Plague of locusts

Locust swarms have devastated agricultural fields in east Africa, parts of the Middle East, as well as India and Pakistan – and continue to spread. So how are scientists responding? Anthony King reports

Desert locusts (Schistocera gregaria) are typically found in Africa and the Middle East and are the most harmful of locusts, wreaking havoc on agricultural crops. A swarm can contain up to 150m individuals/km2. Half a million locusts weigh around 1t and eat the same as 2500 people in one day. These are the insects of biblical plagues, but they have a hidden side.

‘Locusts are grasshoppers, but not all grasshoppers are locusts,’ says Hojun Song, a locust researcher at Texas A&M University, US. Of 7000 species of grasshoppers, only about 20 are locusts. For most of the time, even desert locusts are harmless insects. They live solitary lives in dry regions and eat natural vegetation. Occasionally, environmental conditions turn favourable for them. In 2004, a few days of heavy rains from Senegal to Morocco, a distance of some 1500km, dumped more water than is usual for an entire year. Vegetation boomed, offering prime conditions for S. gregaria to breed.

‘Individual locusts then search for food and gather together. Vegetation dries up and they become clumped together,’ Song explains. The result in 2014 was an outbreak in West Africa that inflicted significant losses on crops.

Once the solitary green and tan S. gregaria become so jammed that their hind legs rub off one another, they undergo dramatic changes. Receptors on their legs transmit signals to the central nervous system, which releases the neurotransmitter serotonin. The insects become more gregarious, and their colour becomes more striking black and yellow. ‘They actually go to a higher metabolic rate, and higher rate of reproduction, so their numbers can grow quickly,’ explains Song. Then they swarm and lay eggs on moist sandy soil, where they can remain for decades. The young, flightless locusts congregate into gigantic marching bands of hoppers. Once mature, they fly off in search of new vegetation.

‘Individuals weigh 2g and eat their own body weight each day. A large swarm can consist of 2bn locusts,’ says Song. The recent outbreak of locusts, which severely impacted eastern Africa and Asia, started in a remote location in Arabia. The origin was two cyclones in the Indian Ocean in 2018 in May and October. ‘It is really odd to have two cyclones in one year, and even odder those two cyclones brought heavy rains to the same place,’ says Keith Cressman, lead locust scientist at the Food and Agricultural Organisation (FAO) in Rome, Italy. ‘Because cyclones are much heavier than normal rains, it allowed conditions to be favourable for three generations of breeding, so that led to an 8000-fold increase.’

In late 2019, the locusts flew across the Red Sea and into Ethiopia. By February 2020, swarms were devouring crops in Yemen, Sudan, Ethiopia, Somalia and Kenya. Into the summer, swarms swept into Iran and on to Pakistan and India, where they consumed 100% of crops in some areas. ‘The current plague is considered as the worst infestation in more than 25 years,’ notes Waqas Wakil, a locust expert in Pakistan at the University of Agriculture, Faisalabad, commenting during that summer. ‘Southwestern districts are heavily attacked by locusts ravaging cotton, chickpea, canola, sunflower, chilli, fodder and other food crops in Sindh and Punjab, province.’

Defences against locust swarms are tried and tested. Outbreaks are defined as two generations of successful breeding, often in a single country or an area of 50km by 100 km. This is a good time to locate and treat the locusts, before their populations take off exponentially. There is good monitoring in countries plagued by locusts, according to Cressman. National teams drive out in all-terrain vehicles, searching for green vegetation in deserts after rainfall, seeking locusts. ‘We’ve got good digital tools, using satellites and models for real time reporting, and all that data is analysed quickly to produce early warnings,’ says Cressman, who delivers forecasts to about 50 countries. Those countries most hit by locusts usually have a dedicated centre with equipment such as vehicles and sprayers. Increasingly, fixed winged drones can locate spots that require physical inspection.

