Comparison of ultra-low-volume electrostatic and high-volume hydraulic application of Verticillium lecanii for aphid control on chrysanthemums

Comparison of ultra-low-volume electrostatic and high-volume hydraulic applicationof VerticiUizam lecaniifor aphid control on chrysanthemums P. I. SOPP, A. T. GILLESPIE AND ANNE PALMER AFRC Institute of Horticultural Research-Littlehampton, Worthing Road, Littlehampton, West Sussex B.Nl7 6LP, UK

ABSTRACT. Using an electrostatic ULV rotary atomizer (APE-80) and a conventional high-volume sprayer, Vertidliumlecaniiblastospores were applied to chrysanthemums infested with Aphisgos.ypii and Macrosiphoniella srmbomi. The deposition of droplets and spores by each sprayer and the subsequent infection of aphids were compared. Four combinations of spore dosage rate and timing of application were made with each sprayer, with untreated plots serving as controls. Using the APE-80 the droplet density, on the abaxial leaf surfaces, was significantly greater than with the hydraulic sprayer. Significantly more spores were deposited by the APE-80, with 3645% of spores being deposited on the abaxial surface compared with 15-23X using the hydraulic sprayer. Infection of aphids occurred earlier on electrostatically treated plots and aphid populations peaked at significantly lower densities. A single full-rate treatment or twelve one-twelfth-rate treatments resulted in lower numbers of aphids compared with two half-rate or six one-sixth-rate treatments, regardless of application method. No significant effect from adding nutrient to the formulation was seen.

KEYWORDS:

Vertidium lecanii; electrostatics; spray; aphids; Aphisgosyfiii; Macrosiphoniella sanbomi;

chrysanthemum

Introduction In recent years, the development of resistance to pesticides in many glasshouse pests has led to increased research on biological control agents. The deuteromycete fungus Verticillium lecanii (Zimm.) Viegas is an entomopathogen, primarily of aphids and scale insects, common in many tropical regions (Viegas, 1939; Baird, 1958). It occasionally causes epizootics in temperate glasshouses (Rombach and Gillespie, 1988). Hall and Burges ( 1979) showed that a single aqueous spray of V. lecanii could control populations of M’j~zm persicae (Sulz.) in chrysanthemum crops. A commercial formulation, Vertalec, was developed which provided satisfactory control of M. per&~ following a single spray of 2.5 kg of product ( W 5 X 10 12 spores) in 500-1000 litres of water per hectare. However, control of two other aphid species, Aphis gossypii (Glover) and Macrosiphoniella sanborni Gill., which are serious pests of chrysanthemums, proved unreliable. Hall ( 1976) showed that, in the laboratory, A. gossypii and M.pe~sicae were equally susceptible to V. lecanii. Hall and Papierok ( 1982) suggested that poor control in the glasshouse was due to differences in aphid behaviour. Contact between the aphid and the spray deposit is essential if infection is to occur; in this respect V. lecanii is analogous to a contact insecticide. A.

gosypii is a sedentary aphid, rarely found away from abaxial leaf surfaces (Sopp, Gillespie and Palmer, 1989). In contrast, M. persicae is very mobile and, thus, contact with spores on dead insects, or from saprophytic growth on nutrient contained in the commercial formulation, is probably common. M. sanborni is found almost exclusively on the stems and lack of control may be due to poor spray coverage and/or low humidity around the exposed stems. Helyer and Wardlow (1987) used repeated low-rate spore applications in high-volume sprays with some success against A. gossypii. By spraying low doses of spores at regular intervals this technique, in effect, improved the chances of spores encountering suitable environmental conditions for germination and subsequent aphid infection and increased the likelihood of spores encountering aphids. However, the longer application times required and the risk of encouraging fungal plant diseases by regular application of large volumes of water to the crop, proved unpopular. The use of an ultra-low-volume application method could overcome both these problems and some success was obtained by Helyer and Wardlow (1987) using the Turbair Fox and the Dynafog. Electrostatic sprayers have been shown to be effective in improving abaxial deposits of chemical sprays

