Why shouldn’t the dragonfly cross the road?: Factors that influence dragonfly vulnerability to vehicular collisions

Background/Question/Methods

Roadway impacts on mammals, amphibians, and reptiles and how to mitigate for them has been widely examined; however, the impacts roadways have on insects has rarely been addressed. Roadways are known to act as barriers to wildlife movement either by eliciting behavioral avoidance or causing direct mortality. This is of special concern for endangered species such as the Hine’s emerald dragonfly (Somatochlora hineana), large numbers of which die along roadways in some parts of its range. One of the simplest ways to reduce mortality is to reduce vehicle speed and likelihood of fatal collisions; however, virtually nothing is known about the relationship between vehicle speed and mortality in insects.  We conducted a controlled experimental evaluation comparing the rates of dragonfly-vehicle collisions at 24 kph (15 mph), 40 kph (25 mph), 56 kph (35 mph), 72 kph (45 mph), and 88 kph (55 mph) on roadways in northern Door County, Wisconsin. Our study evaluated the influence of motor vehicle speed, flight height, and flight behavior on mortality rate of adult dragonflies in ten identified genera. 

Results/Conclusions

Our results indicate that vehicle speed was the most significant predictor (p < 0.0001) of encounter result. There was a strong non-linear structure to the relationship between vehicle speed and mortality, suggesting that reducing vehicle speed is most important at higher speeds and would have fewer benefits at lower speeds. Flight height was not a strong predictor of encounter result (p = 0.72); however, results show a significant difference (p < 0.0001) between the encounter results of straight-flying dragonflies (e.g. Somatochlora, Anax) and all others, with straight fliers most vulnerable and agile fliers (e.g. Pantala, Tramea) least vulnerable.  For dragonflies in general, decreasing the speed limit from 88 kph (55 mph) to 72 kph (45 mph) has the potential to reduce the probability of hit by 17%. For S. hineana, this speed reduction could decrease their probability of hit from 65% to 27%.  Although many wetland insects have short-lived adults, some especially sensitive groups have adults that forage, reproduce, and disperse over an extended period of time. Speed reduction in areas where these sensitive species interact with motor vehicles may be an essential tool in their conservation and may help reduce the overall impact of roadways on wetland ecosystems.