Options for dealing with locust swarms are limited. ‘Over time, many of the pesticides have been banned,’ says Cressman. ‘There is only a list of about ten [chemical pesticides] now.’ In Pakistan, says Wakil, the two main insecticides being sprayed are malathion ULV and lambda-cyhalothrin, which are sprayed by ground sprayers, from the air and by drones. Malathion is an organophosphate insecticide; lambda-cyhalothrin, a pyrethroid. Farmers in Africa use the organophosphates fenitrothion and malathion, both banned in the EU. ‘Many chemical pesticides [sprayed on locusts] are broad spectrum, so not specific for killing grasshoppers, and they are bad for the environment,’ says Song. In Kenya during 2020, most insecticide applications involved fenitrothion, ‘which can negatively impact operators, other people, livestock, birds, terrestrial vertebrates – especially lizards, fish and terrestrial arthropods, including enemies of locusts,’ explains Baldwyn Torto, a locust expert at a national locust research centre in Kenya. It can also contaminate groundwater and wells, he adds.

Non-chemical control

There are some new strategies. ‘For the first time this year, countries are actively using a biopesticide, which is a natural fungus that only attacks desert locusts and other [related] grasshoppers,’ says Cressman. This makes it safe to spray near water bodies, near farms with livestock, beekeeping areas or national parks, which are off-limits for chemical pesticide sprays. The biopesticide is an oil formulation of spores of the fungus Metarhizium acridum, which kills 70-90% of treated locusts within 14-20 days, with no measurable impact on non-target creatures (Ann. Rev. Entomol. 2001, 46, 667). It was developed in an international 13-year research programme that ended in 2002, laying the foundations for Green Muscle, the product based on M. acridum spores. It works when the spores germinate on locusts and hyphae grow into the body of the insects, eventually killing them. It is made by the Elephant Vert Groupe, with a manufacturing subsidiary in Morocco.

Unfortunately, the biopesticide product was not of great interest to the agrichemicals sector. ‘We don’t have these locust problems every year, so it is not a very attractive market for private industry,’ says Cressman. ‘They don’t invest much money in production facilities and there is only one small company that produces it.’ As a result, when demand spikes after locust populations surge, it quickly outstrips supply of the biopesticide.

Another issue with the fungal spores is the perception of farmers and agricultural agents. ‘They are used to seeing an immediate effect from chemical pesticides on locusts. They usually die in a couple of hours,’ Cressman explains. Still, the fungal spores have gained traction. For 2020, Somalia has stopped using chemical pesticides against its numerous swarms, and instead is spraying the biopesticide from the air and by some ground teams. ‘They procured 4000l, which is enough to treat 80,000ha,’ says Cressman. A chemical pesticide would require 80,000l to treat that same area.

The locust problem has intrigued others. When an Egyptian PhD student told Manfred Hartbauer in his insect neurophysiology lab at Karl-Franzens University in Graz, Austria, about the massive damage locust swarms inflict in her country, he became interested. More so when he heard that farmers there burn plastic to try to deter them. His lab then tested various plant extracts to try kill locusts. They reported that a mixture of orange peel, caraway and wintergreen oils, when added to a linseed oil/bicarbonate emulsion, proved lethal (J. Pest Science, doi: 10.1007/s10340-019-01169-7).

Within 24 hours of being sprayed, 80% of desert locusts and 100% of migratory locusts (Locusta migratoria) were dead. ‘If you expose linseed oil to the air, it starts to harden, which is why it is often used to protect wood outside of buildings,’ says Hartbauer. ‘When we add soda, this hardening process is much faster, which means the linseed oil hardens to a stony film that suffocates the locusts.’ Caraway and wintergreen oil caused further neurological problems for the insect, adding to its toxicity. Moreover, Hartbauer says these ingredients could be grown in many countries in Africa plagued by locusts.

The Austrian solution now has sponsors for field studies, says Hartbauer, who complains that it was difficult to get countries to try his recipe, and the FAO did not offer support for field trials. An extra advantage in using linseed with wintergreen and caraway is that, though toxic to insects, it is harmless for people. ‘These are essential oils,’ says Hartbauer. ‘They can be regarded almost as a spice. So, if you treat the locust you can still eat them dead, maybe add some salt.’ This is certainly not possible when locusts are treated with biopesticides or chemical pesticides.

