Combining multiple baiting cycles with digital droplet PCR optimizes description of the distribution of entomopathogenic nematodes in French maize fields

Abstract

Entomopathogenic nematodes (EPNs) from the genera Steinernema and Heterorhabditis are soil-dwelling parasites that feed on insect larvae. They are valuable auxiliaries in the fight against insect pests of crops used in market gardening and through the addition of non-native EPNs. However, a better description of the natural distribution of EPNs in agricultural soils will be required to foster their use in crop management programs. The objective of this study was to develop an optimal methodology for obtaining an accurate picture of the presence and diversity of EPNs in the maize fields around Pau in South-West France. We combined different approaches for the detection of EPNs in 43 maize plots. We optimized a method for isolating EPNs directly from soil samples based on multiple baiting cycles with Galleria mellonella. With this approach, Steinernema and Heterorhabditis were isolated from 25.5% to 2.5% of the plots, respectively. We also extracted the soil nematofauna. An initial morphological identification of the EPNs present in these samples led to the detection of Steinernema and Heterorhabditis in 2.5 % and 7% of the plots, respectively. We then applied molecular detection techniques to the nematofauna samples, focusing on Steinernema. We detected Steinernema in 16.5% of plots by quantitative real-time PCR (qPCR) and in 35% of plots by digital droplet PCR (ddPCR). We propose a combination of multiple baiting cycles on soil samples with soil nematofauna extraction followed by ddPCR to optimize the detection of EPNs for the analysis of their distribution in agricultural soils.