THE CITRUS WHITE FLY: SPREAD. 47 
is almost positive evidence of the presence of the fly in the grove 
during at least the preceding two years unless the infestation is due 
to migrations from China or umbrella trees or from overstocked 
neighboring citrus groves. In such cases infestation may become 
quite general throughout several acres in one season and extensive 
blackening of the foliage may result early in the next season, or in 
about one year after the first introduction. In the case of new infes- 
tation in any locality, however, the beginning doubtless is usually 
the introduction of a few insects by some one of the means herein- 
after discussed. In a mixed grove of tangerine and orange the pest 
is discovered first as a rule on tangerine, and in a grove of seedling 
trees with a few budded trees intermixed usually the latter are first 
discovered to be infested. Many citrus growers who have groves, 
such as those mentioned, and who have watched carefully for the 
appearance of the pest in their groves, have finally found it well 
established on a single tangerine or budded orange tree before any 
evidence of the presence of the insect was observed elsewhere. 
Through the hindrances to successful establishment and the checks 
on multiplication, principally those discussed in connection with 
parthenogenesis and natural mortality, the white fly frequently 
develops so slowly after its first introduction that it may not increase 
to the point where it is usually first observed for three or four years. 
It is a common error to consider that the first discovery of the white 
fly in a grove is an indication of its very recent introduction. This 
may or may not be the case. Usually it is not the case. It should 
be borne in mind in this connection that in the most careful inspec- 
tion, even by a competent entomologist, the failure to discover a 
single specimen of the white fly is not positive proof that it is not 
present. The foregoing generalizations are based upon many 
observations by the agents of this bureau who have been engaged in 
these investigations, more particularly the authors of this bulletin 
and Mr. W. W. Yothers. 
The rapidity of spread into a citrus grove from neighboring infested 
groves is a subject which becomes temporarily important when a 
nonisolated grove becomes infested for the first time. It is a subject 
of more far-reaching importance in connection with fumigation, and it 
is in this connection that the most extensive studies in this line have 
been made. The result of these studies will be published in a final 
report on fumigation. 
The slowness with which the citrus white fly increases in numbers 
and spreads from the first point of infestation has been noted by 
many citrus growers who have been observant enough to discover 
the white fly soon after its introduction into their groves. When 
the rate of spread of the white fly through the grove is affected by 
the presence of migrating adults from China or umbrella trees, the 
