use of insecticides was an especially pleasing 
justification for our interestinnew approaches 
to insect control. Our belief is that sterile 
insects will have growing usefulness in quaran- 
tine and regulatory programs as time goeson., 
STERILE INSECTS AS BARRIERS 
TO SPREAD OF INFESTATIONS 
I briefly mentioned use of a barrier zone 
of sterile screw-worm males to prevent fertile 
females produced in Mexico from reaching 
Texas. The theory behind establishment of 
this zone is that young flies emerging in the 
zone or entering it would soon mate with 
sterile flies and thus be rendered infertile 
before reaching Texas. The width of the barrier 
needed may be critical. Obviously if a fertile 
screw-worm could fly all the way through 
such a barrier, some infestation would be 
expected. This situation occurred in Texas 
early in the screw-worm control effort. A 
wider barrier the following year greatly re- 
duced the number of fertile female flies that 
crossed over it, 
The barrier concept of using sterile insects 
may also serve other special purposes. Some 
insect pests introduced from abroad are still 
present in limited areas only. A band of sterile 
insects at the periphery of such an infestation 
should be effective in preventing further 
spread, or at least in greatly retarding the 
rate of increase of the infested area. The 
width of the band of sterile insects required 
for this purpose would depend on the flight 
habits of the insect. 
In Panama there is a major new citrus 
development of nearly 10,000 acres. This 
acreage is threatened by the Mediterranean 
fruit fly, which has spread from Costa Rica. 
In eradication campaigns in Florida, a bait- 
spray barrier of a mile or even less, usually 
prevented spread of this fly to uninfested 
areas. A barrier Zone of sterile medflies 1 to 
5 miles wide might be surprisingly efficient 
in holding the Panama infestation in check. 
Undoubtedly some normal medflies would get 
through such a zone before attaining sexual 
maturity. However, many of the sterile flies 
would move with them. The normal flies would 
also be so widely distributed that it might be 
difficult for a fertile male and a fertile female 
101 
to be present in the same area for success- 
ful mating, especially since this tropical fruit 
fly has no strong sex attractant, as many 
insects do. 
Often discussions of the possibility of using 
the sterile male release method end on a 
pessimistic note, especially when the insects 
involved are widely distributed throughout 
extensive areas, or even over entire continents. 
It does not seem necessary to exclude con- 
sideration of an insect pest because of the 
size of the infested area. If all other require- 
ments are favorable, a barrier could be used 
to partition the infested area, then sterile 
insects could be applied piecemeal to the 
infested terrain. We believe that certain in- 
sects may someday be eradicated in this 
manner from whole continents. The now nu- 
merous instances of eradication of isolated 
insect infestations in our country, some of 
these extending over large areas, and the recent 
successes with sterilization methods, are 
monuments to the attainability of eradication. 
Impetus should be given to research that will 
furnish ever new and effective eradication 
procedures that do not create objectionable 
side effects. 
There is now worldwide interest in the use 
of sterile males to contain infestations of or 
eradicate entirely highly damaging insect pests. 
A major project to eradicate the olive fruit 
fly is underway in Greece. For some time 
entomologists have been investigating sterili- 
zation techniques for control or eradiction of 
certain costly forest insects and insects that 
attack animals, tobacco, grain, forage crops, 
vegetables, and fruits. Entomologists in 
Australia have had intensive sterilization re- 
search underway on the Queensland fruit fly. 
Studies thus far have yielded exciting suc- 
cesses, sometimes only a promise of success, 
and sometimes dismal failures. The failures 
need not discourage us. Before attaining suc- 
cess in our fruit fly sterilization studies we 
had to learn many things, sometimes at great 
expense, before various components began to 
fit together and the requirements of success- 
ful usage became apparent. 
I have referred to the expense of our steri- 
lization research. Each insect species must 
be considered separately, since its response 
to sterilization may be different than that of 
another insect. No matter what the cost, and 
