Abstract

There are no selective biocides available for unwanted populations of signal crayfish (Pacifastacus leniusculus, Dana), so any use will be limited to sites at which non-selective methods are acceptable. This outdoor tank trial was carried out to test chemicals that have no toxic residues after the treatment, in conditions as close as possible to field conditions. The target was 100% mortalitywithin 24 hours. Ammonia above 20 mg l^-1 was effective in the conditions here, provided pH was elevated to at least pH 9.5 and preferably to pH 10-12. Chlorine was also effective, but rapid loss in field conditions meant initial concentrations of at least 30 mg l^-1 had to be achieved. Chemical deoxygenation killed crayfish within 24 hours, but only if crayfish were prevented from reaching the surface. After 4 hours in dissolved oxygen at 0.5% saturation crayfish recovered when placed in fresh water. Deoxygenation stimulated crayfish to leave their burrows and may be useful as a pre-treatment. A natural pyrethrum insecticide, ‘Pyblast’, was found to be effective as a rapid toxicant (1-4 hours) at concentrations of 0.05 mg l^-1or more. It was also highly effective as a surface spray on crayfish or bare ground. Its toxicity to crayfish was lost in less than 24 hours in field conditions. Crayfish will leave water to escape adverese conditions, so spraying exposed margins with pyrethrum would prevent escapes during biocide treatment of a waterbody.

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