0261-2194/90/03/017748 0 1990 Butterworth-Heinemann Ltd CROP

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(Cayley et al., 1984, 1987; Adams and Palmer, 1986) but there is little information on the application of microbial pesticides using such sprayers. Law and Mills (1980) reported equal or improved control of caterpillar pests on broccoli by Bacillus thuringiensis (Berliner.) when applied through an electrostatic twin-fluid nozzle compared with a conventional hydraulic application. Our early studies (Sopp et al., 1989) showed that application of V. lecanii using an electrostatic rotary atomizer could improve A. gos.&i control. This paper describes an experiment to verify these earlier results with A. gossypii and to extend the study to M. sanborni. Wyatt (1965) reported varietal differences in susceptibility to M. persicae in chrysanthemum cultivars and, following recetn studies, two cultivars believed to show differing susceptibilites to A. gossrpii (R. J. Chambers, personal communication) were included in the experiment. Materials

and methods

lecariii

two chambers (one cv. White Snowdon and one cv. Yellow Westland) received a nutrient dosage rate of 2 kg ha- ’ in each spray while the other two chambers received rates reduced in proportion to the spore rate applied. Hydraulic applications, at 1000 1 ha- I, were made with a compression sprayer (Airflow Ltd, UK) operated at a pressure of 2.5 bar with an 80 degree flat-fan nozzle producing a medium spray (Anonymous, 1986). Electrostatic applications were made using the APE-80 rotary electrostatic sprayer (Arnold and Pye, 1980) operated at 10 1 ha- ’ with a flow rate of 11 ml min- ’. Each spray contained 0.05% Triton X- 100 as a wetting agent. During spraying, and for 10 minutes afterwards, treatment plots were screened by polyethylene sheeting to prevent drift to adjacent plots. All spray applications were made in the late afternoon, usually within the 90 min before blackout. Each r lecanii treatment was applied hydraulically and electrostatically in each chamber, giving four replicates of each treatment. All V. lecanii treatments were started on 10 August, during the first week of blackout (week 1).

The experiment took place in four chambers (each glasshouse at the AFRC 6 x 7 m) of an Alumabrite Institute of Horticultural Research, Littlehampton (IHR-L). Rooted chrysanthemum cuttings were planted in nine plots each of 7 x 14 plants (0.9 x 2.0 m) in each chamber on 29 July 1987. Commercial growing practices were followed, with the plants receiving 2 weeks of natural long daylength ( x 16 h) after planting followed by 6 weeks of short daylengths (10 h) provided by black polyethylene blackouts before reverting to natural daylength. On 7 August 1987, each plot was infested with one adult Aphisgossypii and one adult Macrosiphoniella sanborni on each of the centre 10 plants in the middle row. Verticillium lecanii blastospores were prepared and spore concentration and viability assessed using the methods of Hall and Burges (1979). Viability counts were consistently between 94 and 100%. Treatments of cultivar, nutrient level, spore rate and application method were applied to a split-plot design, with cultivars and nutrients as whole-plot factors and spore dose rate and method of application as sub-plot factors. Two chambers were planted with cv. White Snowdon and two chambers with cv. Yellow Westland. Two treatment factors, hydraulic or electrostatic method of application and dosage rate of spores, were tested in all combinations in each chamber. The remaining ninth plot was untreated. Four V. lecanii dosage rates were used: (1) full rate at week 1; (2) half rate at weeks 1 and 4; (3) one-sixth rate at weekly intervals; and (4) one-twelfth rate at twiceweekly intervals. Hence, all treated plots received the same total number of spores over the course of the experiment (equivalent to 2 x lOI blastospores ha- ‘). Nutrient was applied in the spray as skimmed milk and vegetable fat (Kerrygold Plenty@). The plots in

