Parasitology Today, vot 2, no. 4, 1985 125

Larry K, ncka and Geoffrey Thorpe are at the Wolfson Research Laboratories, Queen Elizabeth Medical Centre, Birmingham B IS 2TH, UK.

I Kricka, LJ. and Carter, T.J.N (1982) Chnical and Biochemical Luminescence, Marcel Dekker, New York

2 Kricka, L.J. et al. (I 984) Analytical Apphcat/ons of Bioluminescence and Chemiluminescence,

Academic Press, New York 3 Kricka, LJ. and Thorpe, G.H.G. (1983)Analyst

108, 1274-1296 4 Weeks, L et o/. (I 983) Chn. Chem. 29, 1474-1479 5 Jablonski, E. (1985) Anal. Biochem. 148, 199 206 6 Kricka, L.J.(1985)lugandBinderAssays:Labelsand

Analytical Strategtes, Marcel Dekker, New York 7 Thorpe, G.H.G. et al. (1985) Clin. Chem 31,

1335-1341 8 Carter, T j .N et o/. (1982) European Patent Pubh-

cat/on 87959 9 Kricka, LJ., Thorpe, G.H.G, and Whitehead, T.P.

(I 983) European Patent Publication I 16454 I 0 Wang, H.X. et at (1985) Steroids 44, 317-328 II Bunce, R.A. etal. (1985)Analyst II0, 657~o63 12 Whitehead, T.P. et al. (1984) European Patent

Publication I 10610 13 Kricka, L.J. and Thorpe, G.H.G. Methods Enzymol.

(in press) 14 Nickless, G.G. et al.j. Wol. Methods (in Dress)

Tsetse Control by Autosterilization

P.A, Langley and M.J.R. Hall

Control or eradication of tsetse flies ns seen by many as the only long term means of combating trypanosomiasis ~n Africa. In the past, tsetse control has been achieved by bush clearance and selective game- culling, but such techniques are no longer favoured because of their enwronmental damage. Modern methods of aerial and ground spraying of ~nsecticides are designed to minimize environmental pol- lutton, but are costly and cannot always be maintained during different seasons of the year. Our ultimate a~m must therefore be to produce highly selective insect~cidal techniques which are safe and can be ~mplemented by the local people at reasonable cost.

Traps, initially designed for sampling tsetse fly populations, have now been developed as effective means of catching out fly populations for control or even eradicatlonL A combination of visual and

olfactory sttmuh can lure flies to a trap or to a s~mple target baited with a persistant ~nsecticide. Low-cost traps can be pro- duced from local materials and installed by local communities ~n areas where they know the fl~es are active 2.

Steri l izat ion versus Kil l in~

Theoretical considerations suggest that sterilization of a proportion of tsetse will be more effective at population reduction than simply kilhng the same number of flies 3. Fig. I. illustrates this approach by computer simulation of a fly population which is doubhng every 105 days, a rate which ~s close to the best that is achieved under field conditions 4. In th~s case the only strategy which leads to a population reduction is the sterilization rather than the killing of I% per day of the flies in the population. Sterilization would also be advantageous because sterile males will

baffle plate

bisazir charged silicone rubber disc with foam plastic spacer above and solid plastic flange below

sterilized fly leaves trap

,J

from sections of y ~ plastic bottles

I I ~ socket o.n apex of ~ ~ r a p netting

fly enters trap trap /

alternative baffle

sterilizing tower made

Fig. 2. Plastic bottle sys~m with Ix~e for delaying exit of tsetse flies entering from trap below in the field. Pos/t/on of praposed disc charged with the chemos~rilant bisazir is indicated.

1.5

1.0

~ 0 . 5 g

A /

/ / c

o / /

j .

~ • • • j l l "O/

e~ - • / "11

2 4 6 8 10 12

months

Fig. I. Computer simulation of changes in numbers of tsetse in a population that is doubling each generation (ie. approximately every 105 days): trapping intensity is I % per day: trapped flies are either killed, or s~rilized and released. (o----o, no control; • - - - • , sterilize males only; Q---Q, kill males and females; B----m, sterilize males and female~ )

have an effect beyond the range of attrac- tion of the sterilizer, thus rendering less important the regular spacing of the sterilizing units in the enwronment~ This attribute will be particularly useful in ter- rain which is not easily accessible for the regular placement of targets or traps, such as in gullies or on escarpments.

