Spotted across the large stretch of highway it can’t be missed, each of your five eyes in utter disbelief as a lush field speckled with wildflowers fills your vision. Stumbling forwards you catch whiffs of lavender and milkweed as you gesture to the swarm around you towards the prospect of sugary delight. According to traditional aerodynamic calculations you should not be able to fly, but in stubborn defiance to the laws of aviation you fly anyways, nearing closer to sweet pursuit. As you continue forwards, the taste of pollen mere feet away, out of nowhere a 4,000 pound piece of metal hurtles towards you and you feel your helpless body go limp.
Unbeknownst to you, you have just contributed to the tragedies of insect road mortality. Despite this singular loss of the Bombus hortorum bumblebee (Ahrné et al. 2009), insect road mortality is responsible for the deaths of billions of pollinating insects annually, with 80-85% of the world’s flowering plants requiring pollinators for reproduction and in combating the rapid decline of global carbon shortage and biodiversity loss (Rhodes 2019). With this devastation being first ignited by the windshield phenomenon, where today’s truck drivers have been increasingly aware of reduced time spent cleaning bugs from their windshields since the early 2000’s. Further inspection revealed this alarming lack of insects around anthropogenic areas became correlated with traffic density and volume (Figure 1), giving rise to the term road insect mortality and its troublesome implication on species abundance (Rhodes 2019).
Figure 1: Vehicles contribute to air pollution, create disruptive turbulence, and cause direct insect mortality through collisions as it crosses areas of high biodiversity. Additionally, roads can fragment populations, isolating pollinators on either side as seen with the red arrow. Negative impacts that contribute to insect and plant mortality are highlighted to show how anthropogenic factors negatively interfere with wildlife such as pesticide spraying and mowing (Meinzen et al. 2024).
Used as a preliminary indicator on the status of remaining wildlife, insect road mortality has increased as recent spikes in urbanization have disrupted breedings sites and pollination. Between 1997 to 2007, the length of paved roads have increased over 6.6 million kilometers, with traffic volume rising 20% across the United States (Meinzen et al. 2024). With the escalating number of vehicles and growing urban sprawl, habitat fragmentation is at large, creating small disconnected spaces that increase genetic isolation between species, and invasion by invasive species when placed in these confined environments.
It’s not only physical restrictions that propel insects towards impending destruction, vehicle speed, aerodynamics, and turbulence play a vital factor in this gruesome collision. Referred to as the boundary effect, vehicles in motion exhibit an environmental change that occurs at the interface of the car and adjacent surroundings (Hucho et al. 1975). This induces a thin layer of air that sticks to the car’s surface in a gradual velocity gradient that starts as a laminar flow before becoming turbulent near the rear, influencing drag and temperature distribution. Greatly interfering with an insects flight path, regions of higher air pressure at the front of the vehicle become compressed, sucking insects forward in a sudden deceleration caused by the stagnation zone where air velocity drops to zero at the point of direct impact (Figure 2). For insects who face vehicles atop the roof, as air travels over the hood, the boundary layer separates and accelerates, creating a sharp velocity change causing disorientation or likely death of many insect species, with the higher the car’s speed, the greater the kinetic energy transfer is during impact (Hucho et al. 1975).
Figure 2: The boundary layer over a flat surface as fluid flows from left to right is shown showing the transition from laminar to turbulent flow. Initially, the flow is smooth, then velocity gradually increases leading to turbulence. The turbulent boundary layer consists of three subregions: the laminar sublayer, the buffer layer, and the turbulent layer as shown on the right. This process influences drag, heat transfer, and surface interactions, all factors in vehicle design and how small particles or insects may behave near objects in motion (Ting 2016).
Often overlooked, the risk of isolating and reducing insect populations through insect road mortality is too detrimental to be ignored. But through mutually beneficial solutions such as insect overpasses for safe migration, modified lighting to lessen attraction, and reduced traffic in ecological areas, we can conserve biodiversity and pollination services, paving a better road towards change.
References
Hucho, W. H., L. J. Janssen, and G. Schwartz. 1975. “The Wind Tunnel’s Ground Plane Boundary Layer – Its Interference with the Flow Underneath Cars.” SAE Transactions 84:380–89. https://www.jstor.org/stable/44681936.
Meinzen, Thomas C, Laura A Burkle, and Diane M Debinski. 2024. “Roadside Habitat: Boon or Bane for Pollinating Insects?” Bioscience 74 (1): 54–64. https://doi.org/10.1093/biosci/biad111.
Rhodes, Christopher J. 2019. “Are Insect Species Imperilled? Critical Factors and Prevailing Evidence for a Potential Global Loss of the Entomofauna: A Current Commentary.” Science Progress 102 (2): 181–96. https://doi.org/10.1177/0036850419854291.
Ting, David S-K. 2016. “Chapter 6 – Wall Turbulence.” In Basics of Engineering Turbulence, edited by David S-K. Ting, 119–38. Academic Press. https://doi.org/10.1016/B978-0-12-803970-0.00006-4.