Biopesticides with complex modes of action: direct and indirect effects of DiTera® on Meloidogyne incognita

Abstract

Abstract Bionematicides may exhibit complex modes of action based on: i) direct lethal or sub-lethal effects on the nematode; ii) promotion of plant growth and damage tolerance; and/or iii) influencing the interaction between the plant and the nematode by inducing plant defences. A suite of assays was conducted to characterise the mode of action of the commercial bionematicide DiTera® (Valent Biosciences, Chicago, IL, USA). Agar-based behavioural assays revealed significant chemotactic responses of second-stage juveniles of Meloidogyne incognita to DiTera with concentrations of 1.0% and 0.1% being repellent and 0.01% being attractive to M. incognita. In addition, exposure to 1.0% DiTera reduced the velocity and increased the meander of nematodes in no-choice tests. These experiments were complemented with soil-based assays. Rhizotron and minirhizotron assays that allowed spatially explicit evaluation of root growth demonstrated root proliferation of Solanum lycopersicum cv. Rutgers in response to localised applications of DiTera. Our results confirm that DiTera can positively affect plant growth and may influence nematode foraging behaviour. However, results of the soil-based assays provided little support for the hypothesis that DiTera has direct or indirect (plant-mediated) effects on M. incognita performance. This is in contrast to some of the published literature, and it may be that the chosen methods were not appropriate to detect these effects.