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Monarch butterflies have been a signature summer sight in Nebraskan prairies for a long time. These insects play a crucial role in the pollination of wildflowers as they search for nectar. But, as cities grow, crop herbicides improve, and land uses change, monarch butterflies find themselves at risk of losing an important resource as they migrate throughout their life. This paper will examine the life cycle of the monarch butterfly and its migration patterns, the role milkweed plays in the butterfly’s life, and current threats to monarchs and milkweed.
Like all insects, monarch butterflies (Danaus plexippus) follow the same general path: egg, larvae, pupa, and adult.
Monarchs must live their lives to the fullest, as they have a very short time here on Earth. For example, some monarch butterflies that are year-round residents in Bermuda, that do not go through diapause, may only survive adulthood for a limited number of weeks (Hillburn, 1989). Hilburn found that these generations that reach adulthood during the summer months may only survive three to four weeks, while those that reach adulthood during the winter months could potentially live up to 13 weeks (1989).
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The shortened lifespan for summer maturing butterflies could be due to the higher availability for mating, as females typically expire after rearing eggs.
There are two populations of migratory monarchs in North America (Brower, 1996). One population starts its reproductive phase west of the Rocky Mountains and travels a relatively short distance in the fall to their overwintering locations on California’s Pacific Coast. The second population breeds to the east of the Rockies and has overwintering locations in the central portion of Mexico, up to 3600 km away (Brower, 1996).
Research by Malcolm, Cockrell, and Brower (1987) suggests that the population of migratory monarchs that occupy east of the Rocky Mountains typically only produce two generations of offspring during their early treks from their overwintering locations in Mexico to when they rear their first eggs in Northern Florida.
Adult monarch butterflies tend to lay between 500 to 700 eggs per generation (Wells, 2010). The data Malcolm, et al. (1987) collected suggests that as mature overwintered adults appear in early April; they produce offspring that will then produce the second generation. This was concluded by the peaks in egg deposition, as well as the appearance of adults inhabiting the vegetation. Butterflies in early April appeared to be pale and weathered as if they had traveled a great distance, while the adults that reared eggs in May were bright and had wings in perfect condition (Malcolm, et al., 1987).
As they followed this population north through their migration, they found an increase in generation production during the later months of summer in Wisconsin. Here, they found that populations of monarchs are able to produce three generations of offspring before the population enters diapause and starts their return to Mexico during the fall months (Malcolm, et al., 1987). Malcolm, et al. (1987) suggests that the reason for these differences in breeding are due to the temperatures found along the path these migratory insects take. By looking at the data provided, it proposes that since the temperatures are cooler in the higher latitudes, the butterflies are able to rear more offspring.
This data was the first in its kind to suggest such a theory. It was contradictory to many other studies including Urquhart (1960) and Eanes and Koehn (1978). They believed, through research and experimentation, that there should be more generations coming out of the warmer, southern breeding sites. Which, for most organisms, one would think that is the normal case. But, monarch butterflies apparently prefer the cooler locations to produce the most generations.
As previously cited, there are two major migratory populations of Danaus plexippus in North America. Through a review of scientific literature, it appears that the east-breeding population is typically published more frequently than the west-breeding population.
There are many different hypotheses about how and when these monarchs migrate. One hypothesis suggested by Brower (1996), indicated that day degrees run the internal migratory clock of monarch butterflies. This hypothesis proposes that the butterflies orientation shifts 1 degree per day, with the solstices and equinoxes marking major directional shifts in the butterflies migration. Brower (1996) states the remigrating monarchs travel to the Gulf region to produce the first set of spring offspring around the vernal equinox (Figure 4). As time progresses, the first spring generation migrates north towards the Great Lakes. At the summer solstice, the summer generations start the migration back towards the Appalachian Mountains. The autumnal equinox marks the start of the migration to the Gulf of Mexico, before crossing the Tropic of Cancer on their way to the overwintering locations in Mexico (Brower, 1996).
This hypothesis aligns well with the data discussed earlier by Malcolm, et al. in 1987. The peaks in egg deposition, measured by Malcolm, et al. (1987), in all of spring, summer, and fall, correlate with the major shifts in migration suggested by Brower in 1996. It is possible, that this could suggest the key shifts in migration and generation production could be linked by temperature and major celestial dates.
One of the most important interspecies relationships that monarch butterflies have is with milkweed. Milkweed, from the genus Asclepias, is found widespread across North America and the Antilles, with 108 species present (Woodson, 1954). Milkweed species are most noted for producing a strongly alkaline compound that makes them undesirable and even poisonous to most herbivores (Wilbur, 1976). While this may ward off most herbivores, monarch butterflies do not seem to mind the bitter taste and are seemingly unaffected by the toxins.
In fact, monarchs infected with a common parasite reared on milkweed species that contain a high concentration of the cardenolides toxin experienced an increase in their fitness & lifespan (de Roode, Pedersen, Hunter, & Altizer, 2008; Sternburg, et. al, 2012). So not only does milkweed provide food and shelter for the maturing monarchs it could also play a role in overcoming parasites.
Monarch Butterfly Life Cycle. (2021, Dec 02). Retrieved from https://studymoose.com/monarch-butterfly-life-cycle-essay
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