Efficacy of carboxymethyl cellulose as an inert water-soluble carrier for formulation of entomopathogenic nematodes, Heterorhabditis bacteriophora and Steinernema carpocapsae

Abstract

The relatively short shelf life of many entomopathogenic nematode (EPN) based products is a major constraint which limits their widespread usage in IPM programs. The two primary factors that affect survival of the infective juveniles (IJs) during storage (i.e. shelf life) are the rate at which the IJs utilize their energy reservoirs during storage and the effect of microbial contamination in formulation. In the present study, the suitability of Carboxymethyl cellulose (CMC) as an inert substrate for EPNs formulation was evaluated. Survival and virulence of dehydrated and non-dehydrated CMC formulated IJs of Heterorhabditis bacteriophora and Steinernema carpocapsae in the presence and absence of an antimicrobial agent was evaluated at two temperatures, 15 and 25 °C, over a six-month storage period. After six months of storage at 25 °C, levels of survival of H. bacteriophora and S. carpocapsae (77% and 93%, respectively) were highest where dehydrated IJs where in CMC which contained an antimicrobial agent. The effect of storage temperature (15 versus 25 °C) with respect to this formulation was species specific: a significant effect was seen in the case of H. bacteriophora, though not for S. carpocapsae. In both species, efficacy of anhydrobiosis on IJs survival emerged as foraging strategy dependent factor; this situation was observed for the efficacy of formaldehyde too. EPNs mortality across all treatments revealed two-stage survival curves, stage I with a steep slope and high mortality rate and stage II with a gentle slope and low rate of mortality. In our results CMC emerged as an efficient substrate for formulation of evaluated EPNs, mainly S. carpocapsae, though the use of antimicrobial agents other than formaldehyde may also make this formulation tool more appropriate for H. bacteriophora. Regarding the two-stage model, we propose that when assessing the suitability of new isolates for mass production, screening IJs from the 2nd stage of this mortality curves could be useful in terms of predicting the long-term viability of EPNs-based products.