Introduction

The red spider mite, Oligonychus coffeae (Nietner) (Acarina: Tetranychidae) is an important pest causing considerable crop loss in tea, Camellia sinensis L. (O. Kuntze) in most of the tea growing countries of Asia, Africa, South America and North-east India (Das, 1959; Banerjee, 1966; 1971; Muraleedharn et al., 2005). Severe infestation of red spider mites leads to defoliation (Selvasundaram and Muraleedharan, 2003). Now day’s different classes of synthetic acaricides as well as botanicals are being used to control the pests (Sarmah et al., 2009). However, use of same molecule for a longer period of time may cause resistance against the pest as a result the rate of mortality reduces. The use of synthetic chemicals for the control of pests leads to problem of pesticide residues in tea. There is an urgent need to overcome the current situation by adopting non-chemical control strategies. Use of biologicals of plant origin has received wide acceptance in many crops including tea. Even though, neem based products are used in many crops, their use is very much limited in tea, except for certain Azadirachtin based commercial formulations. Neem, Azadirachta indica A. Juss (Meliaceae) is a popular tree in India and being indigenous, which products are known to have strong insecticidal properties (Schmutterer et al., 1981; Jacobson, 1986; Schmutterer and Aschar, 1987). Azadirachtin the active ingredient present in Neem belonging to tetranortriterpenoid class and mostly concentrated in the seeds (Saxena, 1981; Stoll, 1996). Now a day’s almost 200 insect pest are reported to be controlled by the neem derivatives (Hamilton, 1992, Babu et al., 2008). Synergists are natural or synthetic chemicals which are considered as nontoxic and it increases the lethality and effectiveness of insecticides. Most of the synergists block the metabolic systems by breaking down insecticide molecules which interfere with the detoxication of insecticides through their action on poly‐substrate monooxygenases (PSMOs) and other enzyme systems. Synergist plays an important role in the insecticide toxicity and mode of action and the nature of resistance mechanism which are even not harmful for non‐target organisms (Bernard and Philogene, 2009). An attempt has been made to evaluate the bio activity of an organic synergist (supplied by the Institute of Pesticide Formulation Technology, Gurgaon, India) to establish the enhanced efficacy of Neem Kernel Aqueous Extract (NKAE) and three recommended acaricides (Table 1) against one of the major tea pest, red spider mite under laboratory conditions.

1 Materials and Methods

1.1 Collection and laboratory culture of red spider mites

The red spider mites were collected from North Bengal Regional R&D Centre experimental plots, (26° 54' 0" North, 88° 55' 0" East longitude) Nagrakata, West Bengal, India. The culture of red spider mite was maintained following the detached leaf culture method of Helle and Sabelis (1985) with slight modifications under the laboratory conditions (25 ± 2°C, 75 ± 5% RH, and 16L: 8D photoperiod) on a susceptible tea clone, TV1. From the stock culture, adult mites were transferred onto fresh tea leaves (approximately 6 cm²) placed on moistened cotton pads (ca. 1.5 cm thick) in plastic trays (38 × 28 × 5 cm). Withered leaves were replaced with new ones at 2-3 days interval. These pure cultures (not exposed to any pesticides) were maintained for several generations (from the F1 to F10) under laboratory conditions (25-30ºC temp. and 75-95% humidity). For different bio-efficacy evaluations, mites were taken from the above mentioned culture.

1.2 Preparation of Neem Kernel Aqueous Extract (NKAE)

1.2.1 Methodology:

1.2.2 Leaf disc experiment

Bioassays were carried out using leaf disc experiment (under laboratory condition). Leaf discs of 2 cm diameter cut from tea leaves were placed on wet cotton kept in petri dish. Moisture in the cotton is maintained throughout the study by wetting it with distilled water every day. Mites were released onto the leaf discs from stock culture which was maintained in the laboratory. Different treatments of NKAE were measured using calibrated measuring jar and dissolved in known quantity of distilled water kept in a beaker for 24 hours. Prepared spray fluid was sprayed onto the leaf disc using glass atomizer. The distance between the atomizer and the leaf disc was maintained constant (one foot) in order to ensure that, only the fine droplets were falling on the disc. Leaf discs sprayed with distilled water served as control. After imposing the treatment, observations were made using a simple compound microscope at a specified magnification of 10X to observe and record mortality of mites at 24h interval for a period of 3 days.

Percentage mortality = (No. of mites dead / No. of mites tested) × 100

1.2.3 Acaricidal activity of NKAE against red spider mites

For evaluation of acaricidal activity of NKAE, 30 healthy adult red spider mites (>24 h old) were released on a healthy detached tea leaf (TV1 clone) from the culture maintained in the laboratory. The adult mites were subjected to spraying with different concentrations of NKAE viz., 5% NKP with soap nut, 5% NKP + Organic Synergist 0.5%, 2.5% NKP + 2.5% Organic Synergist along with a CIB approved acaricide (Hexythiazox 5.45 EC) and an untreated control (water spray). Each concentration was sprayed on both the surfaces of leaf disc using glass atomizer. After imposing the treatment, observations were made using a simple compound microscope at a specified magnification of 10X to observe and record mortality of mites at 24 h interval for a period of 3 days. Each treatment was replicated three times and the per cent mortality was calculated.

