Studies on the ecology and causes of outbreaks of Ectropis bhurmitra Wkr. (Geometridae), the twig caterpillar of tea in Ceylon.
Abstract
The following is substantially the authors' summary. Outbreaks of Ectropis bhurmitra (Wlk.) in Ceylon in 1963-66 developed as a side-effect of dieldrin sprays applied to tea against Xyleborus fornicatus[Euwallacea fornicatus] Eichh. [cf. RAE A 57 1854] during 1961-66. The biology of this Geometrid was studied in the field in 1965-66. The eggs were laid in crevices in the bark of shade trees (Grevillea robusta) and the larvae that hatched mostly reached the tea plant by dropping on threads. The females laid an average of 300 eggs each in several batches averaging 64 eggs each. The distribution of egg-masses on Grevillea was studied, and it was found that about 70% were situated at heights of up to 5 m, predominantly at 1-4 m, from the ground, which is well above the level of the tea. The complete life-cycle occupied 8-9 weeks, so that six generations can occur in a year. Life tables were prepared for the two generations present in November-February and February-March. These showed that most of the population (89.6 and 98.2% in the two generations studied) did not survive beyond the first larval instar. The main loss was caused by wind dispersal of the newly hatched larvae. Other causes of mortality were drowning during rain in the first instar and predadon by birds in all larval instars. Parasitism of the second- and third-instar larvae by Charops sp. appeared to be important. The combined mortality during the second and third larval instars amounted to 91.5 and 67.2% in the two generations studied, and of this 75.8 and 45.8%, respectively, was caused by Charops.