Effect of physical properties and surfactant concentrations on phase separation in tebufenozide droplets: Persistence and bioactivity of the insecticide in spruce foliage

Abstract

A commercial formulation, RH‐5992 2F, of tebufenozide [N'‐t‐butyl‐N'‐(3,5‐dimethylbenzoyl)‐N‐(4‐ethylbenzoyl) hydrazine] was diluted with water to provide an end‐use mix containing 35 g of active ingredient per litre. When sprayed in a laboratory chamber, the mix caused phase separation in the atomizer. Addition of surfactants at 2 to 3% level minimized the problem, although phase separation still occurred in droplets while they were in‐flight. Droplets collected on samplers showed three types of spots: (i) those containing particles in the centre surrounded by a liquid ring (PR), (ii) those containing particles only (P), and (iii) those containing liquid matrix only (L). The number of ‘PR’ spots decreased while the droplets were in‐flight, resulting in an increase in the ‘P’ and ‘L’ spots at the target level. Persistence of tebufenozide in spruce foliage was unaffected by phase separation in droplets, but bioactivity was affected. When the deposits contained mostly ‘PR’ spots, mortality in spruce budworm fed with treated foliage was high, whereas when the deposits contained mostly ‘P’ and ‘L’ spots, mortality was low. The results suggest that when the insect consumed a ‘P’ spot alone (mostly tebufenozide particles) without the liquid phase, the antifeedant activity of the toxicant was more pronounced than when the insect consumed a ‘PR’ spot containing both paniculate and liquid phases together. Thus, phase separation in droplets caused a decrease in foliar consumption, resulting in low intake of the toxicant and reduced bioactivity.

Key words:

• Flowable formulation
• viscosity
• density
• surface tension
• volatility
• spruce budworm bioassay
• antifeedant activity