The effect of sprays on the fauna of apple trees. V. DDT/BHC and lead arsenate/nicotine applied at the green cluster stage.
The following is virtually the author's summary of this part of a series on investigations in south-eastern England [cf. RAE A 57 889, etc.]. The effects of single pre-blossom applications of combined sprays of DDT with BHC and of lead arsenate with nicotine on the fauna of apple trees in an orchard that was otherwise neglected were studied from 1959 to 1963. DDT with BHC was toxic to populations of Psylla mali[Cacopsylla mali] (Schmidb.) and gave good control in some years; lead arsenate with nicotine gave consistently poorer control. The populations of the unsprayed plots were effective colonisers of the sprayed plots, especially as reduced predation allowed numbers to increase. Both insecticides were extremely toxic to the oribatid Humerobates rostrolamellatus Grandjean, and the sprayed plots showed increased growth of the alga Pleurococcus sp., on which the mite fed before its virtual extinction. Very few nymphs of the Mirid predator Psallus ambiguus (Fall.) survived spraying with DDT and BHC, but lead arsenate with nicotine was less toxic. Numbers of this species in the orchard as a whole declined steadily from 1959. DDT with BHC was very toxic to the nymphs or, more probably, the eggs of Phytocoris spp. P. tiliae (F.) appeared to be less susceptible than P. reuteri Saund. Lead arsenate with nicotine had no effect on these predators. The effects of DDT with BHC were slight or indeterminate on Rhopalosiphum insertum (Wlk.), Cicadellids, Lepidosaphes ulmi (L.), Operophtera brumata (L.), Argyresthia spp. and Blepharidapterus angulatus (Fall.). Lead arsenate with nicotine also had slight or indeterminate effects. The effects of DDT with BHC on several species of mites of minor importance varied considerably. Populations of Typhlodromus pyri Scheuten were scarcely affected by it. The apparent discrepancy between this and previous reports of the toxìcity of DDT to this predator is discussed. During the study period, populations of Panonychus ulmi (Koch) and Bryobia rubrioculus (Scheuten) increased greatly, it is believed because of the reduction in predator populations by spraying. The results are discussed in relation to integrated control.