whether the natural population of an insect is 
high or low. But this rule does not apply when 
sterile insects are used, 
Each year Mexican fruit flies that develop 
in mangoes, and perhaps in other fruits trans- 
ported from interior Mexico into the Tijuana 
area, have threatened subtropical fruit pro- 
duction in southern California. Cooperative 
programs conducted by the U.S. Department 
of Agriculture and Defensa Agricola in Mexico, 
in which protein hydrolysate-malathion bait 
sprays have been utilized, have been carried on 
to eliminate the newly emerged flies before 
they could make their way into California and 
become established there. A 3-mile protection 
zone along the border in California was also 
sprayed by the California Department of Agri- 
culture each year to destroy migrating flies 
that crossed the border. The spray programs 
were costly and a public nuisance, even though 
the number of insects present each year prob- 
ably did not exceed more than a few thousand, 
In Tijuana, often the only access to backyard 
patios where host plants to be sprayed are 
located is through the houses of property 
owners. How much longer the spray program 
in Mexico and California might have been 
carried on without serious objection by the 
cooperating public already had become a moot 
question. 
An easier way than applying insecticides 
to take care of the fruit fly problemin Tijuana 
and southern California began to unfold in 
Mexico several years ago. At the Entomology 
Research Division's laboratory in Mexico City, 
studies conducted by R. H. Rhode showed that 
the Mexican fruit fly could be sterilized with 
practical dosages of radiation, without serious 
behavior breakdown. Releases of sterile flies 
in several small-scale pilot tests at semi- 
isolated locations appeared to suppress popu- 
lations or inhibit normal increases in in- 
festations. The first few tests conducted by 
Rhode suggested strongly that the sterile re- 
lease method might be completely effective if 
applied to entire populations, especially where 
a higher ratio of sterile to wild flies could be 
maintained. 
In the meantime, J. G, Shaw, also of the 
Mexico City laboratory, was evaluating chemo- 
sterilants as possible means of sterilizing 
Mexican fruit flies. In tests with several 
dozen candidate materials, one known as tepa 
100 
(tris(1-aziridinyl)phosphine oxide) was effec- 
tive. Shaw and his associate, M. Sanchez- 
Riviello, then developed a highly efficient 
pupal dip technique for sterilizing the Mexican 
fruit fly. The pupae were dipped in a5 percent 
solution of the chemosterilant. Apparently as 
the adult Mexican fruit flies emerged, they 
received a sterilizing dose by coming in con- 
tact with residues of tepa on the outside of 
their pupal cases. The dip method appeared 
to have an advantage over the radiation method 
of sterilization because the males sterilized 
with chemicals were more normal in mating 
competitiveness than males sterilized by 
radiation of pupae. 
With the concurrence of the Dirrecion 
General de Sanidad Vegetal in Mexico, a test 
of the efficiency of chemosterilized Mexican 
fruit flies was arranged in a semiisolated 
mango grove at Bebedero in Morelos State. 
The results were phenomenal. Strong over- 
flooding was quickly achieved, and no in- 
festation was found in the mangoes until some 
immigrating flies began to appear after most 
of the crop had been harvested, An interesting 
thing about this test was the failure of the 
sterile females, introduced with the males, 
to damage the fruit. Their efforts to insert 
ovipositers were largely futile, and the man- 
goes for the most part were unmarked by 
fruit fly stings. 
The next step in the use of chemosterilized 
Mexican fruit flies was an historical one. For 
the first time man employed sterile insects to 
protect agriculture against invasion by a 
foreign insect pest. Weekly releases of sterile 
Mexican fruit flies were initiated in Tijuana 
and its environs early in 1964. Complete 
dependence was placed on the sterile flies, and 
the spray programs were abandoned. Normal 
flies were caught in traps during the summer 
months of 1964, but the 4 3/4 million sterile 
flies that were liberated did their job well. 
There has been no evidence of establishment 
of the Mexican fruit fly either in Tijuana or 
California. Thus, the use of a caretaker popu- 
lation of sterile insects to prevent establish- 
ment of a foreign insect pest became a practi- 
cal reality in 1964. 
The results of this first year of sterile fly 
releases in the Tijuana area again confirmed 
our strong belief in sterilazation techniques. 
Substitution of a biological procedure for the 
