Evaluation of Alternate Insecticides for the Management of Rhinoceros Beetle,(Oryctes rhinoceros L.) in Coconut Ecosystems

Coconut Production and Rhinoceros Beetle Infestation

India is one of the three largest coconut-producing countries in the world, followed by Indonesia and the Philippines. Coconut is cultivated in an area of 21.4 lakh ha in India and has an average productivity of 10,119 nuts/ha (CDB, 2014).

Among the various insect pests causing damage to coconut, rhinoceros beetle, Oryctes rhinoceros (L.), is a serious pest in Southeast Asia (Bedford, 2013), infesting preferably young coconut palms in the age group of one to six years. The adult beetles cause injury to young palms by boring into the central spindle leaf, spathe, and young petioles. An estimated yield loss of 10% is attributed to spathe damage by rhinoceros beetles.

The adult beetle feeds on the soft tissues, and the chewed-up fibrous material is seen protruding from the entry point or boreholes (Nirula, 1955). The spindle leaf is thus prone to breakage and drying up. The damaged spindle leaf, when unfurled, exhibits “V”-shaped cuts on the leaf lamina. Repeated attacks by the pest result in stunted growth or mortality at times (Hinckley, 1966; Giblin-Davis, 2001).

In the majority of cases, rhinoceros beetle attack leads to infestation by red palm weevil, fungal infections, etc., eventually causing the death of coconut palms (Molet, 2013). The female adults oviposit about 50-100 eggs on decaying logs of wood or manure pits (Bedford, 1980). The emerging larva survives in manure pits or decomposing organic matter for three to six months.

The adults, upon emergence, search for young palm crowns for feeding during the night, while remaining in the breeding sites during the daytime. The adults live for another four to six months, during which time they cause enormous damage to younger palms.

The pest could be kept under check by using integrated management options, including cultural, mechanical, biological, and chemical control measures. With regard to the usage of insecticides, phorate and carbofuran are being applied in the crown region by farmers to eliminate the pest. However, due to the toxic nature of these granular insecticides to non-target organisms, the present study was conducted to identify alternate insecticides for the management of rhinoceros beetles.

Field Experiments for the Management of Rhinoceros Beetles

The field experiments for the management of rhinoceros beetles were carried out in the farmer’s fields in the surroundings of Pollachi region. Gardens with young palms infested by rhinoceros beetles were selected for imposing the treatments (Table 1).

Two trials were laid out:

1. Avalchinnampalayam, Anaimalai block (2013-14)
2. Pongaliyur, Anaimalai block (2015-16)

The garden at Avalchinnampalayam was five years old (Var: Chowghat Orange Dwarf), while the garden at Pongaliyur was three years old (Var: Private Hybrid) at the time of laying out the experiments. The treatments were imposed as per the schedule provided in the table.

All the treatments (insecticides/botanicals) were applied on the crown region on the innermost three leaf axils beneath the spindle leaf, along with 100-150 g of fine sand as a carrier.

The experiments were conducted in a Randomized Block Design (RBD) with eight treatments and three replications (4 palms/replication). Observations on the leaf and spindle damage were recorded at quarterly intervals and expressed as level of damage after 12 months. The intensity of coconut rhinoceros beetle was recorded in terms of spindle and leaf damage prior to and after imposing treatments, at quarterly intervals.

The percent leaf damage was calculated using the following formula:

$Per\; cent\; leaf\; damage=\left(Total\; number\; of\; leavesNo.\; of\; damaged\; leaves\right)\times 100$

A damaged spindle corresponds to 100% spindle damage, while a spindle free of infestation is recorded as 0% spindle damage. The data obtained were subjected to appropriate transformation and analyzed using least significant difference.

At the first location viz., Avalchinnampalayam,the initial level of leaf damage ranged from 29.1 to 44.5 per cent in different treatments before imposing the treatments.

Table 1. Particulars of the treatments for rhinoceros beetle management

Table 2. Impact of different treatments on the leaf damage due to rhinoceros beetle in coconut

Table 3. Impact of different treatments on the leaf damage due to rhinoceros beetle in coconut

Table 4. Cost of different treatments used in the experiment

From the above discussion it could be concluded that, chlorantraniliprole and chlorpyriphos could be effectively used as alternate insecticides in the wake of the ban imposed for phorate insecticides. However, an analysis on the cost of insecticides used in the experiments revealed that, chlorpyriphos was comparatively cost effective (Rs.0.30/ palm) followed by phorate (Rs.0.90/ palm) and chlorantraniliprole (Rs.0.98/ palm) (Table 4). Though neem cake and naphthalene balls were effective, they incurred a cost of Rs. 2.50 and Rs. 3.12, respectively. Considering the lesser cost involved, chlorpyriphos and chlorantraniliprole can be used for the management of rhinoceros beetles in coconut ecosystems.

Studies conducted at farmers' fields of Anaimalai block, Coimbatore district, Tamil Nadu, revealed that, chlorantarniliprole @ 5g/palm or chlorpyriphos @5g/palm could be used for crown application at 60 days interval, as alternatives, in place of phorate, naphthalene balls and neem cake which are presently used against rhinoceros beetles. The above mentioned insecticides could be considered for the management of rhinoceros beetles in coconut ecosystem both in terms of efficacy as well as cost effectiveness.