A scientific breakthrough in the battle against insecticide-resistant grain borers
In the hidden world of grain storage, a silent war rages between humans and one of nature's most formidable pests—the lesser grain borer, Rhyzopertha dominica. This tiny beetle, no larger than a sesame seed, is responsible for devastating losses in global food reserves, capable of turning precious harvests into hollow shells.
The lesser grain borer can cause cumulative weight loss of up to 56.9% in wheat over just two months due to feeding by adult borers 3 .
For decades, we've fought back with chemical insecticides, but our weapons are failing. The borers have evolved resistance to traditional treatments, creating an urgent need for new solutions. Enter spinosad, a remarkable biological insecticide derived from soil bacteria that offers renewed hope in this ongoing battle. This is the story of how scientists in Andhra Pradesh, India, are pioneering innovative approaches to protect our food security using lessons from nature itself.
The lesser grain borer is a primary pest that attacks intact grains, penetrating hard outer shells that would deter other insects. Their damage makes grains vulnerable to secondary pests and creates conditions for mold growth.
Australian researchers reported that R. dominica "has developed resistance to the three major groups of grain protectants, i.e. organophosphates, pyrethroids and juvenile hormone analogues" 2 .
In Andhra Pradesh, India, the situation is equally concerning. Research revealed that local populations of lesser grain borers had developed 3.8-fold resistance to malathion and 1.6-fold resistance to deltamethrin at the LC50 level compared to susceptible laboratory strains 1 . These numbers translate to real-world control failures, threatening both food supplies and economic stability in agricultural communities.
Derived from Saccharopolyspora spinosa, a soil-dwelling bacterium first isolated from sugar mill rum stills 5 .
Targets nicotinic acetylcholine receptors and alters GABA receptor function 2 , causing paralysis in insects.
Offers low mammalian toxicity and environmental compatibility 7 , making it safer for use.
"Spinosad has been registered as an insecticidal natural product over the last two decades and has been found quite potent against R. dominica" 2 .
What makes spinosad particularly valuable is its effectiveness at remarkably low concentrations. While traditional chemicals require application rates of multiple parts per million, spinosad often achieves control at doses below 1 mg kg⁻¹ (1 ppm), minimizing residue concerns while maintaining protective efficacy over extended storage periods.
Researchers at the Department of Entomology, Agricultural College in Bapatla, Andhra Pradesh, designed a comprehensive laboratory study to evaluate spinosad's efficacy against local populations of R. dominica that had already developed resistance to malathion and deltamethrin 1 .
Field populations of lesser grain borers were collected from multiple locations across Andhra Pradesh, including Ghantasala, Maruteru, Bapatla, Jangamaheswarapuram, and compared with populations from Nalgonda 6 .
The research followed standard protocols for insecticide evaluation in stored product entomology, including precise insecticide applications, mortality assessments at 24, 48, and 72 hours, and statistical analysis using probit methods 1 .
| Population Source | Malathion Resistance (fold at LC₅₀) | Deltamethrin Resistance (fold at LC₅₀) |
|---|---|---|
| Ghantasala | 3.07 | 0.92 |
| Maruteru | 2.61 | 1.07 |
| Bapatla | 3.80 | 1.60 |
| Jangamaheswarapuram | 2.10 | 1.04 |
| Nalgonda | 1.80 | 1.21 |
The Bapatla population emerged as the most resistant to malathion, tolerating nearly four times the concentration that would kill susceptible laboratory strains. For deltamethrin, the Bapatla population again showed the highest resistance level at 1.6-fold 1 6 .
When the researchers tested spinosad against these resistant populations, the results were impressive. Spinosad demonstrated superior toxicity at the LC99.9 level, proving 1.55 times more effective than malathion and 2.46 times more effective than deltamethrin against the resistant insects 1 . This enhanced efficacy against resistant populations positions spinosad as a viable tool for resistance management.
Stored product entomology relies on specialized materials and methods to evaluate insecticide efficacy and monitor resistance development. The following research reagents and tools are essential for this work 1 2 4 .
Biological insecticide derived from Saccharopolyspora spinosa bacteria; used as grain protectant at rates of 0.5-1 mg/kg.
Organophosphate insecticide; serves as reference standard in resistance monitoring programs.
Synthetic pyrethroid insecticide; often used with synergists like piperonyl butoxide to enhance efficacy.
Insect populations maintained without insecticide exposure for generations; provide baseline susceptibility data for comparison.
Australian researchers established baseline susceptibility data for R. dominica populations and found "very little variation across populations in terms of LC50 and LC99.9," indicating that spinosad resistance had not yet emerged in these regions 2 .
However, history reminds us that continuous reliance on any single control method invites resistance development. Researchers note that "heritable resistance to spinosad has been reported in a wide range of insects of field crops in a short-time frame" 2 .
A 2025 study published in Pest Management Science discovered that maltase enzymes might also degrade spinosad due to structural similarities between the molecules 7 . This represents a novel resistance mechanism not previously documented.
When researchers maintained a spinosad-resistant strain without insecticide pressure for one year, they observed an 86.92% reduction in resistance levels 7 . This suggests that resistance may decline during periods of non-exposure.
Scientists explored the use of maltase inhibitors like acarbose as potential synergists. When applied alongside spinosad, acarbose significantly increased mortality in exposed insects, pointing to a possible strategy for enhancing spinosad efficacy and overcoming resistance 7 .
The evaluation of spinosad against resistant populations of lesser grain borer in Andhra Pradesh represents more than just another insecticide trial—it illustrates a broader shift toward sustainable, biologically-based pest management.
Proven effective against resistant pests
Derived from soil bacteria
Safer for humans and environment
In a world grappling with the dual challenges of food security and environmental sustainability, spinosad offers a compelling solution: effective protection that leverages nature's own inventions rather than relying solely on synthetic chemicals.
As we look to the future, the lessons are clear. No insecticide, no matter how effective, represents a permanent solution. Sustainable grain protection will require integrated approaches combining chemical tools with physical management, biological control, and monitoring. Spinosad, with its unique properties and proven efficacy against resistant pests, deserves a place in this toolkit—helping to protect our harvests while respecting the ecological systems that sustain us.