Student Work Spotlight

Taking Flight: Fall Migration of Monarch Butterflies

Ecology and Population Biology of Monarch Butterflies

By: Jha ’myia Gorley | April 1, 2023

On a warm sunny day in the fall, I find myself in awe of the sight and the fluttering sounds of millions of butterflies making their 4,300-kilometer journey to Central Mexico. Danaus plexippus or more formally known as the Monarch Butterfly, is a six-legged insect. It is known for undergoing the largest migration of any known insect species. Monarchs fly from Northern America to Central Mexico each fall to arrive at their wintering sites in late October and November. Around February, monarchs will mate and return to their breeding grounds and lay their eggs in the spring. One of the spots that is well known for its Monarch population is Cape May, New Jersey. Large numbers of Monarchs migrating along the east coast southward accumulate at this site, and many tourists come to see the monarch butterflies. As a result, scientists such as Robert Walton and colleagues began monitoring the migratory patterns of Monarch Butterflies because not much is known about Monarchs other than the migration patterns, how weather impacts their migration, and that there is a decline in Monarch butterflies that is linked to possibly to deforestation and the displacement of butterflies because they no longer have the food sources such as Milkweed. The researchers focused their study on census data by monitoring the long-term population trends, such as learning the total numbers of migrating Monarchs per year (1991 to 2004) and the seasonal variations.

Robert Walton and his colleagues performed the study in Cape May, NJ. The researchers collected data through a driving census – where the researchers would drive around in a vehicle and count the number of Monarchs seen in an eight-kilometer radius from September 1 to October 31. The researchers included several habitats, such as beaches, fields, and neighborhoods. The census consists of Monarchs in flight, feeding, or resting at approximately 9:00 am, 11:00 am, and 3:00 pm; until October 15, the researchers also surveyed at 11:00 am and 2:00 pm when the seasons changed it started getting darker.

The research team tested variation in Monarch abundance by running a statistical test called the ANOVA (univariate analysis of variance) by utilizing the data collected from the number of Monarchs counted per census. The data was used to examine the frequency and duration of the monarch's migration. From the experiment, a total of 45, 368 butterflies were counted! 1999 had the highest count of Monarchs, followed by years with declining Monarch populations in 2002, 2003, and 2004, with 2004 with the lowest count within those thirteen years. The results were conclusive, with fluctuations in the monarch populations. As a result of this, the scientists believe that long-term studying the migration patterns of Monarchs would be beneficial because we would be able to detect whether there is a positive or negative long-term trend regarding the population.

Since the conclusion of this study, there has been further evidence showing the decline of Monarch Butterflies. As of 2021, Monarch Butterflies are facing the threat of extinction. Concern has been raised about their pending status on the endangered species list. Several articles have been written about the decline, and various studies have been performed. One study, completed by Lincoln Brower and his colleagues, discussed the implications of the fall of the Monarch Butterfly population. The study examined how variability and the abundance of nectar affected the pollinators year to year, even if the weather and climate conditions changed. The researchers made it a point to monitor their food sources and ensure that there was an abundance of places where they could feed because, without those vital plants such as Milkweed, there would be a reduced population of butterflies due to the removal of those essential plants to ensure that the butterflies can return to that area.         

From these two studies, it can be concluded that to monitor the population of Monarchs; we need to focus on long-term studies to be able to describe the migratory patterns of the Monarch butterflies because there is a lack of information on this topic. Also, we should note the fluctuating population size, which is why we should monitor long-term to detect whether there are long-term positive or negative effects on the population of butterflies. However, there are things we could be doing to improve or help with the population of butterflies, such as tracking the migration patterns and helping plant species of milkweed native to the area. Even though we have a species on the endangered species list due to the removal of trees and flowers that are necessary to the growth of the butterfly population, which has led to the reduction of sites to stop for food (nectar) and for when the weather is too windy to fly, scientists have found a solution, so that we will be able to help track the migration of butterflies by completing a census of the number of butterflies spotted, milkweed plants spotted, caterpillars spotted, etc. Several organizations such as Journey North, Monarch Joint Venture, and the World Wildlife Fund have emphasized the importance of planting milkweed as well as tracking the migration of butterflies; however, due to these solutions, there is hope that one day that the butterfly population will remain stable and we will be able to say whether or not we need more intervention or less intervention with the use of further research of tracking the migration patterns of the monarch butterflies.                             

Charlotte called: She wants her web back

By: Zachary Krausman

“If I can fool a bug, I can surely fool a man. People are not as smart as bugs” – Charlotte, Charlotte’s Web. Charlotte was right. While people can be easily fooled, we can learn. Bugs on the other hand, might just be smarter than she expected. A study in 2015 by Takasuka et al. showcased the amazing ability of parasitoid wasps and how they can highjack a spider to do its bidding.

