Abstract
Biological insecticides based on Spodoptera exigua multiple nucleopolyhedrovirus (SeMNPV) can efficiently control S. exigua larvae on field and greenhouse crops in many parts of the world. Spanish wild populations and laboratory colonies of S. exigua are infected by two iflaviruses (SeIV-1 and SeIV-2). Here we evaluated the effect of iflavirus co-infection on the insecticidal characteristics of SeMNPV occlusion bodies (OBs). Overall, iflavirus co-inoculation consistently reduced median lethal concentrations (LC50) for SeMNPV OBs compared to larvae infected with SeMNPV alone. However, the speed of kill of SeMNPV was similar in the presence or absence of the iflaviruses. A reduction of the weight gain (27%) associated with iflavirus infection resulted in a 30% reduction in total OB production per larva. Adult survivors of SeMNPV OB inoculation were examined for covert infection. SeMNPV DNA was found to be present at a high prevalence in all SeIV-1 and SeIV-2 co-infection treatments. Interestingly, co-inoculation of SeMNPV with SeIV-2 alone or in mixtures with SeIV-1 resulted in a significant increase in the SeMNPV load of sublethally infected adults, suggesting a role for SeIV-2 in vertical transmission or reactivation of sublethal SeMNPV infections. In conclusion, iflaviruses are not desirable in insect colonies used for large scale baculovirus production, as they may result in diminished larval growth, reduced OB production and, depending on their host-range, potential risks to non-target Lepidoptera.
Highlights
Recent advances in generation sequencing have led to the discovery of a great diversity of viruses infecting insect populations [1, 2]
As iflaviruses are present in S. exigua populations used to produce Spodoptera exigua multiple nucleopolyhedrovirus (SeMNPV) as the basis for biological insecticides, we examined the consequences of coinfection with SeMNPV and SeIV1 and Spodoptera exigua iflavirus-2 (SeIV-2) on the insecticidal properties of occlusion bodies (OBs) produced in co-infected insects
The effectiveness of baculovirus-based insecticides relies on a diversity of virus-related and environmental factors [14], and on the susceptibility of the pest population to baculovirus applications, which can be altered by cryptic infections involving other types of viruses [23, 41]
Summary
Recent advances in generation sequencing have led to the discovery of a great diversity of viruses infecting insect populations [1, 2]. RNA viruses belonging to the family Iflaviridae (order Picornavirales) were detected during analysis of the larval transcriptome [3] of the beet armyworm, Spodoptera exigua (Lepidoptera: Noctuidae), which is a major pest of horticultural crops in tropical and subtropical and regions worldwide [4]. This pest attacks greenhouse grown vegetable and ornamental crops in temperate regions [5,6,7]. The genome consists of a single ORF which is translated as a single polyprotein that is subsequently cleaved into functional and structural proteins
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