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Assessments Weekly counts of live and infected aphids were made on each of the centre 30 plants from each plot. It was not possible to identify dead aphids to species. Actual numbers of aphids were recorded up to five and thereafter, for practical purposes, a scoring system was used; from six to ten aphids or cadavers per plant a score of 6 was recorded; from 11 to 50 a score of 7; from 5 1 to 100 a score of 8 and > 100 a score of 9. Mean scores were calculated for each plot to produce continuously distributed variates suitable for analysis of variance. The compartments were treated as mainplots and plots within compartments as sub-plots. The residual error variance was estimated by pooling the higher order interactions. It should be noted that a withinestimated on term error residual compartments may not provide an unbiased estimate of between-compartment variability (cultivar and nutrient comparisons). Homogeneity of the data was checked by residual scatter plots. The numbers of other aphid species occurring naturally in the crop were also recorded. At flowering, plants were scored for marketable quality by experienced nursery staff, taking into account the numbers of live aphids and cadavers present. Spore numbers on leaves were assessed by removing leaf discs (1 cm2) and washing them in 5 ml 0.05% Triton X- 100 for 60 s. A sample of the washings was then spread on Sabouraud dextrose agar, containing sulphate and chlor100 pg ml- 1 of both streptomycin amphenicol (Sigma Ltd, Poole, UK), and incubated at 20°C for up to 7 days. The numbers of V. lecanii colonies were then counted. Six leaf discs were removed from the top (top 20 cm) and bottom ( 1O-20 cm from soil) of each of the centre 10 plants on

P. 1. SoPP et al. each plot every week. Leaf discs from individual plots were combined, for practical purposes, for leaf washing to give a mean density of colony-forming units (c.f.u.) for individual plots. In weeks 1 and 4, small sections of stem were removed and assessed in the same way. Data were log transformed before analysis of variance. To differentiate between adaxial and abaxial spore deposition, in weeks 1 and 4, small areas of leaf (adaxial and abaxial) on the full-rate plots were covered with adhesive tape, thus preventing spore deposition on the covered leaf surface. Immediately after application of the spray, the tape was removed, leaf discs taken and the discs washed to assess spore numbers. All spores subsequently recorded from a leaf disc which had the adaxial surface covered must, therefore, have been deposited on the abaxial surface, and vice versa. The density and distribution of spray droplets was recorded using 1 x 2 cm strips of water-sensitive paper (Ciba-Geigy, UK) fixed to leaves on the centre plants using double-sided adhesive tape. Preliminary work, using fluorescent tracers, had showed that such papers were adequately earthed, as droplet densities in adjacent areas of leaf were comparable. In each plot, ten strips of paper were fixed to abaxial and ten to adaxial surfaces at each of two heights within the plant canopy: 15 cm below the top of the plants and 20 cm above soils. Ten strips were also wrapped around stems 15 cm below the top of the plant. The density of impacted drops was measured using a Microsight II image analyser (Digithurst Ltd, Royston, UK). Squirrel data loggers (Grant Ltd, Cambridge, UK) monitored temperature and humidity at 30 min intervals throughout the experiment with probes positioned within the crop and at 10 cm above the crop canopy. To prevent an infestation of the two-spotted spider mite (Tetrunychus urticue Koch.), the predatory mite, Phytoseiuluspersimilis Athias-Henriot, was used prophylactically. Results

The mean temperatures in the four chambers throughout the experiment were in the range of 23.6-24.2”C with 65-73% r.h. Under the blackout the corresponding values were 19.420.2”C with 98.6-100% r.h. The conditions during blackout are within the optimum temperature and humidity ranges for germination and growth of Verticillium lecanii (Mimer and Lutton, 1986). Aphid populations Aphis gosvpii and Macrosiphoniella sanborni were the only

species of aphids recorded on the crop. Aphis gossypii. Two weeks after the first spore application, all the electrostatic treatments had fewer aphids present than their equivalent hydraulic treat-

