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Monarch Butterfly

Why So Many Monarchs This Year?
Many experts are expecting a larger than usual migration this fall. Monarch biologist Dr. Lincoln Brower suspects monarchs may have produced one or two extra broods this year, due to the combination of an early spring arrival from Mexico and good weather conditions during the spring and summer.

While a large population is good news, Brower cautions that there are risks associated with an early arrival. He is concerned that monarchs may be leaving Mexico early due to the thinning of the forest in their winter sanctuaries. If this is a trend, he is concerned about the implicatons:

"Monarchs were lucky this year, because those that got so far north so early in the season by chance encountered a mild spring," stated Brower. "If a major cold front had come through (which is very possible at this time of the year), then all their breeding could have been in vain, because the fresh and delicate leaves on the sprouting milkweeds would have frozen, and the larvae would then have starved."

During August and September, Dr. Brower and his wife, Dr. Linda Fink sampled monarchs visiting a Buddleia butterfly bush garden at Sweet Briar College, in Virginia, just on the eastern edge of the Blue Ridge Mountains. Here is what they observed:

"What is striking is that the big peak in August represented monarchs in very poor condition and with a lot of mating pairs. Spermatophores could be felt in the females, so clearly they were in their sexual phase.

However, these butterflies also seemed to be migrating, as evidenced by their disappearance around 8 September. Their disappearance was then immediately followed by the appearance of an obviously fresh group of monarchs in mint condition. These came in with the southeasterly moving front that I mentioned in my earlier e-mail to you, and it seemes like the front pushed the old ones out and brought the new ones in.

"On another matter, I began collecting adults in this area of Virginia on May 5th, and by 9 May Linda and I were finding many eggs on the freshly sprouting Asclepias syriaca plants. I used our cardenolide fingerprinting technique which indicated that 28 out of 29 butterflies captured between 5 - 18 May had the A. syriaca fingerprint. This is very strong evidence they were spring remigrants that had remigrated from Mexico, and supports our earlier hypothesis that monarchs are leaving Mexico earlier than usual.

"As all will know, I consider this dire. Monarchs were lucky this
year, because those that got so far north so early in the season by chance
encountered a mild spring. If a major cold front had come through (which
is very possible at this time of the year), then all their breeding could
have been in vain, because the fresh and delicate leaves on the sprouting
milkweeds would have frozen, and the larvae would then have starved.

"Putting together these early arrival and breeding data with our
observations this past weekend of 4th and 5th instar larvae still in the
wild here, suggests that at least 4 and possibly 5 broods of monarchs were
produced this year in Virginia.

"If this pattern held for larger parts of the central states to the west of Virginia, then getting in one or two extra broods this spring and summer season may account for what appears may be a larger than usual migration this fall."

Lincoln P. Brower
Linda S. Fink
Department of Biology
Sweet Briar College
Sweet Briar, Virginia
September 16, '97