Virulence and sex-dependent invasion efficiency of entomopathogenic nematodes on developmental stages of Phthorimaea operculella (Lep., Gelechidae)

Abstract

• By IJs concentration established Steinernema in PTM increased. • Sex ratio of S. carpocapsae in pre-pupa was affected by IJ concentration. • An inverse relationship was recorded between penetration rate and IJs concentration. • No correlation was observed between the invasion efficiencies (IE) and LC50 values of EPNs. • Despite higher Steinernema IE into pupae, mortality was lower compared to pre-pupa. Virulence and invasion efficiency of the three entomopathogenic nematodes, Heterorhabditis bacteriophora , Steinernema carpocapsae and S. feltiae against the potato tuber moth (PTM), Phthorimaea operculella was evaluated. Also evaluated were the sex ratio of Steinernema spp. and host stages to determine if 1) the developmental stage of the host affects sex ratio of nematodes; 2) infective juveniles (IJs) concentration affects sex ratio in host developmental stages and 3) the establishment of IJs is affected by developmental stages of host. The PTM pre-pupa and pupa were exposed to IJs in filter substrate petri dish bioassays. By increasing the IJs concentrations, the number of established Steinernema spp. in both PTM stages increased and only decreased at the highest concentration. No reduction in established nematode numbers at the highest concentration was observed for H. bacteriophora . Sex ratio of S. carpocapsae in pre-pupa was affected by IJ concentration. PTM was more susceptible to Steinernema spp. than H. bacteriophora . Pre-pupa were more susceptible to S. feltiae but S. carpocapsae recorded as the most virulent EPN on pupa. Invasion efficiencies were similar for Steinernema and considerably higher than for H. bacteriophora . Despite a higher invasion efficiency of Steinernema into pupae, mortality was lower compared to pre-pupa No correlation was recorded between the invasion efficiencies of the EPNs and mortalities of PTM. The results showed that the invasion efficiency is not appropriate criterion to reflect the virulence of studied EPNs. Compared to H. bacteriophora both tested Steinernema spp. were good candidates for further studies as biocontrol agents of PTM.