Some of the terms used to describe the effectiveness and
side effects of spray application include:
- Coverage: is defined here as the extent
to which a pesticidal spray has been distributed on a target surface.
The biological implication is that good coverage increases the probability
that a pest will encounter a pesticide. A "rule of thumb" guide to desirable
levels of coverage is shown below, (but note that this is primarily
based on what is known from application of older chemicals). Because
of the relationship between the diameter and volume of a sphere, there
will theoretically be a cubic increase in numbers of droplets produced,
in relation to their average droplet diameter.

- Retention:
is the amount of spray liquid retained on (mostly the leaves of) crop
plants. In other words, it is the remaining proportion of pesticide
that has not "run off" (usually high volume spraying) or been eroded
by weathering. There is an interaction between formulation effects on
the tenacity of a deposit and the surface of the leaf to which it adheres.
Droplets often bounce on leaves that are waxy (a property that is often
influenced by age) and poor retention may occur with water-based formulations,
especially those with high dynamic surface tensions. On the other hand,
absorption of active ingredient (a.i.) may occur with oil-based formulations.
Leaf exudates (e.g., in apples and broad beans) may also contribute
to the redistribution of a pesticide.
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- Volume
application rate (VAR): is the amount of formulation applied per
hectare. This table gives a general classification of volume application
rates (in l/ha) for field and tree / bush crops (from Matthews, 2000)
|
Field Crops
|
Tree and Bush
Crops
|
High
volume (HV)
|
>600
|
>1000
|
Medium
volume (MV)
|
200-600
|
500-1000
|
Low
volume (LV)
|
50-200
|
200-500
|
Very-low
volume (VLV)
|
5-50
|
50-200
|
Ultra-low
volume (ULV)
|
<5*
|
<50
|
* VARs of
0.25 - 2 l/ha are typical for aerial ULV application to forest
or
migratory pests
|
The next table shows the theoretical coverage on plants if monodispersed
droplets (i.e., all droplets have the same diameter) in these
size classes were to be evenly applied at the lower limits of the ultra-low,
very-low, low, medium, and high VAR ranges for field crops. It assumes
that all droplets were captured and retained, and the leaf area index
is 1.
% Cover (per ha) for lower limits
of VAR classes:
|
|
|
ULV
|
VLV
|
LV
|
MV
|
HV
|
Mono-dispersed Droplet Size
|
Cross sectional area of deposit
(m2)*
|
droplets per m2
(at 1 l/ha)
|
1
|
5
|
50
|
200
|
600
|
10
|
3.1 x 10-10
|
190,985,932
|
6
|
30
|
NR
|
NR
|
NR
|
50
|
7.9 x 10-9
|
1,527,887
|
1.2
|
6
|
60
|
NR
|
NR
|
75
|
1.8 x 10-8
|
452,707
|
0.8
|
4
|
40
|
NR
|
NR
|
100
|
3.1 x 10-8
|
190,986
|
0.6
|
3
|
30
|
120
|
NR
|
150
|
7.1 x 10-8
|
56,588
|
0.4
|
2
|
20
|
80
|
NR
|
300
|
2.8 x 10-7
|
7,074
|
0.2
|
1
|
10
|
40
|
120
|
500
|
7.9 x 10-7
|
1,528
|
NR
|
0.6
|
6
|
24
|
72
|
985
|
3.0 x 10-6
|
200
|
NR
|
0.3
|
3
|
12
|
37
|
*: single droplets, (spread factor
taken as 2)
NR = not realistic spraying scenario (>100% cover represents
coalescence of droplets).
"Per hectare" application often has very little relationship to the
target area to be sprayed: leaf area indices of crops or weeds can range
from fractions (pre-emergent weeds at the cotyledon stage) to >5
(late stage cereal crops). With bush and tree crops, VAR per hectare
is even more inappropriate, and methods such as the unit canopy row
system (UCR) have been developed where sprayer calibration is based
on canopy size (Furness et al., 1998).
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- Droplet size spectra: generally refers
to the measurement of - and statistics for describing - spray droplet
spectra. Parkin (1992) gives a comprehensive review. At least two important
measures of spray distributions are usually required, and the statistics
used here are:
i) size: the volume median diameter (VMD,
D[v,0.5], or MMD - mass median diameter): where half of the volume
of spray contains droplets larger than the VMD (in µm) and the other
half is in smaller droplets. The number median diameter (NMD)
is the drop diameter such that 50% of the drops by number are of smaller
diameter.
