these flies may attain in some environments. 
Large numbers of sterile flies must some- 
times be produced to adequately overflood the 
wild flies. The hypothetical reduction in fertil- 
ity is seldom, if ever, achieved in laboratory 
cages when untreated flies are overflooded with 
irradiated, sterile flies. We believe such a 
failure is caused by the loss of mating com- 
petitiveness in the treated flies owing to radia- 
tion damage and possibly to behavioral changes 
in the entire group of laboratory-reared in- 
sects. This loss must be compensated for by 
releases of larger numbers of flies, Before 
they attain the ability to mate, many released 
sterile fruit flies die during the week or more 
they must spend seeking proteins having the 
vitamins necessary for sexual maturity. This 
loss, which may be as high as 40 percent, must 
also be compensated for with increased re- 
leases. Fortunately the need for large numbers 
of flies can easily be met. Several species 
of tropical fruit flies can be produced effi- 
ciently and economically at costs as low, or 
lower than $80 per million. 
After releases of more than 20 million 
sterile Mediterranean fruit flies at a semi- 
isolated location in Hawaii by L. F. Steiner 
and associates in the Entomology Research 
Division, infestations in preferred hosts ap- 
peared to be strongly reduced. Releases of 
nearly a half billion sterile oriental fruit flies 
on the 30-square-mile island of Rota, located 
north of Guam in the Mariana Islands, at rates 
as high as nearly 10 million per week, failed 
to exercise control over the wild fly popula- 
tion. However, the target overflooding objective 
of 10 to 1, as established by cage studies, was 
not achieved, Therefore this experiment was 
not considered to be a test of the sterile 
release method but rather one of fly produc- 
tion, logistics, distribution, and sterile fly 
behavior. Unexpectedly large numbers of flies 
developing in breadfruit, previously considered 
to be a marginal host in Hawaii, was a com- 
plicating factor. To adequately overflood popu- 
lations of the oriental fruit fly, we may have 
to achieve an initial ratio of sterile to fertile 
flies of 25-50 to 1. 
The melon fly was the first tropical fruit 
fly to be eradicated on an isolated island, 
again Rota. Experiences with the oriental fruit 
fly on Rota had taught us much, especially 
99 
that scarcity in numbers of the wild population 
is a precious attribute. In further eradication 
efforts, we should take advantage of this con- 
dition whether it occurs naturally or is in- 
duced by man. With this knowledge of the 
importance of low-population density in the 
application of the sterile insect release method, 
a limited amount of insecticide bait spray was 
applied in the vicinity of farm plots where the 
preferred host plants of the melon fly were 
growing. This removed an estimated 75-90 
percent of all adult flies on the island. The 
Hawaii fruit fly laboratory then began to re- 
lease about 10 million sterile flies per week 
and continued the releases for an extended 
period, quickly achieving strong and adequate 
overflooding of the wild flies by sterile flies. 
Infestation in preferred host plants disappeared 
within 4 months after releases began. The 
overpowering effect of a heavy blanket of 
sterility on a wild population again had been 
demonstrated. 
A typhoon in 1963 left a path of devastation 
when it passed through Guam. Among the 
casualties were many favored oriental fruit 
fly host plants. With this help from nature, 
the numbers of wild oriental fruit flies were 
greatly reduced and releases of sterile flies 
in January 1964, again by L., F. Steiner and 
associates, quickly eliminated the remaining 
normal flies from all of Guam. In this experi- 
ment, owing to the low natural population, the 
release of a few million sterile flies each week 
resulted in a ratio of perhaps 100 sterile flies 
to 1 fertile wild fly and quickly eliminated the 
insect from the island. 
An interesting and important use of sterile 
males is their distribution to provide quaran- 
tine barriers against insect invasion and spread 
of established infestations. Occasionally some 
insects are able to avoid detection at ports 
of entry in spite of all precautionary measures 
taken to protect against them, If the new insect 
is allowed to breed unmolested, infestations 
may quickly attain such proportions that profit- 
able production of the crop infested will soon 
be impossible. Eradication programs to elimi- 
nate or contain incipient infestations usually 
have been based on the application of insecti- 
cides, the only means heretofore available. 
The dosage of insecticides required and the 
overall cost of control are essentially the same 
