Effects of simulated highway noise on heart rates of larval monarch butterflies, Danaus plexippus: implications for roadside habitat suitability

Abstract

Developed countries around the world are criss-crossed with vast networks of roadways. Conservationists have recently focused attention on roadsides as possible locations for establishing pollinator habitat, with the monarch butterfly (Danaus plexippus) featuring prominently in such discussions. However, roadsides are inherently loud, which could negatively affect developing larvae. We conducted a series of experiments testing if simulated highway noise stresses monarch larvae, which we gauged by non-destructive monitoring of heart rates. In two replicated experiments, larvae exposed for 2 h experienced a significant increase in heart rate (16 and 17% elevation), indicating they perceive traffic noise as a stressor. Meanwhile, experiments exposing larvae for either 7 or 12 days to continuous traffic noise both showed no heart rate elevation at the end of larval development, suggesting chronic noise exposure leads to habituation or desensitization. Habituation to stress as larvae may impair reactions to real-world stressors as adults, which could be problematic for a butterfly that undertakes an annual two-month migration that is fraught with dangers. More generally, these results could have far-reaching implications for the billions of insects worldwide that develop near roadways, and argue that further study is needed before promoting roadside habitat for butterfly conservation.

Keywords: Danaus plexippus; heart rate; monarch butterfly; pollinator habitat; roads; stress.

Figure 1.

Methods for assessing heart rates of monarch larvae. In year 1 experiments, larvae were visually inspected under a dissecting microscope by one observer, who counted beats of the dorsal vessel (heart) just under the cuticle (video available in the electronic supplementary material). In year 2 experiments, we employed a novel device to electronically monitor heart rates, which involved an infrared emitter fixed to an older-model microscope body, which conveys electronic pulses of the dorsal vessel to a computer screen (further description given in electronic supplementary material, File S2). Comparison of both methods across 10 larvae yielded statistically similar heart rates. (Online version in colour.)

Figure 2.

Heart rates of monarch butterfly larvae in response to exposure to acute or chronic simulated highway noise, replicated over two experiments. In acute exposure trials, we monitored heart rates before and after exposure to either highway noise or a control (no noise). In chronic exposure trials, larvae were exposed to either noise or control and heart rates were recorded after 7 days in experiment 1, after 12 days in experiment 2. All larvae were 5th instar at the time of measurement. Acute exposure (2 h) to traffic noise led to a 16–17% increase in heart rate, but there was no elevation after chronic exposure (7 or 12 days). (Online version in colour.)