179

ment (Figure 1). The full-rate and one-sixth-rate electrostatic treatments gave greater control at week 3 compared with the half-rate and one-twelfth-rate treatments. By contrast, the full-rate and one-twelfthrate hydraulic treatments were superior to the other hydraulic treatments. Four weeks after the first applications, significantly (p < 0.001) fewer aphids were present on the electrostatic treatments compared with the hydraulic treatments, and all V. lecanii treatments had significantly fewer aphids than the untreated controls @ < 0.00 1). Between weeks 4 and 5, epizootics occurred on all plots, including the unsprayed controls, and virtually all aphids were dead by week 6. Macrosiphoniella sanborni. Two and three weeks after the initial spore application, all plots treated with V. lecanii had significantly (p < 0.0 1) fewer aphids than the untreated plots (Figure 2). In addition, the electrostatic treatments had significantly fewer aphids than the hydraulic treatments (p < 0.01). The electrostatic treatments continued to provide greater aphid control throughout the experiment. Only the one-twelfth-rate hydraulic treatment approached the efficacy of the electrostatic treatments up to week 4 (Figure 2). Epizootics occurred on all plots after week 4 and most aphids were dead by week 6. The most effective spray strategy, for both electrostatic and hydraulic applications, was 12 sprays of one-twelfth-rate V. lecanii. M. sanborni increased at a much faster rate than A. gossypii. Cadavers.

Few cadavers were recorded in week 2 except from the electrostatic full-rate treatment (Figure 3). Three weeks after the first application, significantly (p < 0.05) more cadavers were present on the electrostatic treatments compared with the hydraulic treatments (Figure 3). In contrast, significantly @ < 0.01) more cadavers were found on the hydraulic treatments by week 5. This was the result of higher aphid populations in the preceding 2 weeks compared with the electrostatic treatments. Following the development of epizootics, significantly (p < 0.00 1) more cadavers were present on the untreated plots than on treated plots. No significant differences were found that could be attributed to nutrient level. A number of significant (p C 0.05) differences between varieties were detected, but these were not consistent throughout the experiment. The proportion of the crop that was of marketable quality was higher in all the electrostatic treatments than any of the hydraulic treatments (Tuble I). The subjective and unreplicated nature of this assessment precludes statistical analysis. Colony-forming units (c.jIu.)

The number of c.f.u. is the result of a number of factors including initial spore deposition, subsequent spore survival and sporulation from saprophytic growth and cadavers. Consequently the only data that result solely

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Vol. 9 June 1990

Spray a#dication of Verticillium

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FIGURE 1. Effect of electrostatic (a) and hydraulic (b) applications of Verticillium lecanii on Aphis gossypii populations on chrysanthemum. ---, No treatment; 0, full rate; ?? , half rate; A, one-sixth rate; 4, one-twelfth rate. Bars represent s.e.d. See text for details of aphid population scoring system

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(a) and hydraulic (b) applications of Verticiifium iecanii on Macrosiphoniella sunborni populations on chrysanthemum. --- , A, one-sixth rate; ?? , one-twelfth rate. Bars represent s.e.d. See text for details ofaphid population scoring system

No treatment;0, full rate; ?? , halfrate;

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P. I. SoPP et al. =

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of crop for market after different spray applications Crop suitable for market

Spore dosage rate Full l/2 l/6 l/12 No treatment

Electrostatic

application

(%)

Hydraulic

68 48 53 61

application 42 27 36 41

5

from the spray application are from week 1 and from the top of the plants in weeks 2 and 3. After week 3, vertical growth of the plant was much reduced and c.f.u. counts result from all the factors given above. After week 2, samples were divided into those from the upper and lower positions ( 15-20 cm from the top and 2&25 cm from the soil, respectively). In week 1, there were significantly (p < 0.05) more c.f.u. on the electrostatically sprayed plots than on the equivalent hydraulic treatment, and a similar trend continued through to week 5 (Table 2). Numbers of c.f.u. increased on all the electrostatically treated plots up to week 2, compared with the hydraulic treatments where only two spore rates showed an increase. After

I

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(week)

FIGURE 3. Number ofcadavers killed by Verricillium lecanii on the electrostatic (a) and hydraulic (b) treatments. ---, No treatment; rate; A, one-sixth rate; +, one-twelfth rate. Bars represent s.e.d. See text for details of cadaver population scoring system

