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Cocoa (Theobroma cacao) probably originates from the upper Amazon, but is grown throughout the humid tropics, with approximately 70% produced in West Africa: principally from Côte d'Ivoire and Ghana. In this region, cocoa mirids (Sahlbergella singularis and Distantiella theobromae) are the most damaging pests causing crop losses that are second only to black pod diseases caused by Phytophthora megakarya and related species. Fungal control relies heavily on the use of copper compounds and metalaxyl, while currently, mirids are controlled with insecticides. In the past these consisted of cheaper, older compounds such as lindane and endosulfan: whose persistence and fumigant action compensated for any deficiencies in application.

Cocoa production was one of the first of the tropical tree crops to use mistblowers for pest control. This method of application evolved from having a two man team in the 1950s, one with a compression sprayer and the other with a motorised fan to project the spray high into the cocoa trees to control cocoa mirids in West Africa, where the crop typically is very tall by world standards, and it was (and continues to be) not unusual to find trees in excess of 14 meters (Fig.1).

IPARC staff participated in the evaluation of early mist-blower designs (Fig.2), and have maintained interest over the subsequent fifty years, with Clayphon (1971) describing the important criteria for evaluating machines and technical requirements now standardised by FAO (2001). Information is posted on the website www.dropdata.org . The authors are now carrying out research on novel mirid control agents, in collaboration with the Cocoa Research Institute of Ghana and the Natural Resources Institute; sponsored by Cocoa Research UK. An initial part of this work has been an exami-nation of modern commercially available knapsack mistblow-ers as a guide to their effective usage so that those growers able to purchase or lease such equipment are able to achieve maximum benefits.

*International Pesticide Application Research Centre (IPARC), Imperial College, Silwood Park, Ascot, Berks, SL5 7PY

Optimising the use of motorised knapsack mistblowers for control of insect pests of cocoa Nick Jessop and Roy Bateman*

Due to a production error in the last issue of International Pest Control (July/August 2007) a number of tables and one image were omitted from this article. As the tables were integral to the article we are republishing it in full, with our apologies to the authors.

Figure 1 A – Typical West African Cocoa

Figure 1B Stihl 420 in use on cocoa

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If deposition fails to occur in the top 25% due to the limita-tions of the spray equipment then it is probable that a propor-tion of the target species will survive the application and may result in continued damage and re-infection. Behavioural habits of some insects, such as sun basking may exacerbate this problem. Thus the most important parameters tested were droplet size and the vertical throw to assess whether the spray can reach the upper canopy.

The Importance of Droplet SizeThe most common design of nozzle is of the air-shear type, in which thin layers of liquid are introduced into the air stream and thus produce fine sprays. The combination of air assistance and production of relatively small droplets, enable motorised mistblowers to achieve good coverage at low volume application rates (i.e. without “spraying to run-off”). They are typically used to apply water based mixtures to trees at very-low volume (VLV) rates (50-200 litres/ha), but low flow rate ULV adapters are available, even achieving rates of <5 litres/hectare with oil-based formulations. Bateman and Alves (2000) carried out a number of spray measurements and discuss the droplet size distributions for mistblowers, with an optimum thought to be between 40-120μm, depending on the formulation used. Smaller droplet spectra are possible using oil formulations due to reduced evaporative rates as com-pared to aqueous formulations. If droplets are too small, then the risk of losses due to drift and convection increases. In addition, small droplets can lack the necessary momentum to

reach the desired height when spraying. If the droplets are too large then a large proportion will settle out onto the ground, or worse, the operator. Therefore, the ideal sprayer will produce a narrow spectrum of droplet sizes within a desired size range. The droplet sizing was done using a Malvern ‘Spraytec’ laser particle size analyser (www.malvern.co.uk). A typical droplet spectrum is shown in Fig 3.

The sprayers were operated at full engine speed, using manufacturer’s recommended fuel mix, to project the water spray through a laser beam to measure droplet size (Fig. 4). The volume median diameter (also know as Dv50) increases as flow rate increases (Fig. 5). The highest proportion by

Figure 2 - Original Mistblower design

Figure 3 - Average Particle Size Distribution

Figure 4 - testing a Stihl 420 using Malvern Spraytec

Figure 5 - Average Dv50 in response to Flow Rate

Figure 6 - % Spray Volume within Desired Parameters(Desired range set to 54.9 μm to 125.7 μm )

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volume within the sizes 55 – 126 µm expected to remain in the air stream as it is projected into the tree canopy indicates differences between similar mistblowers (Fig. 6) especially at the highest flow rates. The spray nozzle was mounted to ensure consistent analysis of the spray pattern. The resulting data (Fig.6) was analysed to give an average Dv50. We sug-gest that higher the proportion of the spray that occurs within the desired size range (Fig.6), the greater the suitability of the sprayer/setting for applying pesticide to a tree canopy.

