17 months, 3½ billion artificially reared, sterilized flies had been released over Florida and
sections of Georgia and Alabama. The last-known animal wound infes tation that could be
attributed to s crewworms occurre d in February 1959. In the next few weeks s everal adults
were taken in traps. Thereafter no trace of the screwworm could be dis covere d. Its extinction in
the Southeas t had been accomplis hed—a triumphant de mons tration of the worth of s cientific
creativity, aided by thorough bas ic res earch, pers is tence, and determination.
Now a quarantine barrier in Mississippi seeks to prevent the re-entrance of the screw-wo rm
from the S outhwes t, where it is firmly entrenched. Eradication there would be a formidable
unde rtaking, cons idering the vas t areas involved and the probability of re-invasion from
Mexico. Neve rtheles s , the s takes are high and the thinking in the De partment s eems to be that
s ome s ort of program, des igned at leas t to hold the s crew-worm populations at very low levels,
may s oon be atte mpte d in Texas and other infes ted areas of the Southwes t....
The brilliant s uccess of the s crew-worm campaign has s timulated tre mendous inte res t in
applying the s ame methods to other insects. Not all, of course, are suitable subjects for this
technique, much depe nding on details of the life his tory, population dens ity, and reactions to
radiation. Experime nts have been undertaken by the Britis h in the hope that the method could
be us ed agains t the ts ets e fly in Rhodesia. This insect infests about a third of Africa, posing a
menace to human health and preve nting the keeping of livestock in an area of some 4½ million
s quare miles of wooded gras s lands. The habits of the ts ets e differ cons iderably from thos e of
the screw-worm fly, and although it can be sterilized by radiation some technical difficulties
remain to be worked out before the method can be applied.
The British have already tested a large number of other species for susceptibility to radiation.
United States s cientis ts have had s ome encouraging early res ults with the melon fly and the
oriental and Mediterranean fruit flies in laboratory tests in Hawaii and field tes ts on the re mote
is land of Rota. The corn bore r and the s ugarcane borer are als o being tes ted. There are
possibilities, too, that insects of medical importance might be controlled by sterilization. A
Chilean scientist has pointed out that malaria-carrying mos quitoes pers is t in his country in s pite
of insecticide treatment; the release of sterile males might then provide the final blow needed
to eliminate this population.
The obvious difficulties of sterilizing by radiation have led to search for an easier method of
accomplishing similar res ults , and there is now a s trongly running tide of interes t in chemical
sterilants. Scientists at the Department of Agriculture laboratory in Orlando, Florida, are now
sterilizing the housefly in laborato ry experi ments and even in some field trials, using chemicals
incorporated in suitable foods. In a test on an island in the Florida Keys in 1961, a population of
flies was nearly wiped out within a period of only five weeks. Repopulation of cours e followed
fro m nearby is lands , but as a pilot project the tes t was s uccess ful. The Department’s
excitement about the promis e of this method is easily unders tood. In the firs t place, as we have
s een, the hous efly has now become virtually uncontrollable by ins ecticides. A completely new
method of control is undoubtedly needed. One of the problems of sterilization by radiation is
that this requires not only artificial rearing but the release of sterile males in larger number
than are pres ent in the wild population. This could be done with the s crew- worm, which is
actually not an abundant ins ect. With the hous efly, however, more than doubling the
population through releas es could be highly objectionable, even though the increas e would be
only temporary. A chemical sterilant, on the other hand, coul d be combined with a bait
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