TABLE 1. Suitability

I I

0, full rate; ?? , half

week 3, only the one-twelfth-rate treatments showed increases, except at week 4 where the half-rate plots received their second spray. Numbers of c.f.u. decreased on all plots after week 6 when the use of blackouts ceased. The proportion of spores recovered from the abaxial surface of the electrostatically treated plants was 36% in week 1 and 45% in week 4. The corresponding proportions from the hydraulically treated plants were 15 and 23%. Spray deposition

Droplets were collected on four occasions (weeks 1,2,4 and 6) from the top and bottom of the plants. Droplet densities on the abaxial surfaces of the top of the electrostatically treated plants were significantly higher (p < 0.00 1) than those from the hydraulically treated plants (Tuble 3). Abaxial densities were also significantly higher at the bottom of the plants on the electrostatic treatment (weeks 1 and 6, p< 0.01; week 2, p < 0.05; week 4, p< 0.001). At both sampling heights and on all dates, the electrostatic sprayer deposited between 5 and 30 times as many droplets on the abaxial surfaces of leaves than the hydraulic

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Sprayapplication 0fVerticillium

TABLE 2. Mean density of colony-forming

lecanii

units at the top and bottom of the crop determined

by leaf and stem washing

Spores (c.f.u. cm- 2 ) at week Position

Treatment

Rate

1

2

3

4

5

Top of the crop (top 20 cm)

Electrostatic

Full

1572 991 486 372

1761 1480 1243 1007

1950 825 1125 1002

1610 1320 1020 1150

1020 910 980 1220

430 1130

625 200 338 500

1121 698 401 193

945 650 695 565

450 440 670 700

540 950 540 930

420 410 720 860

660 160 645 900

200 162 375 450

0

0

l/2 l/6 l/12 Hydraulic

Full 112 l/6 l/12

None Bottom of the crop (I&20cm)

Electrostatic

Full l/2 f/6 l/12

Hydraulic

Full l/2 l/6 l/12

55

20

85

160

75

1140 640 820 1050

1150 475 730 890

1250 305 540 740

720 275 430 670

980 500 548 1110

430 470 360 800

375 387 300 700

520 295 320 735

340 245 310 545

130

380

400

520

13

Electrostatic

Hydraulic

Full

1098

f/2 l/6 l/12

458 364 246

430 367 230 188

Full

340 234 132 74

86 158 62 48

0

18

f/2 f/6 l/12

TABLE 3. Droplet density (number

cm- ‘) assessed from water-sensitive

990 750

7

2120 780 1020 1240

None Stem (top 20cm)

6

papers fixed to the plants Droplet density (no. cm- z, at week

Plant part/position Leaves: top of plant Adaxial

Abaxial

Leaves: bottom of plant Adaxial

Abaxial

Stems

I

2

4

6

Electrostatic Hydraulic s.e.d.

624 -0

583

620

572

electrostatic Hydraulic se. d

521 24 137.7

482 36 112.3

501 71 130.2

489

Electrostatic Hydraulic s.e.d.

407 _a

330 _a

351 275 66.2

478 318 94.8

Electrostatic Hydraulic s.e.d.

320 38 97.6

215 79 52.7

232 89 46.9

290 56 77.3

Electrostatic Hydraulic s.e.d.

300 38 87.3

340 56 79.2

286 12 81.0

330 49 86.1

Treatment

“Droplets coalesced, thererore impossible to count; s.e.d. not calculated, because

ofcoalescence

sprayer. Adaxial data are not available for the hydraulic sprayer, except at the bottom of the plants in weeks 4 and 6, as the droplets coalesced and could not be distinguished on the water-sensitive paper. There was no significant difference in adaxial deposits between the two application systems in weeks 4 and 6. The number of droplets deposited on the stem by the was significantly higher electrostatic sprayer (p
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29 120.3

Discussion The use of microbial pesticides imposes strict conditions on the performance of the application equipment used, in terms of the deposition site of the spray. Bacterial and viral pesticides need to be ingested to be effective, and therefore deposition in the insect feeding area is essential. In the case offungi, external contact is required for a period of time to allow the germinating hyphae to penetrate the cuticle. For contact to occur,