Vertical Throw:The vertical projection of the five sprayers was measured using an artificial target (water-sensitive card 125mm x 50mm) attached, at one meter intervals, to a short length of high-visibility fishing line. This line was raised vertically using pulleys to a height of 12 meters. (Figures 7 and 8) All tests were performed using the FAO guidelines and conducted in wind speeds below 2 m/s. The deposits on the water-sensitive paper were scanned, converted to Greyscale and a Threshold applied to produce an image containing only black and white pixels. Software ImageJ 1.36b was used to deter-mine the number of black pixels and thus number of droplets within the image. Spray at a particular height had to exceed a median value of 5 droplets/cm2.

The height attained by the spray varied between sprayers and no sprayer succeeded in spraying to the highest measur-able card at 12m (see Table 1). When considering the two sets of results, droplet size and vertical throw, it is possible to gain a more complete picture of the sprayer’s potential. The data from the Guarany sprayer shows an effective distance achieved, but the proportion of the spray within the desired range is poor in comparison with the other sprayers, principal-ly due to the fact that this particular sprayer was supplied with a set of Ultra-Low Volume (ULV) nozzles designed for use

with oil formulations. This demonstrates the importance of having a full understanding of the capabilities of the sprayer in order to implement successful application for control.

ConclusionsIn Ghana, where 45% of the economy and a possible 60% of the population’s income is derived from cocoa (in 2004, FAO statistics put this at a little over US$1 billion), it is vital to maintain an effective pest control strategy. First mass spraying in Ghana, by the government to control capsids, was between 1959 and 1962 and consisted of mobile spray-ing units travelling through the cocoa region to maintain the equipment in good condition, however, this system was discontinued. Thus when a survey by the Cocoa Research Institute of Ghana’s Farming Systems Unit was conducted in 1991 only 0.7% of cocoa areas had been sprayed, and not all

of these cocoa farmers had successfully followed the recom-mendations in terms of spray timing and frequency. In 2003 the Ghanaian government implemented a national coordi-nated spray-programme against mirids (CODAPEC) with the aim of increasing both production and employment within the industry. As small-holders make up a large proportion of the cocoa production, the expenditure on chemicals and application equipment represents a major burden and in some cases is beyond the means of the grower. Capital expenditure on equipment such as motorized mistblowers can be shared across several growers or contracted in when required. This can obviously lead to problems of availability, as demand is naturally high when a mirid outbreak occurs.

However, choice of equipment is only part of the issue. Without a full knowledge of the biology of the target organ-ism and correct timing of treatment, much of the insecticide will inevitably be wasted. Use of thresholds for monitoring will provide growers with early indications of the need to spray. Thresholds can be set using very simple population models based on the life cycle of the target species. By con-trolling the timing and frequency of the sprays the grower should be able to maximise the return on their investment in control. Even the time of day of application can influence the levels of control achieved, with factors such as evaporation rates and insect sun basking to consider. The combination of these two knowledge bases, mechanical and biological, will decrease the risk of yield loss to the grower and therefore must be of economic importance. The successful outcome of a pest management strategy will ultimately lie with the grow-er’s eagerness to implement it on an area-wide basis. Having

Table 1

Sprayer Engine Air Air Volume Vertical Horizontal Speed Speed Flow Rate Throw Throw (rpm) (m/s) (m3/h) (m) (m)

Solo 423 5600 97.7 690 8 12

Solo 450 5500 92.6 654 10 15

Stihl 340 6550 90.6 892 7 13

Stihl 420 6950 98.0 965 11 15

Guarany 7000 96.4 1534 10 1554M ULV

Figure 7- Vertical Throw testing

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Figure 8 - Deposition analysis using water-sensitive paper

the right tools to do the job can go a long way to achieve improved production of cocoa..

References

Bateman, R.P. and Alves, R.T. (2000). Delivery systems for mycoin-secticides using oil-based formulations. Aspects of Applied Biology, 57, 163-170.

Clayphon, J.E. (1971) Comparison trials of various motorised knap-sack mistblowers at the Cocoa Research Institute of Ghana. PANS 17(2), 209-224.

Food and Agriculture Organisation (FAO) of the United Nations (2001) Standards for Agricultural Pesticide Equipment and Related Test Procedures. Volume 2: Portable (operator-carried) sprayers. FAO, Rome 86 pp.

FAO guidelines for testing motorised mistblowers (Data Sheet No. E2)

Matthews, G.A. (1999). Application of Pesticides to Crops

The authors would like to extend their thanks to Mars UK Ltd. for sponsoring this work together with assistance from Prof Graham Matthews (whom we acknowledge for Fig. 2) and Evan Thornhill.

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