
weight and LD 50 . Thus, for virus spraying, the larval weight must be considered.
Rabie et al. (2008) performed an experiment about efficacy of MbNPV against
different larval stages of S. exigua and found that older larvae were more susceptible
than younger larvae.
During 2014–2015, a survey was done to identify the natural enemies of sugar
beet armyworm, S. exigua larva. Different larval stages were collected from beet
fields in Mashhad and Chenaran (Northeast of Iran) and kept in laboratory conditions. Finally, several species of parasitoids and pathogens including virus, nematode and fungus were identified. For virus identification, two genes, lef8 and
Polyhedrin were amplified, sequenced and analyzed. The reconstructed phylogenetic trees inferred from two loci confirmed that two isolates of SeMNPV were
natural enemies of S. exiguae. The identified entomopathogenic virus isolates were
very important because they had high incidence and virulence (Darsouei and Karimi
13 Insect Pathogenic Viruses, Microsporidians and Endosymbionts
507