A n A u t o m a t i c Ster i l izer

Unfortunately the sterilizing devices tested so far have been too complex, expensive and hazardous for ro~ne use 5, The alternative of rearing large numbers of fl~es for sterilization and release is very expensive although it can be effective 6,7. Therefore we consider it something of a breakthrough to find that bisazir*, which has a vapour pressure at 25°C of only 0.005 mm Ug (Ref. 8), will sterilize Glossina morsitans morsitans after only a few minutes exposure to the vapour phase at

~) 1986, Elsevier Science Publishers B.V, Amsterdam 0169~75a/86/$0200

126 Paras/to/ogy Today, vol 2, no. 4, 1986

30°C in an unsealed container consisting of a tower of transparent plastic bottles (Fig. 2). 90% sterility was induced in females after less than 4 minutes exposure, while males required 22 minutes to induce the same degree of sterility. A silicone rubber membrane loaded with bisazir remained active for one month in the laboratory.

Under field conditions, a similar con- tainer with a baffle of small diameter holes or a hollow, black-painted cone (Fig. 2) placed on top of a trap, will delay the pas- sage of flies through the system long enough for a high degree of sterility to be induced if bisazir is present. The sterilizing device is very cheap, although the required traps are more costly to produce than simple targets treated with inse~cidek

Bisazir is a hazardous chemical. Its acute oral LDs0 in rats is 8,5 mg/kg and the acute dermal LDs0 in rabbits is 10,4 mg/kg, Like some other chemosterilants it is probably moderately carcinogenic but as field work on mosquito sterilization in El Salvador indicates, it can be handled safely and

effec~vely (A.B. Borkovec, unpublished). Moreover, bisazir is rapidly destroyed by acid and we envisage that autosterilizers for field use could be made tamper-I~roof by incorporating an acid-based self- destruct system for the bisazir. Replace- ment of the units should not be required more frequently than at monthly intervals, and we hope that this period can be ex- tended following research on controlled release mechanisms for the chemo- sterilant.

At present we can make no compari- sons between the cost of targets which kill and traps which sterilize. However, the efficiency of tsetse traps is continually being improved, both in visual design9, ~0 and in the development of natural and synthetic olfactory attractants ~ ~,~2. Recent research shows that there are interactions between visual and olfactory stimuli in determining the efficiency of a trap for tsetse 13. For the future therefore, there is every likelihood that a low cost means of autosterilizing tsetse can be developed which will be highly selective in terms of target species

and which will be of practical value in the reduction or elimination of tsetse popula- tions. *P,P- bis(aziridinyl)-N -methylphosphinothioic amide kindly supplied by Dr A.B. Borkovec, USDA, Beltsville, Maryland, USA.

Mention of a commercial or proprietary product in this report does not constitute an endorsement of this product by the USDA.

I Vale, G.A. et at. (I 985) Porositol. Today I, 106- II0

20koth, J. (1986) ParasitoL Today 2, (X)-O0 3 Langley, P.A. and Weidhaas, D. Bull Ent. Res. (in

press) 4 Glasgow, J.P. (I 963) The distnbutson and abund-

ance of tsetse 241 pp. Oxford, Pergamon 5 House, A.P.R (I 982) Bull. Ent. Res. 72, 65-70 6 Politzar, H. et al. (1980) International Congress of

Entomology 16, 173 7 Williamson, D.L. et aL (1983) Bull. Ent Res. 73,

391-404 8 Carlson, D.A. and Bailey, D.L. (1981)]. Agricul.

Food Chem. 29, 78-82 9 Flint, S. (1985) Bu11. Ent. Res. 75, 529-534

10 Green, CH, and Flint, S. Bull. Ent. Res. (in press) I I Vale, G.A. (1982) Bull. Ent. Res. 72, 95-106 12 Hall, D,R. et al. (1984)Insect. So. Appl. 5,335-339 13 Green, C.H. Physiol EntomoL (in press)