C R O P P R O T E C T I O N (1987) 6 (4), 226-233
Influence of droplet size, air-assistance and
electrostatic charge upon the distribution of
u l t r a . l o w - v o l u m e sprays on t o m a t o e s
HAMAD A . ABDELBAGI* AND ANDREW
J.
ADAMS* *
Department of Entomology and Insect Pathology, Institute of Horticultural Research,
Worthing Road, Littlehampton, West Sussex, BN17 6LP, UK
ABSTRACT. Scoring systems for efficiency and coverage of spray deposits on tomatoes were used to
assess the influence of droplet size and air-assistance upon the distribution of electrostatically charged
droplets within a crop in relation to their potential to control whitefly. Interactions between the spray
(charge, flow rate and air-assistance) and the crop (height and position) were identified. The best
combination was the lowest flow rate (0.8 ml/min; vmd = 18 ~m; vmd/nmd = 1.13) with the smaller fan
(2.0 m3/min air movement) where deposition on abaxial surfaces at the top of the crop was particularly
good. Spray penetration of the canopy was better with the normal, compared with the smaller, fan. At
the same flow rate (7-9 ml/min) the electrostatic sprayer provided a more efficient distribution of active
ingredient than an uncharged spinning-disc sprayer. The scoring systems may be modified to
accommodate different interactions between crop, pest or disease, application method, spray distribution and mode of action of insecticide, to facilitate improvements in application efficiency.
Introduction
Recent research, using sessile stages of glasshouse
pests and contact pesticides as model bioassay systems,
has demonstrated that the application efficiency of a
toxicant increases as the spray droplet size decreases
(Munthali, 1984). Smaller, additional changes in biological efficacy may be attributed to the use of different
oil-based formulations, the interaction between
droplet and plant surface, the concentration of active
ingredient (Abdalla, 1984; Wyatt et al., 1984) and the
developmental stage of the pest (Palmer, Wyatt and
Scopes, 1983). If these biological requirements, as
determined in the laboratory, can be translated into
crop contexts by improved application techniques, it
should be possible to improve the efficiency of insecticide use--currently estimated at a fraction of 1%
(Graham-Bryce, 1977)--by an order of magnitude or
more.
T h e application of very small spray droplets
(<50/~m in-flight diameter) introduces numerous
practical problems associated with lack of momentum,
low impaction efficiency on foliage, evaporation and
* Present address: Crop Protection Department, Sudan Gezira Board,
Barakat, Sudan.
* * Address for correspondence: Laboratory for Pest Control Application
Technology, Ohio Agricultural Research and Development Center, Ohio
State University, Wooster, Ohio 44691, USA.
0261-2194/87104/0226-08 $03.00 © 1987 Butterworth & Co (Publishers) Ltd
the consequent, but undesirable, likelihood of drift
(Uk, 1977; Spillman, 1984). However, if the spray is
electrostatically charged, the deposition of a small
droplet, with a low terminal velocity, may be
determined by attraction to conductive surfaces (e.g.
foliage) instead of by gravitational forces. Consequently, a greater proportion of the spray may impact
on the undersides of leaves than is possible with
conventional sprays (Coffee, 1979). As many pests are
found on abaxial surfaces, where they are often
sheltered from high-volume (HV) sprays, electrostatic
sprayers should favour deposition on the biological
target (Matthews, 1977).
In some comparative trials, electrostatic sprayers
have given worse pest control than hydraulic sprayers.
This has been attributed, in part, to the failure of the
charged spray to penetrate the crop canopy effectively
(Cayley et al., 1984). Whereas small uncharged
droplets (approximately 20~m in diameter) may be
optimal for the efficient use of insecticides (Himel,
1969), charged drops of this size, or smaller, are
deposited on peripheral foliage only (Adams and
Palmer, 1986). This may be overcome if drop-legs
(Morton, 1982) or air-assistance (Cooke et al., 1986;
Adams and Palmer, 1986) are incorporated into the
application equipment.
