Scientific American - USA (2020-03)

March 2020, ScientificAmerican.com 65

Egg

3–5 days

Larvae/caterpillar

9–14 days

Chrysalis

8–13 days

Population Index (hectares)

Population Index (sum of NABA counts)

Time until adult can lay eggs

5 days

February March

April

May

June

July

September August

October

November

December

January

O

VE

RW

IN

TE

RI

NG

SE

AS

ON

FAL

L (^) M
IGR
ATIO
N SUMM
ER^
M^ I
GR
AT
IO
N
AND
BR
EED
IN
G
AN
D
BR
EE
DI
N
G
SP
RI
NG
(^) M
IG
RA
TI
ON
Annual^
monarch
migration cycle
of four to five
generations
Midwestern
generations
Northeastern
generations
Eastern
monarchs
Spring/Summer Migration
and Breeding
Fall Migration
1st generation
2nd generation
3rd generation
Possible 4th generation
Overwintering super
generation
1993
20
15
10
5
0
350
300
250
200
150
100
50
0
1998 2003 2008 2013 2018
1993 1998 2003 2008 2013 2018
MEXICO
GULF OF MEXICO
CANADA
U.S.
Western
monarchs
?
?
?
?
Midwestern and Northeastern Population Sizes Fluctuate
Overwintering Population Size Trends Down
Graphic by Nigel Hawtin
SOURCES: “MECHANISMS BEHIND THE MONARCH’S DECLINE,” BY ANURAG A. AGRAWAL AND HIDETOSHI INAMINE, IN
SCIENCE,
VOL. 360; JUNE 2018; WORLD WILDLIFE
FUND, MEXICO (
2018 winter population data
); JOURNEY NORTH CITIZEN SCIENCE PROJECT, UNIVERSITY OF WISCONSIN–MADISON ARBORETUM (
annual cycle
)
Monarch Ups
and Downs
Every year millions of monarch butterflies travel
from their winter home in Mexican mountains to
summer breeding fields across the eastern U.S. and
Canada. The winter population has dwindled in an
alarming way. Scientists first blamed the herbicide-
driven loss of milkweeds where butterflies lay eggs
in the Midwest. But recent data show these popu-
lations go up and down, leading some ecologists to
suggest the real threat is elsewhere. Others remain
convinced that vanishing milkweed is the problem.
Differing Trends and Seasons
Since 1993 summer counts of monarchs in the U.S. Midwest
and Northeast, calculated by Cornell University ecologist
Anurag Agrawal and his colleagues, show repeated peaks and
valleys. The cycles lead the scientists to believe milkweed loss
has not threatened the summer butterflies. But monarch
counts at their small winter home in Mexico show a down ward
trend. (That group has rebounded somewhat during the past
three years but has not regained historical abundance.) This
points to an ongoing problem—perhaps loss of forest or nectar
plants—closer to the southern end of the insects’ journey.
Overwintering reserve
(Pine oak forest,
Michoacán, Mexico)
A separate
group of
monarch stays
west of the
Rocky Mountains,
wintering on the
California coast. That
population has dropped
from millions to about
30,000 recently. But
because their habitat is
different from the eastern
monarch, scientists
suspect causes of the drop
are different as well.
One generation lasts approximately one month, except for the overwintering
super generation, which lives for up to eight months.
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