1-s2.0-s1537511002002775-main recent developments in sprayers for application of biopesticide

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Biosystems Engineering (2003) 84 (2), 119125doi:10.1016/S1537-5110(02)00277-5

Available online at www.sciencedirect.com

PAPrecision Agriculture

REVIEW PAPER

Recent Developments in Sprayers for Application of Biopesticides}an Overview

Samuel Gan-Mor1; Graham A. Matthews2

1The Institute of Agricultural Engineering, The Volcani Center, ARO, P.O. Box 6, Bet Dagan 50250, Israel,e-mail of corresponding author: [email protected]

2Imperial College of Science, Technology and Medicine, Silwood Park, Ascot, Berkshire SL5 7PY, UK; e-mail: [email protected]

(Received 11 October 2001; accepted 25 November 2002)

Few biopesticides are currently used commercially as alternatives to chemical pesticides. Part of the problem is

that methods of application of biological agents have not been adequately considered. This paper reviews

current information on the application of biopesticides and concludes that more research and development is

needed to develop effective application technologies so that those biopesticides showing promise, under

laboratory conditions, can be applied by farmers.# 2003 Silsoe Research Institute. All rights reserved

Published by Elsevier Science Ltd

1. Introduction

Increasing public concern about the potential damage

of chemical inputs in agricultural production systems

has challenged industry to develop new and effective

pest management and control strategies against insect

pests, diseases and weeds. These new strategies must be

less harmful to the environment than the current,

chemical-based ones, and they will, therefore, also

safeguard the health of the agricultural workers and

consumers (Weidemannet al., 1995). This environment-

friendly approach still involves using pesticides but only

those of low toxicity. The application of biopesticides

fits the modern strategy of integrated pest management

(IPM) (Matthews, 1999), which combines all suitable

control techniques harmoniously with one another and

integrates them with other crop production practices, to

suppress pest populations below economic injury levels,while maintaining the integrity of the ecosystem. The use

of biopesticides is based on environmental and ecologi-

cal considerations; therefore, aspects such as timing,

location, radius of activity, population density, distribu-

tion, life cycle, persistence and host specificity, are all

very important. The achievement of good efficacy is a

complex problem, but not to a degree that should

prevent implementation of IPM (Fuxa, 1995).

In contrast to most conventional chemical pesticides,

biopesticides have been defined by Steinke and Giles

(1995) as biological products or organisms, which are

produced from a biological source outside the field: they

may include viruses, bacteria, fungi, predators, parasites

and pheromones. These agents utilise a variety of modes

of action, hence their application presents different

problems from those of chemical pesticides. In parti-

cular, biopesticides are living organisms and great care is

needed to maintain their viability.

At present, many have felt that biopesticides should

be applied with existing spray technologies, but their

success has been very limited because of the inappropri-

ateness of the equipment, poor efficacy of the formula-

tion, or a combination of both (Steinke & Giles, 1995;

Navon, 2000a, b; Gan-Moret al., 1996). The argument

for using existing conventional equipment is that farm-

ers will face economic difficulties when purchasing

specific equipment for application of every special

pesticide. One way to overcome this problem is to usethe equipment developed for application of the biopes-

ticide with the highest uniformity and delicacy demands,

for application of all the pesticides. Nevertheless,

effective and efficient application of biopesticides that

are environmentally safe and pose no hazard to non-

target organisms is dependent on the development of

both the formulations and the delivery systems to ensure

adequate handling, metering, dispersal and crucially,

deposition of sufficient amounts of the biopesticide

where the pests are located.

1537-5110/03/$30.00 119 # 2003 Silsoe Research Institute. All rights reserved

Published by Elsevier Science Ltd


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Some alternatives to chemical pesticides, including the

application of entomopathogenic nematodes (EPNs),

are already used intensively to control soil-borne pests

and diseases, where the application technique is less

demanding (Grinstein et al., 1995; Gamliel et al., 1998;

Chet & Inbar, 1994). However, little information has

been reported on the difficulties encountered in thedevelopment of the application techniques for these

materials and organisms. Far more has been reported on

the difficulties involved in foliar application. This paper

discusses these problems in more detail, in an attempt to

promote research and to improve the effectiveness of

biopesticides.