In a preliminary study, the combination of airassistance, electrostatic charge and a narrow droplet
HAMAD A . ABDELBAGI AND ANDREW
a
/
C'
!
..
,"~
~,
227
ADAMS
The design of the two sprayers means that the N and
S treatments are not strictly comparable in terms either
of gross or of localized air movement: whereas the
Ulvafan moved more air, the Tube caused air speeds of
4.5 m/s (IN') and 3" 0 m/s (S), 0.5 m from the nozzle, the
corresponding figures for the Ulvafan being 2.5 m/s
and 2- 0m/s.
b
,.
J.
Spray
Chemicals
f f "bI
~
Spray
,
/
.
/
FIGURE 1. Method of spray application. (a) Top view; (b) side view.
spectrum (vmd= 20/am; vmd/nmd-- 1- 15) resulted in
an even distribution of droplets on adaxial and abaxial
leaf surfaces throughout a tomato crop (Adams and
Palmer, 1986). The aim of the present study was to
establish the effects of charge, air-assistance and
droplet size upon spray distribution, and to relate this
information to the efficiency ofpermethrin application
against the glasshouse whitefly, Trialeurodes vapo-
rariorum.
T h e insecticide used was permethrin (cis:trans 60:40)
supplied at 100g a.i./l in an oil-based U L V formulation JF8133, containing 5g/1 Uvitex OB as a
fluorescent tracer (IC! Plant Protection). This
formulation was diluted to 70 g a.i./1 by the addition of
risella oil to raise the resistivity of the fluid to
4 . 6 x 10- 8 ohm cm so that it would atomize correctly in
the Tube.
Spray application
The spray was directed, judging from the leaf flutter
caused by the air-assistance, up and down each plant
and the gap between each plant, as shown in Figure 1.
Ten treatments were applied (Table 1) including flow
rates a (0.8 ml/min), b (1.8 ml/min), c (3.0 ml/min) and
d ( 7 . 0 - 8 . 8 ml/min). Each plot was surrounded by a
polythene sheet during spraying and for five minutes
after spraying, to prevent contamination of adjacent
plots.
Materials and methods
Sampling
Crop
Each plot was divided into 24 sampling sites for six
vertical levels (each 0.35 m high) and four horizontal
positions (E, ME, M W and W) (Figure 2). One leaf per
site was collected from each of the eight central plants
in every plot. Two leaf discs (1 cm 2) were cut from each
leaf and examined for adaxial and abaxial spray
deposits under ultraviolet (UV) light. Each disc was
assigned to one of six droplet-density classes (Table 2).
These classes were selected on the basis of bioassay
data which indicated that 50-150 droplets/cm 2 are
likely to cause > 50% mortality when first-instar white-
Tomato plants, Lycopersicon esculentum cv. Marathon
were planted in peat bags and allowed to grow to the
top wire (2-1m) in an l l m x l l m
glasshouse
compartment. Each experimental plot consisted of 12
plants arranged in a double row with an interplant and
interrow spacing of 0.5 m (Figure 1).
Spraying equipment
Two hand-held sprayers (Micron Sprayers Ltd) were
used:
1. Air-assisted Microdyne--the 'Tube'. This is an
electrostatic sprayer with a Microdyne nozzle
(based on the design of the ICI Electrodyn)
mounted at the end of a tube and in front of a fan.
The fan could be changed so that either 3.5 m3/min
(normal 'N' fan) or 2-0m3/min (small 'S' fan) air
movement was provided.
2. Ulvafan. This is a commercially available, airassisted spinning-disc applicator comprising a
Micro-Ulva atomizer head mounted in front of a
fan. A voltage restrictor allowed the fan to rotate
at one of two speeds to provide N and S air
movement (4.0 and 3.0m3/min respectively),
without altering the spinning speed of the disc.
