Next to man, the greatest destroyer of forests is insects which damage almost all parts of the forest trees. The insect pests damage about 100,000 ha of forest in India. The annual loss caused by insects to seeds, transplants, standing trees, wood and finished products has been estimated to be about 10 per cent of the total revenue of the forests.
Borers:
The larvae of many beetles and moths cause considerable damage to forest trees by boring into the trunks and feeding on their living tissues. They bore holes and tunnels of various sizes, thus reducing the vitality of the trees and often causing their decay and death. In any case, the quality of the timber is affected and the value of the plantation goes down.
1. Sal Borer, Hoploceramhyx Spinicornis Newman (Coleoptera: Cerambycidae):
The sal borer is known to occur on sal (Shoreci robusta) in Western Ghats, Maharashtra, Assam, West Bengal and Uttar Pradesh. Outside India, this pest has been reported from Afghanistan, Nepal, Indonesia, Singapore Borneo and the Philippines. The adult beetle is dark brown and variable in size measuring 20-60 mm in length and 5-16 mm in breadth.
In the male, antennae are much longer than the body and in the female, antennae are shorter than the body. Eggs are elongate, cylindrical and slightly swollen at the anterior end. The full-grown larva is yellowish and measures up to 9 cm in length.
Life-Cycle:
The beetles appear every year soon after the monsoon rainfall in June or July. The beetles pair soon after emergence and lay eggs about a week later, on cuts or holes in the bark of sal trees. Eggs are laid singly and a female can lay several hundred eggs. Eggs hatch within 5-8 days and the grubs bore into bast and then into sapwood for feeding. They prepare the galleries, which are at first narrow with two or more arms.
Galleries are entirely packed with wood excreta of the grub. In fact, the grub is packed in the galleries with wood excreta ejected by the grub. Grubs further bore into heartwood for preparing pupal chamber. The pupal chamber is covered with white calcareous cocoon. The pupal stage lasts for 2-3 months and the pest hibernates in pupal state in cold weather.
Damage:
The grubs bore into bast, sapwood and heartwood and thus affect the quality of timber and growth of plant in case of standing plants. Grubs can bore into roots, main stem and larger branches of the tree. The cambium layer of the plant is usually completely destroyed. The wood dust ejected by the grules accumulates into heap near the base of the tree. It is the most notorious forest pest in India because of its periodic outbreaks, during which millions of sal trees are killed.
Control:
(i) Remove felled trees from the forest and bark every tree within a week of felling.
(ii) Collect and destroy the beetles when they appear for infestation on the trees.
(iii) Encourage natural enemies like birds, parasitoids and predators. Woodpeckers feed on the grubs by tunnelling the wood.
(iv) Cotton balls soaked in chloroform or petroleum or kerosene is placed in boring holes and sealed with mud for killing the pest inside the tunnel.
(v) Treat felled trees with carbaryl 5 per cent dust.
2. Babul Borer, Celosterna Scabrator (Fabricius) (Coleoptera: Cerambycidae):
The babul borer has been frequently recorded in plantations of babul (Acacia nilotica) in India. However, it is a polyphagous species, attacking saplings of other tree species as well such as Shorea robusta, Casuarina equisetifolia, Acacia arabica, Tectona grandis, Eucalyptus spp., Ziziphus jujuba, Morus Alba, Tamarix indica, etc. The beetle is dull yellowish brown and measures 25-40 mm in body length. The grub is elongated with brown head and black mandibles, and the pupa is reddish yellow.
Life-Cycle:
The adults emerge from the host with the onset of monsoon in June-July. They feed on the bark of the young shoots. Eggs are deposited under bark on stems, 5-23 cm girth, within 15 cm above ground level, usually one egg per stem. The newly hatched larva bores into the stem and as it grows older, it tunnels downwards hollowing out the main root.
The larva ejects the frass through a hole in the stem just above the ground level and the frass accumulates at the base of the stem. The larval period is about 6-8 months and the full-grown grub pupates in the pupal chamber. The grubs block the pupal chamber with fibrous material and the pupal period lasts four weeks. The beetles emerge out through the fibrous material of the exit tunnel. Only one generation is completed in a year.
Damage:
The grubs cause damage by tunnelling down the centre of stem and later the thicker portion of the stem, base of the stem and subsequently the roots. In this process, they cause death of the tree.
In addition to the larvae tunnelling into the stem and root, adult beetles feed on the bark of stems and branches of saplings, in irregular patches, often girdling the shoots and causing them to dry up.
Control:
(i) Collect and destroy the beetles when they freshly emerge from pupal chambers.
(ii) Collect and destroy the infested plant parts along with the pest stages.
(iii) Fill wounds or bored portions with cotton boll soaked with petroleum oil or chloroform or kerosene and plug the same with mud to kill the pest inside the tunnel.
