SYN-A, a naturally derived synergist, restores pyrethroid efficacy against cabbage stem flea beetle but negatively...

The cabbage stem flea beetle (CSFB, Psylliodes chrysocephala) has become the most significant pest of winter oilseed rape (OSR) in Europe following the 2013 ban on neonicotinoid insecticidal seed treatments. Widespread pyrethroid insecticide resistance in this pest has severely limited the primary means of control for many growers, contributing to dramatic yield losses and a decline in OSR cultivation across Europe. This study evaluated SYN-A, a novel natural synergist derived from olive oil unsaturated fatty acids, for its potential to restore pyrethroid efficacy against CSFB while assessing impacts on the parasitoid Microctonus brassicae, a key natural enemy of the adult life stage.

In vitro enzyme assays demonstrated that SYN-A effectively inhibited cytochrome P450 and esterase activity - key metabolic pathways associated with pyrethroid resistance - in both CSFB and M. brassicae in a dose-dependent manner. Glass vial bioassays revealed that SYN-A significantly enhanced efficacy of the pyrethroid insecticide lambda-cyhalothrin against CSFB, increasing mortality more than threefold compared with the insecticide alone. The synergistic effect was sufficiently strong that lambda-cyhalothrin at 20% field rate combined with SYN-A achieved 2.2 times greater control than full-rate lambda-cyhalothrin alone. Semi-field experiments confirmed laboratory findings, with SYN-A + lambda-cyhalothrin treatments increasing CSFB mortality from 20% to 75% and reducing plant damage by at least 50% compared with lambda-cyhalothrin applications. However, SYN-A also synergized lambda-cyhalothrin against M. brassicae with 100% mortality when combined with lambda-cyhalothrin at both 20% and 100% field rates. Sex-specific responses were evident, with female parasitoids showing greater tolerance than males to both SYN-A and lambda-cyhalothrin.

The synergist SYN-A can effectively restore pyrethroid efficacy against metabolic resistant CSFB populations, potentially allowing up to 80% reduction in insecticide application rates while maintaining superior control. However, the severe impacts on beneficial parasitoids highlight the need for careful implementation strategies, including precise timing of applications outside peak parasitoid activity periods and continued evaluation of non-target effects. © 2026 The Author(s). Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.