P. I. SoPP et al. spores must be deposited either on the insect itselfor in an area where the insect is likely to come into contact with them. M. persicae is a very active aphid and probably comes into contact with spores on several areas of the plant. A. gossypii and M. sanborni, however, are far more sedentary and rarely move from the abaxial leaf surface, or the plant stem, respectively. In terms of targeting a spray, both these sites are difficult to hit. Conventional hydraulic systems rarely achieve good deposits at either site (e.g. Pay, 1984). Redistribution of spores after deposition is unlikely to be significant and the majority would not come into contact with either A. gossypii or M. sanborni. In this experiment, the electrostatic sprayer deposited a significantly higher number of spores on the abaxial leaf surfaces and stems than did the conventional application. This increases the possibility of contact between the target aphids and the spores. In addition, the higher relative humidity at the abaxial surface, compared with the adaxial (Jones, 1983), increases the chances of successful spore germination. The combination of increased spore-aphid contact and successful germination probably resulted in the earlier appearance of cadavers and the higher number of c.fu. recovered from the electrostatic treatments. It is extremely difficult to explain changes in c.fu. density on the crop during the experiment (Table 2) because of the number of potential unknown variables involved: deposition, subsequent survival rate and sporulation from saprophytic growth or cadavers. The relative influence of each variable will have varied, to an unknown extent, throughout the experiment. It is clear that application of fresh spores to the one-sixthand one-twelfth-rate treatments had relatively little effect on c.f.u. density after week 3 (Table 2). The half-rate treatment showed an increase in density immediately after the second application (week 4, Table 2) but quickly declined thereafter. However, c.f.u. density is always higher, with one exception, on the electrostatically treated crop. Many differences in c.f.u. density were seen between this experiment and that by Sopp et al. (1989), which used similar spore dosage rates. Densities of c.f.u. were initially much higher in the earlier experiment, but subsequently were much lower. As none of the samples was taken immediately after application (at least a 16 h delay between application and sampling) it is not possible to determine if the initial deposits were the same in the two experiments, as differences in spore survival may have occurred between application and sampling. The most likely explanation for the differences between the experiments is a difference in the local microclimate around the spores. Although the relative humidity of the air around the plant was monitored, the microclimate boundary layer ( < 1 mm) in which the spores were lying cannot be measured. Differences in transpiration rate of the crop may even be significant in spore survival. Detailed studies on the factors affecting spore survival on leaves are urgently required. Similarly, the relationship between c.f.u. density and aphid

183

infection is not clear and studies on the optimum spore concentration and droplet size and density are needed. The primary aim of V: Zecanii treatments is to cause an epizootic among the aphid population; this was achieved on all plots. Epizootics reduced all aphid populations to approximately the same level, although the number of cadavers was significantly higher on the hydraulic treatments. The failure of the hydraulic treatments to inititate early epizootics, allowed aphid populations to increase, thus resulting in large numbers of cadavers by weeks 5 and 6. High numbers of cadavers are cosmetically damaging and resulted in much of the crop being unmarketable. The artificially high numbers of aphids present in this experiment resulted in an unacceptably low proportion of the crop being marketable, even from the most effective treatments. In a commercial situation, fewer aphids are likely to be present and loss due to spoilage by cadavers is expected to be lower. In common with a previous study (Sopp et al., 1989) the single full-rate spray initiated infection of the aphid population while it was still at a low density, which resulted in very few aphids (dead or alive) on the crop at the end of the experiment. The single full-rate treatment and the twelve one-twelfth-rate treatments resulted in the lowest numbers of aphids, regardless of the application method used. However, in the earlier study the twelve one-twelfth-rate treatments gave poorer control of A. gossypii than the six one-sixth-rate treatments for both application methods. This may be related to differing rates of spore survival on the days the treatments were made, caused by the variables discussed above. The experience of the Agricultural Development and Advisory Service (ADAS), using hydraulic methods of applying Vertalec on commercial holdings, indicates that a strategy of several low-rate applications is the most reliable (L. Wardlow, personal communication). In a situation where continual re-invasion of the crop by aphids is occurring, as is likely on a commercial holding, repeated applications of fresh spores, especially to the top of the plants, may prove to be the best strategy, possibly with an initial full-rate application to establish infection of any aphids present on the cuttings. A commercial crop would probably have a lower aphid population and epizootics may be slower to develop. No significant effect of adding nutrient to the formulation was seen in this study. Laboratory studies (R. A. Hall, personal communication) have indicated that a nutrient supply can give a tenfold increase in the number 0fc.f.u. 10 days after application. It is possible that the environmental conditions experienced in this study did not provide sufficiently long periods of high r.h. to enable successful saprophytic growth and subsequent sporulation. The use of electrostatics to apply microbial insecticides is still a largely unexplored area. Dick (1981) used an earlier version of the APE-80 to apply K lecanii for aphid control on chrysanthemums. Control was inferior to that obtained by a conventional sprayer

