D3395: Herbicide use in restoration: Investigating the non-target effects on butterflies and bumble bees in a southern California coastal reserve

Herbicide is a tool used in restoration to increase efficiency of invasive plant removal. While ecological restoration has positive effects on ecosystems, studies have also shown negative effects of herbicides on insects. We tested possible non-target effects of glyphosate on butterfly and bumble bee behavior at a coastal reserve in southern California. We designated paired plots and sprayed one plot in each pair with glyphosate at the beginning of the 2024 and 2025 survey seasons. Surveyors recorded seconds spent inside the plot for each pollinator that entered. We ran linear mixed models on the number of visits per survey and residence time per individual, for butterflies and bumble bees separately. Predictors were standardized before modeling. For butterflies, herbicide presence did not have a significant effect on residence time (β = -0.07, p = 0.38) or visitor count (β = 0.15, p = 0.45). The presence of Eriogonum fasciculatum had a positive effect on visitor count (β = 1.31, p < 0.001). For bumble bees, herbicide presence did not have a significant effect on residence time (β = -0.02, p = 0.83) or number of visitors (β = 0.05, p = 0.79). The presence of Phacelia distans had a positive effect on residence time (β = 0.58, p = 0.001), and presence of Acmispon glaber increased visitors count (β = 1.00, p < 0.001). While we cannot rule out effects of herbicide on pollinator behavior, our study showed no detectable herbicide effects and that key native flora positively influence pollinator visitation.