The absorption and storage of insecticide by the in utero larva of the tsetse fly Glossina Pallidipes Aust

Abstract

The present investigation was carried out to determine whether the susceptibility of the pregnant female tsetse fly to DDT and other chlorinated hydrocarbon insecticides could possibly be influenced by the presence in the uterus of the developing larva. Burnett (1962) has shown that pregnant females of Olossina morsitans Westw. exhibit an appreciable increase in tolerance to topically applied DDT compared with males and virgin females, and has suggested the possibility of detoxication of absorbed insecticide by the developing larva (Burnett, 1963). The increased tolerance that he found was greater than that which could be accounted for solely by increase in weight.Owing to the difficulty of obtaining sufficient numbers of laboratory-reared tsetse flies during this work, it was decided to use wild-caught G. pallidipes Aust., since large numbers of this species, containing a proportion of pregnant females, can usually be obtained by netting around a vehicle moving through its habitat. Materials and methodsTest insects. Wild-caught G. pallidipes from the Lake Manyara area of northern Tanganyika, were used for this work, and catching was carried out over a short period of time to minimise the possibility of fluctuations in susceptibility due to seasonal changes. All flies were given a blood meal on return to the laboratory and were used for experiment 24 hours after feeding. Only those females containing a large third-instar larva were used throughout the investigation. The presence of a third-instar larva was revealed by the dark appearance of the two larval respiratory lobes, visible beneath the integument of the female fly, on the ventral surface and close to the anus.Toxicity tests. Dosage/mortality determinations were carried out on 79 males and 93 females by topical application of unlabelled p-p' DDT in acetone solution from a 1 p.1 capacity Drummond " Microcap " pipette. The insects were anaesthetized with CO 2 for treatment and held for 24 hours before mortalities were recorded. The temperature was maintained at 26°C. during treatment and the holding period.Metabolism of DDT and Endosulfan. It was decided to determine first of all whether the increased tolerance of the pregnant females to DDT could be due to the metabolism of the latter to a less toxic substance. Batches of 50 to 100 male and pregnant female flies were treated with 0-2 fig. per fly of dichlorodi(phenyl-C14(U)) trichloroethane * in benzene solution. The labelled compound had a specific activity of 4 mc/mM. At the same time, a similar quantity of insects were treated with -01 ^g. per fly of labelled endosulfan (1,2,3,4,7,7,-hexachlorobicyclo 2,2,1,2-heptene 5,6-bis(hydroxymethylene-C14) sulphite of specific activity of 0-32 mc/mM. The insects were held individually * U-denotes statistically uniform labelling in a carbon-14 compound.

Condition and fate of larvae from insecticide‐treated female tsetse flies Glossina palpalis

“…The present evidence that non-viability of the offspring of mothers treated, especially, with organochlorines was due to diversion of insecticide from the maternal haemolymph to the gut of the avidly feeding in utero larva supports the suggestion of Irving [1968] and is consistent with the find-249 ing of Kwan et al [1982] that tolerance of endosulfan was maximal in flies treated on the penultimate day of each pregnancy.…”

“…Increased insecticide-tolerance by pregnant tsetse, compared with non-pregnant specimens, was shown for several species and insecticides [Burnett, 1962;Hadaway, 1972;Irving, 1968;Kwan et al, 1982] and Irving [1968] suggested that diversion of absorbed insecticide into in utero larvae protected pregnant flies from lethal action. Davies [1978] showed that all flies (Glossina morsitans centralis Machado) surviving endosulfan-spraying in Botswana were pregnant and mentioned the usefulness of knowing the fate of the progeny of these flies.…”

Activity patterns in pregnant tsetse flies, Glossina morsitans

“…Pregnant tsetse have greatly raised tolerance t o topical applications of insecticide, because a large part of the dose is diverted t o the larva in utero (Irving, 1968). This is assumed to be the reason that the only tsetse flies that consistently survive non-residual, aerosol insecticide sprays in the field are pregnant females (Davies, 1978).…”

The use of ageing techniques to evaluate the effects of aerial spraying against Glossina morsitans centralis Machado (Diptera: Glossinidae) in northern Botswana

“…This gives rise to a further problem: the fate of the larvae that they are carrying. It appears that at least part of a pregnant female's increased resistance is due to the ability to excrete insecticide into the 'milk' supplied to the larva (Irving, 1968). It would be useful to know whether larvae can survive this since, if they can, an aerial spraying programme would have to be adapted not only to eliminate survivors but also the offspring of survivors.…”