Climate change may harm flight of Monarch butterflies

Projected increases in global temperature may reduce the flight performance and alter the wing shape of North America’s Monarch butterflies in a way that may impede their ability to migrate, according to an experimental study published in the Journal of Insect Conservation.

London, 24th September 2020

Projected increases in global temperature may reduce the flight performance and alter the wing shape of North America’s Monarch butterflies in a way that may impede their ability to migrate, according to an experimental study published in the Journal of Insect Conservation.

To investigate the effects of rearing temperature and larval diet quality on the flight ability of Monarch butterflies, a team of researchers at the University of Michigan reared Monarch larvae at either ambient (25°C) or elevated (28°C) temperatures. During rearing, the larvae were fed foliage from one of three species of milkweed, which serve as the main food source for Monarch butterflies in North America. The flight ability, wing shape and wing cardenolide concentrations of adult butterflies were then measured.

The three species of milkweed used in the study - swamp, common or tropical milkweed – each have varying concentrations of cardenolides. These toxic steroids are stored by Monarch butterfly larvae as a chemical defence against predators and a natural antibiotic against parasites, but may be toxic at higher concentrations. Increasing temperatures are contributing to the northward expansion of tropical milkweed, a species high in cardenolides, within the habitat of Monarch butterflies.

The authors found that larvae reared at higher temperatures had reduced flight performance and flew for shorter periods, shorter distances and used more energy per distance measured. Adult Monarchs fed on a diet of cardenolide-rich tropical milkweed as larvae had shorter, wider forewings, compared to those fed a diet of swamp and common milkweed which have low to intermediate cardenolide concentrations, respectively. Butterflies which consumed lower concentrations of cardenolides during the larval stages exhibited longer, narrower forewings.

The results indicate that rising global temperatures could be detrimental to the successful annual migration of Monarch butterflies, as rounder, wider wings are less efficient for flight over longer distances than thinner, elongated wings, which can be used for energy-saving gliding flight.

Dr Abrianna Soule, lead author of the study, said: “Conservation efforts for Monarch populations often involve planting milkweed and it is therefore important to know which milkweed species benefits Monarchs the most. Our results suggest that the preservation of common milkweed should be of central focus, as it appears to be the highest quality food source for Monarchs.”

The authors caution that despite washing the eggs to sterilise them before rearing, many of the Monarchs used in the study were infected with the parasite Ophryocystis elektroscirrha. Although previous research has suggested that this may impact flight ability in Monarchs, infection was found to have no significant impact on flight velocity or distance in the study.

The authors suggest that further research is needed to establish whether increased urbanization of areas in the flight path of migrating Monarchs, which may lead to higher surface temperatures, may impact flight.

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About the journal: Journal of Insect Conservation.

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