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Spring Observations



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Spring Observations
Spring observations (1 March -1 June)
We need your help!

Tracking the return migration of the overwintering Monarchs has become increasingly important. In 1999, 25 million acres of Bt corn were planted within the corn belt (prime Monarch breeding habitat). The rapid adoption of Bt corn, which produces pollen that can kill Monarch larvae or delay their development, has inspired two related lines of research. The goal of one project is to establish the extent to which immature stages of Monarchs are present at the time of the shedding of corn pollen (anthesis). The second project is an attempt to develop a comprehensive understanding of the factors that determine the size of the fall migratory population. The latter study requires knowledge of the birth rates of Monarchs during the growing season as well as the factors that contribute to mortality throughout the year. Both studies will be facilitated by public reports of the following:

  1. First adults of the season
    The following information is needed for these sightings: date, location, weather conditions, observer, behavior of the butterfly(ies) and if possible condition and sex of each specimen seen.

  2. First eggs of the season
    Because Monarchs are often present but not seen, eggs are another valuable indicator that Monarchs have arrived. To observe first eggs, it is useful to identify emerging milkweeds that can be visited on a regular basis to determine if eggs are present on the emerging shoots or the undersides of the new leaves.

  3. First new adults to emerge (under natural conditions) in your area
    Relatively little is known about the dates of emergence of the first Monarchs produced from eggs laid by the overwintering females. We need to know when this first generation emerges and begins to move northward. Observations of first adults can be made by rearing late instar larvae outdoors in cages designed to protect the larvae and pupae while maintaining a temperature which is close to that of the natural environment.

  4. Developmental rates of first generation
    Monarchs Not all Monarch immatures develop at the same rate. Due to subtle differences in microclimate and possible genetic differences, a group of 20 eggs might produce adults that emerge over a 6 day period. Although developmental rates for Monarchs have been determined in the laboratory, the variation of developmental times is not known in the field. These data are crucial to the development of models and to understanding the timing of the arrival of new adults at more northerly latitudes. Determination of developmental rates requires rearing Monarchs in special cages outdoors.

If you are interested in helping us define first adults and first eggs, you can report your observations directly to us at These messages will be posted to Dplex-L, the email discussion list for Monarch Watch. First sightings are picked up from Dplex-L by Journey North and are incorporated into a database they update weekly throughout the spring.

If you would like to assist us with first emergences and/or variation in rates of development, please email us your name, phone number, location, and rearing experience. We will select about twenty cooperators distributed through the southern states and up to 40 degrees north latitude (imagine an E-W line just north of Kansas City). We will provide each cooperator with a protocol and a cage in which to rear the Monarchs over plants outdoors. If you are in the southern tier of states and wish to participate, contact us immediately. Monarchs could be in your area in 2-3 weeks.

Additional background
Although the majority of overwintering Monarchs do not leave the mountains in Mexico until the middle of March, Monarchs are usually reported at many locations in Texas and Louisiana by March 15th. This suggests that some Monarchs leave the overwintering sites in mid to late February. The 600 mile journey from the overwintering sites to south Texas probably takes a Monarch 2-3 weeks under favorable conditions. Based on our calculations, the first of the spring migrants could reach south Texas during the first few days of March.

The overwintered Monarchs or "oldies" can usually be distinguished because of their faded and tattered condition. The females tend to fly low to the ground, moving in a more or less NE direction as they search for newly emerged milkweed.

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