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Spray application

and Dick (1981) noted that abaxial deposits appeared to be low. However, no details of the assessment method or data are provided, making comparison with this experiment impossible. Sopp et al. (1989), using the same electrostatic sprayer as in the present study, obtained results comparable to those given here. The initial spore deposits were higher and better distributed, with respect to the position of the aphid population, than with a hydraulic sprayer. Sopp et al. ( 1989) recorded significantly lower populations of aphids on the electrostatically treated plots in one week only. However, the proportion of the aphid population killed by V. lecunii was higher on the electrostatically treated plots throughout the experiment. It is clear that many questions remain to be answered, especially regarding the best spray rate and timings, the variables affecting c.f.u. density and the relationship between c.f.u. density and aphid infection. Despite these unknowns, the electrostatic spray is more effective in initiating early infection and control of the aphid population. These studies indicate that the use of electrostatic sprayers could be a major aid to the successful use of microbial pesticides. The requirement to achieve good abaxial deposits and the need to minimize wastage of these expensive products makes the use of electrostatics ideal for this purpose.

0fVerticillium

lecanii

British Crop Protection Council Monograph 24, pp. 109-l 17 (ed. by J. 0. Walker). Croydon: BCPC. BAIRD, R. B. (1958). The Artijicial Control of insects by Means of Entomogenous

The authors are grateful to David Griffiths and Barry Pye (IACR-Rothamsted) for the loan of the APE-80, the nursery staff for market assessments and Rodney Edmondson (IHRL Statistics Section) for experimental design and statistical analysis. Vertalec is a registered trademark of Koppert B. V. of The Netherlands, Turbair Fox is a trademark of Pan Britannica Industries, UK and Dynafog is a trademark of Curtis Dyna Corporation, Indiana, USA. References ADAMS, A. J. AND

PALMER,A. (1986). Deposition patterns ofsmall droplets applied to a tomato crop using the Ulvafan and two prototype electrostatic sprayers. Crop Protection 5, 358-364. ANONYMOUS(1986). .No.&e Selection Handbook. Croydon: British Crop Protection Council. 40 pp. ARNOLD, A. J. AND PYE, B. J. (1980). Spray application with charged rotary atomisers. In: Spraying Systems for the 1980s.

Vol. 9 June

1990

of References

with Abstracts.

Crop Protection 6, 365-370.

Journal

of Invertebrate Pathology 27, 4 1-48.

HALL, R. A. AND BURCES, H. D. (1979). Control of aphids in glasshouses with the fungus, Verticillium lecanii. Annals of Applied Biology 93, 235-246.

HALL, R. A. ANDPAPIEROK,B. (1982). Fungi as biological control agents of arthropods of agricultural and medical importance. Parasitology &1, 205-240.

HELYER, N. L. AND WARDLOW, L. R. (1987). Aphid control of chrysanthemum using frequent, low dose applications of Verticillium lecanii. Bulletin SROP/ WPRS

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Received 9 August 1989 Revised 28 November 1989 Accepted 30 November 1989