Explainer: Locust and their Swarms

By: Dr. Chelmala Srinivasulu

What are locusts?

· Locust are a species of short-horned grasshoppers

· Desert Locust (scientifically known as Schistocerca gregaria) is one amongst them

· Desert Locust occurs in two forms – a) solitary (green or brown adults), and b) gregarious (pink immature and yellow adult)

· Desert Locusts are known to change their behaviour with the change in their form

· Gregarious form of the desert locust form swarms (of adults capable of flying) or bands (of wingless nymphs or young, known as hoppers) of large numbers and migrate long distances

· Usually, they occur in a solitary form in their native range in northern Africa to South Asia

· During the right season, with sufficient rains and profuse growth of vegetation in some regions of their range, they breed profusely and their numbers increase manifold

· Highly dense population triggers changes that transform solitary forms to gregarious forms and lead to behavioural changes impacting their feeding and breeding

· Solitary Desert Locusts fly during the night, while gregarious forms fly during the day

· Depending on weather and ecological conditions, the life span of the Desert Locust ranges between 3-5 months

· The life cycle of the Desert Locust consists of three stages: a) egg (hatch in about two weeks, ranges from 10-65 days depending on weather), b) hopper (larval stages of five to six instars spanning 30-40 days), and c) adult (mature in about three weeks to four months, extending up to nine months during extreme weather conditions)

· A solitary female may lay 95-158 eggs, while the gregarious female lays less than 80 eggs.

· Each female can lay up to three times during her lifetime

· Eggs are laid in sandy soils about 10-15 cm below the surface

Where do they occur?

· Desert Locusts are always present in the desert regions, ranging from Mauritania in the west of Africa to India

Why do Desert Locust outbreaks happen?

· In response to good rainfall and subsequent availability of dense vegetation, Desert Locusts breed and increase in numbers rapidly

· Within a short period, a month or two, they gregarize, form small groups of nymphs (known as bands) or adults (known as swarms).

· If left unchecked, they change from solitary to gregarious forms, and a population upsurge happens that affects the entire region

· Subsequent seasonal or unseasonal rains, availability of vegetation and loss of vegetation in the native range of such bands or swarms compel them to migrate to other regions

· Outbreaks are of frequent occurrence, and local authorities usually are successful in controlling them

· A few outbreaks, such as the current one, lead to an upsurge

What is the extent of the current upsurge?

· As of now, Desert Locust swarms have been reported in countries in the horn of Africa region, the Arabian peninsula, Iraq, Iran, Pakistan, and India

· Small swarms of mature and partially mature adults have also been reported from western Africa

How big are the Desert Locust swarms?

· Smallest swarms could be less than a square kilometre, while larger ones could be several square kilometres

· In each square kilometre of a swarm, there could be 40–80 million individuals

How far can the swarm of Desert Locust travel during migration?

· Swarms of adult Desert Locust can fly with the wind at a speed of 16-19 km per hour depending on the speed of the wind

· Swarms have been reported to travel 5 – 130 km in a day

How much does a Desert Locust eat?

· An adult Desert Locust (weighing about 2 gm) can consume roughly its weight of fresh vegetation per day

· A swarm of 40 million Desert Locusts can consume food equivalent to food consumed by 35,000 people (on average 2.3 kg food per day) in a day

Is Desert Locust dangerous to humans?

· There is no evidence that Desert Locusts carry any disease or transmit any disease to humans

· Desert Locust is not known to bite or cause any direct harm to humans

· Desert Locust being insects, like cockroaches and other insects, may cause discomfort to some humans

· Desert Locusts themselves or their activities in an area may release allergens that can cause some discomfort to humans with respiratory problems

How can Desert Locusts be controlled?

· Chemicals, mainly organophosphate in very low volume formulation, spray through

vehicle-mounted or aerial sprayers is most efficient in controlling Desert Locusts

· Small-scale sprayers, including knapsack and hand-held sprayers, can also be effective

· Spraying of organic deterrents, including neem oil suspension, will safeguard the crops and vegetation in the path of migration

· Physical methods, including making noise and erecting barriers, have been reported as effective deterrents

Why is it a challenge to control Desert Locusts?

· Once moving, the Desert Locusts stopover cannot be determined as the distance travelled and stopover selection is dependent on wind directions and speed

· The response of local authorities at the night resting areas determine the success of control measures implemented

How is climate change responsible for the present Desert Locusts upsurge?

· The cyclonic storms and events after Mekunu and Luban cyclones that impacted Oman and Yemen in 2018 converted vast arid regions of these countries into waterlogged areas

· The unseasonal rains provided the required ecological conditions for locusts to breed at a higher rate

· FAO reports that locusts swarms are currently breeding 400 times more than usual

· After the Mekunu and Luban cyclones, the population kept building up through 2019 and swarms began to form and spread

· In November 2019, swarms started to spread to East Africa and by January, 2020, they started to invade Iran and Pakistan

· By the end of April, 2020, they entered India through Rajasthan

What is the situation of Desert Locusts in India?

· As of the end of May, 2020, immature adult groups and swarms that migrated into India from Pakistan through Rajasthan have spread into Maharashtra to Madhya Pradesh

· Wind directions and movements, associated with Cyclone Amphan, has influenced the dispersal of these swarms in India

· It is predicted that the swarms are less likely to move to South India

· In coming months, most of these swarms will become less mobile with the onset of their breeding season

Disclaimer: The above facts have been primarily collected from the FAO reports.

(Dr. Chelmala Srinivasulu is a Senior Assistant Professor, Department of Zoology and Director, Centre for Biodiversity and Conservation Studies at Osmania University, Hyderabad)