Biology Reference
In-Depth Information
to make a good formulation with relatively little
or no diluent. From a toxicity standpoint, it is
desirable to have a very small particle size, since
immediate toxicity is generally inversely propor-
tional to particle size. There are several important
disadvantages to extremely small particle size:
high wind losses, more or less rapid volatilization
and the prohibitive cost of extremely fine grind-
ing. Also, to obtain better toxicant exposure of
technical concentrates absorbed on a carrier, it is
desirable to have the extender or diluent in as large
a particle size as possible and still give good
dusting characteristics. In a 5 % dust effective
toxicant exposure is obtained with the extender
averaging 10 times the size of the toxicant parti-
cles. At present, particle size specifications are
usually 10 to 30
Dusters
Pesticide dusts are most often made with talcs,
pyrophyllite, clays, calcium carbonate, precipi-
tated hydrated silicates and silicon dioxides, syn-
thetic calcium silicate and diatomaceous earth as
the diluents although finely ground plant material
such as tobacco dust or walnut shell flour is
sometimes used.
In some cases, a solution of the toxicant in a
volatile organic solvent such as acetone or
benzene is mixed with the dust diluents, the sol-
vent allowed to evaporate, and the mixture then
ground. A solution of toxicant may be sprayed on
the dust diluent during mixing and grinding or the
toxicant dissolved in a nonvolatile solvent and
mixed with the diluent. When this is done, care
must be taken to avoid an excess of solvent that
might impair dusting qualities of the finished
product. Many technical pesticides in solid form
lend themselves to direct grinding with a sorptive
clay carrier in adequate milling equipment. Field
strength dusts may be produced by diluting or
cutting down dust concentrates which contain
from10 to 50%a.i. (Dust Bases). Because of their
good dusting properties, attapulgites, diatomite,
talc, pyrophyllite, kaolins, and treated calcium
carbonate are used as diluents to provide the
volume per acre needed to facilitate metering of
the dust through the duster mechanism. Since
many formulations contain more than one a.i.,
dry concentrates must have the proper qualities
m for ground dusters and 20 to
m
40
m for aircraft units. For use in fertilizer mix-
tures, granulated powders of 20 to 80 mesh are
prepared by impregnation of Fuller's earth and
bentonite fractions with the desired toxicants.
Some dusts are sold in a can with a shaker top,
meant to be applied like salt, which is certainly
not going to place a fungicide where it will do the
most good. Some dusts are sold in small card-
board cylinders to be used as dusters, which work
for a little while if the cardboard is well
paraffined to slide easily; but the dust soon gets
damp and clogs. Many more dusts are sold in
plastic containers, with the dust supposedly com-
ing out in clouds as you squeeze, but more often it
doesn't after the first few days. Dusts are tricky to
use because of these disadvantages.
m
