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

0

200

400

600

eggs

MW BSW Control

Plant

Laboratory cage egg totals

0

100

200

300

eggs

MW BSW

Plants

Outdoor cage egg totals

0

20

40

60

eggs

MW BSW

Plants

Field count egg totals

ReferencesSheeley, S. E., and Raynal, D. J. (1996). “The Distribution and Status of Vincetoxicumin 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

AcknowledgementsThank you to the University of Rhode Island, URI Insect Biological Control Laboratory and the Partnership for the Coastal Environment.