Submitted to: Symposium Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 1/8/2010
Publication Date: 4/30/2010
Citation: Johnson, J.A., Tang, J., Wang, S., Mitcham, E.J., Monzon, M. 2010. Methyl bromide alternatives for postharvest insect disinfestation of California walnuts. Symposium Proceedings. 861:383-388. Interpretive Summary: About 30% of walnuts produced annually in California are inshell exports. Walnuts may be infested with field pests (navel orangeworm and codling moth) or with storage pests (Indianmeal moth or red flour beetle). Consequently, processors have historically relied on fumigants for disinfestation, particularly methyl bromide. However, the Montreal Protocol has caused the restriction of the use of methyl bromide for non-quarantine purposes, resulting in a need for suitable alternatives. In addition, the rise in demand for organic produce has increased the need for non-chemical alternatives. Non-chemical alternatives to fumigants include the use of radio frequency (RF) energy, low temperature storage, and low pressure or vacuum. While none is a substitute for fumigation, each has advantages and disadvantages. Vacuum treatments in flexible structures are safe and simple to use, have relatively low initial capital costs, and minimum power requirements, but treatments may damage nutshells and treatment times are lengthy at low temperatures. Low temperature storage requires greater initial capital costs, but this becomes irrelevant when processors already use cold storage to maintain product quality. Treatment times to disinfest product with refrigeration (0-5C) require at least 3 weeks, and is not feasible when diapausing larvae are present. Freezing at -18C to kill diapausing larvae may require 2-3 weeks to ensure lethal temperatures throughout bulk-stored nuts. Radio frequency treatments which make use of commercially available, industrial scale RF units show the most promise for quickly disinfesting large amounts of product. While the initial capital costs are significant, the energy cost per kg of treated product is comparable to the current cost of methyl bromide fumigation.
Technical Abstract: Before shipment, California inshell walnuts destined for the valuable export market must be disinfested of both field pests (codling moth and navel orangeworm) and common storage pests (Indianmeal moth and red flour beetle). Until recently fumigation with methyl bromide has been the most common disinfestation method, but access to methyl bromide is now greatly restricted in developed countries, causing walnut processors to seek alternative methods. In addition to alternative chemical fumigants, non-chemical physical treatments are being developed. One treatment uses radio frequency energy to rapidly heat walnuts to an average temperature of 60°C for 5 minutes. Large-scale tests using an industrial radio frequency heater successfully disinfested walnuts of navel orangeworm while maintaining good product quality. A second alternative treatment is the use of cold storage (0-5°C) to either disinfest product or prevent reinfestation of clean product. At temperatures of 0 and 5°C, 95% mortality of test insects was achieved at 10 and 18 days, respectively. Storage at 10°C prevented reproduction, product damage and reinfestation, but 127 day exposures were required to kill 95% of Indianmeal moth larvae. A third alternative treatment exposes product to a low pressure (vacuum) environment of 50 mm Hg in flexible polyvinyl chloride containers. Temperature and moisture content of the product strongly affects treatment efficacy, with complete mortality of test insects achieved at treatment exposures of 48 hours or less under optimal conditions.