How do different adjuvants react at different spray volumes when applied with either Spirotetramat or Pyriproxyfen containing insecticides for the control of California red scale on citrus?

Introduction

It has been well documented that the South African citrus industry relies heavily on medium to high-volume cover crop sprays, especially when managing pests such as red scale and mealybug or diseases such as Alternaria brown spot (ABS) and Citrus black spot (CBS) (Van Zyl et al., 2013). According to Beatie et al (1989) a medium-volume cover spray is defined as 2000-7000 L water/ ha and a high-volume spray as more than 7000 L water/ ha. These defined volumes compare readily with the volumes of 6000 L water to an extremely high 16000 L water/ ha currently reported from South Africa (Fourie, 2013). Although these high volumes have become the market standard and are known to be effective in managing these target pests (Grout, 1997, 2003), conventional airblast sprayers and oscillating boom sprayers capable of achieving them are known to be more wasteful and costly to operate (Furness and Piczewski, 1985). 

Currently the standard spraying volumes in citrus production is indiscriminately determined without taking tree characteristics into consideration (Scapin et al., 2015). However, several studies over the years have focussed on the optimisation of agricultural sprays using quantifiable approaches based on tree characteristics such as tree row volume (TRV), leaf area index (LAI) and surface area index (SAI) (Bataglia, 1999). Of these, TRV has shown definite promise as spray volumes have been established in Brazil for the control of several citrus diseases such as citrus bacterial canker (Scapin et al., 2015), citrus black spot (Silva Junior et al., 2016) and post bloom fruit drop (Soares, 2015), as well as certain pests including the Asian citrus psyllid (Scardelato, 2013) and leprosis virus-transmitting mite (Scihieri, 2018). In which the case of citrus canker management resulted in a 73% reduction in water usage as well as a 40% in savings. 

From the mentioned research it has become apparent that an improved deposition uniformity led to higher levels of control of not only citrus diseases, but also certain pests requiring medium to high-volume cover sprays as well. Unfortunately, this coincided with higher spraying volumes.

Therefore, the challenge is to improve uniformity without the coinciding increase in spraying volume to eventually achieve more efficient spraying volumes without a decrease in efficacy.

Previous studies by Van Zyl et al. (2019), found that certain adjuvants increase the deposition uniformity of spraying applications. Thus, the aim of the research was to determine if TRV, when used in conjunction with several different types of adjuvants, could realise similar levels of biological efficacy against the different target pests at lower spraying volumes compared to standard high volumes as well as the effect that different production formulations would have on this.

Materials and Methods

A Nadorcott citrus cultivar on Carrizo Citrange rootstock with a history of Californian red scale infestation was selected in the Porterville area of the Western Cape Province of South Africa for the trial site. The TRV of the experimental site was estimated on two occasions. Firstly in September before any spraying occurred and then secondly, in the middle of the spraying season late November. The area of 1 ha (10 000m2) was divided

by the tree spacing (2 m between trees and 6 m between tree rows) and then multiplied by the average tree height and width in accordance with the concept developed by Sutton and Unrath (1988). The average values were calculated by measuring the width and height of 10 randomly selected trees throughout each of the experimental sites. The density factor currently standardised by the pome and stone fruit industry as indicated on several registered strobilurin fungicide labels (l water /ha = (Tree height x Tree width x 937) /Row width) of 93.7 litre/1000 m3 TRV was used throughout the study.

Trial Layout

At the trial site, the grower’s spray equipment was calibrated using tractor speed and water pressure to achieve 1: 1728 L water/ ha, 2: 3835 L water/ ha and 3: 6080 L water/ ha calculated TRV based volumes. At the time of the investigation the commercial standard spray volume on the farm for the specific target pest was 6080 L water/ ha. 