The volume average diameter (VAD or D[3,0])
can be a useful single statistic: for describing relatively homogeneous
sprays since it can be used to relate droplet numbers to area dose;
but unfortunately, like the NMD, it is sensitive to errors in measurement
small droplets. In practice, the average droplet volume (ADV): is the
total volume of atomised liquid divided by the total number of droplets,
measured in picolitres (10-12 litres): so VAD is simply the diameter
(in µm) equivalent to the ADV. An approximate value of the VAD can be
obtained by calculating
the square-root of the VMD multiplied by the NMD.
ii) quality: the most reliable statistic is probably relative
span, which is a dimensionless parameter calculated from the
volume distribution only. As with VMD, D[v,0.1] and D[v,0.9] are diameters
representing the points at which 10%, and 90% of the volume of liquid
sprayed is in droplets of smaller diameter; the relative span is defined
as the D[v,0.9] minus the D[v,0.1] and then divided by the VMD (D[v,0.5]).
Some researchers use the VMD / NMD ratio to describe spray
quality, but this is unreliable since number distribution statistics
(including NMD) are seriously affected by the method of spray measurement.

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- Displaying
droplet size spectra
Droplet
size spectra can either be displayed as percentage-in class-distributions
or as cumulative curves (as shown above). The advantage of the latter
is that spectra can most easily be compared - irrespective of the method
of measurement used. The X axis, droplet diameter, is usually transformed
to a logarithmic scale. The cumulative percentage of droplets (Y axis)
is shown as a linear scale, but can be transformed using probit or logit
(more easily calculated) functions. This usually gives approximately
straight-line regressions over the 1-99% range with monomodal spray
size distributions, thus hydraulic nozzle size spectra can be modelled
with a log-normal function.
The slope of this regression line gives a true indication of spray quality.
The spectra of the hydraulic nozzles shown above essentially have similar
relative spans and log-normal regression slopes. Innovatory techniques
such as CDA improve the precision of spray application
and may improve dose transfer efficiency,
but modelling the bi-modal size spectra from rotary atomisers is more
complicated than with hydraulic spectra.
With microbial and other suspended control agents, particle distributions
within droplet size spectra must be considered with respect to droplet
volumes; this is discussed by Bateman (1993) and Chapple et al.
(2000). One method is to display droplet volumes in picolitres with
a juxtaposed X scale. The picolitre scale is identical to the particle
distribution scale when 1 x 10^12 particles/litre
are suspended in the tank mixture; this scale can be shifted right (more
dilute) or left, depending on the particle concentration.
- Spray drift:
With placement (localised) spraying of broad spectrum or toxic chemicals,
wind drift must be minimised, and considerable efforts have been made
recently to quantify and control spray drift from hydraulic nozzles.
On the other hand, wind drift is also an efficient mechanism for moving
droplets of an appropriate size range to their targets over a wide area
with ultra-low volume (ULV) spraying. Himel (1974) made a distinction
between exo-drift (the transfer of spray out of the target area)
and endo-drift, where the a.i. in droplets falls into the target
area, but does not reach the biological target. Endo-drift is volumetrically
more significant and may therefore cause greater ecological contamination
(e.g. where chemical pesticides pollute ground water) and loss
of efficiency.
For most people, spray drift refers to exo-drift: one
of the undesirable aspects of pesticide application that causes contamination
of bystanders, adjacent lands and the environment (see links).
It is caused by the winnowing effect of wind: that is more pronounced
the smaller the droplets concerned (see table below). The displacement
of sprays is in fact highly complex: for a given droplet spectrum, "driftable"
droplets are contained in a number of size classes
- all of which are subject to various rates of evaporation
(which is in turn influenced by formulation). Natural air flows are
also never completely laminar so drift studies are best done empirically
in wind tunnels - or ideally - field tests. Droplets of >100µm
are often described as the "driftable fraction", but as the
table shows, even droplets of this size can drift up to 10m in acceptable
cross winds.
Surface run-off
is another environmental issue, that is linked to endo-drift, and may
account for substantial contamination of water courses, affecting drinking
water quality. Amenity as well agricultural pesticide applications have
been implicated, since run off is especially pronounced with treatments
to hard surfaces, following rain.
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- Survival
of water-based droplets
Small droplets of water (and other evaporative formulations) reduce
rapidly in size, because they have proportionately a larger surface
area than larger droplets. Besides formulation, the rate of evaporation
is determined by ambient temperature and relative humidity (RH) - and
conveniently estimated from delta-T - the difference in temperature
between the wet and dry bulb thermometers. The table below gives the
life time and survival distance from the nozzle of water droplets in
three representative climatic conditions. The method for calculating
RH can be found on Wikipedia.
