Root grubs Basilepta fulvicorne Jacoby

Root grub is a serious subterranean pest of cardamom (Fig. 8.5a,b). It damages roots and thereby obstructs uptake of nutrients, leading to yellowing of leaves and gradual death of plants in case of severe infestation (Gopakumar etal., 1987). The pest is noticed both in nurseries and plantations of Kerala, Karnataka and Tamil Nadu (Varadarasan et al, 1988). However, in Karnataka Thyagaraj et al. (1991) observed it as a serious pest only in the primary and secondary nurseries. Biology and management of the pest was studied by Gopakumar et al. (1987, 1987a, 1988c, 1991), Kumaresan et al. (1989a), Varadarasan etal. (1990a, 1991a,b, 1992) and Thyagaraj et al. (1991).

Figure 8.5 Pests of cardamom: (a) root grubs, Basilepta fulvicerne; (b) roots damaged by the grubs. © 2002 Taylor & Francis

Adult of the pest is a small beetle 4—6 mm in length and of shiny metallic blue, green, or greenish-brown colour. Females are bigger than males. Beetles are polyphagous; jack (Artocarpus heterophylla), Indian almond (Terminalia catapa), mango (Mangifera indica), guava (Psidium guajava), ficus (Ficus indica, F. bengalensis), cocoa (Theobroma cacao), dadaps (Erythrina lithosperma), etc. are certain recorded alternate hosts of the beetle (Anonymous, 1993).

In plantations, infestation of this pest is experienced twice a year. Beetles occur during March—April and August—October, assuming peaks at April and September respectively. Beetles fly about in short distances, alighting on leaves of shade trees and cardamom. Copulation occurs during daytime and the mated females after a pre-oviposition period of 4—6 days extrude eggs in groups to a transparent fluid secreted on dry leaf sheaths or leaves, to which it remains glued. Females lay 124—393 eggs in batches of 12—63 during its oviposition period of 8—71 days. Freshly laid eggs are transparent and gradually turn yellow during the incubation period of 8—10 days (at a temperature of 28-31 °C) or 13-19 days (at a temperature of (19-24 °C) and hatch generally during morning hours liberating small creamy white grubs which fall on the ground, penetrate soil, reach root zone of cardamom and start feeding on the roots. Population of grubs and their infestation is more on cardamom plants under thin shade than under thick shade. As with beetles, grubs too have two periods of occurrence, the first during April-July and the second at August-September to December-January, assuming peaks during May-June and November-December respectively. Larvae feed on roots, become fully grown in 45-60 days and appear pale white, 1 cm long, stout and 'c'-shaped. Pupation takes place in an earthen shell where it remains for 10-17 days. It is seen that at a temperature of 19-24 °C, the pest completes its life cycle within 73-111 days. The development is slightly faster at 28-30 °C taking only 65-102 days for completion.

The pest problem can be effectively managed by a judicious integration of mechanical and chemical methods of control (Gopakumar etal., 1987, 1987a, 1988c; Varadarasan etal, 1990a, 1991a, 1993). Collection and destruction of beetles at the periods of their massive emergence and subsequent insecticidal control of grubs are the two methods incorporated in the strategy.

During periods of adult emergence (March-April and August-October) beetles alighting on cardamom plants can be easily collected using an insect net and destroyed. During peak periods of beetle emergence, 2500-3000 beetles could be collected in a day by a labourer. Such massive destruction of beetles drastically reduces the grub population in soil to a low level, which would otherwise have been enormous and caused heavy root damage. As it becomes impossible to trap and destroy the entire beetles emerging in a field, suitable measures for control of grubs in soil become inevitable in endemic areas. Earlier studies have shown that raking up the soil and subsequent application of granular insecticides, (phorate @ 30-40 g/clump or carbofuran @ 50 g/plant, during May-June and September-October) is an effective method (Gopakumar et al, 1987). Subsequently HCHO 0.2 per cent and aldrin 0.1 per cent were found effective (Gopakumar etal, 1987a, 1991). Varadarasan etal. (1990a) reported the application of 20-40 g of phorate or chlorpyriphos 0.06 per cent effective against grubs. Later, chlorpyriphos at lower doses of 0.04 per cent was found to give adequate control of grubs (Varadarasan etal., 1991b).

The pest has been found susceptible to infection of entomopathogenic fungi both at grub and adult stages. Beauveria bassiana and Metarrhizium anisopliae were isolated

Isolat Beauveria Bassiana

Figure 8.6 Root grubs of cardamom: (a) beetles infected by Beauveria bassiana; (b) grubs infected by Metartizium anisopliae.

Figure 8.6 Root grubs of cardamom: (a) beetles infected by Beauveria bassiana; (b) grubs infected by Metartizium anisopliae.

from naturally infected beetles and grubs respectively (Varadarasan, 1995). The grubs were also infected by the nematode, Heterorhabditis sp. (Varadarasan, 1995). Laboratory studies as well as preliminary field trials with these bioagents have shown convincingly the efficacy of these bioagents, and it is expected that a suitable biocontrol strategy could be developed for management of root grubs (Fig. 8.6a,b).

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