Genetic approach

Meanwhile, in Texas, Song studies the biology behind how an ordinary looking grasshopper transforms into a colourful, ravenous and gregarious desert locust. ‘We can induce the changes in the lab,’ says Song. The forms are so different that they were considered two separate species until a Russian scientist who headed the British Natural History Museum realised that this was a strange case of Dr Jekyll and Mr Hyde in insects. Anywhere in northern Africa, the desert locusts can be found in their solitary green form, causing no trouble. Song says one difficulty for countries in Africa is that locust outbreaks are periodic, and sometimes staffing and equipment seem not so critical and are allowed to deteriorate. Then an undetected outbreak can become a big problem and huge amounts of pesticides seem the only solution. He views biopesticides as a far better approach but wonders if researchers can intervene to stop swarming behaviour. ‘This problem has been going on for millennia, but it is only in the last 100 years that we have dealt with it in a modern way. Still, we don’t have a good enough grasp of predicting or controlling the locusts.’

What is known is that epigenetic modifications to the DNA of locust switch on and off different genes once the grasshoppers undergo the change to locusts. Song hopes that better understanding of the genetics of locusts could help combat outbreaks. A draft sequence of the desert locust genome has just been published in an international effort, (F1000Research, 2020, 9, 775).

Individuals weigh 2g and eat their own body weight each day. A large swarm can consist of 2bn locusts.
Hojun Song Texas A&M University, US

‘Turns out locusts have among the largest genomes of all insects, about three times larger than the human genome. It is very complex and messy,’ Song explains. One approach his lab is investigating is use of RNA interference, which involves using RNA in plants to sabotage critical locust genes. Another strategy is CRISPR gene editing. ‘There is the potential to create desert locusts that do not swarm and then maybe introduce those transgenic insects into the wild population,’ says Song.

Not everyone agrees. ‘I don’t think we should interfere with the life cycle,’ says Cressman. ‘Instead of having 80m insects together in a cohesive swarm, which makes for a fabulous target for control, you could have them scattered over a huge area, which would be far more expensive and harmful to the environment.’

The International Centre of Insect Physiology and Ecology (icipe), Kenya, has done extensive research on locust ecology and physiology, revealing pheromones influence the change from solitary to gregarious phases. The centre has produced an adult pheromone – a blend of four commercially available compounds: phenylacetonitrile (PAN), guaiacol, benzaldehyde and phenol – emitted by mature males. ‘Long-term exposure of gregarious nymphs to the key adult pheromone, PAN, leads to dispersal and very high mortality,’ explains Torto, a senior chemical ecologist at icipe. The centre has carried out field tests in Sudan and observed hopper bands break apart after three days. Predation by natural enemies then rises, including by birds, and cannibalism.

The desert locust is not the only problem species. The migratory Locusta migratoria is the most widespread locust, occurring throughout Africa, Asia, Australia and New Zealand. A draft of its huge genome was published six years ago (Nature Communications, 2014, 5, 2957). In summer 2020, Chinese scientists reported that a pheromone, 4-vinylanisole (4VA), causes gregarious and solitary locusts to aggregate (Nature, 2020, 584, 584). They found the pheromone attracts locusts in the field and suggest synthetic versions could be deployed to attract locust in trapping belts, where they could be killed using a fungus or pesticide, thereby avoiding broader spraying. Also, an agonist to block natural 4VA from locusts could prevent their aggregation and migration. Others argue that these pheromones only have local effects and are pointless once locust numbers are high.

No matter what solutions are developed, locust plagues are predicted to get worse in some parts of the world. East Africa, for example, is expected to experience more rains in future due to climate change, which will encourage locust swarms to form. ‘It is likely we will see locust plagues again in the news in the foreseeable future, largely because of climate change,’ Song predicts.