A lack of food during a critical stage of larval development may impair the growth of adult monarch butterfly wings, with potentially disastrous consequences for the migration success of this species.
Butterflies have a complex lifecycle, undergoing an astonishing transformation from caterpillar to adult butterfly. During the larval stage, the caterpillar must consume vast quantities of food which it will need to form the adult tissues as it metamorphoses within the chrysalis. Previous studies has shown that both the type and amount of food that a caterpillar consumes can alter growth during this crucial process.
Researchers from the University of Jamestown and the University of Georgia have examined the effects of food deprivation during the larval stage on the wing growth of the monarch butterfly (Danaus plexippus). This species is renowned for its epic migrations, as successive generations journey from breeding regions in Canada and northern US to their overwintering locations in Mexico. Efficient flight ability is critical for individuals if they are to cover these huge distances.
The team set up the experiment by limiting the amount of food available to late-stage caterpillars, using the monarch’s main food plant, milkweed (Asclepias spp.). When the adult butterflies emerged, they used image analysis to precisely measure the size of each individual’s forewing. And they found that food restriction had indeed led to a noticeably smaller adult wing size. But of greater significance is the concern that this might be happening in the wild too.
In the last few years, the monarch population has deteriorated at an alarming rate. Where once up to one billion individuals fluttered across the skies each year, the population in Mexico has now plummeted to the lowest point in more than 20 years. Much of this decline can be explained by the widespread destruction of milkweed, largely due to excessive herbicide use in North American breeding areas. Reduced availability of milkweed in late summer affects the migratory generation that flies south to Mexico. Those monarchs with sub-optimal wing size will thus take longer to reach these areas, increasing the likelihood that they will be struck down by early frosts.
Fortunately, there is a great deal of awareness about the plight of these colourful butterflies. Campaigns have begun to encourage people to plant milkweed throughout cities and homes in North America, while organisations such as Monarch Watch and the Monarch Joint Venture (MJV) aim to provide resources to help conserve the butterfly. Such efforts will undoubtedly help to ensure that the monarchs will continue to embark on their spectacular migration every year.
Reference: Johnson H, Solensky MJ, Satterfield DA, & Davis AK (2014). Does Skipping a Meal Matter to a Butterfly’s Appearance? Effects of Larval Food Stress on Wing Morphology and Color in Monarch Butterflies. PLoS ONE, 9 (4) PMID: 24695643
Image: Tarnya Hall (flickr)