eNews

#03 2025

Can rangeland science play a role in managing brown locusts?

By Joh Henschel, NRF-SAEON Research Associate

On Christmas Eve 1963, the Karoo farm Swagersfontein experienced quite a spectacle: the arrival of 50 billion brown locusts, settling five centimetres deep, 10 000 per/m2. Earlier that day, mobile pest control units had tracked seven locust swarms. They were astonished that swarms which had started further than 100 km apart, converged from various directions onto a 500-hectare patch of open veld. So, what drew these brown locusts together? 

This event was by no means unique. Brown locusts convene such kinds of mass meetings every so often. Why? This locust is one of several nomadic species of the Karoo region. However, for some nomads like springbok, their treks were stopped by fences. Then again, many nomadic birds, locusts and other insects, such as Karoo moths and hoverflies, continue to traverse the Karoo. Richard Dean presented a frame of bird nomadism in his monumental works, Where Birds are Rare or Fill the Air and Nomadic Desert Birds. His frame proved valuable in a review on the application of scientific knowledge towards managing brown locusts, Locustana pardalina in the Karoo, as published and presented at a recent SAEON seminar.

Driven by high population growth rates and other factors, such as climate and the genetic propensities to either reside or trek, nomads alternate between sedentary and mobile phases, switching between predictable local and unpredictable remote resources with new potential to thrive. Threshold conditions and their spatial coupling affect the balance of whether to move or not to move. While on the move, nomads face numerous adversaries, including predators, diseases, accidents, thirst and hunger, and cope with high mortality rates. Nomads can have significant positive and negative environmental impacts along their routes and new destinations.

In the early 1900s, scientists realised that brown locusts’ inconspicuous, asocial, resident solitaria generate conspicuous gregaria that form nomadic swarms. They found that it takes about five generations of exponential population growth in favourable conditions to grow from a population of only a few solitaria per hectare to exceeding thousands. Then, as if to “test the waters” before taking the plunge into nomadism, the next generation goes through the first phase-change to transiens, which are larger, more active, social, crowded, resident locusts. The scientists noted that rangeland management methods can influence whether a gregaria or solitaria phase comes after transiens. With light livestock grazing and good veld conditions, the next generation can return to solitaria. By contrast, heavy grazing, land degradation and jackal persecution can tip the next generation to the gregaria phase of hypersocial and hyperactive locusts, who depart and converge with similar departees.

Brown locust egg-bank carpeted with numerous grazed tufts of Enneapogon desvauxii. (Photo: Joh Henschel)

The clandestine exponential population explosions of resident solitaria preceding outbreaks are fuelled mainly by short grasses, especially nine-awn pappusgrass, Enneapogon desvauxii, which is common in many arid regions globally. Karoo farmers know this grass as “haasgras” (hare grass), “kalkgras”, “wondergrass” or “eight-day-grass” as it germinates quickly after small rainfall events, briefly greening landscapes, even during a drought. However, this grass, which proliferates in disturbed areas, is not considered to be of high agroeconomic importance and has largely been ignored, even by scientists.

A solitaria female uses smell transmitted through the antennae to find and graze on E. desvauxii in preference to other lush green grass (right front). (Photos: Joh Henschel)

Also largely ignored has been that the solitaria and transiens and their offspring selectively feed on this grass, which carpets the sandy calcrete soils where the locusts lay their eggs. The prospect of controlling the proliferation of brown locusts by controlling the proliferation of this short grass was recognised in the early research but remains untested, an imperative challenge for future rangeland science.

Some of the solitaria’s eggs remain dormant but viable for several years and thus accumulate, constituting egg banks across several hectares. A burning question is: what triggers the simultaneous hatching of these eggs across several egg banks? Although research has revealed some complex properties of these smart eggs, nobody has yet succeeded in cracking the code of mass hatching. It entails specific sequences of moisture and temperature fluctuations over several months, straddling a cold, dry winter. Perhaps endogenous factors also come into play, for example if the volatile pheromone, locustol, emanating from hopper frass (dung) not only stimulates hyperactivity of hoppers but also induces the hatching of remaining eggs. It may be instructive to compare these factors with the condition and phenology of the short grass.

Locusts mating (left); female laying eggs. (Photos: Joh Henschel)

Moving with the wind 

Brown locust swarms are said to move with the wind. The convergence of the 1963 swarms from opposite directions suggests navigation by mutual attraction, perhaps to enhance safety in numbers. A consequence of convergence is that mega-swarms attract many predators. In the past, many birds, mammals and parasites feasted on the amassed locusts – the 1963 convergence attracted 5 386 white storks. However, the populations of locust predators have been reduced. A century of swallowing toxic insecticides couldn’t have done their health and reproduction much good, but this has not been investigated.

Brown locusts are reputed to threaten agriculture, given that the collective appetites of billions of individuals are reckoned to be disastrous for croplands and rangelands. Locusts are, therefore, persecuted as pests. Notwithstanding determined efforts to annihilate them, locust populations seem oblivious and generate evermore swarms.

The density of brown locusts increases exponentially on an egg-bank over seven generations (after C.J.B. Smit, 1939)

The brown locust is one of several nomadic species of the Karoo region. (Photo: Shutterstock/Roger de la Harpe)

While government-supported pest control units have diligently pursued and destroyed many locust swarms for over a century, nobody recorded their movements except for the 1963 event. No scientific records exist of what locust swarms do in the medium to long term to places where they settle, merely assuming it to be bad. What are the locusts’ nutritional needs, how do they select what they consume, fertilise the land with their dung, and how does the veld regenerate in the short and long term, possibly to the advantage of livestock pastures?

There has never been a study to confirm the extent of economic damage caused by brown locusts that would justify the use of pesticides. To boot, it has not been demonstrated that the purported damage caused by locusts is higher than the collateral damage of pesticides on biodiversity and ecosystem services across the Karoo and beyond.

We in South Africa seem stuck in recurring crisis-combat-apathy cycles. This means locust outbreaks entail scrambling to mobilise human, technical, institutional and financial capacities to dispense pesticides and whip up media hype, leaving no time for research. When outbreaks cease, locust-related matters recede into apathy, likewise the incentive for research. So, the perpetual cycles of pest control continue ad infinitum without anyone the wiser. In the meantime, frequent large-scale applications of insecticides likely diminish Karoo ecosystem integrity over time.

Adaptive management is designed to break such stalemates by incorporating local and international experience and new knowledge into the next cycle. To achieve that, it is imperative to transcend beyond the narrow campaigns of pest control and include brown locusts in rangeland management. The premise of rangeland science is to improve rangeland management by basing it on the growing knowledge of the relationship of herbivores with vegetation and other factors affecting optimal and sustainable productivity. Brown locusts are herbivores thought to affect and be affected by veld conditions. It is time to improve the evidence concerning the role of brown locusts in Karoo ecosystem processes and incorporate these insights into the adaptive management of locust populations.

Further reading 

Henschel, J.R., Duncan, F.D., du Toit, J.C.O., Milton, S.J. & van der Merwe, H. 2023. The brown locust refocussed – Knowns, unknowns and the relevance of Locustana pardalina (Walker) to Karoo ecosystems and rangeland management. Journal of Arid Environments 215. https://doi.org/10.1016/j.jaridenv.2023.105014

BANNER PHOTO of brown locust: Shutterstock