TABLE 1. The spray treatments applied to tomato plots using four flow
rates, charged (Ch) and uncharged (U) sprayers and normal (N) and
reduced (S) air-assistance
Treatment
aChN
aChS
bChN
bChS
cChN
cChS
dChN
dChS
dUN
dUS
Flow rate Spraying
(ml/min) time (s)
0"8
0"8
1" 8
1" 8
3"0
3" 0
7" 0
7" 0
8"8
8" 0
66
70
72
66
65
72
72
68
71
63
vmd*
(/am)
nmd*
(/am)
vmd/
nmd*
18
18
27
27
40
40
56
56
53
54
16
16
25
25
34
34
53
53
21
21
1.13
1.13
1.08
1.08
1-15
1.15
1.07
1.07
2.49
2.53
* Droplet-size data for charged sprays from Malvern 220013300 V2.2 (courtesy,
Micron Sprayers Ltd) and, for uncharged sprays, from Malvern P.S.A. Droplet
Analyser (courtesy, IPARC, Silwood Park)
CROP PROTECTION
Vol. 6 A u g u s t 1987
Electrostatic spraying on tomatoes
228
EIMEIMwlw
Height
i
(m)
2.10
,
I
variance in which the effects of height, flow rate and
position were examined. For the purpose o f the
analysis the E and W samples were treated as replicates
so that the residual error term was derived from interaction terms of E - W samples with height, flow rate
and position in the usual way.
Efficiency scores. A differential scoring system for
adaxial and abaxial leaf surfaces was used to
demonstrate how the distribution of the spray between
the two leaf surfaces affected the efficiency of insecticide usage against whiteflies. For classes A to F
(Table 2), on the adaxial surface, a contact pyrethroid
like permethrin is wasted against whitefly larvae which
are found, almost exclusively, on the abaxial surface.
Hence, the scores for each class are negative and
increase in proportion to the droplet density
(Figure 3a). Conversely, positive efficiency scores are
given to abaxial deposits which increase as the number
o f droplets increases up to 150 droplets/cm 2
(Figure 3a). T h e efficiency scores for classes E and F
are lower than for class D as they are likely to cause
'overkill' which constitutes a waste of active
ingredient--effective but inefficient. Thus, the
6
1.75
5
1.40
4
1.05
3
0.70
.
P
/o
/o
+8
I
0.35
/o/
+4
1
0
0
0
T
i
u ~
u
I
l
I
t
-4
FIGURE 2. Location of the 24 sampling sites at four depths for each of the
six heights.
TABLE 2.
Class
--8
Droplet-density classes
Droplet density(drops/cm2)
A
B
C
D
E
F
<25
25-50
51-100
101-150
151-200
>200
flies are treated with permethrin (Adams, 1986 and
unpublished data).
Scoring systems and method of analysis
A scoring system for 'efficiency' and another for
'coverage' and 'penetration' were designed. Weights
were assigned to the six droplet-density classes and the
resulting indices were then subjected to an analysis of
CROP PROTECTION Vol. 6 August 1987
b
+8
/"
_/m
/ m
or)
m/m
o
,
i
i
i
i
[
A
B
C
D
E
F
Droplet density class
FIGURE3. Scoringsystemsfor: (a) efficiency--scoresfor abaxial(O) and
adaxial(/-q)densitiesin eachclassare summedat eachsite;(b) coverage-depositson both leafsurfacesreceiveidenticalscoresfor eachdensityclass
(m>.
HAMAD A . ABDELBAGI AND ANDREW
+
number of discs in each density class was multiplied by
the corresponding efficiency score. These values were
summed to provide efficiency scores for each height of
the treated plot (see Figure 2).
-
-.!-- 7 5
-
+
50
-
+
25
-
0
-
o
tO
O3
-
25
229
ADAMS
-
+ 100
Coverage and penetration scores. The amount of spray
reaching each site was assessed on a scale which
increased in proportion to droplet density for both leaf
surfaces (Figure 3b). Summing the two surface scores
gives a coverage score for a site from which scores for
the outer (E + W) and inner (ME + MW) positions at
each height were obtained. An index for spray penetration is obtained by dividing the inner by the outer
coverage score.