2. Contrasts between chemical and biopesticideapplication practices

Teske et al . (1995) noted adverse effects on theenvironment, caused by the application of chemicals

and pesticides. However, these effects are strongly

dependent on the actual pesticide applied, the dosage

used and the method of application. Human exposure

occurs at various stages of the application procedure

and is of most concern when the user has to measure a

quantity of the formulated product and mix it

with water, because of the high concentration of

active ingredients in the formulated product. Environ-

mental concerns arise when spray drift occurs, mostly

to areas downwind of a treated area, but also

because of spray failing to reach foliage and

being deposited on non-target surfaces within a

field. These are the exo- and endo-drift, as defined by

Himel (1974). There is a particular concern when traces

of toxic pesticides reach water sources. Further damage

to water sources is liable to occur when spillages

and washings of sprayer tanks are released into the

environment.

Drift is increased if spray is released from a height, so

regulatory authorities have been especially concerned

about aerial applications, which have been reviewed

and modelled by Teske et al. (1995). There is now a

vast database of downwind deposit profiles for

various application technologies and meteorologicalconditions. Models enable interpolation of the database

and assist in assessing the environmental impact of the

application of various pesticides, and the requirements

from buffer zones (Mickle, 1995). Generally, the

application of less toxic and more selective agents can

be applied with narrower buffer zones, the width of

which is related to droplet size and wind speed. Droplet

size needs to be optimised for collection on target

surfaces, but generally drift increases with smaller

droplets, although, improved coverage by small droplets

can be achieved when there is some air velocity to

impact the droplets on foliage.

Biopesticides are distinguished from conventional

chemical pesticides as many are very selective and are

non-toxic towards non-target organisms (Mendelsohn

et al., 1995). While biopesticides are likely to be less

harmful to the environment than the conventional ones,care needs to be taken that wastage is minimised, by

selecting the most appropriate droplet spectrum. A

disadvantage of biological agents relative to chemicals,

is that many are not sufficiently persistent and are

relatively slow acting; therefore, research has been

directed at extending the period of activity (Cornish

et al., 1993; Smits, 1996). However, some such agents

may persist in the field or the forest for many months,

and a riskbenefit analysis should be performed to

establish their environmental acceptability (Sundaram,

1995).

3. Resistance management and integrated pestmanagement programmes

While host plant resistance has been an important

component of IPM programmes, the recent develop-

ment of new techniques for producing genetically

modified (GM) crops is potentially of major signifi-

cance. In terms of insect control, the incorporation

of Bacillus thuringiensis (Bt) endotoxin gene can

significantly reduce the need for applying sprays.

However, the different strains of Bt affect different

insect groups, and the constructs for Bt also vary, so

that different plant selections may exhibit different

degrees of toxicity to certain pests. Owing to the

specificity of the toxin, certain pests are effectively

controlled on some of the Bt transgenic crops, where

they ingest sufficient toxin by direct feeding, while their

natural enemies are allowed to survive. However, in the

absence of broad-spectrum insecticides, relatively minor

pests have the potential to attain greater importance.

Thus, even with GM crops, IPM is essential. Where an

insect becomes a new pest, it will be important to have a

selective insecticide, preferably a biopesticide, although

alternatively, broad-spectrum chemicals could be ap-plied very selectively, to avoid adverse effects on

biological controls.

3.1. Development of biopesticides

Numerous investigations on the development of

biopesticides have been initiated as legislation and

government policy have demanded less reliance on

chemical pesticides and greater adoption of IPM.

S. GAN-MOR; G.A. MATTHEWS120


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In the USA, the aim was to have up to 70% of the farm

area using IPM by year 2000 (Hall & Barry, 1995). In

Europe, some countries have set goals of reducing

pesticide use by 50%. Successes have been achieved

through better timing of applications, so that lower

dosages are effective, and substituting less hazardous

and more active materials, to reduce the number ofapplications.

3.2. Characteristics required from the delivery systems

The design of a system for applying a biopesticide

depends strongly on the type of material and its mode

of action, as well as on the specific shape and density

of the crop canopy. Thus, closer collaboration is

needed between the biologists, formulation specialists

and equipment engineers, to develop specifically de-

signed delivery systems, which are economically accep-table.