3. Semul Borer, Xystrocera Globosa Olivier (Coleoptera: Cerambycidae):
The semul borer is a serious pest of semul (Bombax malabaricum) in Assam, Uttarakhand, Tamil Nadu, Maharashtra and Western Ghats. It has also been recorded from Egypt, Myanmar, Sri Lanka, Indonesia and the Philippines. Other hosts of this pest include Xylia dolabriformis, Albezzia bebbek, Populus cuphratica and Salix spp. The medium-sized beetle, 30-35 mm in body length, is reddish brown in colour with dark blue-green lateral stripes on the prothorax and elytra. The larva is yellowish green and grows up to 40-50 mm in length. Pupa is whitish and exarate type.
Life-Cycle:
The adult beetles appear on the wing in April and lay eggs, preferably in crevices on the stem or branch stubs, generally 3-4 metres above ground. The newly emerged larvae bore into the inner bark and as the larvae grow, they feed on the outer sapwood, making irregular downward galleries, packed with frass. The full-grown larva pupates in the pupal chamber and the beetles emerge from the trees in April. Two generations are completed in a year.
Damage:
The larvae damage bast and sapwood by making galleries, with the result the bark dries and cracks, and the heavily infested trees dry up. Weakened stems are sensitive to wind, particularly during the rainy season. Even when the trees are not killed, borer attack reduces the growth rate and timber quality.
4. Shisham Borer, Aristobia Horridula Hope (Lepidoptera: Cerambycidae):
The shisham borer is an emerging serious pest of Dalbergia cochinchinensis in Thailand and it also attacks D. sissoo in India (West Bengal and Uttar Pradesh). It has also been recorded on Pterocarpus indicus and P. macrocarpus. The adult beetle, 27-32 mm long, is brownish, with bluish hair on the elytra.
A characteristic of the species is the presence of a dense tuft of hair on the distal portion of the first and second antennal segments, those on the second segment being longer. The prothorax has a pair of lateral spines. The full-grown larva is 55-60 mm long and is creamy white.
Life-Cycle:
The female beetle makes a transverse groove on the bark of trees and lays eggs singly. The larva makes irregular, upward galleries in the sapwood initially and finally bores into the heartwood where it pupates. The galleries, 50-75 cm long, are packed with frass and excreta. In D. cochinchinensis, feeding of young larva causes reddish resin exudation from the bark. In addition, feeding around the inner bark causes swelling of bark around the stem.
In D. sissoo, entrance hole exhibits ‘weeping symptom’ (oozing of black fluid). In D. sissoo, the attack is restricted up to four metre height, with maximum attack taking place at about breast height. The life-cycle is annual, with most adults emerging from July to September in India and April to June in Thailand.
Damage:
The grubs bore into the trunk of living trees and borer damage degrades the timber. In India, the borer was reported in epidemic form in all girth classes of D. Sissoo plantation in West Bengal with the incidence ranging from 10 per cent in one-year-old plantation to 80-90 per cent in older plantations.
In Thailand, 33 per cent of 8-year-old D. cochinchinensis plantation was infested. P. indicus is more susceptible, with 100 per cent of 10-year-old roadside plantation being found infested. In P. macrocarpus, about 33 per cent of trees in 8-year-old plantation and 83 per cent of trees in 16-year-old plantation suffered damage.
5. Bamboo Shoot Weevil, Cyrtotrachelus Longipes Fabricius (Coleoptera: Curculionidae):
The bamboo shoot weevil is prevalent in bamboo (Bamboo spp.) in Chittagong hill tracts of India. The adult weevil is 20-40 mm long with long rostrum and legs. Females are much larger than the males. Grubs are legless, curved and whitish, and pupa is whitish in colour.
Life-Cycle:
The adult weevils become active with the onset of monsoon in May-June. They mate soon and the female then finds out young sprouting bamboos for egg laying. They suck the sap of tender shoots and lay eggs on the culm. The larva on hatching bores into the culm, making a long tunnel, passing internally through several internodes and perforating each. The larva pupates within the fallen buried end of the shoot at depth of 8-10 cm. The pupa stays in the chamber for cold and hot seasons, and emerges as an adult at the initiation of rains. The life-cycle is annual.
Damage:
The female damages the shoots by ovipositioning upon them and sucking the sap of tender shoots. The grubs bore into the shoot and affect growth and vigour of the tree. Feeding usually results in death of the culm or sometimes development of multiple shoots of little commercial value. A single larva can destroy a developing culm.
Control:
(i) Collect and destroy adult weevils and other stages.
(ii) Treat the crop with carbaryl 5 per cent dust.
6. Deodar Beetle, Scolytus Major Stebbing (Coleoptera: Scolytidae):
The deodar beetle (also called scolytid beetle) is widespread in Northwest Himalaya in India, as a pest of deodar (Cedrus deodara). The black coloured beetle measures 4.0-4.5 mm in body length. Eggs are spherical and yellowish shining. The grub is whitish, small, curved and legless, and the pupa is whitish.
Life-Cycle:
The mated female prepares an egg gallery mainly in the bast layer. The gallery may be 5-7 cm in length in upward direction in a series of serpentine curves, first to one side and the next to the other. She also prepares egg notches in gallery for deposition of eggs. She fixes the egg in a notch with fine particles of wood dust.
A single female can lay about 70-85 eggs. In each side of the egg gallery, as much as 35-43 eggs are laid in notches. Egg hatching takes place within two days and the larvae on hatching bore into the bast and sapwood. Grubs prepare their galleries away from the egg gallery by feeding on the bast or sapwood.
