The story of Busseola fusca's resistance to Cry2Ab2 maize and what it means for the future of agricultural biotechnology
It began with troubling reports from farmers in South Africa's KwaZulu-Natal province during the 2017-2018 growing season. Despite planting genetically modified maize designed to resist insect pests, their fields showed unexplained damage from maize stem borers. Similar reports emerged from Mpumalanga province years later 2 4 8 .
The African maize stemborer, a major pest of maize crops in sub-Saharan Africa.
A Bt protein engineered into maize to protect against lepidopteran pests like B. fusca.
At the heart of this agricultural mystery was Busseola fusca, the African maize stemborer, which had supposedly been controlled by advanced maize technology containing not one, but two insecticidal proteins. How had this insect managed to survive what should have been certain death? The answer would reveal a critical development in the ongoing evolutionary arms race between insects and biotechnology—incipient resistance to Cry2Ab2 maize 2 4 8 .
Key Insight: This quiet rebellion happening in South African maize fields represents more than just localized crop damage—it signals a potential threat to food security and the sustainability of agricultural biotechnology across the continent.
To understand the significance of this resistance development, we must first appreciate the revolutionary nature of Bt crops. For decades, farmers battled lepidopteran pests like Busseola fusca with limited success using chemical insecticides that often harmed beneficial insects and posed environmental risks. Then came genetically modified crops that incorporated genes from the soil bacterium Bacillus thuringiensis (Bt), enabling the plants to produce proteins specifically toxic to certain insect pests 5 .
Mortality in susceptible insects exposed to effective Bt proteins
Years until first resistance developed to Cry1Ab maize
Generations of Bt maize failed by B. fusca in South Africa
These Cry proteins (crystal proteins) are lethal to targeted insects yet harmless to humans, wildlife, and most beneficial insects. When susceptible insects like B. fusca larvae feed on Bt plants, they ingest these proteins, which bind to specific receptors in their gut, creating pores that ultimately cause death. The technology represented a monumental advance in sustainable agriculture—reducing pesticide use while providing consistent protection against devastating pests 3 .
From the beginning, scientists recognized that insects would eventually evolve resistance to single-toxin Bt crops through natural selection. This led to the "high-dose/refuge" strategy—planting non-Bt crops as refuges near Bt fields to maintain populations of susceptible insects that would breed with any resistant survivors, diluting resistance genes in subsequent generations 5 .
When B. fusca developed resistance to Cry1Ab maize in South Africa just seven years after its introduction, scientists responded with a more sophisticated approach: pyramided Bt crops containing two or more distinct Bt toxins 2 5 . The theory was simple—while an insect might randomly develop resistance to one toxin, the odds of simultaneously developing resistance to multiple toxins were astronomically low. This thinking led to the introduction of MON 89034 maize, producing both Cry1A.105 and Cry2Ab2 proteins, which was commercialized in South Africa in 2011 2 4 .
The current resistance problem in South Africa represents the second act in a longer story of insect adaptation.
Bt maize (MON 810) containing Cry1Ab is first commercially planted in South Africa 5
Just 7 years after introduction, resistance to Cry1Ab is documented in B. fusca populations 2 4
MON 89034 maize, producing both Cry1A.105 and Cry2Ab2 proteins, is introduced to counter Cry1Ab resistance 4
Scientific confirmation of Cry2Ab2 resistance in three problem populations 2
This timeline reveals an accelerating pattern of adaptation, with the pest developing resistance to the second-generation technology in approximately the same timeframe (6-7 years) as it did to the first.
When reports of unexpected damage to MON 89034 maize surfaced, scientists embarked on a systematic investigation to determine whether B. fusca had developed resistance to Cry2Ab2. Their approach combined field observations with rigorous laboratory testing 2 8 .
