The effect of sprays on the fauna of apple trees. II. Some aspects of the interaction between populations of Blepharidopterus angulatus (Fall.) (Heteroptera: Miridae) and its prey, Panonychus ulmi (Koch) (Acarina: Tetranychidae).
These two parts of a series deal with studies in an apple orchard in south-eastern England in 1953-59 on the effect of routine sprays of winter wash (0.1 per cent. DNC in 3 per cent. oil emulsion) followed by lime-sulphur or captan, or of lime-sulphur or captan alone, on populations of Panonychus ulmi (Koch) and its Mirid predator Blepharidopterus angulatus (Fall). The following paragraphs are based almost entirely on the author's summaries, respectively.
During 1953-59, there were two marked fluctuations in the populations of P. ulmi and of B. angulatus, the only predator present in numbers. The highest populations of the mite occurred in 1956 and 1958, while those of B. angulatus occurred in 1957 and 1959 and coincided in both years with a drastic reduction in the numbers of mites. Although both winter wash and lime-sulphur had some acaricidal effect, the full extent of the changes in population was largely determined by the effects of treatment on the predator. Populations of P. ulmi were inversely proportional to those of B. angulatus, which were generally highest on plots treated with captan and lowest where winter wash was followed by lime-sulphur [cf. R.A.E., A 42 190; 50 672]. The number of mites and eggs per leaf recorded at the peak of the second cycle for the treatments with the least and greatest effects on B. angulatus were 20 (captan), 85 (lime-sulphur), 220 (winter wash followed by captan) and 220 (winter wash followed by lime-sulphur). B. angulatus was observed also to feed on Cicadellids, which were less numerous in 1959, when the predator was abundant. Mites of the genus Typhlodromus were observed only on the plots treated with captan; they were scarcer in years in which B. angulatus was abundant. It is concluded that, under certain conditions of orchard management, there is a fluctuating equilibrium between populations of P. ulmi and B. angulatus. Adequate control of the mite has been achieved if its numbers do not exceed the limits of tolerance for the apple variety concerned. Spray programmes harmful to the predator are likely to disturb the mite-predator equilibrium.
The interaction between populations of the predator and its prey following spray applications were studied in 1956, 1958 and 1959. P. ulmi hatched 4-5 weeks before B. angulatus, and, in years in which the latter severely checked the mite, predation was effective at the beginning of the second mite generation. In 1956, after treatment with winter wash followed by either lime-sulphur or captan, the combined numbers of mites and eggs increased at similar rates before B. angulatus hatched, but declined rapidly when the predator appeared, the rates decrease being proportional to the numbers of the predator. There was no evidence of effective predation by any other species before B. angulatus hatched. In 1958, when there were more than 30 mites and eggs per leaf, there was a clear relation between the rate of mite increase and the mean density of B. angulatus. The rate of increase of the mites in the absence of predation was calculated as 5.3 per cent. per day, and the mean number of predators required to stabilise the mite population was 57 per tree. From estimates of total numbers of mites and predators per tree, it was shown that one predator could stabilise a population of approximately 2, 000 mites and eggs, of which 40 per cent. were mites. The numbers of mites decreased where the predator: prey ratio was greater than this and increased where it was lower. The egg density of B. angulatus was low following seasons when the mite was scarce owing to heavy predation and higher when mite density increased. Populations of adult females of B. angulatus on trees sprayed with captan or with winter wash followed by lime-sulphur, which were studied by the capture-recapture method [cf. 46 249] in 1956, had similar daily rates of wastage, by death and emigration, and dilution, by newly emerged adults and immigration, and populations of predator and mite declined at the same rate, though they differed in size. The rate of decline of female populations was more rapid in years in which mites were scarce. There was evidence of some immigration by adults of B. angulatus in 1958, when mite numbers were high. In 1959, there was a movement of adults from areas of low to areas of high mite density.