THE CITRUS WHITE FLY: FOOD PLANTS. 33 
nearly immune than any other of those commonly grown. This was 
first pointed out by Mr. F. D. Waite, of Palmetto, Fla. In this con- 
nection it should be noted that the Koyal variety in its general 
characteristics is not a typical grapefruit. 
The reason for the partial immunity of grapefruit trees to white-fly 
injury is as yet obscure. Several observations on grapefruit and 
orange trees growing side by side give no basis for the supposition 
that it is a matter of food-plant preferences of the adult flies. In 
some cases the differences in the amount of new growth must be 
taken into consideration. Counts of adults, pupa cases, and hatched 
eggs of the citrus white fly on alternating grapefruit and orange 
trees, six in all, located on the laboratory grounds at Orlando, were 
made on June 4, 1909, when no new growth was present on the trees. 
The leaves were selected at random and, with the exception of a few 
upon which adults were counted, they represented the spring growth 
of 1909. The difference between the number of the adults on 500 
grapefruit and 500 orange leaves, 87 and 104, respectively, is not as 
great as would be expected, considering the much greater number 
of insects that had matured on the orange up to the time of the exam- 
ination. There were about twenty times as many pupa cases on 
the 100 orange leaves as on the 100 grapefruit leaves, or 6 and 120, 
respectively. This was offset by the presence of about three times 
as many live pupae on 10 grapefruit as on 10 orange leaves, 41 and 14, 
respectively, making the sum of the pupa cases and live pupae 4.16 
per leaf in the case of the grapefruit and 2.6 per leaf in the case of the 
orange. This is about the same proportion as the number of hatched 
eggs on the two food plants. The condition of the leaves, as shown 
by this data, fails to indicate any cause for the partial immunity of 
grapefruit trees. 
The examinations by Mr. W. W. Yothers of two leaves picked at 
random from each tree in a small isolated grove consisting of 41 
grapefruit and 28 tangerine trees gave rather striking figures, show- 
ing more rapid multiplication of the citrus white fly on the latter 
than on the former. The first examination was made on November 
4, 1908, and the second on June 8, 1909. On the former date the 
average number of live and dead white-fly larvae and pupae per leaf 
was 31.9 on the grapefruit and 96.2 on the tangerine, 16.6 and 80.9, 
respectively, being alive. During the winter a series of fumigating 
experiments reduced the numbers of the white fly so that at the 
second examination the number per leaf was 1.1 on the grapefruit 
and 2.25 on the tangerine. The arrangement of the two kinds of 
trees in the grove was such that they had equal chances of becoming 
reinf ested by the insects which escaped the effects of the experimental 
tests. 
59131°— Bull. 92—12 3 