For example much herbicide spraying in cool temperate zones takes place
in autumn and spring, and droplets are less prone to evaporation: which
has a greater influence on spray drift in summer
conditions.

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-
Controlled
Droplet Application (CDA): is a term probably coined by John Fryer
of the Weed Research Organisation in the UK (G.A. Matthews, pers. comm.).
Bals (1969) stated that "The efficiency of a spraying machine is inversely
proportional to the range of droplets it emits, whilst the suitability
for a specific problem depends on the actual size of the droplets emitted".
No atomiser is commercially available that can produce uniform (monodispersed)
droplets, but rotary (spinning disc and
cage) atomisers usually produce a narrower droplet size spectrum
than conventional hydraulic nozzles. A span which is substantially less
than 1.0 is characteristic of a CDA spray (Bateman, 1993).
CDA is therefore best considered in terms of optimising technology to
achieve a biological objective: delivering appropriately sized droplets
(within practical engineering limits) for maximising the control
of a given pest target, (where this is known). Unfortunately,
the true biological target is often poorly defined and complex in nature,
which when combined with operational variables makes most spraying inherently
inefficient. However, there is often scope for improving existing practice
(Hislop, 1987). Bals (1969) discussed the concept of producing small
uniform pesticidal droplets to achieve adequate control with "ultra-low
dosage" combined with ULV rates of application. Unfortunately,
this is thought to have discouraged many chemical companies from promoting
CDA techniques since sales would be reduced (except when value could
be re-added to an a.i. by proprietary delivery systems such as the ‘Electrodyn’:
Coffee, 1981).
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References
Amsden,
R.C. (1962) Reducing the evaporation of sprays. Agricultural Aviation
4: 88-93.
Bache,
D.H., Johnstone, D.R. (1992) Microclimate and spray dispersion.
Ellis Horwood, Chichester, UK. 238 pages.
Bals,
E.J. (1969) The principles of and new developments in Ultra Low Volume
spraying. Proceedings of the 5th. British Insecticide and Fungicide
Conference, 189-193.
Bateman,
R.P. (1993) Simple, standardised methods for recording droplet measurements
and estimation of deposits from controlled droplet applications. Crop
Protection, 12: 201-206.
Chapple,
A.C., Downer, R.A., Bateman, R.P. (2000) Theory and practice of microbial
insecticide application. In L. Lacey & H. Kaya (eds.) Field Manual
of Techniques for the Evaluation of Entomopathogens, 1-37.
Coffee,
R.A. (1981) Electrodynamic crop spraying. Outlook on Agriculture,
10: 350-356.
Furness,
G.O., Magarey, P.A., Miller, P.H., Drew, H.J. 1998. Fruit tree and vine
sprayer calibration based on canopy size and length of row: unit canopy
row (UCR) method. Crop Protection, 17: 639-644.
Graham-Bryce,
I.J. (1977) Crop protection: a consideration of the effectiveness and
disadvantages of current methods and of the scope for improvement. Philosophical
Transactions of the Royal Society, London B, 281: 163-179.
Himel
C M (1969) The optimum drop size for insecticide spray droplets. Journal
of Economic Entomology 62: 919-925.
Himel,
C.M. (1974) Analytical methodology in ULV. In: "Pesticide application
by ULV methods" British Crop Protection Council Monograph No. 11,
112-119
Hislop
E.C. (1987) Can we define and achieve optimum pesticide deposits? Aspects
of Applied Biology, 14: 153-165.
Matthews,
G.A. (2000) Pesticide Application Methods 3rd Edition. Blackwell
Science, Oxford. 432 pages.
Matthews,
G.A., Bateman, R.P. (2004) Classification criteria for Fog and Mist
application of pesticides. Aspects of Applied Biology 71:
55-60
Parkin,
C.S. (1992) Methods for measuring spray droplet sizes. In: Matthews,
G.A. and Hislop E.C. (Eds.) (1993) Application technology for crop
protection. CAB International, Wallingford, UK, pp. 57-84.
Spackmann,
E., Barrie, I.A. (1982) Spray occasions determined from meterological
data during the 1980-81 season at 15 stations in the UK and comparison
with 1971-80. Meteorological Office Agricultural Memorandum No. 933.
Meteorological Office, Bracknell, UK.
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