It’s becoming spring again very soon, which could mean a return of the locust swarms we saw last year in Saudi and across the Middle East, as well as Africa. You may be asking why this still happens, and what causes grasshoppers to form swarms and turn to locust behavior. Well for one, we as humans have not had effective ways to control swarms. We’ve used pesticides, fire, etc. and it hasn’t really made much of a dent. Weather and climate are a major factor on whether there are swarms or not. If we get a rainy season then there tends to be more eggs that will produce nymphs and nice juicy food resources. The more nymphs around, the larger the population density becomes, which can effect their behavior.
I had written a short paper last year for an entomology class on just this topic. It was fascinating to learn about this insect and their ability to change from one color, diet, behavior, to quite another. Scientists continue to study this most interesting insect to learn more. Our food security for the future depends on controlling their numbers to mitigate their negative effect on agriculture. You can read my paper below. Use the references in the “Reference” section below to write your own paper, instead of plagiarizing. Thank you.
(I had originally posted an article in alarabiya.net talking about the spring swarm in the Middle East in 2020. My post is called “Locusts are the Focus –Spring 2020” on Sept. 7th 2020. Here is the newest article from today’s Feb. 6, 2021 arabnews.com., “Locust swarms pose new threat to Middle East and Africa’s food security”) https://www.arabnews.com/node/1804536/middle-east
Swarming of Non-Social Grasshoppers and Locusts by Desert Gardener
Locusts are in the insect order Orhoptera, family Acridoidea. They are shorthorned grasshoppers that have the ability to become social and migrate en masse. “Several species of grasshoppers swarm as locusts in different parts of the world, on all continents except Antarctica and North America.” (Wikipedia.com 2020) Locusts usually live solitary lives on grasslands or open fields. They have large hind legs for jumping, have wings in the adult stage and are not predatory insects. Their mouths are made to cut and grind food from plants such as leaves, flowers, stems and seeds.
They camouflage within the environment to avoid predators and complete their simple metamorphosis in one year. This means they emerge from their egg in the spring, and molt throughout nymph stage to become the adult form. Then they breed and deposit the eggs directly in the ground. However at times when the weather happens to be advantageous, many locusts congregate and form swarms, turning form solitary to social insects. Not all solitary locusts, such as the American Grasshopper Schistocerca Americana, do this. What factors are present to change solitary behavior in a way that form swarms?
Scientists have been trying to understand this phenomenon for at least 100 years. “Up until 1921, it was thought that the Desert Locust was actually two different species of locusts.” (FAO, UN) Locusts actually have different coloring when solitary than when social. It is called phenotypic plasticity, a term that refers to environmental conditions and the change produced in the animal. “The creatures undergo a remarkable transformation. ‘They change their physiology. Their brain changes, their coloration changes, their body size changes, …Instead of repelling one another, they become attracted to one another — and if those conditions persist in the environment, they start to march together in coordinated formations across the landscape…” (Baskar 2020)
The coloration difference from solitary phase to gregarious phase has led researchers to ask questions about spacial grouping, hormones and diet to understand what contributes to swarm behavior. A study in 1999 concerned grouping/swarming and color differentiation as well as insect diet. In The Sword Lab at Texas A&M University, research carried out found that nymphs in low density populations were typically green as they needed to blend in with their environment as a survival strategy. However nymphs that molted in a dense population, knew to eat toxic plants that actively affected their coloring to exhibit warning colors of black and yellow. They changed their survival strategy from avoidance of predators, by blending in with surroundings while solitary, to exhibiting warning colors visibly to predators, warning them to stay away as they were toxic to eat. (Texas A&M U)
In 2006, another study was performed at the University of Sydney, Australia, particularly addressing spacial relations. Researchers experimented with groups of desert locusts, Schistocera gregaria in restricted spaces. They put a certain number of them together in a controlled area and increased the numbers of locusts by increments. They found that in low numbers they exhibited solitary behavior. “Once a certain number was reached, in this experiment once the number was about 30 individuals they started to group together and follow the same direction. Without an apparent leader they all moved as one large group socially.” (Goudarzi 2006) This study demonstrates that locusts bond together when there is a population density sufficient to be advantageous for finding food and avoiding predators, instead of creating hostility and competition in the species due to resource scarcity. What are the advantages of being in the gregarious stage for locusts? “When population density increases to the extent that you can no longer remain inconspicuous on your own, you are safer from predators in a crowd. Once you are in a crowd, you must keep moving to find food—or become food.” (Goudarzi 2006)