When an insect pest is feeding on a sprayed leaf, the

efficacy of the insecticide is primarily determined by

the quality and amount deposited and its toxicity, as

well as the movement of the pest and its feeding

pattern (Hallet al., 1995; Navonet al., 1991; Alchanatis

et al., 2000). Typically, high uniformity of deposition,

to cover all the surfaces reached by a pest, has

been regarded as essential. However, efforts to do so

have led to high volumes of spray being applied

and once a surface has been wetted, the surplus is

lost by dripping off the foliage. Laboratory studies

have indicated that an even distribution is not always

the most effective in relation to the application of Bt

as a spray (Hallet al., 1995). As pointed out by Chapple

et al . (2000), when applying biopesticides, such

as chemical insecticides, cover is important, especially

where the pest is located, the amount of active material

present in any droplet is proportional to its size and

the efficiency of application is indirectly proportional

to the droplet size, until a droplet contains less than

a 95% lethal dose (LD95) or similar critical factor.

Some studies refer to biorational agents, i.e. chemicals

which are considered less damaging to non-target

organisms. For example, Sundaram (1995) presentedan overview on research conducted on four biorational

insecticides, insect growth regulators diflubenzuron

and tebufenozide, a botanical azadirachtin, and Bt

applied as a spray, in order to evaluate the roles of

formulation and delivery technology on field perfor-

mance in forests. Factors affecting field deposition, such

as distribution, coverage, application volumes, droplet

spectra, droplets per unit area, and other aspects such as

type of foliage, ageing period of the deposit and

formulation ingredients, were examined.

4. Experiments utilising commercial and speciallydesigned sprayers

4.1. Experiments on application with commercially

available sprayers

Steinke and Giles (1995) refer to several reports of themore successful field applications of biopesticide utilis-

ing commercially available sprayers. Chapple et al .

(2000) have recently discussed many of the problems

associated with application of microbial insecticides,

while Bateman and Alves (2000) have described equip-

ment that has been used to apply biopesticides.

4.1.1. Bt application

Sprays of Bt are biorational rather than biopesticidal

as it is the toxin produced by the bacterium that

provides the control, but as this insecticide has led the

way in terms of an alternative spray especially for those

developing IPM systems and organic crop production, it

is important to include it in this discussion.

Perezet al. (1995) conducted laboratory and field tests

to determine the effect of application technology, plant

age and Bt subspecies on the mortality of one susceptible

and one resistant population of diamondback moth.

There was less variation among different sections of

plants when sprays were applied with an electrostatic

sprayer (ESS, Watkinsville, GA, USA) than with a

knapsack sprayer (Solo, Newport News, VA, USA) or a

hydraulic sprayer fitted with hollow-cone nozzles. The

hydraulic sprayer was fitted with drop-legs so that each

row was sprayed via three nozzles, one above the rowand two on the drop legs on either side of the row.

In contrast to a spray when granular particles,

approximately 70mm in diameter, containing Bt was

applied on cotton and dates by Navon et al. (1997,

1999), a relatively high percentage of the material was

deposited on the plant with the aid of electrostatic

charging. Laboratory tests had determined that at least

1500 particles cm2 were needed to provide good con-

trol, but it was difficult to achieve such a high density in

the field, even when electrostatic charging was used. The

results were sufficiently encouraging and worthy of

further investigation.

4.1.2. Entomopathogenic nematodes

As EPNs are large organisms, relative to the spray

droplet diameters, Chapple et al. (2000) have argued

that there is no optimum droplet size. Certainly,

there is a minimum droplet volume that can hold a

nematode. Large numbers of small droplets in a spray

will simply not convey a nematode. While for soil-

applied treatments, a high volume may be advantageous

RECENT DEVELOPMENTS IN SPRAYERS 121


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to get nematodes beyond the soil surface and provide

moisture for survival, high volumes applied to foliage

are wasteful. In an effort to develop a technique to apply

EPNs to foliage, more recent studies have included

investigations on the use of spinning discs (Mason et al.,

1998a, b, 1999). Two types of spinning discs were

considered. The addition of a number of adjuvants,containing nematode juveniles, to sprays was also

examined. Both types of spinning disc produced similar

spray spectra, that were found to be unaffected when

various adjuvants were added to the spray solution. The

mean number of infecting nematodes was significantly

enhanced by the use of some of the adjuvants. The

mortality among the target pests generally increased as

the flow rate was increased. Subsequent studies by

Piggott (2000) revealed the separation of nematodes

from the spray liquid with a grooved disc, so a smooth

disc was needed. Piggott et al. (2000) also showed an

advantage in adding a polymer (polyacrylamide) to thespray to improve the EPN survival on foliage.

In other experiments, the viability of an EPN

was significantly decreased as the pumping period of a

high-pressure hydraulic sprayer increased (Nilsson &

Gripwall, 1999).

