agronomy Article New Technology for Desert Locust Control Graham A. Matthews Faculty of Life Sciences, Imperial College, Ascot SL5 7PY, UK; [email protected] Abstract: Locust outbreaks usually begin in remote unpopulated areas following higher than average rainfall. The need to survey such areas has suggested that unmanned aerial vehicles (UAVs), often referred to as drones, might be a suitable means of surveying areas with suitable detection devices to survey areas and detect important locust concentrations. This would facilitate determining where sprays need to be applied at this early stage and would minimise the risk of swarms developing and migrating to feed on large areas of crops. Ideally, a drone could also spray groups of hoppers and adults at this stage. To date, tests have shown limitations in their use to apply sprays, although it has been suggested that using a fleet of drones might be possible. The use of biopesticide in these areas has the advantage of being more environmentally acceptable as the spray has no adverse impact on birds. Keywords: locust; drone; unmanned aerial vehicle; early warning; preventive control; biopesticide 1. Introduction Since biblical times, vast numbers of desert locusts (Schistocerca gregaria Forskål, 1775) have periodically increased to such an extent that the plagues cause extensive damage to major crops. Locust upsurges occur infrequently and during a recession period, inter- Citation: Matthews, G.A. New national organisations have not prioritized research, nor have governments in countries Technology for Desert Locust Control. subject to locust plagues maintained in-country research, due to years of under-funding, so Agronomy 2021, 11, 1052. https:// they are not prepared for locust surveillance and the control of locust swarms when they doi.org/10.3390/agronomy11061052 do occur. Over the last 60 years, the onset of a plague has been detected at an early stage, but Academic Editors: Michel Lecoq, in 2019, the early onset of locusts was following cyclones in 2018 which resulted in heavy Arianne Cease and Beatriz Gámiz rain in the inhospitable deserts of Arabia. This allowed locusts to breed unseen in the wet sands. Strong winds in 2019 blew the growing swarms into the Yemen where they were Received: 20 April 2021 undetected as the country was beset with a war. Accepted: 21 May 2021 Initially, the solitary insects typically occur at low densities across a recession area, but Published: 24 May 2021 following a period of good rainfall, the locusts thrived and soon aggregated and formed swarms, which spread both eastwards to Iran, India and Pakistan, and westwards to Publisher’s Note: MDPI stays neutral East Africa, with Kenya experiencing its worst outbreak in 70 years. Back in the 1950s, with regard to jurisdictional claims in published maps and institutional affil- despite vast areas being invaded by swarms, it was possible to use aircraft to spray iations. insecticides, including dieldrin to protect crops and eventually reduce the plague [1] (Figure1). Dieldrin was used to kill hopper bands that crossed barrier strips and as a very low dose in very fine sprays to kill swarms of desert locust. However, due to increased concern over persistent insecticides affecting non-target species, the use of dieldrin was banned at meeting organised by FAO in 1988 [2]. Following this meeting, the FAO Pesticide Copyright: © 2021 by the author. Referee Group was established to advise which insecticides could be used. At the same Licensee MDPI, Basel, Switzerland. time, CABI organised the international LUBILOSA Programme project, which led to the This article is an open access article Metarhizium acridum, distributed under the terms and production of a biopesticide, based on to control locusts. This major conditions of the Creative Commons development provides control of locusts that does not harm people or the environment [3,4]. Attribution (CC BY) license (https:// Today, we need to bring together new technology to improve the detection of locusts in creativecommons.org/licenses/by/ remote areas and use biopesticide to control locusts before populations build up and 4.0/). swarms invade extensive farming areas. Once swarms arrive in new areas, the control Agronomy 2021, 11, 1052. https://doi.org/10.3390/agronomy11061052 https://www.mdpi.com/journal/agronomy Agronomy 2021, 11, x FOR PEER REVIEW 2 of 9 Agronomy 2021, 11, 1052 2 of 9 swarms invade extensive farming areas. Once swarms arrive in new areas, the control of ofadult adult locusts locusts is israpidly rapidly needed needed before before crop crop damage damage occurs. occurs. Biopesticide Biopesticide acts acts slowly slowly so soinsecticides insecticides that that can canachieve achieve high highmortality mortality within