Current adequacy of earth observation satellites for locust surveillance
The desert locust, Schistocerca gregaria (Orthoptera: Acrididae), a type of grasshopper, in accordance of their habitat, adopt one of two life forms—that of a lonely individual (solitary phase) or join other conspecifics to form swarms of migrating locusts (gregarious phase). This type of phenotypic plasticity is called density-dependent phase polyphenism (Cabej 2019 and Simpson et al. 2008). Changes in the local population density will reflect striking differences of morphology (brain-body size and colour), physiology (neuro-endocrinological status), food-eating habits, reproductive physiology, metabolism, immune responses and behaviour. During the solitary phase (occurring at low population densities) they exhibit shy, well-camouflaged demeanour and avoid one another. The gregarious phase sees the formation of huge groups that embarks on spectacular mass migrations, travelling as marching bands of flightless juveniles or vast flying swarms of winged adults (Cabej 2019, Simpson et al. 2008). The swarm size can be of one square km to several hundred square km and contain a population of 40 to 80 million adult locusts in each square km. The carpet of the swarm can fly for about 100 to 150 km, and evidence exists that they can fly across continents too (Showler 2008; Cressman 2013). A single adult of the highly voracious locust swarm can consume vegetation equivalent to its weight (approximately 2 gm) in a day, and a small-sized swarm (about 40 million locusts) can demolish an equivalent amount food that 35,000 people would eat in a day (Cressman 2013, Lazar et al. 2016, Latchininsky et al. 2016).
The locust events usually alternate between periods of low numbers (recessions) and very high numbers (plagues) (Magor et al. 2008). A locust outbreak occurs when consecutive rainfall events influence soil conditions and vegetation status that become optimal for breeding and gregarisation, in which there will be an increase in hopper bands and swarms over several months. Substantial widespread rains lead to locust upsurge with a massive increase in locust numbers in two or more successive seasons of transient-to-gregarious breeding. This complimentary seasonal breeding affects several countries (Cressman 2013). If an upsurge continues with heavy infestations for more than a year, a plague can develop (Hemming et al. 1979).
Desert locusts are usually active only in the semi-arid and arid deserts of Africa, the Near East and South-West Asia that receive less than 200 mm of rain annually. Approximately, 16 million km2 of the area in 30 countries, ranging from Mauritania through the Sahara and Arabian Peninsula into western India provides an optimal habitat to the locust. During an upsurge, locusts can reach as many as 60 countries, covering an area of 30 million km2. The Indo-Pak region containing the Thar Desert acts as a potential breeding area, and many researchers (Cressman 2013, Hemming et al. 1979, Woldewahid 2003, Bhatia 1961 ) observed the presence of a solitarious population. Parts of semi-arid regions of Rajasthan and Gujarat act as summer breeding points and have witnessed numerous outbreaks when the environmental factors have triggered swarms that have spread to other parts of the country (Hemming et al. 1979, Rao 1945, Steedman 1990).
Cyclones Mekunu and Luban during May 2018 and October 2018 brought heavy rains to the Arabian Peninsula and allowed at least three generations of unprecedented and undetected locust breeding. Swarms emigrated from these areas for spring breeding in the Central and Eastern regions from January to March 2019. Two generations of spring breeding occurred that spread the swarm to the Horn of Africa and the Indo-Pakistan border in June 2019. Along with this, the best monsoon rain in 25 years in the Horn of Africa allowed more generations of breeding well into June 2020 (FAO 2020).
There is a strong emerging need to monitor, control and prepare for locust upsurges to ensure food security. Currently, earth observation (EO) systems can also provide a synoptic view of the inaccessible regions with very high spatial and temporal resolution. Remote sensing satellite data plays a vital role in assessing meteorological parameters, vegetation status and soil conditions. Synchrony of EO data and in-situ field parameters can help with early warning systems and geo-alert trigger mechanisms for efficient locust control and management at various landscape levels.