Insects are the most abundant animals on the planet. Among insects, wasps, especially parasitoids are thought to be one of the most diverse animal groups. A parasitoid is an organism that grows and develops within (or on) a host until maturity, eventually killing the host. In our case, researchers observed a parasitoid wasp, Reclinervellus nielseni. It is part of a group of wasps that parasitize orb weaver spiders. There are over 4000 species of orb weavers on the planet and they live all across the planet. In fact, Charlotte is an orb weaver. Our orb weaver, Cyclosa argenteoalba is the unfortunate victim. 

Many orb weavers have two types of webs. There is the namesake orb-shaped web used for catching food and general spider activities (chilling around). This is generally where you see the elaborate sticky webs with lots of insects trapped for the spider to eat. The other web type is a rudimentary, stout structure. This kind of web is bare bones with attachment points that are less sticky so the spider can molt uninterrupted. Interestingly, this is the time where spiders don’t want to catch prey. If an insect flies into the web, the spiders can get hurt while waiting for their new exoskeletons to harden.

Researchers studied the building behaviors and webs of unparasitized C. argenteoalba and compiled data on web shape, time it took to construct, and tensile strength of the web. Takasuka et al. explain that the behaviors associated with building each type of web design are well observed and were generally consistent with other orb weaver spiders. What is interesting about our spider-insect interaction is that our wasp parasitizes C. argenteoalba: the insect gets the spider. The insect effectively captures the spider by parasitizing it and then it uses preexisting spider web-building behaviors to create a safe and stable environment for the wasp to grow. Researchers wanted to find out how the parasitoid wasp affected web shape and development from the norm to benefit the wasp.

What scientists discovered was that R. nielseni in fact does utilize pre-existing spider web building behaviors. However, what was so intriguing is that the infected orb weavers designing webs for wasp cocoons made webs that were far stronger the normal orb shaped webs or molt webs. It was like the wasp was mind controlling the spider to do its bidding! While it was astounding that they discovered the webs were much stronger, it came with a cost. Web development for the wasp cocoon took significantly longer than a typical molting web set up. All the extra strength came from repeated web laying to ensure the cocoon was robust. Scientists deduced that it took extra time to build and needed to be robust because pupating wasps take much longer to develop than it takes for spiders to successfully molt.

While this discovery is a bit perplexing, it is easy to rule out magic or mind control. Admittedly, Takasuka et al. called for further research to be done to explain exactly how the wasp managed to make the spider create a web for the wasp without any new instructions. Fortunately, they do have a leading hypothesis. They think the wasp won its bidding by utilizing hormone dose-dependent manipulation.

Hormone manipulation is a rather common phenomenon among insects. Perhaps the most common way is during mating when males often use hormones to stop females from running away or mating with other males. While the mechanism for use of hormones by insects on other organisms may be similar, the execution is different. The general idea is that the wasps are manipulating spider hormones to make sure that the spider enters web building behavior but does not leave it until the spider completes the wasp cocoon web and then perishes. Normally, once the molting web stage is complete, other hormones would take place allowing the spider to molt and then continue its way to catching more food. Unfortunately, our parasitized C. argenteoalba never gets to.

While scientist might not know the answer yet, they are on their way and maybe you can help. This interaction between R. nielseni and C. argenteoalba showcases an interesting spin on host-parasite interactions and behavioral ecology. The groundwork has been laid by scientists, but they need help. Going outside and getting involved with citizen science can be as simple as taking a picture or reporting that you saw an organism. Citizen science cooperation is a great way to learn more about the world around you and help scientists collect data to find out more about organisms around us. You can check here to find local projects, as well as searching for local nature centers and university initiatives. Our world is constantly changing with new stressors and insects are quick to adapt. Science often cannot match the pace, but with your help, it is possible to observe just a little bit more and expand our connected web of knowledge.

Citations

Hammond, G. (n.d.). Critter Catalog - Araneidae. BioKIDS. Retrieved April 1, 2023, from http://www.biokids.umich.edu/critters/Araneidae/#:~:text=Orb%2Dweaving%20spiders%20are%20found,many%20still%20unknown%20to%20science.

Smith, M. A., Rodriguez, J. J., Whitfield, J. B., Deans, A. R., Janzen, D. H., Hallwachs, W., & Hebert, P. D. (2008). Extreme diversity of tropical parasitoid wasps exposed by iterative integration of natural history, DNA barcoding, morphology, and collections. Proceedings of the National Academy of Sciences, 105(34), 12359–12364. https://doi.org/10.1073/pnas.0805319105

Takasuka, K., Yasui, T., Ishigami, T., Nakata, K., Matsumoto, R., Ikeda, K., & Maeto, K. (2015). Host manipulation by an ichneumonid spider ectoparasitoid that takes advantage of preprogrammed web-building behaviour for its Cocoon Protection. Journal of Experimental Biology, 218(15), 2326–2332. https://doi.org/10.1242/jeb.122739

U.S. General Services Administration. (n.d.). Citizenscience.gov. CitizenScience.gov. Retrieved April 17, 2023, from https://www.citizenscience.gov/#