Acetaldehyde and Carbon Dioxide for
Postharvest Control of Arthropods on Strawberry Fruit
Final Report - October 2001
Principal Investigator:
Elizabeth Mitcham
University of California, Davis
Dept. of Pomology, 1045 Wickson Hall
University of California
One Shields Ave.
Davis, CA 95616
Telephone: (530) 752-7512 Fax: (530) 752-8502
Email: ejmitcham@ucdavis.edu
Cooperators:
Doug Gerdts, Calif. Controlled Atmosphere
39138 Road 56
Dinuba, CA 93618
Telephone: (559) 591-8874
Rob Webb, Driscoll Strawberry Assoc.
1750 San Juan Road
P.O. Box 111
Watsonville, CA 95077
Email: rwebb@driscolls.com
Bill Biasi, Dept. of Pomology
1045 Wickson Hall
One Shields Ave.
University of California
Davis, CA 95616
Telephone: (530) 752-1826 Email: wvbiasi@ucdavis.edu
Location of project: University of California, Davis, Yolo County
Funding:
| UC SAREP | UC IPM | |
| 1999-2000 | $47,662 | -- |
| 2000-2001 | $28,324 | $14,162 |
Objectives
- Determine the efficacy of acetaldehyde fumigation alone and in combination with carbon dioxide to kill western flower thrips and two-spotted spider mites.
- Determine the affect of fumigation with acetaldehyde and carbon dioxide on strawberry fruit quality and postharvest life.
- Demonstrate the commercial feasibility of the treatment within existing methyl bromide fumigation facilities.
Summary
Methyl bromide fumigation is used prior to shipment of California strawberries to Japan and Australia. Methyl bromide will be phased out for soil fumigation in 2005 under the Clean Air Act and the Montreal Protocol. While there is currently an exemption for postharvest and pre-shipment uses, methyl bromide will likely be more difficult and expensive to use in the future. The value of the export market to Australia is more than $1.3 million and to Japan is more than $18 million. Alternatives to methyl bromide for postharvest insect and mite control on strawberry fruit are limited because of the perishable nature of the commodity. Natural fruit volatiles have been tested for efficacy against various insect pests.
Plant volatiles such as acetaldehyde (Aa) and ethyl formate (EF) have been shown to have varied effects on fruit quality parameters and have been demonstrated to have fungicidal and insecticidal properties. This study explored the possibility of using Aa and EF for postharvest disinfestation of western flower thrips and two-spotted spider mite on harvested strawberries.
Dose response curves for western flower thrips and two-spotted spider mites were developed for exposure to Aa. Strawberry fruit treated with 0, 1, 2, 3, or 4% Aa in air or in CO2 and stored at 0ºC or 20ºC were evaluated for changes in fruit quality. Volatile compounds in strawberry juice after treatment was also quantified. A repeated exposure technique was developed to determine if low concentrations of Aa had less impact on fruit quality.
Western flower thrips were susceptible to Aa; however, quarantine levels of control were not achieved. Two-spotted spider mites were more resistant to Aa than western flower thrips and concentrations necessary to elicit high mortality were well above those tolerated by strawberry fruit.
Acetaldehyde concentrations >=3% caused calyx browning and drying. Initially, fruit exposed to 2, 3, or 4% acetaldehyde in the presence of 20% CO2 showed slightly less calyx damage than fruit exposed to acetaldehyde in air, however, after 24 h, there were no significant differences.
Repeated exposures to low concentrations of Aa improved fruit tolerance to the treatments but did not maintain the same level of target pest mortality as a single, high dose of Aa. Acetaldehyde is readily absorbed and metabolized by strawberry fruit and was rapidly reduced to ineffective concentrations for control of target pests in the presence of strawberry fruit under the conditions of our experiments.
Strawberry fruit and target pests were exposed to varying concentrations of EF in treatments utilizing both single and multiple exposures. Although EF was toxic to both target pests, concentrations necessary for complete control of two-spotted spider mite were well above those tolerated by strawberry fruit.
While neither Aa or EF appear particularly promising for postharvest insect control in strawberry, the information gained in the research may lead to a new quarantine treatment for other commodities.
Specific Results
Objective1: Determine the efficacy of acetaldehyde fumigation alone and in combination with carbon dioxide to kill western flower thrips and two-spotted spider mites.
Thrips mortality increased with increasing concentration of Aa in the presence of strawberry fruit. There was no significant difference in mortality between western flower thrips exposed to Aa in air or Aa in 20% CO2, except with 1% Aa when thrips mortality was 57 and 75% following exposure to 1% Aa in air or CO2, respectively (P <0.05) (Table 1). Complete mortality was not achieved, even with 4% Aa. In separate tests without strawberry fruit the addition of CO2 did not result in higher mortality with 1% Aa (Table 2). In fact, mortality for treatments with 1% Aa and >= 10 %CO2 was reduced. When CO2 was combined with 0.5% Aa, mortality was reduced with 20 and 40% CO2 but increased with 5 and 10% CO2 as compared with 0.5% Aa in air.
Western flower thrips were more susceptible to Aa than two-spotted spider mites (Table 3). Complete control was not achieved for either target pest. Exposure to Aa with a cumulative concentration >=2% resulted in >89% mortality for western flower thrips, and multiple exposures to Aa produced mortalities comparable to that of a single exposure to the same cumulative concentration. However, for two-spotted spider mites, multiple exposures to Aa did not achieve the same mortalities as a single exposure to the same cumulative concentrations (Table 3). A single exposure to 4% Aa was necessary to elicit >90% mortality for two-spotted spider mites.
The response of target pests and strawberries to EF was similar to Aa. Western flower thrips were more susceptible to EF than two-spotted spider mites, and concentrations above those tolerated by the strawberry fruit were necessary for complete control.
