The effect of hydraulic nozzles on the deposition of Steinernema feltiae through a crop canopy

Growers strive to minimise the impact of crop pests in order to increase yield and quality of horticultural products supplied to an exacting market.  A wide array of crop pests such as Frankinella occidentalis, Delia radicum and Thrips tabaci can be controlled at the larval stage with soil applications of entomopathogenic nematodes.

Two techniques exist for the application of nematodes to the soil: drip irrigation and boom spraying. Growers are recommended to apply at 1000L/Ha and 2.5 x 106 nematodes/ha by overhead boom spraying allowing drip down through the crop to the soil but the effectiveness of this technique is constrained by poor penetration of the crop canopy.

In contrast, drip application systems deliver nematodes directly and more effectively to the soil. This system requires much lower volumes of water and does not have as great an influence on humidity but requires large capital investment to install the infrastructure, unlike the spray boom which is present for many other pest control tasks.

The current technique for soil application through a dense crop canopy is to increase the volume of application via an increased flow rate. This technique however is highly dependent on the capacity of the water tank of the grower’s sprayer. Also, increased application of water can raise crop humidity which encourages plant pathogens.

The majority of nozzles are designed for the application of fluid to the foliage with a minimum of run-off, not to by-pass the foliage with the soil as the principal target. Improvement in the technology for nematode application through improved nozzle design to achieve better canopy penetration is imperative.  A wide array of nozzles exist which are cheap, easily accessible, create diverse spray patterns, and are technically able to apply nematodes, but not necessarily optimally.

This investigation aims to determine which of a range of nozzles will apply nematodes to the soil surface most effectively and potted chrysanthemums were selected as a test plant because their dense canopy provides a high level of spray interception.

Methods

Seven boom-mounted nozzle designs (see Table 1) were trialled using three nozzles of each type mounted 50cm apart on the 1.5m boom. A motorised knapsack sprayer (Shindaiwa Motors, Ltd) connected to the end of the boom generated 3 bar pressure regulated with a constant flow valve (GATE LLC. FL, USA).

Chrysanthemum growers are recommended to apply nematodes at 1,000 l/ha ≈ 0.1 L/m² at a concentration of 2.5 x 106 nematodes/l ≈ 2.5 x105 nematodes/m². The various nozzle designs created a range of flow rates requiring individual nozzles to be calibrated and a walking speed determined to ensure the standard application rate across all nozzles. For calibration, the boom was held 50cm above a 1x1m tray and the volume applied during 60sec was recorded. A Patternator (Lurmark, Ltd) was used to check a consistent application rate across the width of the spray. A mechanical pacemaker was designed to ensure the accuracy of the walking speed for each nozzle type. Three pieces of water-sensitive paper (Teejet Ltd) were also placed randomly at the base of the crop to capture and visualise deposited droplets. The volume median diameter (VMD) of the spray droplets was measured using a droplet analyser (Malvern Instruments Ltd, Worcestershire,UK).

Mature 4-weeks old un-flowered potted Chrysanthemum indicum were used at a density representative of commercial conditions and spaced evenly in frames (1m x 2m) holding 32 pots, each pot containing 5 plants. The potted Chrysanthemums were randomly selected from a stock of 200 pots for use in each application. Each nozzle was tested 3 times.

Petri dish lids (5.7cm diameter) were used to capture the spray fluid and nematodes deposited, chosen for their low vertical profile thus accentuating collection rates. Beneath the foliage, one set of three lids were randomly placed at soil level and another set at 7cm above soil level. A final three lids were placed on soil filled pots outside of the experimental frame to be exposed to the spray without interference from the foliage. Deposition results from Petri dish lids are later referred to as “Base”, “Middle” and “Control.”

Each Petri dish lid was weighed before and after each spray, the weight gain recorded and converted to L/m². Each lid was then washed in 25ml of water to collect the deposited nematodes. Three measured aliquots were removed from the 25ml sample and the number of nematodes were counted under a light dissecting microscope and converted to nematodes/m². Each sample was analysed within 3 hours of collection and only live nematodes were counted.

A single control was used in the experiment placed at soil height to reduce exposure time of deposited fluid to evaporation. To ensure that there was no significant difference in average deposition of fluid between a control Petri dish at soil level and a control petri-dish at 7 cm higher an assay was performed to determine whether a calibration was required to control for any difference in pick-up. Three Petri-dishes were placed at soil level and another three positioned at 7cm mimicking the positioning of the experimental Petri-dishes in the crop. Factors such as boom height and application rate were kept the same. No significant difference was found between the two heights (df=6, t=-0.359, p>0.05).

Results

The average spray deposit collected in the control plates was 0.09 L/m² (± S.E. 0.0045) with a mean number of nematodes of 6.8 x 105/m² (± S.E. 7.9 x 104). To correct for differences in nozzle output in each nozzle replicate, the fluid and nematode deposition rates at the plant base and at an intermediate height were adjusted proportionately according to the deposition measured in the controls on the basis that the theoretical deposition rate was 0.1L/m² and 2.5 x 105 nematodes/m². Adjusted and unadjusted values of fluid deposition are shown in Fig. 1 and nematode deposition in Fig. 2. A 60% (df=40, t=-13.5, p<0.001) and an 86% (df=40, t=-43.45, p<0.001)decrease was found in the spray fluid volume deposited at the middle and soil level of the crop relative to the controls (Fig. 1) The number of nematodes deposited decreased by 82% ( df=40, t=-82.5, p<0.001) and 93% (df=40, t=-35.5, p<0.001) at the Middle and Soil level respectively (see Fig. 2).

