Phosphine and its effect on some common insects in cut flowers

The most effective fumigant for insect disinfestation of cut flowers is currently methyl bromide, which will soon be unavailable in several countries. The toxicity of an alternative fumigant, phosphine (2% PH₃ and 98% N₂), was tested at 24 °C on adult greenhouse thrips (Heliothrips haemorrhoidalis), adult aphids (Myzus persicae) and lightbrown apple moth larvae (LBAM; Epiphyas postvittana). These are commonly found as insect pests on many cut flower crops. Thrips were exposed to phosphine concentrations ranging from 20–600 μl/l for 1 or 2 h. All thrips were killed within 18 h of exposure after a treatment of 300 μl/l phosphine for 2 h. Adult aphids and fifth instar LBAM larvae were more resistant to phosphine, and trials were therefore conducted using higher phosphine concentrations (> 500 μl/l) combined with atmospheric (0.035%) or elevated (33%) CO₂. The most effective treatment for aphids was 1000 μl/l phosphine + 33% CO₂ for 4 h, which killed all insects within 36 h of exposure. Under atmospheric CO₂, levels, 92% of aphids were killed within 36 h after exposure to 1000 μl/l phosphine for 6 h, with 100% kill attained after exposure to 5000–8000 μl/l phosphine for 6 h. Elevated CO₂ levels did not improve the efficacy of phosphine on LBAM larvae. The optimal treatment was 2000–2500 μl/l phosphine for 4 or 6 h, which killed 96 or 100% of the larvae, respectively. Under atmospheric CO₂ levels, 4000 μl/l phosphine killed 74% of LBAM larvae after 4 h, and 94% after 6 h exposure.