The experimental results provided clear and compelling evidence of Cry2Ab2 resistance emerging in South African B. fusca populations.
| Population Type | Number of Populations | Mortality on Cry2Ab2 Diet |
|---|---|---|
| Problem populations | 3 | Significantly less than 100% |
| Other populations | 5 | 100% |
In diet-based assays incorporating Cry2Ab2 protein, all populations except the three problem populations showed 100% mortality when exposed to Cry2Ab2, demonstrating that the protein itself remained highly efficacious against most B. fusca populations 2 . The significantly reduced mortality in problem populations indicated these insects had developed a genetic resistance to the Cry2Ab2 toxin.
Perhaps the most striking finding emerged from the plant-based assays, which more closely simulated real-world conditions:
| Population Type | Number of Populations | Survival Rate Range |
|---|---|---|
| Problem populations | 3 | 75% - 91% |
| Other populations | 5 | 0.4% - 9.6% |
The dramatic difference in survival rates—75-91% for problem populations versus 0.4-9.6% for others—provided undeniable evidence that resistance to MON 89034 maize had evolved in specific regions of South Africa 2 8 .
An unexpected discovery came from testing with Cry1A.105, the other Bt protein in MON 89034 maize. Assays with this protein did not cause significant mortality in any of the B. fusca populations tested, including larvae from a susceptible reference population 2 . This revealed a critical vulnerability in the pyramid strategy—against B. fusca, MON 89034 essentially functioned as a single-toxin product rather than a true pyramid, with all activity coming from Cry2Ab2 4 8 .
Understanding how resistance is detected and monitored requires familiarity with the essential tools and methods employed by researchers in this field.
| Tool/Method | Function in Resistance Research |
|---|---|
| Artificial diet bioassays | Allows evaluation of insect responses to specific Bt proteins independently of plant factors |
| Plant-based bioassays | Assesses insect survival on actual Bt plant tissue under controlled conditions |
| Insect colony maintenance | Provides standardized insects for comparison (susceptible and resistant strains) |
| Bt protein purification | Produces the toxic proteins used in diet bioassays |
| Statistical analysis software | Determines significance of survival differences between populations |
| Field sampling protocols | Ensures representative collection of insects from problem and reference areas |
These tools enable researchers to move from initial field reports of unexpected damage to scientifically verified cases of resistance, following established criteria that require demonstrating reduced efficacy of the Bt toxin in controlled experiments 2 .
The development of resistance to Cry2Ab2 in B. fusca represents more than just a local management issue—it reveals fundamental vulnerabilities in current resistance management approaches:
Previous research on other pests has shown that cross-resistance between Bt toxins can occur, particularly when insects have low inherent susceptibility to begin with 1 .
Evidence suggests that non-compliance with refuge requirements and other insect resistance management strategies likely accelerated resistance evolution in South Africa 5 .
The South African experience offers crucial insights for other regions implementing or considering Bt crop technology:
Resistance patterns can be species-specific and region-specific, requiring tailored management approaches 5 .
Integrated Pest Management (IPM) remains critical: Overreliance on any single technology, no matter how advanced, invites eventual failure .
The detection of incipient resistance in Busseola fusca to Cry2Ab2 maize in South Africa represents a significant development in the ongoing relationship between agriculture and insect pests. It demonstrates that even our most advanced genetic solutions remain vulnerable to evolutionary countermeasures. Yet this is not a story of defeat, but rather a reminder that sustainable pest management requires continuous innovation, vigilant monitoring, and adaptive strategies.
As new technologies emerge—including three-toxin pyramided systems and novel insecticidal proteins like Vip3A—the lessons from South Africa's experience with B. fusca become increasingly valuable 3 . By understanding how and why resistance develops, we can design more durable solutions that maintain their effectiveness while reducing environmental impacts.
The quiet rebellion in the maize fields continues, and science is responding with new strategies, new technologies, and a deeper understanding of the evolutionary forces that shape our agricultural systems. In this ongoing dialogue between human ingenuity and biological adaptation, each challenge overcome moves us closer to a more sustainable and productive agricultural future.