Integrated Tse-tse Fly Control

R. Cavalloro

• Publisher: CRC Press
• Year: 2020
• eBook ISBN: 9781000162561
• Topic: History
• Subtopic: African History
• Index: History

This book discusses methods which might appropriately be employed under various circumstances. It is concerned with control of insect pests and tse-tse flies, and considers prospects for integrated control of the African trypanosomiases, in which vector control is envisaged as playing a key role.

Session 1
The safety and efficaciousness of chemical control

Chairman: S.Geerts

Is resistance developing in tsetse flies? Susceptibility to two chlorinated hydrocarbon insecticides in sprayed and unsprayed populations of Glossina pallidipes in Kenya

D.A.Turner & T.K.Golder

International Centre of Insect Physiology and Ecology, Nairobi, Kenya

Summary

An investigation into the possible development of insecticide resistance in tsetse flies was carried out by comparing the susceptibility to topical applications of endosulfan and dieldrin between two populations of Glossina pallidipes Austen in Kenya; one from the Lambwe Valley, where there has been a long history of spraying operations, and the other from Nkruman which has never been sprayed. Dosage-mortality regression lines and LD data showed that wild-caught, non-teneral males and females from Lambwe were significantly more tolerant of both insecticides than their counterparts from Nkruman, with the exception of males dosed with dieldrin. The differences were insufficiently great, however, to be incontestable evidence for the development of resistance in the Lambwe strain, although there were no obvious sources of variability between strains adequate to account for the differences in susceptibility.

Introduction

Given the genetic potential, which seems to be a feature of all natural populations, the development of insecticide resistance is primarily a matter of sufficient selection pressure over a sufficient period. Selection pressure is applied through the frequency of application and the dosage of insecticides, and is conditioned by population density and distribution, reproductive capacity and movement capability of the insect. Prom a consideration of some of these factors as they apply to tsetse flies Burnett (9) concluded that resistance might develop but only very rarely. The continuing high levels of kill obtained in control operations would appear to bear this out, and where there have been failures in eradication attempts, these have mostly been accounted for by re-invasion and/or the survival of residual populations resulting from enhanced tolerance of insecticide by pregnant females (1, for review). The fact remains, however, that no resistance studies have ever been carried out, presumably because there has been no compelling reason to suspect it.

The subject has not been altogether neglected, nevertheless. In the process of insecticide toxicity trials and studies aimed at providing practical guidelines on dosages and formulations (1, for review), a body of comparative susceptibility data has been acquired which could serve as base-line information for resistance studies. More recently, an investigation by computer simulation predicted that there were circumstances in which high frequencies of resistant genes could be attained in tsetse populations subjected to fairly typical spraying regimes (18). Biochemical studies have also shown that tsetse can metabolise DDT (18) and the dieldrin analogue. HEOM (4), by reductive dechlorination into less toxic forms, indicating a potential for resistance in two insecticides commonly used for tsetse control.

Insofar as insecticidal pressure and population isolation are concerned in promoting resistance, the influence of these factors has been expressed, perhaps more intensely than elsewhere, on a population of Glossina pallidipes Austen inhabitating the Lambwe Valley in western Kenya. This population was therefore considered to be a prime candidate for an investigation of possible development of resistance, and the justification for this is enlarged upon in the following description of the history of spraying operations and other relevant factors. An additional justification was the suspicion of control workers that one of the insecticides being used in the Lambwe Valley, namely dieldrin, was giving less than the desired effect, though a number of other factors could also be responsible.

As tsetse fly-belts go, the G. pallidipes infestation in the Lambwe Valley is small in area (approx. 300 km²) and isolated (the nearest other infestation being 70 km away), but in parts contains one of the densest fly populations ever recorded (2). Whereas livestock trypanosomiases have always been present, Rhodesian sleeping sickness also became endemic in the area in the early 1960’s, and from 1968 to the present, various measures, mainly insecticidal, have been carried out, initially to contain the diseases and later to eliminate them through vector eradication. During the late sixties and early seventies tsetse control was aimed at confining the fly to its principal habitat of thicket within the Lambwe Valley Game Reserve in the southern half of the valley, by clearance of lowland thicket in adjacent settlement areas and ground spraying in the form of a series of four monthly applications of residual dieldrin (Dieldrex 15T 1.8% e.c.) to thickets on hillsides flanking the valley. Also in the period 1968-1971 a series of five aerial spray trials, employing first helicopters then fixed-wing aircraft, were conducted in the valley thicket (17, 2). This thicket consists of fairly discrete large blocks, and each was either blanked sprayed or strip sprayed on one occasion or another with residual deposits of dieldrin invert emulsion, at dosages which varied between 54 and 200 g a. i./ha. Post-spray monitoring recorded reductions in the G. pallidipes population of 95-99.9%, which were maintained for six to eight months afterwards.

Soon after the completion of the aerial spray trials the indications were that the tsetse population was rapidly recovering (3). Because the disease had been rendered quiescent, however, tsetse control measures were sporadic thereafter, and aimed at preventing re-infestation of scrub and thicket in settlement areas, chiefly by ground applications of residual dieldrin. These measures seem to have been discontinued altogether in the late seventies.

A resurgence of the disease occurred in 1980. In response, the authorities, in 1981, undertook to eradicate the fly from the whole valley and adjacent hillsides, by a programme of sequential aerial applications of endosulfan aerosol spray, supplemented by ground application of dieldrin 1.8% e.c. and limited bush clearance in areas of difficult terrain for aircraft. Despite nine aerial sprays at 12-day intervals of relatively heavy dosages of insecticide (19-38 g a.i./ha.), eradication was not achieved, but the population was reduced by over 99.9% by the end of operations in May, 1981. A model of spray effectiveness suggested, however, that this final reduction was the cumulative effect of only about a 90% kill per spray application, and that several thousand flies possibly remained after spraying. Post-spray monitoring later showed that the population in...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Foreword
• Table of Contents
• Opening session
• Session 1. The safety and efficaciousness of chemical control
• Session 2. Biological, biotechnical and other control methods
• Session 3. Present and future programmes of international and national organizations
• Session 4. General aspects on Glossina
• Closing session
• List of participants
• List of authors