In assessing the effectiveness of foliar sprays of the

EPN Steinemema carpocapsae against the apple sawfly

Hoplocampa testudinea and the plum curculio Cono-

trachelus nenuphar, two early season pests in apple

orchards, Belair et al. (1998) found significantly less

damage at harvest where the nematode had been

sprayed via a commercial handgun sprayer. However,

where a commercial air-blast orchard sprayer was used

no significant difference was observed. The differences

here may be that the handgun applied a much higher

volume and with the air-blast sprayer many of the

smaller droplets failed to carry sufficient EPNs.

4.1.3. Baculoviruses

One of the major difficulties with applying baculo-

viruses is that they are liable to be inactivated by

sunlight, so formulation with sunscreens has been

investigated by Killick (1986) and Arguer and Shapiro

(1997). Nevertheless with a suitable formulation, it is

important that the deposition occurs primarily wherethe target pest is feeding. In attempting to control cotton

bollworms, Parnell et al. (1999) compared the distribu-

tion of a baculovirus applied at a medium volume (MV)

using a motorised knapsack mistblower with a very low

volume (VLV) application from a spinning disc. Field

tests showed that the spinning disc VLV application

gave better control than the commonly used mistblower

MV application, presumably because the more uniform

droplet spectrum deposited more at critical areas where

young larvae would feed. In similar trials in Egypt

againstSpodoptera littoralis, better results were obtained

when nozzles were placed between the rows and directed

up to the lower leaf surfaces where eggs were laid.

4.1.4. Mycoinsecticide

One of the largest research programmes has sought a

biological control of locusts. For logistical reasons,preference has always been given to ultra-low volume

sprays, so the first study was to determine whether the

conidia ofMetarhizium anisopliae var acridum could be

applied in an oil formulation. Fortunately, the spores

are lipophilic and successful formulation led to effective

applications using rotary atomisers to optimise droplets

at 70100mm, even under arid conditions in Africa

(Bateman, 1997). Other strains of the fungus have since

been effective in sprays formulated for dilution in water,

but where an oil adjuvant has been included in the spray

(Bateman & Alves, 2000). Backpack sprayer application

of a fungus against Colorado potato beetle wassignificantly more effective than a conventional insecti-

cide treatment, especially in comparison with the no-

treatment control (Poprawskiet al., 1997). Although the

specific mechanism remained unclear to the researchers,

the significant reduction in larval densities provided

substantial foliage protection.

4.1.5. Fungicides

Very little field development of antagonistic fungi has

been reported. An early use ofTrichoderma viridewas by

means of specially adapted secateurs so that a suspension

was applied to the cut surface (Jones et al., 1994). The

difficulties in developing a biofungicide are discussed byHofstein and Chapple (1999), giving the development of

a hyperparasitic antagonistAmpelomyces quisqualisas an

example. In relation to application, they point out that

the equipment used in small trials must match that of the

end-user as the size distribution of particles in the spray

tank was markedly different after passage through a

pump. Any clumping of particles will decrease the

efficiency of an application. They also note the difficulty

in selecting a surfactant system in which to suspend the

spores that is not itself fungitoxic.

4.2. Development of application of parasitoids andpredators

Giles and Wunderlich (1998) developed an electrically

controlled delivery system for spraying beneficial insect

eggs in liquid suspensions. Large droplets (ca. 2 mm

diameter) were released via a pulse-width modulated

valve which monitored the application rate and the

spacing between the discharged eggs. A uniform sus-

pension of eggs in liquid was delivered with no signi-

ficant reduction in viability (Wunderlich & Giles, 1998).



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Giles et al. (1995) developed a delivery system especially

for the gentle release of predatory mites, for biological

pest control in strawberries. The mites were easily injured

by the agitation of the mixture, so the handling system

consisted of an insulated storage reservoir that kept the

chilled mixture stationary, and a rotating metering plate.

As this plate rotated, the formulation filled cylindricalcells in the plate and each cells contents were released as

the cell passed above an opening.

Bouse et al. (1981) and Bouse and Morrison (1985)

applied Trichogramma pretiosum pupae aerially by

incorporating a refrigeration system to prevent wasp

emergence before delivery, but with this technique many

pupae are lost on the soil.

4.3. Bait sprays using pheromones

Typically, the research in which agricultural engineers

are involved does not extend into the formulationaspects related to the problem. However, sometimes

the formulation and the suspension in which the agent is

carried have an important role, and they affect the

application technology. Such a case was reported by

Atterholt et al. (1994), who found that the carrier and

the application technology significantly affected the

release and degradation rates of the insect sex pher-

omones, which formed the control agent.

