#02 2020

Putting the brake on locust outbreaks

By Joh Henschel, Manager, SAEON Arid Lands Node

“For too long, the world has operated on a cycle of panic and neglect. We throw money at an outbreak, and when it’s over, we forget about it and do nothing to prevent the next one.”

Tedros Adhanom Ghebreyesus, Director-General of the World Health Organization (WHO), was talking about pandemics like the Coronavirus, but could just as well have referred to the desert locusts, Schistocerca gregaria, swarming across his home country, Ethiopia.

Locusts are polyphenic – they have two distinct phases that differ in terms of diet, ecology, reproduction, behaviour and morphology: antisocial Solitaria at low abundance and social Gregaria at high densities. Solitaria initiate Gregaria, whose phenology is, in turn, geared towards quickly propagating many migrant Gregaria.

Most research efforts have focused on curbing the billions of Gregaria by applying insecticides at high costs. By contrast, getting to understand the reclusive Solitaria has been considered to be less critical than destroying masses of Gregaria, even with significant collateral damage to ecosystems.

The reverse is true, as it is the Solitaria that trigger outbreaks.

Swarming locusts in Kenya (Picture: Jen Watson, Shutterstock)

Sahel researchers found that part of the problem emanates from the management of locust populations. Administering financial and human resources reactively cause them to lag behind the problem. One ends up trying to control what is already somewhat out of control.

By contrast, preventative management of locust populations, applied consistently and comprehensively over time and space, especially when densities are low, would be far more effective.

Furthermore, scientists found that overgrazing by livestock decreases the nutritional quality of grass, which boosts the population irruptions of locusts. Solitaria favour grasses that are low in nitrogen (proteins) but rich in carbohydrates, which allows the locusts to enhance resistance against pathogens and build up fat reserves that promote egg production and fuel flight.

Good rainfalls at the right time bring about the rapid growth of ephemeral grasses and trigger synchronised hatching of locusts from egg banks. Crowding stimulates the production of serotonin, a neurotransmitter (“happy chemical”) that changes locusts from being antisocial to social.

Gregaria eat indiscriminately, and when they have depleted their food, they fly away in search of more. Disturbances caused by insecticide application on locust swarms animates surviving Gregaria to move even further. Given their extraordinarily high reproductive potential, especially when they reach superabundant crops, swarms escalate, famines follow.

Thus, poor rangeland management in arid lands impacts crop production in distant areas. This connection clarifies why locust plagues have shadowed humanity since the First Agricultural Revolution.

Local impact

In South Africa, the last significant outbreaks of brown locusts, Locustana pardalina, were a decade ago. Many may have forgotten their existence, and the Locust Research Unit of the Agricultural Research Council (ARC) shut down after a century of its existence.

Preliminary analyses by SAEON of ARC records show that two-thirds of locust hopper bands and swarms occur in the Bushmanland region of the Nama-Karoo. The mean annual precipitation in this area is only about 100 mm but is highly variable (CV=49%).

Figure 1: Incidence of brown locust swarms in South Africa, ranging from absent (0), present (1), moderate (2), to abundant (3). Information sources were a) 1797–1909, the number of municipal districts with swarms; b) 1910–1988, costs of insecticide application; c) 1988–2006, records of swarms subjected to control measures. Source: ARC

Figure 2: ARC records of brown locust swarms and hopper bands (n=469,900) in different Karoo vegetation types and out of the Karoo (“Out”) indicate an anticlockwise progression of outbreaks. Source of map and codes for vegetation types: SANBI

Drought, heat waves and wet or warm winters inhibit hatching and allow egg banks to grow over many years. Mass hatching of Bushmanland eggs is preceded by good late summer rainfall, a dry cold winter and intermittent rainfalls in early summer. Such outbreaks can expand across South Africa and beyond.

It remains impossible to predict what triggers mass outbreaks of brown locusts, and when and where these will occur, even though years with and without outbreaks correlate with subcontinental climatic phases. That makes it compelling for SAEON because of the potential indicator value of ecosystem status and because locusts offer us natural data loggers.

Solitaria deploy “smart eggs” into egg banks that record previous weather events over months to years. They remember sequences of stimulatory and inhibitory events of soil moisture and temperature conditions until they match “Open Sesame”.

Currently, I am sitting in my office at home in Covid-19 lockdown. The world is experiencing the enormous costs of controlling the outbreak of the Coronavirus and realising that prior research could have avoided this outbreak at a fraction of the costs and risks.

The same goes for locusts. A paradigm shift is called for to manage potential problems without first letting them become actual problems, to brake outbreaks before they break us.