Principal Investigators: Krista C. Shellie, Research Plant Physiologist, and Robert L. Mangan, Research Entomologist, Crop Quality and Fruit Insects Research Unit, USDA–ARS, Weslaco, TX

The Mexican fruit fly, Anastrepha ludens (Loew), is undesirable in citrus production regions. Citrus fruit harvested from regions infested with fruit fly could contain unhatched eggs or larvae of various ages, depending upon the elapsed time between fly ovipositing and fruit harvest. Because infested fruit cannot be easily eliminated by external inspection, regulatory agencies in many countries have established phytosanitary quarantine protocols. Quarantine protocols include preharvest techniques, such as sterile fly release, non-host status, and pest-free growing periods, as well as postharvest commodity treatments, such as methyl bromide fumigation and cold storage. "All postharvest problems pale in comparison to . . . quarantines placed on fruit shipments due to an outbreak of exotic fruit flies" (Citrograph, 1992).

Among permitted treatments, methyl bromide is the only available fumigant, yet it can cause losses of fruit ranging up to 60 percent of loads treated at destination. If the citrus industry were forced to fumigate with methyl bromide before loading and shipment, losses in exports would be even higher, making methyl bromide fumigation an economically unacceptable treatment (Citrograph, 1992). Storage at 33 to 35 °F for 18 to 22 days is approved by the Animal and Plant Health Inspection Service of the United States Department of Agriculture as a quarantine treatment against Mexican fruit fly for citrus imported into the United States from Mexico or Central America. However, this treatment is not used commercially because of problems with fruit developing chilling injury. The objective of this research was to assess whether storage under a nonchilling temperature in an atmosphere with modified levels of oxygen or carbon dioxide could provide quarantine security against Mexican fruit fly without damaging fruit quality.

Tolerance of 'Rio Red' grapefruit, Citrus paradisi Macf., to storage at 50 °F in a hypoxic or hypercarbic atmosphere was evaluated in a series of small experiments. Grapefruit quality was evaluated after 14 or 21 days of storage in air, ultra low oxygen (0.05, 0.10, or 0.15 percent), or high carbon dioxide (20, 40, or 60 percent). Results from these experiments demonstrated that storage in either 40 or 60 percent carbon dioxide (balance air) caused a breakdown of flavedo tissue, and that fruit tolerated storage for up to 21 days in ultra low levels of oxygen (0.05, 0.10, or 0.15 percent) or 20 percent carbon dioxide (balance air). Grapefruit stored in 0.05 percent oxygen were rated for flavor as acceptable yet inferior to fruit stored in air or in 0.10 percent oxygen. Grapefruit stored in ultra low levels of oxygen had a lower incidence of decay, a higher amount of titratable acid, and a lower ratio of soluble solids to titratable acid than grapefruit stored in air.

The mortality of Mexican fruit fly larvae after storage in a refrigerated controlled atmosphere was also evaluated in a series of small experiments. The mortality of third instar larvae that were artificially infested into grapefruit and then stored for 14 or 21 days in air, 0.05 percent oxygen, 0.10 percent oxygen or 20 percent carbon dioxide (balance air) was found to be highest after storage in 0.05 percent oxygen. Six life stages of Mexican fruit fly were stored for 21 days on laboratory diet at 50 °F in 0.05 percent oxygen or in air. The number of surviving individuals were counted after removal from cold storage and holding under optimum rearing conditions. Late third instar larvae and eggs were found to be the life stages most likely to survive cold storage in 0.05 percent oxygen.

Responses observed in this study for grapefruit were similar to that reported by other researchers. Ke and Kader (1990) found that 'Valencia' orange (citrus sinensis (L.) Osbeck) tolerated exposure to 0.02 percent oxygen at 50 °F for up to 20 days without detrimental effects on external and internal appearance, but developed skin browning and poor external appearance after storage in 60 percent carbon dioxide. They also found that 'Valencia' oranges developed an acceptable, yet detectable off-flavor during storage in ultra low oxygen and they associated this off-flavor with an increase in tissue concentration of ethanol and acetaldehyde. Davis et al. (1973) suggested that development of off-flavor under anaerobic conditions may also be attributed to a shift in equilibrium toward reduced forms of flavor compounds. Intolerance of grapefruit to storage in elevated levels of carbon dioxide has been reported by Stahl and Cain (1937) and Scholz, et al. (1960). Grapefruit from Florida (Stahl and Cain, 1937; Chace, 1969), and Texas (Scholz et al.,1960) have been shown to tolerate refrigerated storage in a low oxygen atmosphere for up to 4 weeks.

Low oxygen was also found to be more efficacious than elevated carbon dioxide for killing Mexican fruit fly larvae during heating in forced-air (Shellie et al., 1997). Grapefruit heated in 1 percent oxygen required 30 percent less exposure time than grapefruit heated in air to obtain 100 percent larval mortality. Results from these experiments suggest that storage in ultra low levels of oxygen at specific temperatures has potential for disinfesting grapefruit of the Mexican fruit fly.

Although quarantine uses of methyl bromide amount to only 5 to 8 percent of total U.S. consumption, this application will be especially difficult to replace. There are few candidate fumigants that have the necessary toxicity to the target pest, volatility to be useful in the required range of temperatures for treatment and acceptable phytotoxicity effects. The U.S. General Accounting Office reported in 1994 that $431,510,000 in U.S. exports were treated with methyl bromide as a condition of entry to receiving countries. Moreover, methyl bromide is currently the only emergency fumigant available to disinfest commodities from growing areas quarantined as a result of the invasion of exotic pests, such as the Mexican fruit fly. The loss of methyl bromide in 2001 will have significant negative impact on U.S. domestic and international trade. The research presented herein demonstrates the importance the Agricultural Research Service places on developing alternative commodity treatments to ensure a highly competitive, economically viable U.S. agricultural production system.