Calcium-alginate beads as a formulation for the application of entomopathogenic nematodes to control rootworms

Entomopathogenic nematodes (EPN) have great potential as biological control agents against root-feeding insects. They have a rapid and long-lasting mode of action, minimal adverse effects on the environment and can be readily mass-produced. However, they have a relatively short shelf-life and are susceptible to desiccation and UV light. These shortcomings may be overcome by encapsulating EPN in Ca²⁺-alginate hydrogels, which have been shown to provide a humid and UV protective shelter. Yet, current Ca²⁺-alginate formulations do not keep EPN vigorous and infectious for a prolonged period of time and do not allow for their controlled release upon application. Here, we introduce solid Ca²⁺-alginate beads which we supplemented with glycerol to better retain the EPN during storage and to ensure a steady release when applied in soil. Glycerol-induced metabolic arrest in EPN (Heterorhabditis bacteriophora) resulting in quiescence and total retainment of EPN when added to beads made with 0.5% sodium alginate and 2% CaCl₂·2H₂O solutions. More than 4,000 EPN could be embedded in a single 4-5-mm diameter bead, and quiescence could be broken by adding water, after which the EPN readily emerged from the beads. In a field trial, the EPN beads were as effective in reducing root damage by the western corn rootworm (WCR, Diabrotica virgifera virgifera) as EPN that were applied in water. Although further improvements are desirable, we conclude that Ca²⁺-alginate beads can provide an effective and practical way to apply EPN for the control of WCR larvae.

The online version contains supplementary material available at 10.1007/s10340-021-01349-4.