- Plocaederus ferrugineus L.
- Plocaederus obesus Gahan
- Batocera rufomaculata DeGeer
- Cashew stem and root borer (CSRB) is an another important pest of cashew, capable of killing even the grown up trees. It is a kind of hidden pest, where aerial symptoms of damage become visible only at the later stages of infestation.
- It is serious both in East and West coasts of India.
- Among the two species of Plocaederus, P. ferrugineus is the commonly occurring species.
- Mango stem borer, B. rufomaculata attacks the cashew trees at later stage of CSRB infestation.
- Eggs are small, slender, ovoid, smooth, creamy white in colour and look similar to rice grains measuring about 4.5 × 2.0 mm.
- The grubs are creamy, with 3 pairs of thoracic prolegs and about 6 – 10 cm in length.
- Larva spins calcareous cocoon inside the tree bark and hardwood itself for pupation. Adult beetle makes a circular exit hole of 1.5 cm width for its emergence.
- The adults of P. ferrugineus are dark reddish brown, medium sized beetles (25 – 40 mm in length) with long antennae.
- Adults of P. obesus are chestnut coloured, longicorn beetles, measuring about 4.0 cm with a waxy cuticle.
- Adults of B. rufomaculata are greyish, measuring 50 cm in length and have yellowish or orange spots on the forewings. The grubs of this species are apodus (legless) and pupate without forming any calcareous cocoon.
Grub of CSRB (P. ferrugineus)
- Adults are sluggish on the day of emergence, mating starts on the second day, and repeated matings occur during the life time of the adults.
- Eggs are usually deposited in the crevices of the bark of main trunk up to one metre height from ground level and also on the exposed roots as well as in soil close to collar region of the tree.
- Eggs are pale white, ovoid and smooth measuring about 4.5 x 2.0 mm.
- Young grubs hatch in 5-7 days and immediately start boring into bark. The nascent first instar grubs feed on the tissue near the site of oviposition and extrusion of fine dusty frass is noticed within few days of hatching.
- The larval period continues for 6-7 months. The fully grown grub measuring about 100 mm in length enters into heart wood for pupation and makes a circular exit hole of 1.5 cm width for adult emergence.
- Pupation takes place inside a calcareous cocoon. The adults form within 40-60 days, but lie quiescent within the cocoon and emerge out after 45-60 days.
- Adults are normally not seen during day time in the field as they are active during night and rest below dried leaves.
Season of occurrence
- Pest damage symptoms are noticed during December to May in different cashew growing tracts of the country. Emergence of adult beetles starts around November-December.
- But, different stages of infestation are generally seen all round the year. During the onset of monsoon, the healhty trees turn dark green, whereas, the infested trees remain yellowish.
Symptoms of damage
- During the initial stages of attack, exuadation of gum and frass present in small quantities at the base of CSRB infested trees.
- During later stages of attack, the infested tree canopy show a sickly appearance with yellowish leaves and premature dropping of leaves occurs.
- In the severe stages of attack, the twigs dry off and the bark on the trunk starts splitting. At this stage, large quanity of chewed fibres, gum and frass can be seen as huge lumps at the base of infested trees and the tree finally succumbs to death.
- Sometimes, the unexposed stout lateral and taproots will be extensively damaged without any external symptoms and such trees may look very healthy or with sickly appearance and suddenly die without any yellowing of leaves.
- A single grub can tunnel around one square foot area of bark wood tissue. In young trees of 1-3 years, infestation mostly occurs at collar region with damage below the soil surface and a single grub is sufficient to kill those trees.
- P. ferrugineus – Sapota (Manilkara zapota), Bombax malabaricum, Boswelia serrata, Buchanania latifolia, Buchanania lanzan, Diospyros melanoxylon, Hardwickia binata, Lannea grandis and Holigarna spp.
- P. obesus – Shorea robusta
- B. rufomaculata – Mango (M. indica), silkcotton (Ceiba pentandra), jack (Artocarpus heterophyllus), rubber (Hevea brasiliensis), fig (Ficus carica), guava (P. guajava), pomegranate (Punica granatum), apple (Malus domestica) and walnut (Juglans regia).
Controlling this pest is a tough job, as the borer remains in a concealed condition in the interface of bark and hard wood, and normally it escapes from the attack of the natural enemies.
- Later stages of CSRB grubs are infected by an entomopathogenic fungi namely Metarhizium anisopliae and Beauveria bassiana, however, the intensity of natural infection is very less.
- The eggs of CSRB were recorded to be parasitized by Avetianella batocerae Ferriere (Encyrtidae: Hymenoptera) which, however is not commonly encountered.
- The entomopathogenic nematodes belonging to Steinernema and Heterorhabditis are found to be effective in inducing mortality of grubs in controlled studies, which need to be further evaluated under field.
- Phytosanitation of the cashew plantations help to reduce the pest population in a given location and leads to lesser fresh incidence of the pest in the subsequent years.
- The newly planted grafts should be trained to have branching at a height of 0.75-1.0 m from ground level for facilitating better inspection and adopting pest management techniques effectively.
- Once the larvae entered into heartwood for pupation, it is difficult to locate and get them. Hence, it has to be killed by inserting a gear wire/ any bending metal wire.
- Several insecticides have been evaluated at various research centres, for over two decades. It is to be noted that any insecticidal treatment without removing the pest stages will not be effective as the grubs remain inside a thick protective layer.
- The pest stages have to be carefully removed by careful chiseling of the tunnels in the infested portion especially with the fresh frass and destroyed.
- Then, the chiseled portion should be swabbed thoroughly with chlorpyriphos 10 ml/lit (proposed to be banned) or Fipronil 2 ml/lit, and subsequently the tree base should also be drenched.
- Repetition of the treatment should be done, if fresh pest infestation symptoms occur after 30-45 days. More than 50 per cent of the bark circumference should not be damaged while treating, if not, girdling and death of the treated trees occur later.
- In case, more than 50 per cent of the bark circumference has been damaged by grubs or the leaf canopy has yellowed, trees have to be uprooted and the pest stages should be destroyed.
- The volatiles emitting from the frass of the CSRB infested trees attract female beetles for oviposition in and around those infested trees.
- Though male as well as female sex pheromone activity prevails in CSRB, attraction of egg laden females towards kairomone (plant volatiles) is recorded as very strong. If effective compounds can be identified and synthesized, this can be used for managing CSRB.