Findings From a 16-Year Study of Migrating
Monarch Butterflies in Southern PA
Holtwood
Muddy Run
Gayle Steffy
Aerial view of the tagging areas in South Central PA
Average Forewing Length(mm) of Migrating Monarch
Butterflies by Date Range
Nectaring monarch butterfly. Forewing
measurement is illustrated by the white line.
Introduction
Migrating monarch butterflies often stop to feed at two sites along
the Susquehanna River in Southern Pennsylvania. Both sites,
located approximately two miles apart, feature an abundance of
Buddleia. Monarch butterflies were captured and tagged by a
single individual at one of both of these locations during the fall
migration from 1992-2009, excluding 1996 and 2004. Over 11,000
butterflies were tagged, and forty-nine were recovered in Mexico,
the majority from the fall of 2003. Wing measurements were taken
from 1998 on. Females were tested for the presence of
spermatophores by abdomen palpation from 2000 on.
53.0
52.8
52.6
52.4
52.2
52.0
51.8
51.6
51.4
51.2
51.0
The sex ratio of the nectaring butterflies in this study was
skewed toward males throughout, with only 28% of the total
being female. The yearly percentage ranged from a high of
35% in 1999 to a low of 15% in 2007. The lower percentage of
females than males is consistent with other studies of nectaring
monarchs (McCord, 2012). Fourteen percent of female
migrants were found to have spermatophores upon abdomen
palpation. The percentage of female migrants increases as the
season goes on (Fig. 1).
Percent Female Monarch Butterflies Migrating Through
Southern PA by Date Range
45
40
35
30
25
20
15
10
5
0
52.8
52.6
F
M
ALL
10/2-10/20
Fig. 1 The percentage of female migrants increases as the season
goes on.
Forewing Length
Forewing measurements were taken with a transparent ruler
from the apex to the white spot on the thorax nearest to the
base of the wing (see photo upper left). Using these
measurements as an indicator of overall size, the data show
that monarchs migrating later in the season are smaller on
average than those migrating earlier. The average female is
smaller than the average male (Fig. 2).
52.4
All Migrating
All Recovered
52.2
52.0
51.8
51.6
8/20 - 9/9
9/10-10/1
10/2-10/20
51.4
Fig. 2 Using forewing length as an estimate of size, the average female is
smaller than the average male, and the average size of both sexes
decreases as the season goes on.
Less than one percent of the butterflies tagged were recovered in
Mexico. More early season migrants (0.7 %) than mid season
migrants (0.4%) were recovered. No late season migrants were
recovered. Forty-seven percent of the early season recoveries
were female, which is more than twice the percentage of female
migrants (22%) (Fig. 3). A similar higher percentage of female
recoveries than migrants was found from inland VA. (Brindza,
2008). The average forewing size of the recovered monarchs is
similar to the average forewing size of the migrants (Fig.4). When
the results are split by sex, it becomes evident that the average
recovered male is larger than the average migrant for the early
season, and slightly smaller mid-season. The average recovered
female is about the same size as the average migrant for the early
season, and larger than the average migrant mid-season. (Fig. 5).
Percent Female Migrating vs. Recovered Monarch Butterflies
by Date Range
%F
9/10-10/1
53.0
Recoveries
Sex Ratio
8/20 - 9/9
Average Forewing Length(mm) of Migrating Monarch
Butterflies vs. Recoveries from Mexico
50
45
40
35
30
25
20
15
10
5
0
%F Migrant
%F Recovered
8/20 - 9/9
9/10-10/1
10/2-10/20
Fig. 3. Recoveries of early season migrants include a higher percentage
of females than mid season recoveries.
Acknowledgments
I would like to thank Chip Taylor for taking the time to provide valuable advice,
and Lincoln Brower for providing many of the tags used in this study.
8/20 - 9/9
9/10-10/1
10/2-10/20
Fig. 4. Overall, the average recovered monarch was slightly larger than
the average migrants early in the season, and slightly smaller mid-season.
No late season migrants were recovered.
Forewing Length(mm) of Migrant vs. Recovered Monarch
Butterflies by Sex and Date Range
53.4
53.2
53.0
52.8
52.6
52.4
52.2
52.0
51.8
51.6
51.4
Migrating F
Recovered F
Migrating M
Recovered M
8/20 - 9/9
9/10-10/1
Fig. 5. The size difference between migrants and recoveries is most
apparent for early season males and mid season females.
.Conclusions
This study shows that the percentage of female migrants
nectaring at the study sites increases later in the season, and
that the size of both sexes decreases as the season goes on.
One possible explanation for these differences is that larger
monarchs migrate faster than small ones. While the overall
size of the recovered monarchs is similar to that of the
migrants, splitting the data by sex shows that recovered males
are larger for early migrants, and recovered females are larger
for the mid-season. Since it is difficult to determine whether
the recoveries represent the butterflies most likely to make it
to Mexico , those most likely to die there, or some combination
thereof, more research is needed to fully understand these
trends.
Literature Cited
Brindza, L.J,. L.P. Brower, A.K Davis, T. Van Hook. 2008. Comparative Success of
Monarch Butterfly Migration to Overwintering Sites in Mexico from Inland and Coastal
Sites in Virginia. Journal of the Lepidopterists’ Society 62(4), 189-200
McCord, J.W. and A.K. Davis. 2012. Characteristics of monarch butterflies (Danaus
plexippus) that stopover at a site in coastal South Carolina during fall migration.
Journal of Research on the Lepidoptera 45:1-8