54 
FLORIDA STATE HORTICULTURAL SOCIETY 
cability of controlling thrips with dust. 
But we have demonstrated that the lime- 
nicotine dust will kill them. 
UNDER WHAT CIRCUMSTANCES WILL IT 
PAY TO SPRAY FOR THRIPS? 
This is a question that we have kept 
constantly before us. As with all spray¬ 
ing, there are several factors involved in 
the answer to this question. At least five 
are of prime importance. 
(1) 'Abundance of Thrips. —How 
many thrips per bloom before spraying 
will pay? We have heretofore set this at 
about two dozen, and now see no reason 
to change it; i. e., under average condi¬ 
tions as to other factors, if spraying for 
thrips alone is to pay they should average 
about two dozen per bloom. 
(2) The Amount of Bloom. —When 
the bloom is scanty, spraying for thrips 
becomes correspondingly more important, 
as an effort should be made to save as 
much of the bloom as possible. 
(3) The Variety of Citrus. —A small¬ 
er number of thrips will damage oranges 
than grapefruit. 
(4) Quality of Fruit. —Fruit which is 
fancy in other respects will repay more 
care than a lower grade. If one is raising 
small fruit on white-fly and scale-infested 
trees, and russeted by mites and melanose, 
a few thrip marks in addition will be a 
minor matter. 
(5) Possibility of a Combination 
Spray .—If one is spraying with other in¬ 
secticides at blooming time it will usually 
pay to add the nicotine sulphate, where 
spraying for thrips alone oftentimes 
would not pay. For instance, in one 
grapefruit grove the owner was spraying 
to control citrus scab. There was an av¬ 
erage of only ten thrips per bloom. Yet 
we estimated that the improvement of the 
appearance of the fruit on a block where 
nicotine sulphate was added to the lime- 
sulphur paid twice over for the cost of 
the insecticide; but it would not have paid 
for the entire cost of spraying. We have 
had similar results in groves which were 
being sprayed with bordeaux oil. 
DESTROY WEEDS ABOUT THE GROVE 
As a preventative measure against an 
infestation of thrips, it is important that 
weeds whose blossoms are breeding- 
grounds for thrips, should be kept down 
in and about the citrus grove during win¬ 
ter. Most growers practice clean culture 
in their groves during winter. But some 
low-hammock groves are never cultivated 
and in some groves on drained land there 
are often waste places, especially along 
ditch banks, which grow weeds. Other 
growers are inclined to allow weeds to 
grow under the trees where the cultivator 
does not reach and also in fence corners. 
One of the most common weeds in such 
places is the white-blossomed Spanish 
bayonet, Bidens leucantha (L) Wild. A 
blossom head of this plant will harbor as 
many thrips as a citrus blossom and, when 
allowed to bloom in a citrus grove, is a 
source of continuous infestation to the 
citrus. Indeed, experience during 1921 
demonstrates that it is useless to spray cit¬ 
rus groves for thrips where these weeds 
are allowed to bloom. In one grove ob¬ 
served, these weeds were present only in 