Another factor that contributes to social behavior in locusts are chemical hormones and aggregation pheromones. “The neurotransmitters serotonin and dopamine are also involved in the phase transitions. Serotonin can initiate gregarious behavior in the migratory locust, whereas dopamine is able to induce and maintain gregarious behavior. Thus, dopaminergic and serotonergic systems play synergistic roles in regulating behaviors.” (Shi et al. 2014) Hormones are important regulators of behavior and influence locust behavior transition from solitary, shifting to social behaviors. Solitary insects will fly at nighttime and gregarious ones will fly during the day. “The principal chemical stimuli are provided by a mixture of semiochemicals released from locusts and their fecal pellets, collectively known as aggregation pheromones. Fecal aggregation pheromones are the product of hindgut bacteria.” (Zhang 2019)
Locust diet is being researched by Professor Marion Le Gall at Arizona State University. In 2017 she conducted a study involving migrating locusts, using millet fields in Senegal. Fertilizer was used on a portion of the fields while other fields were not fertilized. Her findings show that adult female locusts prefer the non-fertilized millet, as it has more carbohydrates, than the fertilized field containing more protein. “What the researchers think is happening is locusts prefer food with more sugar to fuel migration, because locusts are grasshoppers that fly long distances. When migrating, they can travel up to 350 kilometers in one night. But insects are not very efficient flyers, and for them, it’s very energy-consuming to fly long distances.” (Arizona State U 2020)
While the ability to go from solitary to gregarious state is part of what gives these insects a survival edge, it can be very disruptive to the ecosystem and agricultural areas where swarms occur. As the swarms consume large amounts of edible plants and span large areas, food insecurity for animals and humans is the effect of this behavior. Climate change is facilitating some of the more recent swarms. “When the weather has a lot of rainfall and moisture then the eggs that were laid underground in sandy bare soil, release the nymph. ‘They increase in numbers, and as they do so, they sense one another around them’…” (Baskar 2020) “Locusts tend to live in areas where resources that they need are very unpredictable …The strongest hypothesis is that these crazy, unpredictable dynamics select evolutionarily for this ability to go through these dramatic changes, to respond when you can capitalize on a rare opportunity and also have capacity to migrate.” (Baskar 2020)
Will we be able to more easily control locust plagues in the future? That can depend on if we can apply biological controls other than using insecticides that aren’t good for the environment. We may see with increased land cleared for crops and grazing cattle, just how agricultural practices and climate change may affect them. Perhaps locusts in certain parts of the world will go the way of the Rocky Mountain locust, now extinct, whose overwintering egg fields were destroyed by agriculture in the US. (Wikipedia.com 2020) We can learn something from locusts’ strong cohesive behavior, as it has power in numbers. To me locusts are like Clark Kent becoming Superman in the insect world, going from solitary and hidden, to a super swarm organism with an unapologetic appetite.
Baskar, Pranav. “Locusts Are A Plague Of Biblical Scope In 2020. Why? And …What Are They Exactly?”, NPR.org, 14 June 2020, https://www.npr.org/sections/goatsandsoda/2020/06/14/876002404/locusts-are-a-plague-of-biblical-scope-in-2020-why-and-what-are-they-exactly. Accessed 9 September 2020.
Food and Agriculture Organization of the United Nations, Locust Watch,
http://www.fao.org/ag/locusts/en/info/info/faq/index.html. Accessed 9 September 2020.
Goudarzi, Sara. “Why Locusts Swarm: New Study Finds ‘Tipping Point’”, 1 June 2006, https://www.livescience.com/10500-locusts-swarm-study-finds-tipping-point.html. Accessed 10 September 2020.
“Like marathon runners, locusts carbo-load before a long journey”, Arizona State U, 13 August 2020, https://asunow.asu.edu/20200813-discoveries-marathon-runners-locusts-carbo-load-long-journey. Accessed 11 September 2020.
“Locust”, Wikipedia.com, 9 September 2020, https://en.wikipedia.org/wiki/Locust. Accessed 19 September 2020.
“Phenotypic plasticity and the evolution of warning coloration”, Texas A&M U, The Sword Lab, https://swordlab.tamu.edu/research/phenotypic-plasticity-and-the-evolution-of-warning-coloration/. Accessed 11 September 2020.
Shi et al. “Unveiling the mechanism by which microsporidian parasites prevent locust swarm behavior”, PNAS, Vol 111 (4) 1343-1348, 28 January 2014, https://doi.org/10.1073/pnas.1314009111. Accessed 12 September 2020.
Song, Hojun and John W. Wenzel. “Phylogeny of bird-grasshopper subfamily Cyrtacanthacridinae (Orthoptera: Acrididae) and the evolution of locust phase polyphenism”, 7 August 2008, https://doi-org.ezproxy2.library.colostate.edu/10.1111/j.1096-0031.2007.00190.x. Accessed 11 September 2020. Zhang et al. “Locust and Grasshopper Management”, Annual Review of Entomology, Vol. 64:15-34, January 2019, https://doi.org/10.1146/annurev-ento-011118-112500. Accessed 12 September 2020.