Both scoring systems may be used to provide
information per site, height, position or an overall
figure for each treatment.
125
J.
/
o
/
J\oJ °
o
n
I
°j °
~
~
o~n..__--
i
i
i
1
2
3
O-
i
4
_n
---'-" n
I
J
5
6
Height
Results
FIGURE 5. Efficiency scores at each height of the crop for charged, small
fan, sprays at flow rates a (©), b ([7) and c (A).
Arrangement of data
The spray deposition data was grouped into outer and
inner regions of the crop to simplify the analysis.
Droplet distribution
The treatments were compared on the basis of
efficiency, coverage and penetration scores. In
addition, allowances were made for the effect of
droplet size upon the droplet density (LNs0) and
quantity of a.i. (LQs0) required for 50°7o mortality:
halving the droplet diameter doubles the LNso but uses
25°7o of the insecticide (from Adams, 1986). Further-
A
+
75
+
50
+
25
0
O
O
O3
-
25
-
50
-75
-
100
/
m
L
O
I
I
i
I
I
I
1
2
3
4
5
6
Height
FIGURE 4. Efficiency scores at each height of the crop for charged sprays,
with the normal fan, at flow rates a (O), b (F-l) and c (A).
more, the most susceptible developmental stages of T.
vaporariorum are the egg and first-instar larva (Palmer
et al., 1983), which generally occur at tomato heights 5
and 6 (1.40-2.10m).
Charged (Ch) droplets. Efficiency. The efficiency
scores for each height at flow rates a, b and c, with the
normal fan, are presented in Figure 4. With the
exception of the score for a flow rate of 1" 8 ml/min
with the normal fan (bChN) at height 1 (0-0.35 m), the
scores tend to increase with increasing height. Positive
efficiency scores occur at the top half of the crop where
bChN is the most consistent combination, although
the best score at height 6 (1.75-2.10m) was recorded
with a flow rate of 3.0ml/min and the normal fan
(cChN). The analysis showed that height was the most
important factor influencing the scores in the treatments (P<0.001) and accounted for 30% of the
variation. A significant effect of flow rate was also
found (P<0.001).
When the results with the smaller (S) fan are
examined (Figure 5), it is evident that the efficiency
scores are generally higher than those recorded with
the normal fan. The scores for the lowest flow rate,
0.8ml/min (aChS) are positive at every height, and
increase towards a maximum at height 6. Conversely,
the scores are all negative when the flow rate is 1.8 ml/
min (bChS), but do not vary much between heights.
The analysis revealed that flow rate was the most
significant factor (P<0.001), accounting for 28% of
the variation. Position (outer and inner scores have
been summed for clarity in Figure 5) and its interaction
with height also showed significant effects (P<0.05).
The spray distribution between leaf surfaces was
most efficient in aChS at every height and, most
importantly, deposition was clearly biased towards the
abaxial leaf surface at heights 5 and 6. This combinaCROP PROTECTION Vol. 6 August 1987
Electrostatic spraying on tomatoes
230
200
150
0
tO
100
tO
50
0
I
I
I
I
I
200
]
n
150
Charged (Ch) v. uncharged (U) sprays. The Tube and
the Ulvafan were compared, directly, at a flow rate of
7 . 0 - 8 . 8 ml/min (d).
==
0
0
100
0~
0
/
~ 0
GO
r-/~'
When the normal fan was used, penetration at each
flow rate was lower at heights 2-4 (0" 35-1" 40 m) than
at heights 1, 5 and 6 (Table 3, section a). As the foliage
becomes less dense towards the top of the plants, better
penetration at heights 5 and 6 is to be expected, while
the high indices at height 1 may be due to the wraparound effect of small charged droplets. In most cases,
penetration at each height improved as the flow rate
(droplet size) decreased. The indices in section b of
Table 3 are, generally, equal to or below the corresponding figures in section a of Table 3, indicating that
reducing the air-assistance reduces penetration by the
spray. This is particularly marked at heights 5 and 6
where the smaller fan was less able to push the droplets
upwards into the canopy.
