Ecology is by conducting a standardised rearing experiment, in which these grasshoppers Locust swarms: the are reared both in isolated and crowded conditions from hatchlings, and the resulting phenotypes (colour, behaviour, and gene expression patterns) are quantified at evolution of a powerful the last nymphal instar stage. Using this framework, Dr Song and his students have been able to quantify density-dependent force of nature phenotypic plasticity across several species of Schistocerca. The results are very Locust swarms are a severe agricultural threat. The destruction of crops interesting because there seems to be a lot and pasture jeopardises many livelihoods and adversely impacts global of variation in terms of the species’ ability to respond to density. For example, two food security. To address this issue, Dr Hojun Song, a distinguished non-swarming Schistocerca species, native Entomologist at Texas A&M University, is conducting ground-breaking to Florida (Schistocerca americana and S. research, focused on locust swarm evolution. Understanding how and serialis cubense) exhibit morphological and why locust swarms occur is crucial if we are to develop effective control behavioural changes, similar to those of the mechanisms. desert locust. Two other sedentary species (S. rubiginosa and S. lineata) develop black patterns when crowded, but behaviour does not change. Another sedentary species (S. ocust swarm invasions are swift and swarms in Mexico, is now devastating other nitens) from Texas does not change either incredibly destructive. Black swarm countries in Central America. colour or behaviour when crowded. clouds race across the sky, creating an ominous solar eclipse-like effect. Losses caused by locust swarms greatly Dr Song has studied the evolutionary Moments later, the plague of locusts impact food security – an already vulnerable relationship among Schistocerca species and descends, attacking every crop, tree and area due to the increasing global population. found that the desert locust is the earliest Lblade of grass in their path. Dr Song and his team are dedicated diverging species in the genus. What is more to improving our understanding of the interesting is that the desert locust is the Central American Suffice to say, locusts are a major global pest, evolution of locust behaviour and the genetic locust swarm only species found in Africa, and the rest threatening the agricultural industry and mechanisms behind swarming with the aim of of the genus (about 50 species in total) are affecting the livelihood of at least one tenth developing an effective method of controlling found throughout North, Central, and South of the world's population. Desert locusts these pests, to protect defenceless crops. America. Dr Song suggests that the ancestral (Schistocerca gregaria), typically found in Africa phase' (high population density), individuals Schistocerca, which was probably similar to and the Middle East, are one of the most ALL LOCUSTS ARE GRASSHOPPERS... are attracted to each other, forming large the current-day desert locust, originated harmful species because of the great distances ...But not all grasshoppers are locusts. cohesive units. They then grow larger, have in Africa and colonised the New World by they can cover, wreaking widespread havoc. It Locusts are a type of grasshopper, capable an increased metabolic rate and become transatlantic flight about 6 million years is estimated that 20% of the world is affected of undergoing reversible changes to their darker in colour, with striking yellow and black ago. The rearing experiments reveal that by desert locusts. A large desert locust plague physical appearance and behaviour based on markings. most Schistocerca species have the genetic can contain up to 150 million individuals per population density – a phenomenon known capacity to change colour when crowded, km2. Half a million locusts weigh approximately as 'locust phase polyphenism'. This is an Interestingly, the desert locust belongs because it is an ancestral physiological trait 1 tonne, and 1 tonne of locusts eat as much example of phenotypic plasticity, whereby an to the genus Schistocerca, which includes for the genus, but the ability to change food in one day as about 10 elephants, 25 organism can alter its appearance/behaviour both swarming locust and non-swarming, behaviour has been lost and regained camels, or 2,500 people. in response to environmental changes. sedentary grasshopper species. This raises a multiple times throughout the diversification number of questions: why do certain species of Schistocerca. Most sedentary species do In addition to the desert locust, there are Locust phase polyphenism was first observed exhibit this swarming behaviour whilst others not