Each of the spraying volumes were used to apply either a systemic insecticide: Movento® (from Bayer (Pty) Ltd.; active ingredient: 240 g/ L spirotetramat; Reg. No. L8559 of Act 36 of 1947) at 10 ml/ 100 L water and a contact insecticide: Nemesis® 100 EC (from Philagro South Africa (Pty) Ltd.; active ingredient: 100g/ L pyriproxyfen; Reg. No. L6378 of Act 36 of 1947) at 30 ml/ 100 L water registered to control Californian red scale. Label instructions regarding the recommended programme were followed. This entailed both product programmes to start either between 80% and 100% petal fall or at the first sign of crawler movement with follow up applications of the Movento and Nemesis 100 EC programmes respectively 4 and 6 weeks later. 

The first sign of crawler movement ended up being the determining event to initiate sprays. Two sprays were conducted. One on 7 November 2022 and another on 12 December 2022.

Furthermore, each of the TRV based spray volume systemic/contact insecticide combination programmes have been amended by adding one of the following Act 36 of 1947 registered adjuvants:

• • Nu-Film 17 (extender sticker-spreader from Miller® Chemical & Fertilizer, LLC; active ingredient: 905 g/L di-1-p-Menthene; Reg. No. L 2981) at 600 ml/ ha.
  • Entrée (non-ionic activator enhancer from Miller® Chemical & Fertilizer, LLC; active ingredient: 819 g/L vegetable oil; Reg. No. L 8055) at 1200 ml/ ha.
  • Super wetter at 100 ml/ 100 L water.
  • Mineral oil at 300 ml/ 100 L water.

The objective was to determine the effect of the different adjuvants on the efficacy of the insecticides at the different Calculated TRV’s. The untreated in this regard was left unsprayed while a treated control received a TRV 1, 2 and 3 TRV and 3 with a systemic or contact insecticide programme without the addition of an adjuvant (water only). Each treatment consisted of 3 randomly selected five tree plots of which the three centre trees served as data trees. An untreated buffer row was left between each treated row.

Assessment of Insect Pests

On 5 June 2023 (176 days after last application) 10 fruit per tree position (inside and outside of the canopy) on each of the 3 data trees per plot were inspected for the presence of red scale according to an infestation scale of 1 being infested and 0 not infested.

Results and Discussion

From Figures 1 to 4 it can be clearly observed that all the treatments numerically reduced the percentage of red scale infected fruit compared to the untreated control. With both Entrée and Nu-Film 17 (Fig 1b and 2b) there was an increased efficacy from the 1728L/ha volume to the 3835L/ha application when applied with the contact insecticide pyriproxyfen. There was however a slight reduction in efficacy at the highest spraying volume of 6080L/ha. This can be due to Entrée and Nu-Film 17 having a per hectare dosage. However, when in combination with the systemic insecticide, spirotetramat, the efficacy increased with an increase in spraying volume when applied with Nu-Film 17 (Fig 1a), while Entrée (Fig 2a) had a similar effect as with the pyriproxyfen. Mineral oil (Fig 3) in both cases had an increased effect on efficacy with an increase in spraying volume when applied with both insecticides. The super wetter (Fig 4) on the other hand had a reduced effect on red scale control with both registered insecticides as the spraying volume increased. This can be attributed to spray run-off. In both cases where Entrée and Nu-Film 17 were applied with either the systemic or the contact insecticide it gave numerical similar results as the market standard of mineral oil. The addition of Entrée, Nu-Film 17 and mineral oil to both insecticides in water (at all spray volumes) managed to improve control when compared to the insecticides in water only (Figures 1 to 3).

The results from these investigations indicate that Entrée and Nu-Film 17 have the ability to improve the control of California Red Scale on citrus at various application volumes when applied with two types of insecticides respectively. The results were as favourable when applying the insecticides with mineral oil. Control was however compromised when the super wetter was applied to the insecticide solutions as the application volume increased.

(Written by Charl Kotze and Herman Walters – FertAgChem Division of Hygrotech South Africa)

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