Objective 2: Determine the affect of fumigation with acetaldehyde and carbon dioxide on strawberry fruit quality and postharvest life.
The concentration of Aa in the treatment jars during fumigation decreased rapidly in the presence of strawberry fruit (Fig. 1). The rate of decline in Aa concentration was determined by the weight of the fruit in the jar (load factor). A load factor of 13% (volume by weight) resulted in approximately a 70, 84, and 95% decrease in Aa concentration after 0.5, 1 and 2 h, respectively, when 3% Aa was applied.
Firmness, color, soluble solids and berry damage were not significantly different between treated and untreated fruit at any evaluation (Table 4). However, calyx damage was higher in fruit treated with 3% or 4% Aa (Table 4, Fig. 2). Fruit treated with Aa in 20 % CO2 had slightly less calyx damage immediately after treatment than fruit treated with Aa in air (Table 4, Fig. 2). This effect was most pronounced for the 3% and 4% treatments, but did not persist beyond the initial evaluation.
Immediately after exposure to Aa, treated strawberry fruit had increased concentrations of Aa and ethanol compared with that of the untreated control (Table 5). However, fruit held for two days at 20ºC after treatment had similar concentrations of Aa and ethanol as the control (Fig. 3). Acetaldehyde and ethanol concentrations in fruit held at 0ºC after treatment decreased over time, but remained higher in treated fruit than in the control (Fig. 4). Strawberry fruit treated with Aa in 20% CO2 exhibited higher ethanol and Aa concentrations than fruit exposed to Aa in air (Table 5, Figs. 2 and 3). This effect was most apparent for the initial observations, and remained significantly different for fruit stored at 0°C after treatment, but not for fruit stored at 20ºC after treatment.
Methanol concentrations were similar between treated and untreated fruit for the initial evaluation, and remained relatively constant during storage for fruit held at 0ºC (Table 5). Only fruit treated with 4% Aa had higher concentrations after 0°C storage. Untreated fruit held at 20ºC showed a sharp rise in methanol levels on day 2, but treatment with Aa inhibited the increase in methanol (Table 5, Fig. 3). Treatment atmosphere did not have an effect on methanol levels.
Acetone concentrations in treated fruit were lower than in the control fruit (Table 5). Acetone levels in fruit held at 0ºC decreased during storage, while those of fruit held at 20ºC increased (Table 5, Figs. 3 and 4). However, the relative differences between treated and untreated fruit were maintained during storage. Immediately after treatment, fruit exposed to Aa in CO2 exhibited slightly higher levels of acetone as compared to fruit exposed to Aa in air; but this difference did not persist.
Ethyl acetate was present at elevated levels in all treated fruit at all evaluation periods (Table 5). For treated fruit stored at 20ºC, ethyl acetate levels were elevated on day 1, but declined sharply by day 2. (Fig. 3). Ethyl acetate increased during storage in all treated fruit held at 0ºC, and then declined after day 2 (Fig. 4). Treatment atmosphere had no effect on ethyl acetate concentration.
In multiple exposure tests with strawberry fruit, calyx damage increased with increasing concentration and with repeated application of the same concentration (Table 6). Fruit were exposed to Aa for 1 h, then vented in air for 1 h, and this sequence was repeated for the desired number of exposures. Although fruit treated four times with 1% Aa (1-1-1-1%) or two times with 2% Aa (2-2%) each had a cumulative exposure of 4% Aa, calyx damage ratings for the 1-1-1-1% treated fruit were significantly lower than for fruit treated once with 4% Aa, and 2-2% Aa was intermediate. Berries treated with 4% Aa, either as a single application of two applications of 2% Aa (2-2%) were softer, and had more berry damage, than berries from other treatments (data not shown).
Strawberry response to EF was similar to those for Aa. There was no effect on berry damage, firmness, soluble solids or color; however, strawberry calyx damage increased with increasing concentration of EF (data not shown). Repeated exposure to lower concentrations of EF resulted in lower damage scores than a single exposure to a high concentration (data not shown).
Objective 3: Demonstrate the commercial feasibility of the treatment within existing methyl bromide fumigation facilities.
Our research, thus far, did not result in a commercially feasible treatment, and therefore has not been tested in a large scale fumigation facility.
Potential Benefits/Impacts on Agriculture
Our results indicate that acetaldehyde is too phytotoxic to strawberry fruit to be of commercial value. Fruit quality results with EF indicate that phytotoxicity may also be a barrier to using this fumigant on strawberry fruit.
Additional work with EF, including a direct comparison with methyl bromide fumigation which can also cause calyx damage, is recommended. There are few other potential alternatives for postharvest insect control except irradiation. If social issues regarding irradiation can be resolved, irradiation may prove a viable option.
Dissemination of Findings
We will be presenting our research, "Effects of acetaldehyde on fruit quality and target pest mortality for harvested strawberries" in the poster session at the 2001 International Research Conference on Methyl Bromide Alternatives and Emmisions Reduction in San Diego, CA, November 5-8, 2001. Results will also be presented in Korea on November 2, 2001 at the symposium "Advances in drying and storage technology". Our results were reported in the 2001 annual report for the northeast 103 multi-state research project on postharvest physiology of fruits.
The following manuscripts are in progress:
Simpson, T., V. Bikoba and E. J. Mitcham. 2001. Effects of acetaldehyde on fruit quality and target pest mortality for harvested strawberries. In preparation for Postharvest Biology and Technology.
Simpson, T., V. Bikoba and E. J. Mitcham. 2001. Control of western flower thrips and two-spotted spider mites with ethyl formate on harvested strawberries. In preparation for Journal of Economic Entomology.