Hot Water Immersion Alternative to MeBr Fumigation of Limes

W.P. Gould and R.G. McGuire, USDA–ARS, Subtropical Horticulture Research Station, Miami, Florida

Mealybugs are major pests of many agricultural commodities. Limes which are imported into the United States from the Bahamas are inspected for mealybug pests. When unidentifiable early stages, or actionable species of mealybugs are found, limes are fumigated with methyl bromide. This quarantine treatment is necessary to prevent spread of new pests which do not occur in the United States. During cold weather, higher doses are required, which cause injury to the limes. Alternative treatments for commodities infested with hitchhiker pests are required, to prevent interruptions in commerce. The mealybugs found in this study were identified as Planococcus citri Risso (50%) and Pseudococcus odermatti Miller & Williams (50%).

A number of possible treatments were considered, including insecticidal coatings. Tests with coatings did not reach the high pest mortality necessary for a quarantine treatment. Hot water was found to be the most likely alternative to replace methyl bromide in this case. Hot water has been used to disinfest commodities of a variety of surface pests including mealybugs. Hot water treatment fits well as a rapid treatment on a packing line. It was proposed that a hot water treatment in the range of 46–57 °C for 5–20 minutes would disinfest the surface of the limes of pests. The pest insects are not allowed into the United States so limes were treated at the port of entry, held and examined for pest mortality, then returned to their origin for disposal.

Preliminary tests indicated that limes would tolerate 49 °C for up to 15 minutes without showing damage, so this temperature was chosen for the dose/mortality study. Limes with feral mealybug populations were dipped in 49 °C ± 0.5 °C hot water for times of 1 to 16 minutes. Limes were removed from hot water and hydrocooled at 25 °C ± 2 °C for 10 min. The limes were then held for 2–3 days at 24 °C ±2 °C. Limes were then examined under a stereo-microscope for insect mortality. An untreated control was used to assess natural mortality. Hot water treatment of limes showed little effect on insect mortality until 5 minutes, then mortality increased linearly until no survivors were recovered after 12 min of treatment. Statistical analysis gave 99.9968% mortality (probit 9) predictions of 21 and 13 minutes. We chose 20 minutes at 49 °C as a conservative treatment which would have no survivors and initiated a large scale test with groups of 1,200 fruit. The test was continued until over 1,000 mealybugs had been treated and killed with no survivors. During this test over 7,200 limes were treated with 1,308 insects killed and no survivors.

The effect of short hot water dips on lime quality was also studied. Limes were immersed for 10, 15, or 20 minutes in water at 46, 49, and 52 °C. One group of fruit was not treated and served as a control. All fruits were cooled in water at about 24 °C for 10 minutes after hot water immersion. All fruits were weighed initially. Control fruits were measured for color, firmness, pH, % acids, % solids, and ascorbate. Treatment and control fruits were measured for these characters after 10 days storage at about 24 °C. This experiment was repeated four times and data were analyzed using analysis of variance.

The temperature of the hot-water treatment and the time of immersion significantly affected the fruit quality. Treatment at 52 °C was significantly more damaging than treatment at 46 or 49 °C. At 52 °C injury and weight loss were greatest, and the fruits were less firm. The fruit color was lighter, more intense, and less green, and the pH of the juice and acidity increased slightly. The effect of treatment time was less significant. Firmness and juice characteristics were not affected by time of treatment between 10 and 20 minutes at these temperatures. Injury and weight loss were greater after a 20 minute treatment, and surface color was lighter, more intense, and less green with increasing time. Treatment at 49 °C for 20 min did not significantly affect quality when treated fruit were compared with untreated control fruit.

The 20 minute 49 °C hot water immersion treatment was effective in killing mealybugs and all other arthropods found externally on limes, or under the calyx in this study. We did not see any surviving insects or mites after the 20 min treatment. This treatment is comparable to 10 minutes immersion in 49 °C hot water for two scale insects on fruit and 12 minutes immersion in 49 °C water eliminated 95% of ants, aphids and mealybugs on red ginger flowers in Hawaii. Also the longtailed mealybug required an estimated 19 minutes to reach 99% mortality on persimmons dipped in 49 °C hot water in New Zealand. Fuller rose beetle eggs were killed by 8 minutes at 52 °C in a California test on lemons. Researchers in New Zealand found that two spotted spider mites were more resistant to heat and required 40 minutes at 48°C to reach 99% mortality.

The treatment time of 20 minutes is conservative and is based on the theoretical prediction of survivors if large numbers of insects are treated. It has been shown that long hot water treatments damage some citrus, but limes in this study tolerated the relatively short 20 minute treatment with no loss in quality. We intend to conduct further work with mealybugs to expand the range of temperatures that can be used to treat commodities. We feel that hot water is a technologically simple and robust technique which may be valuable for other commodities as well as limes.

Last Updated: February 24, 2000