While many monarch conservation efforts focus on increasing breeding habitat availability, conservation efforts can also target improving the monarchs’ fall migration. Even though some estimates of the monarch summer breeding populations have been relatively stable in recent years, the overwintering numbers continue to experience a significant decline (Davis et al., 2024). This disconnect between summer numbers and overwintering size has led many to investigate what happens to monarchs during their fall migration. Recent research has found that migrating monarchs experience a steep decline in the size of fall migratory roosts (groups of resting monarchs) along their southern migration route (Davis et al., 2024). These findings provide clues as to why there might be a disconnect between summer and overwintering numbers. Below, are a few of the major threats fall migrating monarchs face, along with actions people can take to help protect this important link for monarch conservation.
Shifting breeding range: With increasing spring and summer temperatures, the range of suitable breeding habitat for monarchs has been gradually expanding northward (Davis et al., 2024). While this is good news for breeding habitat availability, it does mean that these northernmost monarchs will have a longer migration distance to reach their overwintering grounds. This increased distance could put a strain on migrating monarchs, making it more difficult for them to successfully migrate.
How to help: One way to lessen the strain migrating monarchs face is to ensure that they have abundant access to native fall blooming plants. Monarchs rely on native plants blooming September through late-October to fuel their migration journey. By providing a variety of native fall-blooming species like goldenrods, asters, and blazing stars, we can help ensure monarchs have the resources they need along their migration route.
Widespread planting of non-native milkweed: One widespread threat monarch butterflies face is the OE parasite (Ophryocystis elektroscirrha) which can weaken a monarch’s ability to fly, leading to lower migratory success and even death (Davis et al., 2024). OE is highly transmissible and can be passed onto healthy monarchs through spores left behind on milkweed plants. Non-native milkweed species like tropical milkweed (Asclepias curassavica) and balloon milkweed (Gomphocarpus physocarpus) can increase the presence of OE. Their extended fall growing season allows non-native milkweeds to accumulate OE spores which are then passed on to migrating monarchs.
Non-native milkweed species pose additional risks to migrating monarchs. Their tendency to grow later in the fall, and even throughout the winter in some areas, supports winter breeding monarch populations and disrupts the fall migration (Davis et al., 2024). These winter breeding populations experience much higher levels of OE infection which can be transmitted to migratory individuals. Research has also shown that caterpillars that feed on non-native tropical milkweed have lower migratory abilities and decreased survival (Davis et al., 2024).
How to help: It is important that we remove non-native milkweed from our landscapes and replace them with native species. By restoring natural sources of milkweed, we can help reduce the abundance of OE found in our landscapes and help migrating monarchs be healthier along their journey. When designing pollinator gardens and habitats, make sure to select only native milkweed species.
Release of Captive-reared monarchs: Many people will captive rear caterpillars in their homes to release as adults. While well-meaning, these actions can have a negative impact on the monarch population as a whole. Captive-reared monarchs often have reduced migratory navigational abilities and are often weaker than wild monarchs (Davis et al., 2024). Captive-reared monarchs can also experience higher OE loads if raised in large colonies. When released into the wild, captive-reared monarchs could spread OE to other monarchs or pass on their weaker migratory genes to the next generation.
How to help: The best way to support monarchs is to provide native habitat and let nature take its course. Planting pollinator gardens in your yard or larger pollinator habitat is a great way to provide long-term support for monarchs. After that, allow monarch caterpillars to grow naturally outside so that they have the best chance to complete their migratory journey. The more diverse, native habitat you provide, the more caterpillars you can support.
Final Thoughts: To best assist monarchs along their migration, we need to enact three important steps. First, we need to plant a variety of native nectar plants that bloom throughout the fall (September through late October). These late-blooming species are key food resources during the monarchs’ long migration. Second, we need to remove existing non-native milkweed species from our yards and landscapes and replace them with native ones. Talk to your neighbors and local gardening centers about removing non-native milkweed and providing native species instead. Finally, we need to move away from large-scale captive-rearing of monarchs. Instead, provide monarchs and other pollinators with diverse, native habitats that will allow them to grow naturally outside and provide them with long-term conservation.
References:
Davis, A. K., Croy, J. R., & Snyder, W. E. (2024). Dramatic recent declines in the size of monarch butterfly (Danaus plexippus) roosts during fall migration. PNAS, 121(43), 7. https://doi.org/10.1073/pnas.2410410121