Locust Plague and Climate Crisis

Esha Haldar

A locust swarming is one of the age old pest attacks in the history of the world that leaves country after country barren from its destruction of crops. But to many of us, it has for the first time materialized from the pages of school text books into reality as the South Asian countries like Pakistan, India face one of the gravest locust upsurges since 2003, that has transformed into a plague besides the middle east countries and horn of Africa. There have been periodic outbreaks of locust swarming in the African countries before that remained confined regionally, but a plague of this massive form has been attributed to global warming that has changed the climate pattern of the world and is pushing the poverty stricken countries towards extreme climatic crisis like this. 20.2 million people in the countries of middle and west Africa are already facing acute food security crisis and threat of famine as the locusts are wreaking havoc over around 1 million ha land. In India and Pakistan, there is expected to be a continuum of attacks till the end of July 2020 which if not mitigated can result in food crisis amidst this corona pandemic. The Food and Agricultural Organisation (FAO) of United Nations coordinates mitigation of such outbreaks between the countries and provides most of the data for use in public domain and by the Govts. But the pressing question remains: why such a sudden outbreak and what is to be done?

Figure 1 (from FAO) - Map of the breeding grounds of locusts showing recession and invasion area

Locust Facts: Scientists working on locust swarming continuously try to provide predictive models through which an outbreak can be anticipated in future and measures can be taken well in advance. By now we have an idea that locusts are a type of grasshoppers belonging to the family Acricidae, which during recessions or quiet periods lead normal solitary life. The particular desert locust (Schistocerca gregaria) is found in the deserts and semi desert regions of 30 countries of Asia, Africa and Europe, but when ecological conditions become favourable as in the deserts receive more than 25mm of rainfall in 2 consecutive months, they breed at enormous rate, change their morphology and become gregarious in nature. This phenotypic plasticity in behaviour is what sets the locusts apart from other grasshoppers.

Figure 2 (from FAO) showing shortening of incubation period of eggs with increase in temperature

Bands and Swarms: The adult desert locust always lays eggs in moist desert sandy soil which has received rainfall as the eggs absorb moisture for development of the nymph inside. The viability and development of the egg is always positively correlated to moisture content of the soil and reasonably with soil and air temperature. Hence these variables can be used to predict egg development rate for control measures.

Eggs hatch into wingless nymphs called hoppers which moult many times before becoming adults within 4-5 weeks. This hopper stage is when the phase change from solitarious to gregarious takes place. This phase change is density dependent or crowding-induced: as the concentration of nymphs arises they become gregarious faster and form bands of hoppers. The growth rate of nymphs is also positively temperature dependent.

Winged adults become mature after 2-4 months, even faster when conditions are favourable (higher temperature, more vegetation). Their congregation results in swarms which may sweep over 50,000 sq.km, flying downwind and moving easily at a rate of 100-200km/day. The speed and direction of wind are used to calculate the spread and progress rate of a swarm. The swarms move during the day time while at dusk they settle down on vegetation to feed gregariously. Hence the night time is used by pest controllers to spray pesticides on them.

Figure 3 (from FAO) showing rate of development of nymphs is faster with increase in temperature

Gregariousness: Collett et al. in their 1988 PNAS article have shown that solitarious locusts achieve gregariousness within 4hrs of crowding and can revert back to normal phase upon isolation. A typical swarm of 1sq.km contains around 150 million locusts which can consume crops that can feed 2500 people for one year. In 1988 plague, locusts reached Italy, UK and had even crossed Atlantic Ocean.

Climate Change and the Crisis: Global warming and climate change is the root cause behind these increased locust outbreaks which has this time intensified into an unimaginable dimension. Abram et al. in their Nature article published in March 2020 showed that the Indian Ocean dipole, a mode of interannual climatic variability in the Indian Ocean has positively intensified in the twentieth century due to global warming which is leading to extreme climatic disturbances like cyclones, floods, especially in countries surrounding Indian Ocean. In 2019 alone, there had been 8 cyclonic storms in the Arabian Sea and Bay of Bengal which brought heavy rainfall in the Arab and African horn countries providing perfect breeding grounds for the locusts. It all started with poor governance and unchecked control of locust breeding in the Empty quarter of Arab peninsula from October 2018 which has gradually led to this massive plague.

Studies all over the world are showing that with increasing warmer climate, feeding and breeding ecology of insects are changing immensely resulting in outbreaks of insect population and damage of crops globally. From our understanding of the biology of locust, we know that higher the temperature, higher is the rate of development of all life stages of locust and so their gregarious nature. Another important aspect of locust swarming is that locusts do not flow with any prevailing wind but wait for specific warmer winds that carry more moisture and congregate in regions of wind convergence. In 2019, the strong westerly winds associated with cyclone Amphan brought most of the swarms further inwards of Indian subcontinent.

The immature locust swarms that had arrived in Rajasthan of India came from the winter breeding grounds of Iran. Now it is anticipated that the swarms which have extended to interiors of UP, Madhya Pradesh will flow downwind with the monsoon winds and go back to Rajasthan desert areas which will be their summer breeding grounds. More swarms are expected to arrive from Africa horn in the month of July for breeding in Rajasthan which will again result in outbreaks if not controlled properly.

Figure 4 showing trajectory of path expected to be followed by swarms of locust in June-July 2020

Mitigation: Developed nations have to acknowledge the impacts of global warming urgently as time is not far when locust as well as other insect outbreaks will be felt there too. Studies have shown that crop infesting insects are moving pole wards 7 kms per year and food crisis will cross the boundaries of poor African countries and Asia and reach the north and west coast too.

Chemical spray of organophosphate pesticides like Chloropyrifos is the most extensive method of locust control. However, chloropyrifos use is dangerous for other animals as well as for the soil. Hence an integrative approach using biological pest control method like use of fungicide and repellent plant species are encouraged along with chemical treatment. But poor investment in sustainable agricultural research in countries like India as well as uncontrolled conversion of forested land areas into agricultural plots are resulting in massive loss of food crops from these climatic crisis.

Experts believe that global warming is changing the rainfall patterns and increasing aridity in other parts of the world where locusts will gradually extend their geographic range. Destruction of the Amazon, rampant coal and petroleum mining, privatisation of coal fields and clearing of forests in India – all are culminating into worst global warming era. If we do not say NO to this capitalist greed now, the smallest soldiers of the earth will arise in numbers, from locusts to beetles and guide us to our doom.

Sources: FAO of United Nations, Nature, PNAS, www.insideclimatenews.org