within 24 h are 24 needed. h are needed. Nevertheless, Nevertheless, the least thehazardous least hazardous insecticides insecticides should be should selected be selectedin terms inof termsmammalian of mammalian toxicity and toxicity environ- and environmentalmental protection. protection. Figure 1. The number of countries in which desert locusts were detected since 1950, with fewer countries having an onset Figure 1. The number of countries in which desert locusts were detected since 1950, with fewer countries having an onset of of major desert locust activity (i.e., major upsurges and plagues) since 1965. Control has been achieved sufficiently early major desert locust activity (i.e., major upsurges and plagues) since 1965. Control has been achieved sufficiently early to to avoid a major plague, but in 2019/20 the locusts spread without early detection (updated from [5,6]). avoid a major plague, but in 2019/20 the locusts spread without early detection (updated from [5,6]). 2. The Initial Signs of a New Upsurge 2. The Initial Signs of a New Upsurge During a recession, locusts survive in small numbers as solitary insects in arid areas in sparselyDuring populated a recession, areas locusts from survive the Atlantic in small Ocean numbers to Northwest as solitary India, insects but in particularly arid areas inin sparselythe Middle populated East. After areas a period from the of Atlanticabove average Ocean rainfall to Northwest that provides India, but the particularly vegetation, inthe the locust Middle population East. After in these a period areas of can above increa averagese rapidly, rainfall which that translates provides into the vegetation,spectacular the locust population in these areas can increase rapidly, which translates into spectacular hopper band movements and swarms which migrate. Using phenomenological models hopper band movements and swarms which migrate. Using phenomenological models using gridded monthly data, Tratalos et al. [7] suggested that desert locust dynamics are using gridded monthly data, Tratalos et al. [7] suggested that desert locust dynamics are influenced by endogenous factors and rainfall, and that broad patterns of locust upsurges influenced by endogenous factors and rainfall, and that broad patterns of locust upsurges and declines can be forecast with some degree of success using data on only these factors. and declines can be forecast with some degree of success using data on only these factors. Locust monitoring has relied on ground-based surveys, which have required indi- Locust monitoring has relied on ground-based surveys, which have required individ- vidual searches in areas with a history of locust activity, albeit at long and irregular inter- ual searches in areas with a history of locust activity, albeit at long and irregular intervals vals to notify the national locust control units and share the information with neighbour- to notify the national locust control units and share the information with neighbouring ing countries and international agencies [8]. As the population increases, and desert lo- countries and international agencies [8]. As the population increases, and desert locusts custs often aggregate to lay eggs, and as they are extremely sensitive to density changes, often aggregate to lay eggs, and as they are extremely sensitive to density changes, this this rapidly triggers their phase transformation, resulting in their movement in swarms to rapidly triggers their phase transformation, resulting in their movement in swarms to new areas.new areas. The phase The phase transition transition is a continuous, is a continuous, cumulative, cumulative, and easilyand easily reversible reversible process process that canthat take can placetake withinplace within a short a period short (fromperiod 4 (fro h tom 32 4 h) h into the 32 deserth) in the locust desert [9,10 locust]. Studies [9,10]. in ChinaStudies have in China indicated have the indicated presence the of presence an aggregation of an aggregation pheromone pheromone in the migratory in the locust,migra- Locustatory locust, migratoria Locusta(Linnaeus, migratoria 1758) (Linnaeus, [11], but 1758) as Vosshall [11], but [12 as] discusses,Vosshall [12] it is discusses, not clear whether it is not pheromoneclear whether traps pheromone could assist traps in detecting could assist the presence in detecting of locusts, the presence or whether of alocusts, chemical or couldwhether be founda chemical to block could the be receptor found to preventblock the aggregation. receptor to prevent The latter aggregation. may not be desirableThe latter asmay it would not be not desirable reduce as locust it wo populationsuld not reduce but ratherlocust dispersepopulations large but numbers rather ofdisperse locusts large