Autumn applied pyrethroid insecticides : consequences for the natural enemies of cereal aphids

Abstract

The thesis reports the findings of a series of field trials and semi-field experiments examining the effects of single autumn applications of pyrethroid insecticides on the non-target polyphagous predatory fauna associated with the cereal ecosystem. Autumn applications of the pyrethroids cypermethrin (25g a.i./ha) and deltamethrin (6.25g a.i./ha) induced post-treatment reductions in all three of the major polyphagous predatory arthropod groups; the Carabidae, Staphylinidae and Linyphiidae. The organophosphate demeton-S-methyl (243.6g a.i./ha) produced significant reductions in the apparent abundance of a number of species of linyphiid spider but not in members of the Carabidae or Staphylinidae. Populations of carabids exposed to autumn applications of pyrethroids typically underwent 70-80% reductions in apparent abundance with recovery occurring within 1-2 weeks in the case of cypermethrin (2 ha plots) and 3-5 weeks for deltamethrin (1-4 ha plots). Staphylinids appeared to exhibit only limited susceptibility to autumn applications of cypermethrin, deltamethrin and demeton-S-methyl. All three autumn applied aphicides typically produced significant decreases in the effective abundance of linyphiid spiders which persisted until late in the spring of the following year. Cypermethrin also induced significant reductions in the abundance of linyphiid spiders when applied to barriered plots of winter wheat during the summer. The pyrethroid-induced reductions in the abundance of spiders were frequently preceded by bouts of hyperactivity. The relative abundances of the alternative prey groups Acar, Collembola and Diptera exhibited only slight and extremely transient reductions following autumn applications of deltamethrin and DSM. Autumn applications of deltamethrin and DSM did not significantly impair the capacity of the predator complex to restrict the development of summer infestations of cereal aphids. During the summer, higher peak aphid densities and an associated reduction in grain quality were recorded in plots which had not been treated with an autumn aphicide. The thesis also includes sections relating to the sublethal effects of the aphicides on predatory capacity and on carabid fecundity.

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