Â鶹ÒùÔº

Monarch butterflies (Danaus plexippus) have long been known for their impressive migration abilities, with North American species traveling around 3,000 miles from the United States and Canada into Mexico or Southern California to overwinter. However, there has been a noticeable decline in these overwintering populations over the last few decades. This decline has been thought to be at least partially caused by habitat loss, a decline in milkweed availability, and a parasite called Ophryocystis elektroscirrha (OE), which is known to increase monarch mortality by weakening migrants' ability to fly.

Several studies indicate climate change could affect summer and winter monarch populations, but few address the possibility that warming temperatures could affect their migration, which occurs in autumn. But recently, a group of researchers has attempted to remedy this by conducting an experiment meant to determine how different autumn temperatures affect monarch butterfly activities during their migration period. Their research was recently in Royal Society Open Science.

In the summer, monarch butterflies typically only live for about a month, during which they mate and reproduce, but the last generation of summer monarchs live much longer—around eight months. These monarchs enter a stage called diapause, which enables them to put off reproduction and fly south to overwinter, then reproduce in the spring. The researchers involved in this study suspected that warmer autumn temperatures interfere with diapause.

They say, "Warming autumn temperatures may destabilize the diapause phenotype. Diapause induction and metabolic rate in insects can be heavily influenced by temperature. Thus, pre-migratory and migratory monarchs in warmer environments may invest in fewer lipids, storage proteins or cryoprotectants needed for migration/overwintering due to muddled diapause signaling (e.g., warm days with declining photoperiod), higher metabolic demands or poor lipid storage efficiency.

"Moreover, higher temperatures may cause some autumn monarchs to begin migration in a less recalcitrant state of diapause, making them prone to abandon migration and attempt reproduction."

To test out their theory, the researchers exposed 499 wild-caught monarchs to simulated "warm" and "cold" migratory temperatures in a lab for 30 days. Then the survivors were exposed to "warm" or "cold" overwintering temperatures and their body condition, reproductive development, mating, and mortality were analyzed. Effects from OE parasites were also observed.

In the warmer environment, they found that the butterflies tended to end diapause prematurely and mate instead of overwintering. This is problematic because the diapause state allows the monarchs to live longer, while premature mating ultimately causes them to die long before the arrival of spring.

They also found that the warmer temperatures led to reduced male body condition and an increased risk of male mortality by 88%. The researchers also noted that the monarchs that experienced the warm migratory treatment during the first part of the experiment exhibited, on average, a 28% greater risk of mortality, leading to the conclusion that the temperature during migration has a significant impact on overwintering survival.

"The impact of migratory temperature also extended into the overwintering phase, with those from the warm migratory treatment exhibiting the greatest mortality risk and reproductive development. Furthermore, reproductive development during overwintering and/or OE burden were the greatest predictors of mortality. Thus, our results suggest that warming temperatures in the field should have an impact on monarch migration and overwintering success," the study authors write.

This study provides more insight into the cause of monarch decline in their normal overwintering locations. The authors note that the research can be built upon by tracking wild monarchs to confirm lab findings.