The Duration of Efficacy of the Insecticide Dieldrin Against the Chigger Vectors of Scrub Typhus in Malaya 1

Abstract

Dieldrin, when applied to the surface of the ground and to low-lying vegetation by means of a high pressure spray or Swing-fog apparatus at the rate of 2.5 pounds to the acre, effectively controlled the chigger vectors of scrub typhus, Trombicula (Leptotrombidium) deliensis Watch and T. (L.) akamushi Brumpt, for at least 26 months. In experiments in a field of lallang (kunai grass), Imperata cylindrica , at Subang, Selangor, Malaya, the Chigger Index or average number or chiggers per wild-caught rat, Rattus rattus jalorensis (Bonhote), before application of insecticide, was more than 360 per rat, and 10 days after treatment the index was 9 in the sprayed area and 13 in the Swing-fogged area, respectively. The index ranged from 2 to 12 in the sprayed plots and from 4 to 25 in the Swing-fogged plots during checks made over a period of 52 weeks, while the mites remained prevalent in the control area. At the termination of the experiment at 113 weeks, the reduction in the numbers of chiggers on rats in the treated areas as compared with the controls was 92% or greater. The actual duration of efficacy has not been determined. The use of dieldrin for disinfestation of terrain in hyperendemic areas of scrub typhus is recommended where relatively large numbers of non-immunes lire a risk, as in military camps, rubber plantations and the like. A severe grass fire drastically reduced the numbers of chiggers, found on rats, but the effect was only temporary, and in 2 or 3 months the chigger index was as high as ever. The fire did not appreciably affect the rate at which live rats were trapped even 1 week after the grass was burned. The lallang although completely burned over, started to grow rapidly from unaffected rhizomes within a few days of the fire. Shrubs were apparently killed, and were replaced by lallang.