They work in a direction more or less right angle and in an upward or downward direction, and hence they have a special type of damage pattern. The larval period lasts for four weeks. Grubs bore down sapwood in poles, large trees and saplings. The full-grown gurb prepares a pupal chamber in sapwood and pupates in it. The pupal stage lasts for about two weeks. The pest completes its life cycle within 6-7 weeks and there are four generations in a year.
Damage:
This is a serious pest of deodar and damages the tree by making galleries in the bast and sapwood. The beetle can attack trees of all sizes from smallest sapling to the largest tree. Upward flow of sap is affected due to tunnelling the plant, which adversely affects the vitality of the plant. Beetles can cause damage to newly felled unbarked trees by making galleries. The pest can attack the green standing healthy and sick trees. The pest is responsible for causalities up to 50 per cent.
Control:
(i) The infested plant parts should be collected and destroyed along with the pest stages.
(ii) Inner face of bark should be turned outward so as to expose pest stages to sun for killing grubs and pupae.
(iii) Green standing trees or newly felled green trees should be used as trap crop for attracting and killing the beetles.
(iv) Treat the crop with carbaryl (0.15%) or azadirachtin (0.03%).
Sap Suckers:
Suckers include aphids, thrips, coccids, psyllids and bugs which suck the sap of foliage of forest trees and adversely affect their vitality. Some of the species secrete honeydew, over which the sooty mould develops which interferes with photosynthesis.
1. Subabul Psyllid, Heteropsylla Cubana Crawford (Hemiptera: Psyllidae):
The subabui psyllid is distributed throughout India, Sri Lanka, Indonesia, Thailand, Taiwan and the Philippines. It is mainly a pest of subabui (Leucaena leucocephalci). The adult psyllid is light yellow with 4-5 antennal segments and measures about 0.32 mm in body length and 0.15 mm in width. Nymphs resemble the adults except that they are smaller in size and devoid of wings.
Life-Cycle:
Females lay eggs on tender leaves of subabul twigs, which hatch in 2-3 days. The newly emerged nymphs wander here and there in search of tender portions of the plant and start sucking the cell sap from tender leaves. There are five nymphal instars and the nymphal period lasts for 8- 10 days. The life-cycle is completed within 10-13 days and the adults can survive for 7 days. During a year, as many as 8-10 generations are completed on subabul.
Damage:
Both the nymphs and the adults suck the cell sap from tender portion of the plant especially leaves buds, stems, flowering and fruiting bodies. The psyllids inject toxins into the plant body when they suck the cell sap. This results in curling and yellowing of leaves, and dropping of flowering bodies. The nymphs and adults secrete honeydew on the leaves and other plant parts, which attracts sooty moulds to develop. The sooty moulds on leaves affect the photosynthesis of the crop, and affect the growth and quality of the plant.
Control:
(i) Collect and destroy the infested plant parts along with the pest stages.
(ii) The fungus, Beauveria bassiana causes white muscardine disease in the pest and controls about 64-100 per cent of pest population,
(iii) A ladybird beetle, Menochilus sexmaculatus (Fabricius) feeds on nymphs and adults of the psyllid, and can be manipulated to control the pest.
(iv) Grow pest resistant varieties such as K6, K8, K500 and K636.
(v) Spray the crop with malathion (0.03%) or DDVP (0.03%) or phosphamidon (0.03%) or carbaryl (0.15%) or monocrotophos (0.05%) or quinalphos (0.05%).
2. Gamhar Lace Bug, Tingis Beesoni Drake (Hemiptera: Tingidae):
The gamhar lace bug is an occasionally serious pest of young gamhar (Gmelina arborea) seedlings in India, Myanmar and Thailand. In India, it is prevalent in Uttarakhand, Madhya Pradesh, Uttar Pradesh, Maharashtra and Kerala. The small, dark, lace bugs (4.5 × 1.7 mm) aggregate in large numbers on the stems and branches of saplings and feed gregariously.
Life-Cycle:
Eggs are inserted in a vertical row into the tender shoot tissue. The nymphs congregate at the base of the leaf lamina and the axilla. There are five nymphal instars and the life-cycle is completed in 11-40 days, between April and October, depending on the temperature. There is a considerable overlap between generations; up to seven generations may be completed in a year. The eggs laid in the cold weather overwinter, hatching only in the following March.
Damage:
Both the adults and the nymphs feed gregariously at the base of the leaf blade, sucking the sap from the larger veins. As a result of feeding by the insect, the leaves become spotted and discoloured, and wither. Eventually, the saplings show dieback of shoots and epicormic branching. Heavy infestation may result in total defoliation. The insect attack may be followed by fungus infection, characterized by black necrotic lesions at leaf bases, followed by defoliation and drying of young shoots.
Control:
(i) Collect and destroy the nymphs and adults.
(ii) Remove and destroy the insect infested plant parts along with the pest stages.
(iii) Spray monocrotophos (0.02%) or deltamethrin (0.005%). In case of additional fungal infection, spray a mixture of the insecticide and the fungicide, carbendazim (0.1%).