The desert locust, Schistocerca gregaria, remains one of the most destructive transboundary pests on Earth, threatening agriculture across arid and semi-arid zones from North Africa to the Indo-Pakistan border. During plague periods, roughly 60 countries face risk, and traditional control has leaned heavily on fast-acting organophosphate insecticides such as chlorpyrifos. These chemicals work, but mounting environmental and human health concerns have increasingly restricted their use, pushing agencies toward integrated approaches that include the entomopathogenic fungus M. acridum. Two commercial formulations based on this fungus, Green Muscle and Novacrid, are currently available, and during the 2020–2021 Horn of Africa outbreak the biopesticide was successfully applied across several hundred thousand hectares, achieving mortality above 80 percent within two weeks.

Despite that track record, operational adoption has been hampered by two practical uncertainties. First, spore viability during storage in remote field conditions is difficult to guarantee, since control campaigns are sporadic and stockpiles may sit for years between uses. Second, efficacy under variable environmental conditions—fluctuating temperatures and structurally diverse vegetation—has remained incompletely quantified. Previous studies suggested that vegetation might either help or hinder the fungus, potentially shielding spores from solar radiation while complicating contact with locusts, but the influence of plant size and cover under real operational conditions had not been clearly resolved. The Mauritanian team set out to address both questions simultaneously, and, according to the authors, no previous published study has monitored a stored biopesticide formulation for as long as ten years.

The storage trial centered on a batch of fifty one-kilogram bags of Green Muscle powder, based on the IMI330189 strain of M. acridum, kept in a refrigerator set at 4 degrees Celsius at the National Desert Locust Control Centre. The refrigeration was far from ideal: power outages were recurrent throughout the decade. Every one to two years, the researchers withdrew a sample bag and ran germination tests, inoculating Petri dishes of malt extract or Sabouraud medium with diluted spore suspensions and incubating them for 18 to 20 hours before counting at least 300 spores under a microscope at 400 times magnification. Viability hovered consistently above 90 percent across all ten sampling points, from 98.31 percent at reception in 2014 to 96.53 percent in 2024, with no statistically significant variation over the entire storage period.

Two stress tests made the results especially compelling. One bag removed from refrigeration after two years was deliberately kept at ambient room temperature, averaging around 31 degrees Celsius, for a full year—and its spores still germinated at 96.03 percent. Another bag, taken from the fridge after three years, was transported and applied in the field over five consecutive days without any cold chain, after which residual formulation left under tree shade retained 97.53 percent viability after a further month. Notably, the field trial product had already passed the expiry date printed on its packaging by three years, yet it performed robustly. Laboratory bioassays confirmed that viability translated into virulence: nymphs treated at reception, after five years, and after ten years of storage all suffered high mortality, and even a drastically diluted dose of just 50 spores per insect—a million-fold dilution—eventually killed as many locusts as the standard dose, only with a delayed onset.

The field component took place in a desert locust breeding zone in northern Mauritania, about 70 kilometers north of Zouérat in the Tiris Zemmour region, in an area locally known as Waïd Jerjir—the wadi of Schouwia—named for the dense stands of the shrub Schouwia thebaica that dominate the landscape. As part of a campaign by a specialized Green Team focused on biopesticide applications, seven plots totaling 20 hectares were treated using hand-held Ulva+ spinning-disc sprayers calibrated to deliver 50 grams of formulation in 2 liters of gasoil per hectare, equivalent to 2.5 trillion spores per hectare. Applications ran in the early morning and late afternoon across five days, under temperatures of 23 to 26 degrees Celsius, relative humidity of 34 to 48 percent, and wind speeds of 3 to 5 meters per second—conditions the researchers judged suitable for fungal activity.

To probe the vegetation question, the team selected two contrasting three-hectare plots. Plot A held large, widely dispersed S. thebaica plants, while Plot B contained small, closely spaced individuals, a difference confirmed statistically by measurements of plant length, width, height, and spacing. Nymphs collected from both plots two days after spraying were placed in mesh cages, 50 insects per treatment, fed with uncontaminated vegetation, and monitored daily for two weeks under partial tree shade. The outcome was unambiguous: mortality after 14 days reached 82.8 percent in Plot A and 83.9 percent in Plot B, compared with just 4.3 percent among control nymphs from an untreated zone roughly ten kilometers away, where no fungal symptoms appeared in dead insects. The post-hoc analysis found no significant difference between the two plots, demonstrating that neither plant size nor cover meaningfully altered biopesticide performance.

Field observations reinforced the cage data. Nymph bands, predominantly third and fourth instars occupying areas of 3,000 to 5,000 square meters, rapidly vacated treated plots, often on the same day as treatment, likely because gregarious locusts perching on contaminated vegetation quickly accumulated lethal spore doses through direct deposition and contact. Two weeks after application, locust populations in the treated zone were visibly depleted, while in the untreated zone most nymphs had progressed to fifth instar and winged adults, forming cohesive and active bands. The authors note that the fungus typically needs suitable temperatures to act, with optimal growth between 25 and 32 degrees Celsius, and that temperatures during the study, ranging from 9 to 36 degrees Celsius, fell within the moderately favorable window described in earlier work. They also cite a comparable trial on the Italian locust that achieved similar mortality despite daytime temperatures reaching 40 degrees Celsius.

The broader implications are considerable for preventive locust management. If properly stored formulations remain effective well beyond their labeled expiry dates, national programs could stockpile biopesticides with far greater confidence, a critical advantage in regions where campaigns are irregular and cold-chain infrastructure is unreliable. Vegetation independence likewise matters, because desert locust habitats span everything from sparse desert scrub to lush wadi vegetation, and the fungus now appears dependable across that spectrum. The authors caution that challenges remain: fungal spores can persist on vegetation for months, but residual activity in the field is still poorly characterized, and improving persistence is a recognized priority for wider adoption. A deeper obstacle, they note, is the lingering perception among decision-makers that biopesticides are less effective than chemical insecticides—a perception this study, with its decade-long storage data and consistent 80-percent-plus field mortality, directly challenges. The team calls for future work to define more precise profiles of efficacy and persistence under diverse operational conditions, paving the way for the systematic integration of M. acridum into routine desert locust control.

Subject of Research: Long-term storage stability and field efficacy of the fungal biopesticide Metarhizium acridum for desert locust control

Article Title: Ten-year storability and field efficacy of the biopesticide Metarhizium acridum in desert locust management

Article References: Etheimine, M. O., Maeno, K. O., Ely, S. O., Benahi, A. S., Jaavar, M. E. H., & Ebbe, M. A. (2026). Ten-year storability and field efficacy of the biopesticide Metarhizium acridum in desert locust management. Journal of Orthoptera Research, 35(1), 209-214. https://doi.org/10.3897/jor.35.180737

Image Credits: AI Generated

DOI: 10.3897/jor.35.180737

Keywords: Metarhizium acridum, desert locust, biopesticide, biological control, spore viability, Schistocerca gregaria, Mauritania, Green Muscle, entomopathogenic fungi, locust management, storage stability, field efficacy