Submitted to: International Conference on Methyl Bromide Alternatives and Emissions Reductions
Publication Type: Proceedings
Publication Acceptance Date: 9/1/2009
Publication Date: 11/13/2009
Publication URL: http://mbao.org/2009/Proceedings/076JohnsonJetalMBAO2009final.pdf
Citation: Johnson, J.A., Wang, S., Tang, J. 2009. Radio Frequency Heat Treatments to Disinfest Dried Pulses of Cowpea Weevil. International Conference on Methyl Bromide Alternatives and Emissions Reductions, November 10-13, 2009, San Diego, CA. p. 76.1-4. Interpretive Summary: Dried pulses are valuable export commodities in the US Pacific Northwest. A major problem in the marketing of these products is infestation by insect pests, such as cowpea weevils (Callosobruchus maculatus) which may cause importing countries to require phytosanitary treatments before shipment. Typically, chemical fumigants are used to disinfest product, but regulatory issues, insect resistance, environmental concerns and the increase of the organic market have caused the industry to explore non-chemical alternatives. One possible alternative is the use of radio frequency (RF) energy to rapidly heat product to insecticidal levels. This paper reports preliminary results from investigations into the potential of the non-chemical treatment method to control insect pests in dried pulse products. Results from heat block studies showed that cowpea weevil pupae are the most heat tolerant stage. Although adult weevils are the most heat susceptible, they were more tolerant than most previously studied dried fruit and nut pests. At frequencies commonly used by industry for RF heating, dielectric loss for both insect stages was higher than that for lentils, chickpeas and green peas, suggesting that cowpea weevils would heat at a faster rate than the product. Preliminary studies indicate that chickpeas, green peas and lentils can tolerate RF treatments to 60C for 10 minutes without adverse effects on quality. This, along with the deep penetration depths of these products found at RF frequencies suggest that practical large-scale industrial treatments should be possible. RF treatments should provide a practical, effective and environmentally friendly method for disinfestation of postharvest legumes, thereby allowing processors to maintain access to critical export markets.
Technical Abstract: To explore the potential of radio frequency (RF) heat treatments as an alternative to chemical fumigants for disinfestation of dried pulses, the relative heat tolerance and dielectric properties of different stages of the cowpea weevil (Callosobruchus maculatus) was determined. Among the immature stages, the most heat tolerant stage appears to be the pupal stage. Eggs were quite susceptible; however, this may be due to the eggs being more directly exposed to heat. Larval and pupal stages, being internal to the bean, were more insulated from the heat, experiencing a shorter exposure to lethal temperatures. Adults were less tolerant than immature stages; at the temperatures and exposures used to treat immature stages, there was no survival of treated adults. When comparing adult mortality to larval and pupal stages, the difference may again be due to the insulating effect of the bean. However, adults were also less tolerant than eggs, which were more directly exposed to heat. Mortality from less extreme temperature-time combinations showed that adult weevils, while less tolerant than immature cowpea weevils, were more tolerant than most dried fruit and nut pests. The dielectric constant and dielectric loss for cowpea weevil immature stages (larvae and pupae), adults increased with increasing temperature. At frequencies commonly used by industry for RF heating, dielectric loss for both insect stages was higher than that for lentils, chickpeas and green peas, suggesting that cowpea weevils would heat at a faster rate than the product.