.A~
[]
50
0
I
I
I
I
I
I
1
2
3
4
5
6
250
a
0 ~O
200
Height
FIGURE 6. Coverage scores on (a) outer (E+ W) and (b) inner (ME + MW)
positions at each height for charged, normal fan, sprays at flow rates a (O),
b (El) and c (A).
150
o
0
0
tO
~
°
~
o
/
°
100
50
tion of droplet size and air-assistance is, therefore,
likely to be particularly good for whitefly control.
On the same basis, the consistency of the two remaining small fan treatments is outweighed by the higher
efficiency scores at heights 5 and 6 when normal airassistance was employed at each flow rate.
Coverage. T h e analysis of the coverage scores for
outer and inner positions, when the normal fan was
used (Figure 6a, b), indicates that height and position
are the most significant factors ( P < 0 . 0 0 1 ) and they
account for 33°7o and 32% of the variation, respectively. Thus, the coverage score increases with increasing height, and is greater on the outer than on the inner
position, at each height. Flow rate, and the interactions
between height and position, and height and flow rate,
were also significant (P<0.05). Similar results are
revealed by the analysis of coverage scores with the
small fan (Figure 7a, b) where position and height,
between them, accounted for over 63% of the variation
(P<0.001). Significant effects due to flow rate
( P < 0 . 0 1 ) and the interaction of height and position
(P<0-05) were also recorded. As before, coverage
improved towards the top of the crop, and more
droplets were deposited on the outer position at each
height.
Penetration. The penetration indices from the
charged spray treatments are presented in Table 3.
CROP PROTECTION
Vol. 6 August 1987
0
I
200
I
I
I
I
b
[3
o/o
150
0
I
100
O9
50
0
0 "'" ...--.-"0
I
I
I
I
1
I
1
2
3
4
5
6
Height
FIGURE 7. Coverage scores on (a) outer and (b) inner positions at each
height for charged, small fan, sprays at flow rates a (O), b ([]) and c (A).
TABLE 3. Penetration indices for the charged spray treatments
Height
Flow rate
Fan type
(ml/min)
1
2
3
4
5
6
(a)Normal(N)~n
a 0"8
b 1-8
c 3.0
1"0
0.7
0'6
0"6
0.6
0.4
0"7
0-4
0-4
0"7
0.5
0"4
0"8
0-8
0-7
0"7
0.9
0"9
(b) S m a l l ( S ) ~ n
a 0.8
b 1-8
c 3"0
0.5
1.1
0.7
0.3
0.5
0.4
0.6
0.5
0.3
0"6
0.5
0.4
0.4
0.5
0.4
0"5
0.8
0.6
HAMADA.
+ 50
-
+ 25
-
0
-
o
0
CO
--
25
-
--
50
-
ABDELBAGI AND
75
o~•
...~-/
n. . . . o - \ • /
--
%
%
--
100
%
-
/
%am
--
125
231
worse than at the corresponding outer positions. The
exception was the combination ofdUS where the outer
and inner scores were almost identical. Overall, height
and position, and the interaction between position and
charge, were the most significant factors (P<0-001).
The interactions between height and charge, height
and air-assistance, and position, charge and airassistance were also significant (P<0.05). The
combination of electrostatic charge with normal airassistance provided the most efficient spray with the
best coverage scores, particularly at heights 5 and 6.
%
--
ANDREWJ. ADAMS
~,
/ /
~'m...,~.....
n
Penetration. The penetration indices in Table 4
demonstrate that, at this flow rate, a greater proportion
of the spray reached the inner position at each height
than with most of the lower flow-rate treatments
-
I
I
I
I
I
I
I
1
2
3
4
5
6
250
a
Height
•
. ok--o
200
FIGURE 8. Efficiency scores, at each height, for sprays at flow-rate d using
the Tube with normal (O
O) and small ( 0 - - -- - - 0 ) fans, and the
Ulvafan at normal ( n - - - - - - - I )
and reduced ( U - - -- - n )
150
fan speeds.
