Shot-hole borer (Xyleborus fornicatusEichh.) of tea in Ceylon. I.—Chemical control and population dynamics

Abstract

In 1960–63, fifteen field trials were carried out on tea estates in Ceylon to find whether dieldrin at 1–5 or 3 Ib. per acre applied as a spray to the tea bushes immediately after pruning would afford control of the shot-hole borer,Xyleborus fornicatusEichh. Koutine sampling by a method that is described was carried out for three years after spraying, and fluctuations in the numbers of live stages and galleries of the Scolytid were recorded.In unsprayed plots, the beetle population left in the old wood of the tea bushes after pruning at first declined, but began to rise about nine months later. Sampling was then continued in the new wood that grew after pruning. In this, the numbers of galleries and live stages increased to a major peak late in the second or early in the third year and then declined; there was a smaller increase later in the third year in several of the trials. By comparison, dieldrin at 1·5 lb. per acre greatly delayed the build-up of attack and effected an average reduction in gallery numbers of 81 per cent, by the eighteenth month and 70 per cent, by the twenty-fourth month. The main peak of attack was delayed until the early part of the third year, and the maximum level averaged only about half of that on unsprayed plots; but control in the third year was less satisfactory and more variable than in the second year. Spraying with dieldrin at 3 lb. per acre did not give better control, and no difference in effectiveness was found between 1·5 lb. applied as a high-volume (90 gal. per acre) and a low-volume (8 gal.) spray.Special sampling in two trials revealed that reinfestation of sprayed plots began at edges adjacent to infested unsprayed plots and continued progressively inwards. It is concluded that the duration of control from spraying will be influenced by the size of the area sprayed and the proximity of infested tea fields, and that on estates, where fields of 20–40 acres would be sprayed, the level of control will be at least as good as that observed in the trial plots of 5–10 acres.The main drawback to the use of dieldrin is that it induces outbreaks of tea tortrix,Homona coffearia(Nietn.), by killing off the parasiteMacrocentrus homonaeNixon. The possible use of less persistent insecticides is discussed, together with the timing of chemical control measures in relation to cultural practices.The numbers of open galleries (entrances visible) were closely correlated with those of live stages of the beetle up to the time of peak attack and less closely afterwards, probably because the entrance holes healed up at varying rates. The proportion of immature stages (eggs, larvae and pupae) in the population fell from the first to the third year; the fall was most rapid in unsprayed plots after the second-year peak of attack. One-tenth of adult beetles counted were males, but the true proportion in the broods was estimated to be between one-third and one-fifth. The frequency distribution of gallery numbers showed that invasion of new wood starts at random and becomes increasingly over-dispersed (more uniform) as the density rises. This would be expected if there is a maximum density of galleries that is much the same in all the tea bushes. There was some evidence for a density-dependent regulation of borer numbers brought about by intra-specific competition for wood suited to gallery formation and brood-rearing. Other possible factors influencing density are discussed.