Cydia pomonella granulovirus Genotypes Overcome Virus Resistance in the Codling Moth and Improve Virus Efficiency by Selection against Resistant Hosts

Abstract

Cydia pomonella granulovirus (CpGV) has been used for 15 years as a bioinsecticide in codling moth (Cydia pomonella) control. In 2004, some insect populations with low susceptibility to the virus were detected for the first time in southeast France. RGV, a laboratory colony of codling moths resistant to the CpGV-M isolate used in the field, was established with collection of resistant insects in the field followed by an introgression of the resistant trait into a susceptible colony (Sv). The resistance level (based on the 50% lethal concentrations [LC 50 s]) of the RGV colony to the CpGV-M isolate, the active ingredient in all commercial virus formulations in Europe, appeared to be over 60,000-fold compared to the Sv colony. The efficiency of CpGV isolates from various other regions was tested on RGV. Among them, two isolates (I12 and NPP-R1) presented an increased pathogenicity on RGV. I12 had already been identified as effective against a resistant C. pomonella colony in Germany and was observed to partially overcome the resistance in the RGV colony. The recently identified isolate NPP-R1 showed an even higher pathogenicity on RGV than other isolates, with an LC 50 of 166 occlusion bodies (OBs)/l, compared to 1.36 ؋ 10 6 OBs/l for CpGV-M. Genetic characterization showed that NPP-R1 is a mixture of at least two genotypes, one of which is similar to CpGV-M. The 2016-r4 isolate obtained from four successive passages of NPP-R1 in RGV larvae had a sharply reduced proportion of the CpGV-M-like genotype and an increased pathogenicity against insects from the RGV colony.

Evidence for a Second Type of Resistance against Cydia pomonella Granulovirus in Field Populations of Codling Moths

“…First, with regard to inheritance and susceptibility to CpGV-M and CpGV-S, populations 1 to 4 (BW-FN, BW-HI, BW-HA, and HE-WI) showed similar patterns of resistance, as previously reported for CpRR1. Most resistant CM field populations in Europe were expected to follow the CpRR1 type of resistance, since resistance in these populations is sex-linked and dominant and can be broken by CpGV isolates lacking the 24-bp insertion in pe38 (4,14,19,26). Only two families, K1f and R27f, deviated from this model, since zero mortality was observed in CpGV-M infections after 7 days, which is not in accordance with Z-linked inheritance.…”

“…For CpRR1, systemic resistance in all instars was observed, and an early block in virus replication was found to be inherited in an incompletely dominant monogenic mode linked to the Z chromosome (13,19,20). It was further found that CpGV resistance had developed only against the commercially used CpGV-M (genome group A), whereas other CpGVs from genome groups B to E were able to break resistance (4,14,21,22). Strikingly, the genomes of all resistance-breaking CpGVs of genome groups B to E had only a single common difference from CpGV-M, which was the insertion of a 24-bp repeat motif in the open reading frame (ORF) pe38 of CpGV-M (4).…”

“…From 2005 on, however, the emergence of field resistance of CM populations against CpGV products was detected in Germany and France (11,12). Meanwhile, CpGVresistant CM populations have been documented in 38 orchards, mainly organic apple orchards from different European countries, including Germany (22 orchards), France (3 orchards), Switzerland (22 orchards), Italy (6 orchards), Austria (2 orchards), The Netherlands (2 orchards), and the Czech Republic (1 orchard) (13)(14)(15)(16)(17). Some of these field populations were 1,000 to 100,000 times less susceptible to CpGV-M than were typical susceptible larvae (13).…”

Neo-sex chromosomes and adaptive potential in tortricid pests

“…The CpGV resistance is mediated by a major gene with concentration-dependent dominance linked to the Z chromosome (26). Although other CpGV isolates were shown to overcome CpGV resistance (27,28) caused by an early blockage of virus replication (29), its genetic basis remains elusive possibly due to false assumption of conserved gene content of the Z chromosome between B. mori and C. pomonella.…”

Recent Advances in Our Knowledge of Baculovirus Molecular Biology and Its Relevance for the Registration of Baculovirus-Based Products for Insect Pest Population Control

“…In nature, baculoviruses consist of mixtures of different genotypes of the same species and it has been shown that this diversity is naturally favored in wild type virus populations (Clavijo et al, 2011) where these genomic variants are known to impact virulence in the target organism (López-Ferber, 2003;Simón et al, 2008). The different viral genotypes may compensate for variations that occur in the larval host and/or its environment (Berling et al, 2009;Hitchman et al, 2007;Hodgson et al, 2002). For this reason, the evaluation of baculoviruses should be carried out at the species level rather than at the level of a single isolated genotype (Hauschild et al, 2011).…”