show behavioural plasticity in their natural more than a dozen species of locusts, by the Russian entomologist Sir Boris Uvarov do not? What are the fundamental molecular environment, because it is not adaptive affecting every continent. In 2016–2017, in 1921. He primarily studied the migratory mechanisms underpinning the behavioural for them to do so. Furthermore, three the South American locust (Schistocerca locust (Locusta migratoria) and desert locust and physical changes? And how does the swarming species within Schistocerca are cancellata) had a massive population upsurge and discovered that during the 'solitarious environment influence this polyphenism? not evolutionarily close to each other, which in Argentina and Bolivia for the first time phase' (low population density), individuals, Using a series of innovative experiments in suggests that swarming behaviour evolved in 60 years. The Central American locust vibrant green in colour, are repelled from a comparative framework, Dr Song and his repeatedly within the genus. (Schistocerca piceifrons), which normally each other. However, during the 'gregarious research team investigated each of these questions. MOLECULAR MECHANISMS UNDERPIN PHENOTYPIC PLASTICITY REARING EXPERIMENTS Swarming occurs due to positive feedback Dr Song's research has enhanced our While we know a lot about the desert processes. A combination of sight and locust, it is not clear whether other species smell or touch alone can trigger behavioural understanding of the evolution of in Schistocerca can respond to changes in Ryan Selking gregarisation. When locusts come into feeding hoppers swarming locusts density. The only way to address this question contact with each other, tactile stimulation 18 www.researchfeatures.com www.researchfeatures.com 19 Ecology of mechanosensory receptors located on the hind leg activates sensory pathways. Detail These transmit signals to the central nervous system, resulting in the release of RESEARCH OBJECTIVES the neurotransmitter serotonin, which is Dr Song’s research focuses on locust responsible for initial aggregation behaviour. What are the adaptive advantages of locusts will expand their range accordingly. swarms. The molecular mechanism In fact, following initial physical stimulation, swarming in locusts? In fact, recent upsurges of locusts in Central behind locust’s ability to form swarms serotonin levels rapidly increase three-fold, Locusts show an extreme form of and South America are probably somewhat in response to change in population over a period of just a few hours. However, phenotypic plasticity, which typically related to changes in environmental density is poorly understood, and is an this increase is transient: serotonin levels only evolves as an adaptation to heterogeneous conditions. area that Dr Song and his research team remain elevated for around 24 hours and environmental conditions, and swarming is particularly consider. then decrease. Maintenance of swarming one extreme of this plasticity. Gregarious How can we effectively control locust behaviour and the physiological changes nymphs are brightly coloured, and there swarms? FUNDING associated with gregarisation is controlled is evidence that at least in the early This is a million dollar question. Locusts are National Science Foundation (NSF) by a variety of molecular and biochemical stage of gregarisation they feed on toxic very difficult to control because they are pathways. From an ecological point of view, plants, which makes their colour warning extremely mobile and honour no political COLLABORATORS locusts generally live in a harsh environment, Central American locusts marching colouration. Being in a group that all show boundaries. If you spot adult swarms today, • Mario A. Poot Pech (Comite Estatal de where availability of resources is often warning colouration can be adaptive it is already too late to control because Sanidad Vegetal de Yucatan, Mexico) unreliable and food shortages can occur. because it can provide protection from the swarm will migrate to somewhere else • Maria Guadalupe Galindo Mendoza Interestingly, food deficiency can cause visual predators. Locusts are typically tomorrow. Locusts also happen to affect (Universidad Autónoma de San Luís cannibalism in desert locusts and research in a nutritionally poor state, and being many developing countries without proper Potosí, Mexico) suggests that collective movement may in a crowd provides an opportunity to infrastructure to survey and manage them. • Maria Marta Cigliano and Martina have evolved because of this behaviour. cannibalise each other to obtain necessary Therefore, early warning systems and Pocco (División Entomología, Museo de Much of what we know about locust phase nutrients, which has been shown