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The Effect of the Invasive Weed Black
Swallowwort (Vincetoxicum nigrum)
on Monarch Butterfly Populations (Danus
plexippus).
Dr. Richard Casagrande and Jennifer Dacey
Abstract:
The egg laying habits of Monarch Butterflies were observed in
laboratory and field settings to determine what effect the presence of
Black Swallowwort had on the behavior. It was discovered through
laboratory experiments, that Monarch butterflies lay their eggs on
the invasive weed. The eggs laid on Black Swallowwort
hatched,however, all first instar larvae died after feeding on the
leaves. A similar experiment was conducted in a large outdoor cage
under more natural conditions. The results supported the findings of
laboratory tests. To discover if this behavior was occurring in
nature, a field study was conducted throughout various sites in
Rhode Island. Eggs were found on Black Swallowwort in areas
were it grew in close proximity to Milkweed.
Introduction
Black Swallowwort (BSW):
- A member of the Milkweed (MW) family (Asclepiadaceae)
- An invasive weed from Southern Europe.
- Escaped cultivation as an ornamental in Massachusetts.
- Invades new areas through seed dispersal and deep rhizomes.
- Found in natural and disturbed areas where it out competes and
eliminates native vegetation.
Monarch Butterflies:
- Monarch adults lay eggs on Milkweed, which their larvae feed
on exclusively.
- Adults have 5 generations a season, each living 3-5 weeks.
- Each generation completes their lifecycle on Milkweed and heads
north during the spring migration, laying eggs along the way.
- The last generation heads south to overwinter, and will begin the
spring migration the following year.
1. Black Swallowwort
2. BSW distribution
3. Monarch Butterfly
Material and Methods
I. For the laboratory experiment, a 2’ X 2 X 3’ flight cage
was constructed to house the butterflies. Two males and two
females were exposed to various combinations of plants to
attract egg laying behavior. Each combination (a total of four
plants) was left for a 24 hour period and repeated multiple
times. Eggs on BSW were left on the plant to observe their
development. Eggs laid on Milkweed were reared and used for
out outdoor cage experiment.
4. Monarch egg on BSW
5. Indoor laboratory cage
II. The outdoor cage experiment involved exposing five mated
pairs of butterflies to 5 BSW and 5 Milkweed plants over a fifteen
day period. Plants with eggs laid on them were removed and
replaced on a daily basis.
6 a,b. Outdoor field cage
III. Our field study involved making random egg counts on both
BSW and MW plants, at various sites throughout South County,
RI. During the months of August and September. Three of the five
sites had both plants growing in close proximity. The remaining
two sites consisted of Milkweed only and BSW only.
7. MW and BSW plants
Data
Laboratory cage egg totals
600
eggs
400
200
0
MW
BSW
Control
Plant
Field count egg totals
Outdoor cage egg totals
300
200
eggs
100
0
60
40
eggs
20
0
MW
BSW
Plants
MW
BSW
Plants
References
Sheeley, S. E., and Raynal, D. J. (1996). “The Distribution and Status
of Vincetoxicum
in Eastern North America”. Bulletin of the Torrey Botanical Club.
123(2): 148-156
Urquhart, F.A., and Stegner, W. R. (1966). “Laboratory techniques
for maintaining
cultures of Monarch Butterflies”. Journal of research on the
Lepidoptera. 5(3): 129-136
Acknowledgements
Thank you to the University of Rhode Island, URI Insect Biological
Control Laboratory
and the Partnership for the Coastal Environment.