Viability and efficacy evaluation of entomopathogenic nematodes applied with two spray application techniques under controlled lab-conditions

Components and working parameters of pesticide application equipment (PAE) such as nozzle type, pumping system, spray pressure, and temperature can be detrimental for entomopathogenic nematodes (EPN) used as bio-control agents, thus affecting their viability and consequently the pest control efficacy. Due to limited literature, trials were conducted to test the influence of two different PAE on Heterorhabditis bacteriophora formulation viability for controlling Popillia japonica larvae to prevent vineyards colonization. To this extent, a Heterorhabditis bacteriophora formulation was sprayed by a conventional small-boom sprayer and an uncrewed aerial spray system (UASS) used in static laboratory-controlled conditions. The conventional sprayer and UASS were operated with four and two active flat fan nozzles at 0.6 and 0.3 MPa, respectively. Due to the different spray application techniques, the volume rate was 400 and 105 L ha-1 for the sprayer and UASS, respectively. In both cases, suspensions of water and Heterorhabditis bacteriophora were prepared to comply with the dosage equal to 250,000 nematodes m-2. The spray mixture temperature in the tanks was recorded in different positions for the entire trial duration by using thermocouples. Directly from the nozzles, the spray mixture samples were collected at five and three times for the sprayer and UASS, respectively. Immediately after the collection, EPN viability per each sample was assessed by microscope counting of alive and dead nematodes. Furthermore, to evaluate the pest control efficacy, batches of three larvae of Galleria mellonella were placed in a Petri dish and inoculated with the samples (5 replicates). The larvae mortality percentage was evaluated after 48 h. The data analysis showed that, for both PAEs, the EPN viability decreases progressively from the first to the last sampling timing. Interestingly at the beginning of the trials, the nematodes mortality was close to 48 and 35% for the conventional sprayer and UASS, respectively. At the end of the trials the two PAEs reached mortality equal to 88 and 69%. Even if the nematode mortality reaches very high values, the mortality of larvae was mostly above 80%, with higher lethal effect using the conventional sprayer. Results provide first information about the influence of different PAE on nematodes viability, suggesting that the choice of the PAE and their working parameters deeply influence the viability from the beginning of spray application, already when the spray mixture is prepared.