Locusts are specialized short-horned grasshoppers defined by their capacity to shift from a harmless, solitary phase to a gregarious, swarming phase. This behavioral change, often triggered by dense populations and favorable weather, transforms them into a devastating agricultural pest. A single swarm can contain billions of individuals, traveling vast distances and consuming massive amounts of food daily. Controlling these outbreaks is critical for global food security, and modern integrated pest management increasingly emphasizes sustainable, non-chemical approaches that exploit the locust’s natural vulnerabilities.
Pathogenic Biological Control Agents
The most promising non-chemical method for large-scale locust control involves naturally occurring pathogens, particularly the entomopathogenic fungus Metarhizium acridum. This fungus is the active ingredient in biopesticides, such as Green Muscle, and targets the locust externally. It is highly host-specific, primarily affecting locusts and grasshoppers, which minimizes harm to beneficial insects and vertebrates.
Infection begins when fungal spores, called conidia, land on the locust’s cuticle. The spores germinate only after encountering specific chemical stimuli found on the locust’s waxy surface. The growing hyphae then penetrate the exoskeleton and multiply within the body cavity, leading to death, typically within seven to fifteen days. The time taken to kill the insect depends on ambient temperature, with warmer conditions accelerating the process.
Another biological agent is the microsporidian parasite Nosema locustae, a unicellular organism ingested by the locust. This parasite sets up a chronic infection, primarily in the midgut, spreading throughout the host’s tissues. While Nosema locustae does not cause rapid death, its long-term effects include reduced fecundity, stunted growth, and a shorter lifespan, making it a valuable population regulator. The parasite also interferes with neurological pathways, inhibiting aggregation behavior and encouraging gregarious locusts to revert toward a less destructive solitary phase.
Ecological Role of Natural Predators
A wide variety of animals naturally prey on locusts throughout their life cycle, helping to maintain low population densities during non-swarming periods. Avian predators, such as storks, gulls, and various raptors, consume enormous numbers of flying adults. These birds often target the larger, reproductive female locusts, disproportionately reducing the population’s future breeding potential.
On the ground, reptiles and amphibians, including lizards and frogs, feed heavily on locust nymphs, known as hoppers. Mammals, particularly small rodents, also raid the soil to consume egg pods buried underground by the females. These vertebrate predators are generalist feeders, hunting a wide range of insects, and their populations cannot increase quickly enough to match the explosive growth of a massive swarm.
Insect parasitoids also target the most vulnerable life stages. Tiny parasitic wasps, such as those from the genus Scelio, locate egg pods buried in the soil and lay their eggs inside the locust eggs, killing the developing embryo. Parasitic flies, including Blaesoxipha species, deposit larvae directly onto hoppers; the larvae burrow inside, often killing the nymph or severely weakening the adult. While these natural enemies are crucial for ecological balance, they are insufficient for controlling a plague due to the sheer size and mobility of swarms.
Habitat and Physical Management Strategies
Physical management strategies target the non-flying life stages of the locust to prevent the formation of destructive swarms. One historical and relevant method is trenching, which involves digging ditches across the path of a marching hopper band. The flightless nymphs fall into these trenches, which are designed to prevent their escape. Once trapped, the concentrated hoppers can be buried or burned, effectively eliminating large numbers of the developing population.
Targeted management of egg beds is another preventative measure, as females lay their eggs in moist, sandy soil, often in dense, localized areas. Plowing or deep tilling these identified egg beds exposes the egg pods to the air. This exposure causes them to dry out and die from desiccation, or makes them accessible to ground predators. This method is labor-intensive and requires accurate forecasting and identification of the egg-laying sites.
Water management is a long-term strategy addressing the environmental conditions required for successful breeding. Locust outbreaks are often preceded by drought followed by rains, which create the damp soil necessary for egg-laying and subsequent vegetation growth. In managed areas, carefully controlling irrigation and drainage systems prevents the creation of consistently moist patches of sandy soil. This limits the potential breeding grounds that attract females to lay their eggs.