1.2.4 Bioactivity the organic synergist in enhancing the efficacy of NKAE and three recommended acaricides against red spider mite

The bioactivity of an organic synergist (supplied by the Institute of Pesticide Formulation Technology, Gurgaon, India) was assessed to establish the enhanced efficacy of NKAE and three recommended acaricides such as (a) Fenpyroximate 5 EC, (b) Ethion 50 EC and (c) Spiromesifen 22.9 SC against red spider mite under laboratory conditions. Leaf discs of 2 cm diameter cut from tea leaves were placed on wet cotton kept in petri dish. Moisture in the cotton is maintained throughout the study by wetting it with distilled water every day. Ten mites were released onto the leaf discs from stock culture which was maintained in the laboratory. Different treatments included: T1: Recommended acaricide (stand alone) (100%), T2: Acaricide (100%) + Organic Synergist, T3: Acaricide (75%), T4: Acaricide (75%) + Organic Synergist, T5: Acaricide (50%), T6: Acaricide (50%) + Organic Synergist, T7: Acaricide (25%), T8: Acaricide (25%) + Organic Synergist, T9: Organic Synergist @ 2.5 gm/100 ml and T10: untreated control. Leaf discs sprayed with distilled water served as untreated control. After imposing the treatment, observations were made using a simple compound microscope at a specified magnification of 10X to observe to record mortality of mites at 24h interval for a period of 2 days. The experiments were replicated 3 times and the percent mortality was calculated. The efficacies of the selected three different acaricides, with recommended doses were compared with that of NKAE. 

2 Results and Discussion

2.1 Acaricidal activity of NKAE against red spider mites

The results of the experiment on acaricidal activity of NKAE against red spider mites carried out in the lab are presented in Table 2. After 72 h of treatment NKP with soap nut powder@ 5% (T1) exhibited 46.7% mortality. The mixture of NKP @5% with the Organic Synergist @ 0.5% (T2) exhibited 66.7% mortality and when NKP @ 2.5% mixed with the Organic Synergist @ 2.5% (T3) it exhibited 90% mortality. Comparatively 82.1% mortality was recorded in case of the acaricide Hexythiazox 5.45 EC. This study revealed that the organic synergist enhanced the efficacy of NKP (even at 2.5% of NKP) to the tune of almost 45%. Babu et al., (2008) reported a maximum of 72.33% mortality of red spider mites after 72 h of spraying of NKAE and based on the laboratory and field experiments they reported that, NKAE at the concentrations of 5% was effective in controlling red spider mites infesting tea.

Table 2 Acaricidal activity of NKAE (Post treatment observation (Mortality %)) Note: * Values represent mean ± SE of three replications

2.2 Bioactivity the organic synergist in enhancing the efficacy of NKAE and three recommended acaricides against red spider mite

The study revealed that, after 24 hrs of treatment, when the Fenpyroximate was mixed using distilled water and used as standalone (Fenpyroximate at recommended dosage (0.5 ml/L), the mortality observed was 73.4%, on the other hand, when the spray solution was prepared using the extract of the organic synergist, the mortality rate of red spider mites was recorded as 96.7% which was 31.7% higher than that at the recommended dose. Further, even at lower doses of Fenpyroximate (at 25% of the recommended dose with Organic synergist, the mortality rate was recorded as high as 83.6%, which was more than 13.8% (Table 3). In the case of Ethion 50EC (stand alone) at recommended dose (2.5 ml/L), the mortality rate was observed up to 66.6%. When the Organic synergist was added with Ethion 50EC (in a recommended dose) the mortality rate of red spider mites has increased up to 93.3%. Similar trend was observed when the Organic synergist was added even in lower doses of Ethion also (at 75% and 50% of Ethion with Organic synergists mortality rate were 83.3% and 70% respectively) (Table 4). When the Spiromesifen 22.9 SC was mixed using distilled water and used as standalone (1 ml/L), the mortality observed was only 54.3% after 24 h, on the other hand, when the spray solution was prepared using the extract of the organic synergist, the mortality rate of red spider mites was found to increase by 25% at the recommended dose. Lower doses of Spiromesifen 22.9 SC (at 25% of the recommended dose with Organic synergist), the mortality rate achieved was as high as 60%, when compared to other treatments (Table 5).

The study revealed that, the organic synergist enhanced the efficacy of each acaricides to the tune of almost 20-40% compared to the individual treatments which could be further exploited for reducing the pesticide load on tea. Synergists are being using in the different agricultural practices to control different pest which have been contributing significantly to improve the efficacy of insecticides, especially when the problems of resistance have arisen over several decades (Bernard and Philogene, 2009). NKAE is non-phytotoxic, safer to beneficial insects like predators and it does not leave any undesirable residues on black tea (Babu et al., 2008). Application of organic synergist with the recommended acaricides can minimize the pesticide load and even can minimize the dose of eco-friendly NKAE to control the tea red spider mites.

Acknowledgements

Authors are thankful to the Ministry of Chemicals & Fertilizers, Govt. of India, New Delhi for the financial support rendered in the form of a project. The technical support received from the Institute of Pesticide Formulation Technology, Gurgaon, Haryana is also acknowledged.

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