Production Practices and Quality Assessment of Food Crops. Vol. 1

90 Sally A. Bound
pests are not always clear and there are conflicting requirements in relation to safety,
coverage or cost (Heijne, 1980). For example, Himel et al. (1971) suggested that
if the spray is driven into the plant canopy, the optimum size for insecticide spray
droplets is substantially smaller than 20 µm, while Smith et al. (1975) recommended
a droplet size between 140 and 200 µm for controlling boll weevil, because smaller
droplets were subject to excessive drift.
4.4. Spray retention
Spray retention is a measure of the percentage of the spray volume applied that is
deposited on the target canopy. In most spray systems, but particularly in conventional
air-blast application with hydraulic nozzles, only a portion of the spray is
actually physically deposited where it is required. Large droplets over 300 µm end
up on the ground, leading to soil and ultimately ground water contamination. Small
droplets under 30 µm drift out of the target area. This drift is increased by droplets
falling below the critical size through evaporation in transport, particularly with
water based sprays. Bals (1984) reports that in the UK alone, at least 16 million
gallons of spray liquid contribute to general environmental contamination every year
through long distance drift.
The deposition of spray droplets is also influenced by air velocity and turbulence,
size of the target, angle of the target surface, morphology and physiological state
of the target. The optimum droplet size for maximum retention for aqueous solutions
is reported to be 100 µm or less (Heijne, 1980) and such a reduction in
droplet size would also improve coverage due to an increase in the number of
droplets at the same volume application rate. Droplets less than 100 µm tend to
adhere to most target surfaces after contact, with oil based droplets being more
adhesive than water based droplets. Droplets greater than 100 µm with high contact
angles are often deflected off the target (Broadley et al., 1986).
4.5. Coverage
Coverage describes the percentage area of target (e.g. foliage) bearing spray deposits
and how evenly these are distributed across the target surface. Coverage is influenced
by tree size, growth stage, density and spacing, and is the result of the way
the sprayer outputs interact with the canopy being sprayed. Furness (2000) has
produced an aid for estimating spray coverage and deposit with agricultural spray
equipment and describes the droplet rating chart as the basis of a simple, rapid
field technique to ensure that a sprayer is achieving adequate coverage and chemical
dose throughout the canopy.
Banks et al. (1990) report that studies with lepidopterous larvae have shown
that droplet density and size are important after deposition has occurred. Larvae
that had not been contacted directly by spray droplets were able to avoid large
droplets of pesticides on the leaves. When droplet density was increased, the number
of contacts made by larvae also increased. A similar problem exists with control
of fungal diseases. As fungal spores are immobile, the greater the droplet density,
the more likely that spores will be targeted.