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9. Mosquitoes. Wet pits encourage mosquito breeding, although in
Zimbabwe this is not generally a severe problem. The ventilation system of
the VIP latrine reduces mosquito breeding but not to the extent that fly
breeding is reduced. Covering the surface water in wet pits with polystyren:7
balls has been found to be an effective mosquito control strategy L/. This
work has been recently confirmed in Zimbabwe, where 1 kg of 4-6 mm diameter
2
polystyrene balls added to wet pits of 1.76 m cross sectianal area achieved
substantial mosquito control 2/; however the long ter- efficacy and
practicality of this method of mosquito control and its effect on sludge
accumulation rates in pits 'subject to seasonally variable groundwater levels
remain to be determined. Recent work in Tanzania 3/ siggescs that mosquito
control can also be achieved by placing a suitably designed trap over the
squatting plate hole; such a strategy is necessary since mosquitoes are not so
phototropic as flies and so may emerge through the squat hole, especially in
the evenings 4/. Further research is underway on oLosquito control in wet pits.
10. The vent pipe thus performs three vital functions: it eliminates
odors in the superstructure, prevents most flies from entering the pit and
.. traps newly emergent adults. It is important that air circulation through the
latrine is not impeded in any way, for example by placing a cover over the
squat hole. Such covers used to be recommended to control flies, but in VIP
latrines they are not only unnecessary but also detrimental and their use
should be discouraged 5/.
1/ P. Reiter (1978). Expanded polystyrene balls: an idea for mosquito
control. Annals of Tropical Medicine and Parasitology, 72(6), 595-596.
Experimental results are given in Annex III.
3/ C.F. Curtis (1981). Insect traps for pit latrines. Mosquito News,
40(4), 626-628,
S4/ Recent work in Botswana and Tanzania has shown that approximately
two-thirds of emerging mosquitoes try to leave via the vent pipe and one-third
leave via the squat hole (C.F. Curtis and P.M. Hawkins, "Entomological studies
of on-site sanitation systems in Tanzania and Botswana," Transactions of the
Royal Society of Tropical Medicine and Hygiene, 76(1), 99-108; 1982).
5/ An exception to this rule may be in areas where it is culturally
unacceptable to have a dark superstructure interior and therefore a squat hole
cover is needed to reiduce the amount of light entering the pit from the
superstructure. Research is required to compa.e the effect on fly control of
having no cover (thus maximizing air flow but permitting light to enter
through the squat hole) as against having a cover (thus impeding air flow but
restricting the light which is needel to encourage young flies to try to exit
up the-vent pipe). If a cover is used, it should be raised from the slab so
that air cirdulation is not unduly inhibited.