Chandler and Sutter (1997) reported on a develop-

ment of a high-clearance field sprayer and spraying

methods for the application of baits. Semiochemical-based baits were applied in cornfields using water

volumes of 19 and 37 lha1. An over-the-canopy nozzles

sprayer was compared with two drop-line sprayers, one

having nozzles mounted on drop lines on every row and

the other having drop lines on alternate rows. No

significant differences in pest population reduction were

found among the nozzle configurations, therefore, the

over-the-canopy spraying technique is preferable be-

cause of its mechanical robustness.

5. Summary

The combined technologies of control agents and

application systems, referred to in the text, involved

large variety of biopesticides and sprayers. Table 1

Table 1Sprayer technology, pests and control agents referred to in the text

Author Sprayer or application technology Pest Control agent

Perez et al. (1995) Electrostatic knapsack mistblower;hydraulic, top application; hydrau-lic, drop legs

Diamondback moth Bt

Navon et al. (1997, 1999) Electrostatic pollen applicator;electrostatic, drop tubes

Lesser date moth,Helicoverpa

Bt

Mason et al. (1998a, b,1999)

Spinning discs Diamondback moth Nematode

Piggott (2000), Piggottet al. (2000)

Grooved and smooth spinningdiscs

Leaf miner Nematode withpolyacrylamide

Nilsson and Gripwall(1999)

Hydraulic; knapsack; coldfogger

Control-agent viabilitytested

Nematode

Belair et al. (1998) Hydraulic handgun; orchard airblast

Apple sawfly plumcurculio

Nematode

Parnell et al. (1999) Knapsack mistblower; Spinningdisc

Cotton bollworms Baculovirus

Bateman (1997) Spinning disc Grasshoppers andlocusts

Mycoinsecticide in oilformulations

Poprawski et al. (1997) Knapsack mistblower Colorado potato beetle Beauveria bassianaJoneset al. (1994) Knapsack with tailboom Spodoptera BaculovirusGiles et al. (1995) Refrigerated, applicator with cells

in a plateMites damage Predatory mites

Bouse and Morrison(1985)

Refrigerated aerial spraying Trichogramma Predator pupae

Atterholt et al. (1994) Dispersing pheromones inparaffin-wax capsules

Fruit moth Insect sex pheromones

Chandler and Sutter(1997)

Hydraulic}over the top and droplines

Corn rootworm Semiochemical-basedbait (SLAM)

Sundaram (1995) Laboratory}spinning discs;field}aerial/MicronairAU4000

Persistence in laboratoryand forest canopy

Diflubenzuron; Tebufen-ozide; Azadirachtin; Bt



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summarises these technologies to facilitate the selection

of the control agents and the application systems, which

may perform best for the investigated conditions. The

droplet size strongly influences the cover uniformity and

also the handling sensitivity of the beneficial insects.

Thus, Table 2 provides this crucial design data.

6. Conclusions

(1) Transition from the optimised conditions of a

laboratory experiment to the harsh conditions

experienced in the field has so far proved more

difficult for application of biopesticides in contrast

to chemicals. This has undoubtedly been due to lack

of investment in the development of effective

formulations and delivery systems, in order to

commercialise more potential biopesticides. The

relatively small effort invested in target-specific

sprayers, compared with the investment in labora-

tory studies, has led to unbalanced development,and exemplifies the need for closer integration

between formulation and engineering research.

The challenge is to get effective formulations so

that biological control agents can be easily applied

by farmers.

(2) Formulation of control agents as dry granular

particulates and baits frequently improves their

stability, which ensures a longer period of activity

of an active ingredient. Since electrostatic deposi-

tion technologies, in conjunction with air assistance,

have been developed and improved significantly, the

use of granular formulations for application of

biorational pesticides should be reconsidered, in

spite of some past and present reservations among

certain authorities.

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Table 2Typical droplet sizes of sprayers referred to in the text

Sprayer Droplet size, mm

ESS}air/liquid atomiser with electrostatics 1525Coldfogger 822Grooved spinning disc 70150Smooth spinning disc 90150Hydraulic nozzles 110200Air-blast orchard sprayer 130200Knapsack mistblower 100200



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1-s2.0-s1537511002002775-main recent developments in sprayers for application of biopesticide

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