Factors affecting the efficacy of methyl bromide fumigation to control Liriomyza trifolii in imported chrysanthemum cuttings

Laboratory investigations were made into the effect of cultivar type, prior cold storage, fumigation temperature and methyl bromide concentration, on the efficacy of a fumigation treatment to control Liriomyza trifolii in chrysanthemum cuttings. The tests related to the standard quarantine treatment used in the UK to control Spodoptera littoralis on imported chrysanthemum cuttings: cold storage for 2 days at 1 – 2°C followed by methyl bromide fumigation at 15°C with a concentration time product (CTP) of 54 g h/m³. L. trifolii larvae, within detached leaves, and 1 – 2 and 2 – 3 day old pupae, were treated. Methyl bromide concentrations of 6·75 or 13·5 g/m³ were used to achieve a range of CTPs and thus obtain accurate dose-response lines and estimates of the LD99 and LD99·9 for each insect stage. Fumigation temperatures were 8, 11 or 15°C. Efficacy of the standard treatments differed between the three cultivars tested, but the LD99 for larvae remained below 54 g h/m³. Decreasing fumigation temperature to 11°C or less increased LD99 values for larvae and pupae and substantially increased variability. There is therefore little scope for using fumigation temperatures of less than 15°C for quarantine purposes. Omitting the cold storage treatment prior to fumigation did not significantly affect efficacy of fumigation. Reducing the methyl bromide concentration from 13·5 to 6·75 g/m³ did not significantly affect the LD99 for larvae but significantly reduced LD99s for pupae.