Evaluation of two viral isolates as a potential biocontrol agent against the Egyptian cotton leafworm, Spodoptera littoralis (Boisd.) (Lepidoptera: Noctuidae)

Wael Elmenofy,Aya Zaki,Aya El-Sayed,Salma Amin,Sally Tamim,Norhan Yasser,Lamiaa El-Gaied,Engy Osman,Engy Hanafy,Maryam Saleh,Aml El-Bakry,Reda Salem,Alaa Abdelmawgod

doi:10.1186/s41938-020-00276-3

Abstract

Two different Spodoptera littoralis (Boisd.) (Lepidoptera: Noctuidae) nucleopolyhedrovirus (SpliNPV) isolates were obtained from natural infected S. littoralis larvae in Egypt. The phylogenetic analysis of the partial nucleotide sequence of the polyhedrin gene (polh) showed that both isolates, Spli-6 and Spli-7, were closed and had a common ancestor with S. littoralis NPV isolate 1263 polyhedrin gene, partial coding sequence with accession number AGE90003.1 that belongs to group II NPVs. This indicates that the natural host of Spli-6 and Spli-7 must be S. littoralis. The virulence of these isolates was tested against S. littoralis larvae in a laboratory. The LC50 against 1st instar larvae was significantly different, 3 × 104 OBs/ml and 9.5 × 104 OBs/ml for Spli-6 and Spli-7, respectively. However, the LC50 for the mixed infection of both isolates was 3.9 × 104 OBs/ml. On the other hand, the ST50 was 96 h for both isolates, while it was 120 h for the mixed infection, which was higher than that observed of each single isolate. Upon digestion of viral DNA with ScaI endonuclease, the restriction profile showed one more fragment of about ~ 25 kbp and 3 fragments of about ~ 19, ~ 5.8, and ~ 5.3 kbp in Spli-7 isolate compared to Spli-6 as well as the reference strain SpliNPV-AN1956. The differences in the fragment size and number suggest the existence of genotypic variants between both isolates. Spli-6 and Spli-7 proved to possess promising insecticidal properties for the development of virus-based biopesticide for the control of S. littoralis.

Highlights

The cotton leafworm, Spodoptera littoralis (Boisd.) (Lepidoptera: Noctuidae), is one of the most economic insect pests in Egyptian fields
Viruses’ isolation and propagation In this study, two baculovirus isolates denoted SpliNPV isolate Spli-6 (Spli-6) and SpliNPV isolate Spli-7 (Spli-7) were characterized. Both isolates were obtained from diseased S. littoralis larvae and exhibited viral-like symptoms which were collected from Giza district in Egypt
In the present study, two different S. littoralis nucleopolyhedrovirus (SpliNPV) isolates, denoted as Spli-6 and Spli-7, were obtained from natural infected S. littoralis larvae in Egypt

Summary

Introduction

The cotton leafworm, Spodoptera littoralis (Boisd.) (Lepidoptera: Noctuidae), is one of the most economic insect pests in Egyptian fields. In Egypt, it is considered as a destructive pest that causes economic losses for the cotton plants and for other crops and vegetables (Hosny et al 1986), orchard trees, and ornamentals (Mohamed et al 2019). Many biological agents have been used for the control of insect pests, such as parasitoids, predators, and entomopathogens (bacteria, fungi, and viruses) (El-Gaied et al 2020 and Atia et al 2016). Among these biopesticides are baculoviruses, which have been considered to be insect-specific viruses

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