Mass Die-Off of Western Monarch Butterflies Linked to Pesticides, Study Finds

A
monarch
butterfly
in
the
butterfly
pavilion
at
the
Natural
History
Museum
of
Los
Angeles
County
in
Los
Angeles,
California
on
June
1,
2022.
David
Crane
/
MediaNews
Group
/
Los
Angeles
Daily
News
via
Getty
Images

A
new
peer-reviewed
study
has
linked
pesticides
as
a
likely
cause
to
a
mass
die-off
of
Western
monarch
butterflies
that
occurred
in
2024.

In
January
2024,
researchers
found
hundreds
of
dead
or
dying
monarch
butterflies
near
the
Pacific
Grove
Monarch
Sanctuary
in
California,
where
Western
monarch
butterflies
typically
overwinter.

As
The
Guardian
reported,
researchers
found
the
butterflies
showing

signs
of
neurotoxic
pesticide
poisoning,
leading
to
further
testing
and
analysis
that
has
now
been

published
in
the
journal

Environmental
Toxicology
and
Chemistry.

[embedded content]

The
researchers
tested
the
dead
butterflies
using
liquid
and
gas
chromatography
and
mass
spectrometry,
which
led
the
team
to
find
a
mix
of
15
insecticides,
herbicides
and
fungicides
present
on
the
butterflies.

“We
found
an
average
of
seven
different
pesticides
per
butterfly,
including
multiple
insecticides
that
are
highly
toxic
to
insects,”
Staci
Cibotti,
lead
author
of
the
study
and
pesticide
risk
prevention
specialist
at
Xerces
Society
for
Invertebrate
Conservation,

said
in
a
statement.
“Although
a
review
by
Monterey
County
could
not
determine
the
source
of
the
chemicals,
the
high
levels
detected
suggest
that
insecticides
were
likely
responsible
for
the
monarch
deaths.”

According
to
the
study,
three
human-made
pyrethroid
insecticides,
including
bifenthrin,
cypermethrin
and
permethrin,
were
found
at
or
near
their
lethal
doses.
Further,
every
sample
included
bifenthrin
and
cypermethrin,
and
all
but
two
samples
contained
permethrin.

Western
monarch
butterflies
overwinter
along
the
Pacific
coast,
but
they
are
vulnerable
to
pesticide
residue
and
drift
from
nearby
farms
and
urban
areas,
Cibotti
explained.

According
to
the
Xerces
Society
for
Invertebrate
Conservation’s
annual
Western
Monarch
Count,
monarch
populations
dropped
to
the

second-lowest
number
ever
recorded
in
2024,
and
by
2025,
overwintering
Western
monarchs
totaled
just

9,119
individuals.

There
were
already
reasons
to
suspect
pesticides
for
the
death
of
hundreds
of
monarchs
at
an
overwintering
site
in
2024,
but
our
new
research
provides
clear
evidence
of
what
happened.
Each
monarch
had,
on
average,
7
different
pesticides,
many
at
lethal
doses.
➡️
xerces.org/press/study-…

[image
or
embed]

—
The
Xerces
Society
for
Invertebrate
Conservation
(@xercessociety.bsky.social)

July
23,
2025
at
3:31
PM

The
Western
monarch
butterfly
population
has
declined
by
almost
95%
since
the
1980s,
Xerces
Society
reported.
Migratory
monarch
butterflies
are

listed
as
endangered
by
the
International
Union
for
Conservation
of
Nature
(IUCN),
and
the
U.S.
Fish
and
Wildlife
Service
estimated
that
Western
monarch
butterflies
have
a

99%
chance
of
becoming
extinct
by
2080,
the
Los
Angeles
Times
reported.

As
such,
preventing
the
deadly
effects
of
pesticides
is
a
priority
for
monarch
butterfly
conservation.
Following
the
study
results,
Xerces
Society
has
recommended
several
actions,
including
increased
education
about
pesticide
risks
and
safer
alternatives,
establishment
of
pesticide-free
zones
around
overwintering
sites,
greater
pesticide
exposure
protections
in
conservation
and
recovery
plans
for
butterflies,
and
stronger
coordination
and
tracking
for
pesticide
risks
by
public
officials.

“Protecting
monarchs
from
pesticides
will
require
both
public
education
and
policy
change,”
Emily
May,
co-author
of
the
study
and
agricultural
conservation
lead
at
Xerces
Society,
said
in
a
statement.
“We
are
committed
to
working
with
communities
and
decision-makers
to
ensure
that
overwintering
sites
are
healthy
refuges
for
these
butterflies.”

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