Left Fig. 1. Spray fluid volume deposited (adjusted and unadjusted) at soil level in pots with potted chrysanthemum plants (Base) or without plants (Control). Significant differences compared to the column on the left are displayed on the graph; ***(p<0.001)
Right Fig. 2. Nematodes (S. feltiae) deposited (adjusted and unadjusted) at soil level in pots with potted chrysanthemum plants (Base) or without plants (Control)

The Syngenta Vegetable flat fan nozzle produced the highest fluid deposition rates at an intermediate height in the canopy and at soil level of 0.056 ± 0.0016 L/m² and 0.03 ± 0.0016 L/m² at an adjusted application rate of 0.1L/m² (see Fig. 3) and the highest nematode deposition rate at soil level of 2.8 x 104 ± 2.2 x 103 nematodes/m² at an adjusted application rate of 2.5 x 105 nematodes/m² (Fig. 4).

Fig. 3. Spray fluid volume (adjusted to an application rate of 0.1 l/m²) deposited at soil level beneath potted chrysanthemum plants from a range of spray nozzles. Significant differences between the Syngenta Vegetable Nozzle (SV) and other nozzles are displayed on the graph * (p<0.05); **(p<0.01); ***(p<0.001).

Fig. 4. Nematode deposition rate at soil level beneath potted chrysanthemum plants from a range of spray nozzles delivering an adjusted application rate of 2.5 x 105 nematodes/m². Significant differences between the Syngenta Vegetable Nozzle (SV) and the other nozzles are displayed on the graph * (p<0.05).

There was a significant relationship between fluid and nematode deposition rates at the base (df=19, t=-0.31, p>0.05). There was no significant relationship between flow rate and nematodes/m² (df=19, t=0.623,p>0.05) or flow rate and L/m² (df=19, t=0.430,p>0.05). There was no significant relationship between fluid or nematode deposition rates and the order plants were used in for experimentation (df=19, -1.031, p>0.05) and (df=19,t=-1.14,p<0.05) respectively. There was no significant relationship between fluid or nematode deposition rates and walking speed (df=19, t=-0.474, p>0.05) and (df=19, 0.202, p>0.05) respectively.

There was no significant relationship between flow rate or nematode deposition (nematodes/m²) and VMD for all nozzles (df=5, R²=0.29,t=1.861, p>0.05) or for nozzles with a vertical spray pattern only (df=3, R²=0.58, t=2.589, p>0.05) (see Fig. 5)

Fig. 5. The relationship between the VMD and the volume deposited at the base of each vertical spray pattern; 1. XR-TT-03, 2. XR-TT-05, 3.LD-03, 4.LD-05, 5. SV, 6.HT-04, 7.HT-06. The blue coloured points are nozzles with a vertical spray orientation. The red points are the nozzles with a 30˚ angled spray.

Discussion
This study has demonstrated the importance of nozzle type on soil applications through a dense crop canopy. There was a 46% difference in fluid deposition and a 66% difference in nematode deposition between nozzles. The “Syngenta Vegetable Nozzle” (SV) was the most effective for penetrating the canopy layer with the highest VMD, L/m² and nematodes per m² reaching the soil. Syngenta’s description of the nozzle is “A high impact fan nozzle which pushes spray into the crop canopy by increasing the velocity of fluid exiting the nozzle, designed to get spray to the crown of carrots.” In use, this nozzle was seen to open the canopy, allowing the spray to penetrate the foliage. The suggested increased velocity of fluid could not be quantified, but the increased size of the droplets (VMD 647µm) increased the momentum of the spray from the SV nozzle and thus improved its penetrative capabilities. A relationship was apparent between the penetrative capabilities of a nozzle and its VMD, however due to limited number of nozzle types, the relationship was not significant. In combination with the highest application rate, the Syngenta Vegetable nozzle had a fast nozzle velocity; therefore the simple adoption of this nozzle in IPM programs would help improve pest suppression with a reduction in effort.

With low replication and the high variation between applications of the different nozzles, nematode deposition was only significantly different between the Syngenta Vegetatble (SV) and the Low drift – 05 nozzle. The concentration of nematodes deposited in the controls was higher than expected in comparison with the delivered concentration.

Foque and Nuyttens, (2010) investigated the influence of spray angle on the ability of a nozzle to penetrate the crop foliage, concluding that to use a vertical nozzle is essential. This idea has been supported by the current investigation. The lowest deposition rates to the soil came from those nozzles at a 30° angle as these nozzles are designed to increase foliage application.

This preliminary study has illustrated that improved application of pesticides and bio-pesticides through a dense canopy is possible. It has also highlighted that multiple factors will influence optimum application. This study hypothesises that vertically orientated applications, with high velocity and high VMD will significantly improve the effectiveness of delivery to the soil. This area warrants further research.

Bibliography
Foque, D. & Nuyttens, D. (2010). Pest Management Science. 67, 2, p199-208

Authors:-

2 – Becker Underwood Ltd, Littlehampton,

Published in International Pest Control – January/February 2013 issue.

Category: Agriculture