0
0
09
Efficiency. Figure 8 indicates that the charged
droplets were distributed more efficiently than
uncharged droplets, with the score tending to increase
towards the top of the crop. The combination of an
uncharged spray with the normal fan speed (dUN)
gave good scores at heights 4-6 which are comparable
to bChS and cChS (Figure 5). However, the most
striking feature is the extremely poor efficiency of the
uncharged spray applied using the slower fan speed
(dUS). Air-assistance, charge and their interaction
were the most significant factors (P<0" 001). Position,
and its interaction with air-assistance, also caused
significant effects (P<0.01). Efficiency scores were
lower on the inner compared with the outer position,
at each height, and with the reduced compared with
the normal air-assistance.
100
50
0
200
I
I
I
V
0
0
I
I
•
0
b
150
°
100
3"a
cO
50
0
Coverage. Coverage scores for charged sprays were
generally better on the outer canopy than within the
crop (Figure 9a). There was also a tendency for the
charged scores to increase towards height 6--the
notable exception being a sharp decline in coverage
score between heights 5 and 6, on both outer and inner
positions when the combination of a charged spray
with the smaller fan (dChS) was applied (Figure 9b).
This may be due to the greater influence of gravity
upon 56/am droplets compared with droplets that are
40/am (flow rate c) or smaller (see Figure 7) such that
the air-assistance fails to push the droplets far enough
into the topmost foliage, resulting in enhanced
deposition at height 5 from droplets directed there by
the sprayer, supplemented by droplets falling from
height 6. Within the crop, there was little difference
between the four treatments, but coverage was usually
I
I
I
I
I
I
I
1
2
3
4
5
6
Height
FIGURE 9. Coverage scores on (a) outer and (b) inner positions, at each
height, using flow-rate d applied through the Tube with the normal (©)
and small (O) fans, and through the Ulvafan at normal (A) and reduced (A)
fan speeds.
TABLE 4. Penetration indices for charged (Ch) and uncharged (U) sprays
applied at flow rate d (7-9 ml/min) using either the normal (N) or reduced
(S) air-assistance
Height
Treatment
dChN
dChS
dUN
dUS
1
2
3
4
5
6
0.7
0.6
0.8
1.0
0.7
0.7
0.6
0.9
0.5
0.4
0.7
1.2
0.6
0.5
0.7
1.1
0.5
0-7
0.6
1.2
0.7
0.5
0.8
1.1
C R O P P R O T E C T I O N Vol. 6 August 1987
232
Electrostatic spraying on tomatoes
(Table 3). The interaction between position and charge
is illustrated in Table 4 because the charged sprays had
lower penetration indices than the Ulvafan sprays.
Indeed, although the combination dUS provided the
worst efficiency scores, penetration of the crop was
extremely good. This emphasizes that different spray
combinations are likely to be suited to each application
problem, depending on the distribution of the pest or
disease, and the mode of action of the pesticide.
Discussion
The most efficient distribution of droplets, likely to
provide effective control of T. vaporariorum, occurred
with the Tube when the lowest flow rate, producing
18/am droplets, was combined with the small fan in
treatment aChS (Figure 5). Bioassay data suggest that
twice as many 18/am droplets would be required to
exert the same biological effect as 36/am droplets (,--flow
rate c) (Adams, 1986) and coverage with aChS was
worse than that with the other charged sprays
(Figure 7). However, the exceptionally high proportion of droplets which had impacted on abaxial leaf
surfaces, to provide the high efficiency scores at
heights 5 and 6 (Figure 5), should compensate by
providing the required control of the most susceptible
pest stages without increasing either the application
time or the concentration ofpermethrin. In some other
treatments, particularly with the small fan, where
there was little difference between efficiency and
coverage scores for flow rates b (vmd= 27/am) and c
(40/am) (Figures 5 and 7), the smaller droplets would
use chemical more efficiently, but this would be associated with increased spraying time to improve droplet
density. Alternatively, in view of labour costs, a slight
increase in the concentration of a.i. in the smaller
droplets could still economize on chemical costs,
without altering the time factor.