The lifespan of a desert locust varies between three to five months—however, weather and ecological conditions influence substantially (WMO-FAO 2016). The life cycles consist of three stages—egg, hopper or nymph and the (winged) adult. Hatching of eggs can take 10 to 65 days depending on the temperature and soil conditions. Throughout the hopper stage of 30 to 40 days, the size of the nymph will increase, and at various instar breaks, they will shed their skin (moulting)—a process that is highly dependent on conditions such as day and night temperatures (Chapman 1965). Adults may remain immature for six months if the environmental conditions are dry and cool. However, during supportive environmental determinants, solitary individuals and sexually immature adults become mature and can participate in the copulation process and lay eggs (WMO-FAO 2016). Table 1 represents the environmental conditions for the life cycles of the desert locust and associated remote sensing data that has applications for monitoring the breeding sites, mapping the locust distribution and predicting the swarm migration (synthesised from the works of Cressman and WMO-FAO) (Cressman 2013, WMO-FAO 2016).
Table 1: Environmental conditions for various life cycles of desert locust and remote sensing data used in the model
In-situ field data (like locations of breeding sites, presence of hoppers, solitary and gregarious adults) play a pivotal role for kick-starting the model and predicting the spatial locust distribution. Soil moisture conditions (surface and root-zone), temperature, wind speed and direction, rainfall data and vegetation status will be highly useful to find out the optimal conditions for various phases of desert locust’s life cycles.
The issue of desert locust’s presence and migration is an inter-continental concern, and thus, studies using remote sensing need to be at a continental level. Normalised Difference Vegetation Index (NDVI) will convey synoptic vegetation status and global vegetation monitoring platforms like MODIS (Earth Resources Observation and Science EROS MODIS), and PROBA-V collections disseminate vegetation products. Geophysical products from Soil Moisture Active Passive (SMAP) sensors provide surface soil moisture and root zone soil moisture in the top 5 cm and 1 m of the soil column respectively (Entekhabi et al. 2014). Level 4 geophysical products from SMAP disseminates leaf area index (LAI) and land surface temperature (LST) products which are useful in retrieving the ground conditions from the existing location of egg pods, hoppers and adult locust. Geoportals like the National Information system for Climate and Environmental Studies (NICES) under Bhuvan platform disseminates geophysical products about land, ocean, atmosphere and cryosphere. These data sets play a pivotal role if we need to correlate swarm migration and environmental conditions. Similarly, Meteorological and Oceanographic Satellite Data Archival Centre (MOSDAC) provides downloadable meteorological data like rainfall, wind speed/direction (along with forecast) and temperature (Giribabu et al. 2018).
Synthesis of meteorological data, vegetation status and soil moisture details are essential inputs in monitoring various stages of locust and also for the preparation of threat maps. Figure 1 represents a map comprising locations of swarms, wind direction and vegetation status as on June 29, 2020. As represented in Figure 1, the wind direction has influenced the desert locust to favour a trajectory towards Nepal between June 30, 2020, and July 1, 2020.
Fig. 1: Prevailing wind direction, locations of locust swarms, and vegetation status as on June 28, 2020.The environmental conditions have influenced the locust migratory towards Nepal on June 30, 2020.
Fig. 2: Integrated set of various remote sensing data used for heuristic prediction of locust and to generate locust threat maps. (a) False color composite, (b) Normalised difference vegetation index, © land surface temperature, (d) wind vectors, (e) surface soil moisture, (f) root-zone soil moisture, (g) accumulated rainfall, (h) points of existing locust, and; (i) locust threat maps
Early detection of the locust is one of the prime needs for effective management. Integrating various remote sensing data can be a useful tool in assisting locust management. Figure 2 shows the integration of remote sensing data for performing heuristic prediction of locust and estimation of swarm trajectories. Mobile apps also play an essential role in updating the location of locust sightings, which enables to enhance field data. Advancements in the remote sensing, WebGIS and mobile apps can further enable precision and timeliness in early warning and better locust management.
Authors would like to thank Locust Warning Organising (LWO)–Jodhpur, India for providing in-situ data which have been used in the heuristic prediction models. We also thank the Director, National Remote Sensing Centre and scientists from EDPO, ISRO Hq. for providing institutional support. Authors would like to appreciate the scientists of RRSC-West, Jodhpur for their valuable and constructive suggestions.
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