Other Threats

It will freeze soon in my area, and I'm still seeing monarch caterpillars. Will they survive? What should I do?

In order for an adult monarch to fly, temperatures need to be above 55 degrees Fahrenheit. While monarch eggs and caterpillars can survive some exposure to cold, these cooler temperatures slow their development time and prolonged exposure may cause sub-lethal effects (e.g. monarchs may take longer to develop, leaving them more vulnerable to disease and predation). 

Since there is not a distinct boundary separating breeding and migratory generations, there will be caterpillars that are developing late into the season as temperatures decrease and milkweed ages. Not all of them will survive. In many cases, the milkweed plants (caterpillar food source) may be more vulnerable to freezing temperatures than the caterpillars themselves. Similarly, killing frosts can eliminate remaining nectar plant species that are in bloom, which feed fall migratory adult monarchs. These late season monarchs have a lower chance of surviving the long-distance migration due to a number of additional stressors that they face, including temperatures too cold for them to fly and lack of available nectar to fuel their journey. For monarchs, other pollinators, and thousands of other organisms, these cycles and stressors are part of the natural cycle of life and the seasons.

Do wind turbines kill monarchs or disrupt their migration?

Little research has been done to understand the effects of wind turbines on monarch migration. However, migrating monarchs have been recorded as flying as high as 12,000 meters, depending on the wind patterns, so they are capable of avoiding turbines.

What is the most appropriate time to cut tropical milkweed back?

It is recommended to prune the milkweed stalks to about 6 inches in height during the fall and winter months to discourage monarchs from establishing winter-breeding colonies. Cutting back the milkweed will also help to eliminate OE spores that may be present on the plant. Re-cut the milkweed every few weeks as leaves re-sprout. Tropical milkweed might pose fewer problems in the northern monarch breeding range because it dies back naturally when it freezes.

These recommendations are not applicable in south Florida (south of Orlando), where a distinctive, non-migratory population of monarchs has long been established. However, native milkweed planting is still encouraged in this area.

For more information on tropical milkweed, read the our handout on the topic.

Are invasive plants a threat to monarchs?

Invasive species do not typically pose a direct threat to monarchs, with the exception of two species of invasive swallow-wort, which can serve as a "sink". These species, black swallow-wort (Cynanchum louisea) and pale swallow-wort (Cynanchum rossicum) confuse egg-laying females; if a female monarch oviposits (lays eggs) on these species, mistaking them for milkweed, the caterpillars will not survive because the swallow-wort is not a viable food source for them. While not a widespread invasive across North America currently, it is important to understand and control these invasive species to minimize their risk to monarchs. Other invasive plants can also cause concern for monarchs. While some invasive species may provide an abundance of nectar for monarchs and other pollinators during certain times of the year, if left uncontrolled invasive species can be extremely detrimental to the long-term viability of a habitat. Invasive species may out-compete other natives and decrease the overall diversity at the site. This could reduce or eliminate milkweed that is present at a site and could decrease the diversity of nectar resources that are available throughout the growing season. Control of non-native invasive species in a habitat is recommended to ensure monarch habitat exists for a long time! 

In places where tropical milkweed is native, do monarchs have a high incidence of OE as well?

The short answer is 'sometimes but not always.' The long answer is that monarchs breeding year round on tropical milkweeds in some places like Australia, Hawaii, Bermuda, South Florida do have very high infection rates with OE.  In Hawaii it is very variable - some local sites on some islands have low prevalence and some have nearly 100%.  In Australia it can be variable too. Other locations such as Belize and Costa Rica, where monarchs breed on A. curassavica, are on the lower end of the spectrum with about 20-30% heavy infection rates.  Further research is needed into why this variability exists.

When planting pollinator habitat adjacent to a road, how big of a concern is mortality by vehicle traffic?

While vehicle traffic is a contributor to adult monarch mortality, more research is needed on this issue to know the full extent. However, given the extreme loss of habitat that monarchs and others have faced, we feel confident that the benefits of planting pollinator habitat along roadsides outweigh the cost. You can read more on roadside threats and opportunities for pollinators in this literature review  for the Federal Highways Administration conducted by the Xerces Society. 

What are the potential problems associated with planting tropical milkweeds?

Research by Satterfield et al., 2015, shows very clearly that monarchs breeding on tropical milkweed throughout the winter have higher levels of protozoan infection (caused by Ophryocystis elektroscirrha, OE) compared to monarchs in the migratory cycle. Please see the Monarch Joint Venture Fact Sheet and our Q&A with monarch and tropical milkweed experts for more information about the potential risks of growing exotic milkweed for monarchs.

How big of an impact do logging and other threats to the winter habitat have on the population?

In 1986 a decree was set in place to protect the Monarch Butterfly Biosphere Reserve, which has reduced the amount of illegal logging significantly. Read more about threats to the overwintering sites in the North American Monarch Conservation Plan (pg 24).

How do hurricanes and other extreme weather events impact monarchs and their habitat in the eastern US?

Monarch butterflies generally avoid direct impacts from Atlantic hurricanes, as most migrate through hurricane zones in October and November, after peak storm activity in September. The US Fish and Wildlife Services’ Species Status Assessment review of monarch research found very little documented direct monarch mortality from storms during migration (USFWS 2020). However, with the potential for longer hurricane seasons, there is a rising concern that early spring hurricanes could impact monarchs during egg-laying in Texas, harming developing caterpillars (Reis et al. 2018).

Indirect hurricane effects, such as increased fall vegetation growth due to heavy rainfall, could influence monarch migration positively or negatively. Enhanced nectar availability, as vegetation regenerates after a storm, could aid their lipid reserves for winter, improving migratory success. Conversely, out-of-season plant growth, including milkweed, might disrupt migration by encouraging monarchs to pause and even break reproductive diapause, which can lead to disease and increased mortality (Reis et al. 2018).

Severe winter storms can also potentially impact monarch and other butterfly populations. One study found that a spring (April) winter storm in southern Texas had a more negative impact on butterfly populations than a low-level (Category 1) hurricane during July (Zerlin et al. 2023).

In Mexico’s overwintering sites, storms have caused severe mortality events (up to 80%) due to wet butterflies freezing at warmer temperatures. Historical winter storms in 1980-2016 led to high mortality rates at these sites, where monarchs are more vulnerable due to deforestation and climate change. While some models predict fewer freezing events due to warming, deforestation and habitat degradation reduce the protective forest cover, making overwintering monarchs more susceptible to freezing. Thus, storms at overwintering sites remain a high catastrophic risk for the eastern monarch population, especially with decreasing population sizes (USFWS 2020).

Future research priorities include understanding hurricane impacts on resource distribution and the adaptability of migrating species to increasing hurricane frequency and intensity. Monarchs may serve as a model for studying these impacts on migratory behavior in the context of global change.