Heat as a phosphine resistance breaker: Is this feasible?

“Phosphine and heat are applied at different types of facilities, which means that heat treatment cannot be considered as an actual replacement of phosphine. Heat is applied in empty storage and processing facilities, while phosphine is usually applied for product disinfestation. Still, the data underline the importance of heat to kill insect populations. Hence, heat and phosphine may not be that far away in terms of their target facilities and heat treatment can be used by the industry as an alternative method for the control of stored product insects that are resistant to phosphine.”

C. G. Athanassiou
Laboratory of Entomology and Agricultural Zoology
Department of Agriculture, Crop Production and Rural Environment
University of Thessaly, Greece
Currently, heat is one of the most widely adopted methyl bromide alternatives for the control of post-harvest insects in durable agricultural commodities. Heat treatment is an environmentally-friendly technology that can be used in conjunction with chemical and non-chemical methods for insect management (Subramanyam et al., 2011). Heat treatment is based on the application of high temperatures (between 50 and 60 oC) for a relatively short time period (24-36 hours or even shorter periods) for the control of various stored product insects without affecting product quality or the structures that are heated (Mahroof et al., 2003a; Wang et al., 2006).

Heat equipment is based on gas, electric or steam heaters which are used to gradually increase air temperature to the desired levels (Mahroof and Subramanyam, 2006). There are published papers on the successful use of elevated temperatures against stored-product insects, with different susceptibility levels to conventional control methods (Boina and Subramanyam, 2004; Yu et al., 2011; Agrafioti et al., 2019). Moreover, the temperature can be recorded, by using wireless sensors, providing an algorithm that can give predictions of the success of the treatment in a given facility and for given target species and life stages.

In a thorough evaluation of heat treatments in “real world” applications, Mahroof et al. (2003b) reported that in laboratory conditions heat treatment was also effective and that adults of the red flour beetle, Tribolium castaneum (Coleoptera: Tenebrionidae) required ≤1.8 h at 50 oC, which is indicative of the rapid effect of this method against major stored product insects. Based on various studies, it seems that heat is viable alternative of methyl bromide in empty facilities and processing plants. In this context, despite the fact that there are more to be done for the improvement of the method, heat treatment with continue to play a key role for space disinfestation.

Heat vs phosphine resistance
The withdrawal of methyl bromide, as expected, increased the reliance from phospine in commodity treatments. There are numerous paradigms that underline the wide uses of phosphine in various types of commodities and facilities. Still, in most cases, phosphine is applied on commodities and not on empty facilities, so, in this sense, it is not considered as a “real” alternative of phosphine, as each gas has a different target. On the other hand, heat is a real alternative to methyl bromide, given that it can be easily applied in empty facilities for space treatment, and, at the same time, is really fast (very often <24 h), which means that heat is directly comparable with methyl bromide. Conversely, phosphine requires several days to act, can not be applied in machinery/equipment due to its corrosiveness and needs a meticulous sealing preparation before application. From a first view, phosphine and heat do not meet in the stored product world, as they cover different target areas. But they do. The magic word here that explains this complementarity is resistance.

The application of phosphine in the commodity often produces resistant insect populations that are able to spread throughout the facility. This means that resistance is likely to be present even in areas where no phosphine is applied, such as flour mills and semolina factories. These populations occasionally end up in the final product, e.g. pallets with pasta etc. Consequently, even if the final product is fumigated, insect survival may be eventually high. Recently, Agrafioti et al. (2019) studied the efficacy of heat treatment on phosphine resistant and susceptible populations of the lesser grain borer, Rhyzopertha dominica (Coleoptera: Bostrychidae) and the sawtoothed grain beetle, Oryzaephilus surinamensis (Coleoptera: Silvanidae), at 23 facilities in Greece. They found that complete control was observed for both resistant and susceptible populations at all facilities. In general, in the few cases where survival and progeny production was recorded, there was no specific trend towards specific species or population. This clearly suggests the role of heat against insects that have developed a considerable level of resistance to conventional insecticides, incl. phosphine. To our knowledge, this is the first report on which heat was examined against populations with different susceptibility to phosphine and proved successful against both primary and secondary colonizers in a wide range of treated facilities.

Phosphine and heat are applied at different types of facilities, which means that heat treatment cannot be considered as an actual replacement of phosphine. Heat is applied in empty storage and processing facilities, while phosphine is usually applied for product disinfestation. Still, the data that are mentioned above underline the importance of heat to kill insect populations. Hence, heat and phosphine may not be that far away in terms of their target facilities and heat treatment can be used by the industry as an alternative method for the control of stored product insects that are resistant to phosphine. Also, heat may alleviate the problems arising from the application of phosphine in “sensitive areas”, such as flour mills etc., that have equipment that can be damaged by phosphine. Like phosphine, heat can easily penetrate the treated areas, and has a very good ovicidal effect, which is an additional reason to use heat where phosphine is currently used, given that resistant populations are usually survive fumigations at the egg stage.

References
Agrafioti, P., Athanassiou C.G, Subramanyam, B., 2019. Efficacy of heat treatment on phosphine resistant and susceptible populations of stored product insects. J. Stored Product Res. 81, 100-106.
Boina, D., Subramanyam, B., 2004. Relative susceptibility of Tribolium confusum life stages exposed to elevated temperatures. J. Econ. Entomol. 97, 2168-2173.
Mahroof, R., Subramanyam, B., Throne, J.E., Eustace, D., 2003a. Temperature and relative humidity profiles during heat treatment of mills and its efficacy against Tribolium castaneum (Herbst) life stages. J. Stored Prod. Res. 39, 555-569.
Mahroof, R., Subramanyam, B., Throne, J.E., Menon, A., 2003b. Time-mortality relationships for Tribolium castaneum (Herbst) life stages exposed to elevated temperatures. J. Econ. Entomol. 96, 1345-1351.
Subramanyam, B., Mahroof, R., Brijwani, M., 2011. Heat treatment of grain-processing facilities for insect management: A historical overview and recent advances. Stewart Postharvest Review, 7, 1-11.
Wang, D., Collins, P.J., Gao, X., 2006. Optimizing indoor phosphine fumigation of paddy rice bag-stacks under sheeting for control of resistant insects. J. Stored Prod. Res., 42, 207-217.
Yu, C., Subramanyam, B., Flin, P.W., Gwirtz, J.A., 2011. Susceptibility of Lasioderma serricorne (Coleoptera: Anobidae) life stages to elevated temperatures used during structural heat treatments. J. Econ. Entomol. 104, 317-324.

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