The effect of sprays on the fauna of apple trees. V. DDT/BHC and lead arsenate/nicotine applied at the green cluster stage.
The effects of single pre-blossom applications of DDT/ BHC and lead arsenate/nicotine on the fauna of apple trees in an orchard which was otherwise neglected were studied from 1959 to 1963. DDT/BHC was toxic to populations of Psylla mali[Cacopsylla mali] and gave good control in some years; lead arsenate/nicotine gave consistently poorer control. The populations of the control plots were effective colonizers of the sprayed plots, especially as reduced prédation allowed numbers to increase. Both insecticides were extremely toxic to Hitmerobates rostrolamellatus, and the sprayed plots showed increased growth of the alga Pleurococcus sp., on which H. rostrolamellatus fed before its virtual extinction on these plots. Very few larvae of Psallus ambiguus survived spraying with DDT/BHC, but lead arsenate/ nicotine was less toxic. Numbers of this species in the orchard as a whole declined steadily from 1959 to 1963. DDT/BHC was very toxic to larvae of Phytocoris spp., or more probably the eggs. P. tiliae appeared to be less susceptible than P. reuteri. Lead arsenate/nicotine had no effect against Phytocoris spp. The effects of DDT/BHC and of lead arsenate/nicotine were slight or indeterminate on Rhopalosiphum insertum, Cicadellidae, Lepidosaphes ulmi, Operophtera brumata, Argyresthia spp. and Blepharidopterus angulatus. The effects of DDT/BHC on several species of mites of minor importance varied considerably. Typhlodromus pyri populations were scarcely affected by DDT/BHC. The apparent discrepancy between this result and previous reports of the toxicity of DDT against T. pyri is discussed. During the period of study populations of Panonychus ulmi and Bryobia rubrioculus increased greatly, probably because of the reduction in predator populations by spraying. [For part IV see H.A., 37: 2684.]-E. Malling Res. Stat., Maidstone, Kent.