 |
| Monarch Butterfly Life Cycle: Egg to Caterpillar to Pupa to Adult |
What’s Natural
Saving Our Monarch
Butterflies, part 1
With their clownishly colored caterpillars and bold black
and orange adults, monarch butterflies get featured in most children’s nature
books. Monarch’s ability to migrate thousands of miles, is one of nature’s
greatest wonders. But worrisomely, monarch abundance plummeted by 90% over the
past 2 decades. Fearing monarchs could be vulnerable to extinction, the US Fish
and Wildlife was petitioned
in 2014 to list monarchs as “Threatened” under the Endangered Species Act.
But due to several contentious issues, more extensive studies were needed. A
determination is now expected
by the end of 2020. So, what is killing monarchs?
In the 1970s scientists discovered that virtually the entire
population of monarchs that breed east of the Rocky Mountains, migrate to extremely
small patches of high mountain forests in central Mexico. There they spend the
winter from November to March. Since the early 1990s, scientists began estimating
monarch abundance by measuring the areas occupied by wintering butterflies. The
greatest winter abundance, estimated in 1997, was confined to an area equal to
40 football fields. By 2013, wintering monarchs occupied less than 2 football
fields.
In January 2002, a winter storm brought cold rains followed
by clear skies. Without the clouds’ greenhouse effect, clear skies allowed
temperatures to plummet to 23°F (- 4°C). Still damp, millions of butterflies simply froze in
place. Many millions more fell to the ground creating an eerie carpet of dead
and dying butterflies several inches deep. Altogether,
500 million butterflies died that winter, killing 80% of the entire eastern
population. That the survival of the entire eastern monarch population could
hinge on conditions affecting such a small area became a huge concern.
 |
| Carpet of Dead Monarchs from Brower (2002) |
Such devastating effects from freezing storms emphasized the
need to protect the forests where monarchs spend their winters. The intact forest
canopy creates a microclimate that had protected monarchs for hundreds of
thousands of years. A
closed canopy inhibits freezing. But recent logging opened the canopy and enhanced
rapid cooling. The Mexican government eventually agreed to ban all logging wherever
the butterflies overwinter. Nonetheless, there has always been significant winter
storm fatalities. So, a few degrees of global warming would minimize those cold
weather massacres.
(In contrast, monarchs breeding west of the Rocky Mountains
migrate to forests
along the coast of California each winter where freezing is not a concern. The
bad news, populations are still collapsing, and monarchs choose to winter in
introduced Eucalyptus trees that many people try to eradicate. It remains to be
seen how Eucalyptus will be managed.)
Every scientist agrees 2 key factors are reducing monarch
abundance. First is degradation
of wintering habitat. Second is the loss of milkweed, the caterpillars’
only food plant. The good news is humans are working to restore landscapes to
benefit monarchs. However, media outlets hyping a climate crisis, falsely claim
climate
change is thwarting our attempts to protect the monarchs. But whether
global warming is natural or man-made, warmth benefits monarch survival.
Despite horrific winter losses, monarch populations can
rapidly rebound. Surviving adults leave their Mexican wintering grounds in
March, and soon arrive to breed in Texas and other Gulf Coast regions. They lay
eggs, then die. One female can lay up to 1100 eggs. However, for each female,
perhaps 40 eggs survive to produce the next generation of females. Depending on
temperature, the transformation from egg to adult takes 30 days. Wherever
temperatures are favorable, 3 to 4 more generations can be produced throughout
the summer. So, a single female arriving in Texas can eventually give rise to
6400 adults by the 3rd generation.
Temperature controls much of monarch growth. Overall, warmer
temperatures increase the speed of development, with an optimal
temperature approaching 84°F. If temperatures fall below 53°F then eggs, caterpillars
and pupa stop growing. If temperatures exceed 91°F, they also stop growing.
But research shows if exposed to higher temperatures for just a few hours,
there are no detrimental effects.
Monarchs also actively control their body temperature. Caterpillars
feed on the top of milkweed leaves during cool weather to enhance warming by
the sun, but feed underneath the leaves as temperatures rise. If midday
temperatures get too hot, caterpillars seek shelter in shaded leaf litter.
Monarchs linger in their Mexican winter habitat waiting for optimal
spring-time temperatures to develop in the USA’s Gulf Coast states. As summer
conditions become too warm along the Gulf Coast, monarchs then migrate
northward. Favorable warm temperatures, in places like Kansas, allow 4 new
generations each year. Further north in cooler Minnesota, only 2 generations
are possible. Thus, favorably warmer
temperatures allow more generations per year, and more generations allow
the monarch’s abundance to multiply and quickly rebound from their winter
losses. The 2019
winter count determined wintering monarchs tripled their abundance from their
2013 low point.
Of course, each generation is also dependent on their food
plant abundance, which landscape changes and pesticides greatly affect; a topic
for part 2.
Jim Steele is Director emeritus of San
Francisco State’s Sierra Nevada Field Campus and authored Landscapes and
Cycles: An Environmentalist’s Journey to Climate Skepticism
