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ARE NATURAL COMPOUNDS THE FUTURE OF FUMIGANTS?

Researchers from South Korea and The University of Queensland have investigated the fumigant properties of methyl benzoate – with promising results.

 

Typically known to infest grains, cereals, dried fruit, and seeds, the Indian meal moth (Plodia interpunctella) poses both health issues, due to the introduction of allergens, and an economic threat to stored product operations. Over the past few decades, control of P. interpunctella has relied heavily on the use of synthetic pesticides (e.g. organophosphates and pyrethroids) and fumigants (e.g. methyl bromide or phosphine) but the growing concern over safety has led to alternative options being explored.

The use of naturally occurring toxins is becoming more desirable in pest management programs because these compounds generally show rapid environmental biodegradation and lower toxicity to non-target organisms. Monoterpenes are considered suitable compounds with which to develop new insecticides because they are typically safe, effective, and fully biodegradable. This is in stark contrast to traditional fumigants, which typically have high mammalian toxicity and the growing issue of insect resistance. However, the naturally occurring ester, methyl benzoate, could also be a potential alternative.

In an academic paper published in Insects in December 2020, researchers from South Korea and The University of Queensland evaluated the fumigant toxicity of methyl benzoate – a plant-derived, food safe, natural insecticide – against adult Indian meal moths.

Methyl benzoate is a sweet-smelling compound found in many plants, including snapdragons and petunias. It biodegrades slowly in the atmosphere and in recent studies has been shown to have contact toxicity against various insect pests including spotted wing drosophila, tobacco hornworms, brown marmorated stink bugs, diamondback moths, red imported fire ants, white flies, aphids and mites. The potential fumigation toxicity of methyl benzoate has been demonstrated on bed bugs and some stored product insect pests.

The researchers of this study assessed the effectiveness of methyl benzoate as a fumigant against adult P. interpunctella in two ways: using glass bottles (replicating a small, contained environment) and large cardboard boxes sealed with tape, (replicating a large, spacious environment).

In the glass tube tests, various methyl benzoate concentrations were loaded onto small cotton balls that were dropped to the bottom of the glass tubes, then live moths were added and mortality recorded over a four-hour period. A similar protocol was used for the cardboard boxes but mortality was assessed after 24 hours. The efficacy was compared to a range of commercially available monoterpenes: citronellal, linalool, cineole, and limonene, as well as the ester ethyl formate, a commercially available fumigant.

The glass tube studies demonstrated that methyl benzoate not only outperformed all the monoterpenes but also delivered the same level of mortality as the commercially available ethyl formate (100% after four hours), but at far lower concentrations. Further analysis revealed that the LC50 (concentration required to kill 50% of the insects) for methyl benzoate as a fumigant treatment against adults of P. interpunctella was 0.1 μL/L air. Based on this value, methyl benzoate showed greater toxicity than ethyl formate (which had an LC50 of 3.2 μL/L air).

In the cardboard box tests, methyl benzoate was assessed against the best performing monoterpene from the glass tube trials, citronellal. Again, methyl benzoate showed stronger fumigation toxicity than citronellal against adults of P. interpunctella. After 24 hours of exposure to methyl benzoate at a concentration of 0.01 μL/cm3, 100% mortality was observed. In contrast, for the same concentration and exposure time, citronellal produced only 86% mortality.

Compared with commercially available monoterpene compounds used in pest control, methyl benzoate showed the highest fumigant toxicity.

The findings show that methyl benzoate has high fumigant toxicity against adults of P. interpunctella. The study also illustrates that methyl benzoate stands out amongst other potential natural biofumigants as a strong candidate to protect stored products against insect pests (here P. interpunctella); thus, methyl benzoate may be a safer and perhaps more effective alternative to traditional fumigants. The findings are similar to those of other tests conducted on stored product pests, including lesser grain borer and red flour beetle.

However, compared to most other gas fumigants, methyl benzoate is much less volatile and may therefore have less capacity to penetrate products such as vegetables and stored goods. However, Yang et al. reported that methyl benzoate is useful as a fumigant against insects on both fresh and stored products and showed that methyl benzoate fumigation could combat post-harvest pests. Moreover, they also found that methyl benzoate fumigation had no adverse effect on the quality of apples in terms of colour, weight, and firmness.

Although these results are promising, further research is required to assess the natural evaporation of methyl benzoate in commercial-scale trials, improve control efficiency, and develop protocols for large-scale treatment.

 

Further reading: Mostafiz et al. (2020). Methyl Benzoate Is Superior to Other Natural Fumigants for Controlling the Indian Meal Moth (Plodia interpunctella). Insects. 12. 23. 10.3390/ insects12010023.

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