1a. Objectives (from AD-416):
In order to identify cold storage treatments that are efficacious against spotted wing drosophila (SWD) the project seeks to determine the most cold-tolerant stage of SWD, develop lethal exposure (LT) times necessary to produce 95, 99 and 99.9863% mortality of the most cold tolerant stage of SWD exposed to commercial cold storage conditions (0.5-2.2C and 90-95% RH), and determine rate of survival of SWD after SO2/CO2 fumigation and 21 day exposure to commercial cold storage conditions.
1b. Approach (from AD-416):
All SWD life-stages in artificial diet will be treated at 0.5C for 5, 10, 15, 20, and 25 days following the guidelines for optimal cold storage of table grapes. Probit analysis will be used to calculate LT values and identify the most cold tolerant stage, which will be used to estimate LT values for 2.2C, using the methods described above. Large-scale confirmatory tests using grapes infested with the most cold tolerant SWD stage and held in standard commercial shipping boxes will be done at 0.5 and 2.2C using the optimal treatment times derived above. Grapes infested with SWD will be treated with SO2/CO2 at the levels and times currently used for export to Australia/New Zealand and then held at 0.5 and 2.2C for 7, 14, 21 and 28 days. In all tests, treatment temperatures will be recorded with data loggers.
3. Progress Report:
Progress was made under objective 3.B, by developing a non-chemical disinfestation treatment using low temperature storage, which falls under NP308. Spotted wing drosophila (SWD), Drosophila suzukii (Matsumara) is known to infest ripe fruits, including grapes. Australia and New Zealand, concerned that this insect may be introduced into their agricultural areas on table grapes imported from California, require some method of quarantine treatment. Methyl bromide, although still available for quarantine applications, may eventually be restricted. Cold storage has been used as a quarantine treatment for fresh fruits. Alone or in combination with SO2/CO2, cold storage shows potential as an alternative to methyl bromide fumigation for SWD on California table grapes. Adults exhibited a relatively high survival rate after 6 days of exposure to 33°F. However, most surviving adults were unable to fly or walk normally, and reproduction was negatively affected. Adults treated with sulfur dioxide (SO2) at 100 ppm/hour (a standard industry practice to reduce mold growth) prior to cold treatment showed little difference in their cold tolerance from adults treated with cold alone. SO2 dose response studies indicated that 99% mortality of adults would require 352 ppm/hour. Preliminary studies on artificial diet indicated that 0-24 hour old SWD (eggs and newly-hatched larvae) may be the age most tolerant to 33°F (0.5°C). Tests of immature SWD stages on grapes show that either 0-24 hour old or 120-198 hour old SWD (third instar larvae and pupae) were more tolerant. At the numbers of test insects treated so far, exposures of 8-10 days at 33°F may be sufficient to provide quarantine security. This treatment time will be confirmed with larger scale tests. This research will benefit the viticulture industry by providing a cold storage treatment against spotted wing drosophila. Such a treatment will maintain access for the table grape industry to foreign markets such as Australia and New Zealand.