Research Project

Are all migrating monarchs in reproductive diapause?

Mayra Arias, Kristin Duncan, Dayton Kring, Jenny Saik, Mary Kennedy,
and Sandra Moore
Texas Military Institute
San Antonio, TX

Abstract

In the autumn of the year, monarch butterflies enter a period of arrested development that has evolved as an adaptation to survive the annual flight to and from Mexico. As the monarchs pass through Texas in October, they are thought to be in reproductive diapause. We compared the reproductive status of early arriving monarchs to monarchs in mid-migration by determining presence of spermatophores by palpating them. To be sure of our observations, we dissected a random sample of ten monarchs. We found that sixty percent of the female monarchs collected in early October did have spermatophores, but not a single butterfly collected in mid-October showed signs of reproductivity. Therefore we concluded that there is a higher percentage of reproductive females in early October than in mid-October.

Introduction

As day length declines and temperatures decrease in the northern United States and Canada, monarch butterflies (Danaus plexippus) reportedly enter "reproductive diapause" and begin to migrate to Mexico (Goehring, 1999). When a female monarch enters reproductive diapause, her eggs do not mature and she is unable to lay eggs, but this does not mean that she does not mate with males. She may save the sperm for later to fertilize her eggs. Monarchs do not reproduce until four or five days after they have emerged from their chrysalis. But for migrating monarchs in diapause, reproduction does not take place until after they have over-wintered in Mexico. Eggs and larvae have been observed in Texas during the fall. One possibility is that monarchs arriving early in August and September remain reproductive or break out of diapause and become reproductive, while those arriving in mid to late October remain in diapause (Calvert, 2000).

The purpose of this study was to determine if there are female monarchs still laying eggs as they pass through Texas and if there is a difference in reproductivity in early and mid-migration. To answer these questions we caught butterflies, palpated migrating females for spermatophores, dissected a sample of migrating females, and asked other monarch watchers to send us reports from their areas of Texas.

Hypotheses

H₀: There will be no difference in reproductive females between the early and late migrating butterflies.
H_A1: There will be a higher percentage of reproductive females in early migrants than in later migrants.
H_A2: There will be a higher percentage of reproductive females in later migrants than in early migrants

Method

We netted monarchs at Garner State Park, Uvalde County, Texas, in a pecan grove with abundant frost weed, a nectar source for the butterflies. On October 9, 12–13, 1999, we collected 50 females, and on October 24, 1999, we collected 58 females. We palpated these migrating butterflies for spermatophores and measured their wing length, mass, and wing condition. We also checked the butterflies for spores. To palpate them, we pushed the butterfly’s abdomen out and squeezed it gently. A spermatophore felt like a small rock-like ball inside the abdomen.

We performed a dissection on a random sample of 10 butterflies. The abdomen was cut off the body of the butterfly and split open. Spermatophores appear as small yellow-tinted balls.In addition to our monarch observations we received e-mails from monarch watchers from different parts of Texas indicating the presence of egg laying monarch females in early October. We performed a chi-square test.

Results

Sixty percent of early migrating females collected at Garner State Park in early October had spermatophores. None of the females collected in mid-October had spermatophores (see Figure 1). Ten females from the mid-October netting were dissected. All of these were in reproductive diapause.

kennedygraph.gif (4820 bytes)

Figure 1. Number of butterflies observed with and without spermatophores during two observation periods.

We performed a chi-square test and, with a 99% confidence level, we reject our null hypothesis.

Reports from Texas Monarch Watchers located in the blackland prairie (Bronzebow1, 1999), the coastal prairie (Aschen, 1999) and the south Texas brush country (Oberhauser, 1999) indicated that reproductive monarchs were present and laying eggs in September and throughout October.

Discussion

According to Dr. Karen Oberhauser (1999) female monarchs enter a state of reproductive diapause when they travel down to Mexico. Our study shows that there are more female monarchs in reproductive diapause in mid-October than in early October. Perhaps the reproductive migrating monarchs are depositing eggs on the supply of milkweed that grows in the fall in the Texas hill country as a "back-up" in case the monarchs that are in diapause do not make it all the way to Mexico for the winter.

According to Dr. William Calvert, "The early population, which may arrive as early as late August, may differ from the main migrant body in its breeding activity and non-communal roosting. It may not be part of the migratory movement to Mexico. Instead these may be breeding butterflies that have dispersed southward in the same manner that they disperse northward in the spring, stopping to lay eggs as the opportunity presents itself."

Since we found that early migrating female monarch were reproductive and other Texas Monarch Watchers reported females laying eggs in October, we feel that further study is needed.

Next year we will continue our monarch study. We will palpate migrating females, place them in cages with milkweed, and hold them for twenty four to forty eight hours to see if they lay eggs on milkweed. We will also net monarchs in late September and throughout October.

Acknowledgments

We thank Dr. Karen Oberhauser and Dr. William Calvert for their advice and support. The National Science Foundation (ESI-9731429), McNutt Foundation, Southwest Foundation Forum, and Texas Military Institute supported our research.

Literature Cited

Aschen, Harlen. Personal communication by e-mail. October 27, 1999.

Bronzebow1. Personal Communication by e-mail. October 10, 1999.

Calvert, W.H., 1999. Patterns in the spatial and temporal use of Texas milkweeds (Asclepiadaceae) by the monarch butterfly (Danaus plexippus L.) During fall, 1996. Journal of the Lepidopterists' Society. 53(1). pp. 37-44.

Calvert, W.H., 2000. Personal communication by e-mail, November 13, 2000.

Goehring, Elizabeth. Environmental factors involved in reproductive diapause in monarch butterflies, Danaus plexippus. Master’s Thesis. September, 1999.

Oberhauser, Karen. Personal communication by e-mail. October 10, 1999.

Oberhauser, Karen. Lecture notes. July, 1999.