# Primary Investigation on the Synergistic Effects of Methyl Bromide and 1 °C Cold Treatment for Two-Spotted Spider Mite (Tetranychus urticae) and the Citrus Mealybug (Planococcus citri)

## TL;DR

This study explores combining methyl bromide with cold treatment to control pests more effectively while reducing chemical use.

## Contribution

The study introduces a synergistic method combining methyl bromide and cold treatment to reduce pesticide dosage.

## Key findings

- Tetranychus urticae nymphs and adults showed synergistic effects with a ratio of 1.4 and 2.6 when combined with cold treatment.
- Planococcus citri required less methyl bromide when followed by cold treatment due to its low cold tolerance.

## Abstract

Methyl bromide has been designated as an ozone depleting substance under the Montreal Protocol since 1987; however, single treatment with methyl bromide has long been used in plant quarantine, and some products still require methyl bromide during plant quarantine. In this study, we focused on reducing the methyl bromide dosage by developing a combined treatment method of methyl bromide and cold treatment. Tetranychus urticae was more tolerant of low temperature than Planococcus citri, and the egg stage of T. urticae was the least tolerant of both methyl bromide and cold treatment. Compared with T. urticae, P. citri was more tolerant of methyl bromide, but all growth stages of P. citri showed very weak tolerance to cold treatment. The synergistic effect ratios for nymphs and adults of T. urticae were 1.4 and 2.6, respectively, while P. citri showed higher values. These results demonstrate the potential for combined application in the control of pests. The combination of methyl bromide and cold treatment is expected to be applicable to various pests and crops, and further studies on other pests are needed to reduce methyl bromide usage.

Since the use of methyl bromide has been prohibited globally because of environmental concerns, several alternative fumigants have been newly developed and applied to fresh products. However, single treatment with a methyl bromide alternative fumigant cannot completely replace methyl bromide treatment for some products because of issues associated with long treatment times and phytotoxicity. In this study, we compared the mortality of two agricultural pests, Tetranuchus urticae and Planococcus citri, after single treatment with methyl bromide and combined application of methyl bromide and cold treatment to confirm the synergistic effects of chemical and physical treatments. The combined application of methyl bromide and cold treatment was effective against the nymph and adult stages of T. urticae, but no synergism was observed at the egg stage. For P. citri, the required dosage of methyl bromide decreased when methyl bromide treatment was followed by low temperature, possibly because of the susceptibility of P. citri to cold treatment. These results indicate that the synergism of fumigants with cold treatment can differ by pest species and growth stage, and further studies on other pests are needed to reduce methyl bromide usage.