Quantitative and qualitative characteristics of alginate-based formulation of a native isolate of the entomopathogenic nematode Steinernema carpocapsae (Rhabditida: Steinernematidae)

Entomopathogenic nematodes (EPNs), in the genera Steinernema and Heterorhabditis, are potent biocontrol agents that are widely used against various insect pests. The non-feeding and non-developing third stage infective juvenile (IJ), or the dauer juvenile, is the survival stage of EPNs that is able to survive outside the host and is used for formulation. In recent years, alginate has been recognized as the most promising biopolymer in the formulation of living cells and microorganisms. According to the results of scientific research, native isolates of each region have a greater impact than non-native and foreign isolates due to high ecological compatibility.

The native isolate of Steinrnema carpocapsae IRMoghan1 from the Moghan region, with accession number MF187616 in the NCBI database, was selected for use in the alginate-based formulation owing to its high potential of survival and pathogenicity. The best and most cost-effective capsule composition was selscted by evaluating 0.5%, 1%, 2%, and 3% alginate concentrations with 0.5%, 1%, and 2% concentrations of a calcium chloride solution. Formulation characteristics, the survival rate of the nematodes inside the formulation, exited water volume, escaped nematode rate, and pathogenicity of the formulated nematodes against Galleria mellonella larvae as the laboratory host were measured and recorded during 4 to 90 days post formulation. The analysis of variance (ANOVA) and comparison of means by Duncan’s test were performed on data using SAS software.

Based on the results, the best concentrations for sodium alginate and calcium chloride were 2% and 0.5%, respectively, which were therefore selected for the next experiments. The diameter of the formed capsules ranged from 3.5 to 4.5 mm. The ANOVA of the amount of water leaving the capsules, the number of nematodes escaped from the capsules, and the survival rates of the nematodes inside the capsules were significantly different at various time intervals post formulation (P ≤ 0.01). Statistical analysis showed that the formulated nematodes had a higher survival rate than the unformulated congeners in all the time points from (4 to 90 days) after formulation. The results of the ANOVA for bioassay experiments showed significant differences between different time points in the percentage of mortality of G. mellonella (P ≤ 0.01).

The present study showed the high potential of S. carpocapsae IRMoghan1 native isolate for sodium alginate formulation and, the successful formulation of an entomopathogenic nematode as a biocontrol agent, which is a valuable finding as it increases the shelf life without refrigeration. This finding should be used as the basis of further studies for the commercialization of this isolate.