It is extremely difficult to compare the results of the
charged v. uncharged treatments (Figures ~3and 9), at
the same flow rate, in terms of biological effect.
Table 1 shows that spray uniformity with the Tube at
flow rate d (7.0-7.2ml/min) was very good (vmd/
nmd= 1-07), so deposition results can easily be related
to bioassay data, but the Ulvafan produced a much
wider droplet-size spectrum (vmd/nmd~2.5). Consequently, the overall droplet density consists of a range
of droplet sizes where the proportion of large and small
drops is, intuitively, likely to vary with the height and
position of the sampling site. A possible solution is to
combine droplet density data with quantitative
analysis of the a.i. present, which has been used to
evaluate spray deposits on cotton--although biological
effects were not reported (Uk and Courshee, 1982).
However, the results with the charged sprays indicate
that flow rate had a significant effect upon efficiency
and coverage scores, although the droplet size changed
only from 18/am to 40/am. This will, inevitably,
complicate the interpretation of deposition data--even
from an electrostatic sprayer producing a narrow
CROP PROTECTION Vol. 6 August 1987
droplet-size spectrum--unless deposits on leaf surfaces
can be translated to in-flight droplet sizes. Even then,
some allowances for in-flight evaporation and consequent changes in the concentration of a.i. will be
necessary, especially for aqueous sprays.
Scoring systems, such as those presented in
Figure 3, can simplify comparison of spray deposition
data, and provide a means to assess the likely efficacy of
a spray against a pest or disease on a crop. However, a
prerequisite for a scoring system is extensive fundamental data for the numerous spray parameters (e.g.
concentration ofa.i., droplet size, etc.), combined with
knowledge of the distribution and susceptibility of the
pest on the crop, in space and time. Data of this type
are available for the major glasshouse pests,
Tetranychus urticae (Munthali, 1984; Munthali and
Wyatt, 1986) and T. vaporariorum (Palmer et al., 1983;
Wyatt et al., 1984; Xu, Zho and Zhang, 1984; Adams,
1986) and the droplet-size uniformity of the Tube
sprays simplifies comparison of deposition and bioassay data. For example, 20/am droplets ofdicofol were
most efficient against T. urticae eggs at a concentration
of 11.8gll (Munthali, 1984), and the LNs0 was 250
droplets/cm2, whereas only 130 drops were required if
the concentration was doubled (Munthali and Wyatt,
1986). If a spray of 40/am droplets deposited 100
droplets/cm, > 50°70 kill occurred if the concentration
was 10g/l, or more. Similarly with 40/am droplets of
100g/l permethrin against first-instar T. vaporariorum,
the LNs0 was 80drops/cm2, which increased to
90 drops/cm2 if 31 lam droplets were applied (Adams,
1986). In this way, compromises can be made to help
the application suit the problem in an educated,
efficient and economic manner.
Using the Tube in a glasshouse, it is possible to
combine the virtues of small droplets (efficient use of
a.i.; ultra-low volume) with the virtues of electrostatics
(underleaf coverage; minimal drift) and superimpose
improved targeting of the spray with the directed airassistance. Thus, the chemical may be restricted to the
portion of the crop where the pest is found. In some
cases it may be possible to separate broad-spectrum
insecticides spatially from areas where beneficial
insects are found. In addition, apical spraying of
tomatoes could occur without direct spray contamination of fruit--although the mode of action and
persistence of the chemical may, to some extent, offset
the improvements in application specificity.
Acknowledgements
We are grateful to John Fenlon, for help with statistical
analysis, and Anne Palmer for technical assistance.
The first author also thanks the British Council and
the government of Sudan for financial support.
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Received 12 January 1987
Revised 18 February 1987
Accepted 26 February 1987
CROP PROTECTION Vol. 6 August 1987