Vacation season and butterfly season coincide. Vacationers in cars and butterflies sometimes collide. Conservationists wonder about the impact of roads on insects. In a study published in 1992 Use of Road Verges by Butterfly and Burnet Populations, and the Effect of Roads on Adult Dispersal and Mortality two British scientists investigated the abundance and diversity of butterfly species along roadways. They identified factors that had the greatest influence on butterfly populations. They made several conclusions and recommendations about roads that are of interest to conservationists.
The authors found that the variation in species was influenced by the numbers and types of breeding habitats on the “verge” (a British term that applies to the maintained landscape next to the road). The authors recommended that the verge be made as wide as possible. Wider verges had higher densities and numbers of species than narrower ones. Diversity is correlated with the abundance of nectar. The authors recommend planting a diverse collection of plants that supply abundant nectar.
Surprisingly, maximum mortality due to collisions with traffic was 7 percent, and considered insignificant compared to natural mortality factors. Mowing the verges was the largest factor in reducing butterfly abundance. Mowing reduces the amount of nectar available to insects that must migrate from the mown area or die. Staggered mowing to leave some nectar plants would likely increase populations but is impractical in most instances. Bottom line: Worry less about those butterfly car collisions. Worry more about the roadside and its maintenance.