0739-B1

Heartwood Borer Epidemics in Central India: A Threat to Shorea Robusta Forest Ecosystem

U. Prakasam^[1], A. P. Dwivedi and Anil Oberoi

Abstract

Sal (Shorea robusta Gaertn.f.), belongs to the family Dipterocarpaceae, and is widely distributed from Sri Lanka and India through Myanmar and other countries of Southeast Asia up to the Philippines. It is native to Northeast India. Sal forests occupying an area of 105 790 km² in India, are very important from both ecological and economical points of view as they not only harbour maximum biodiversity but are also a source of livelihood for millions of people.

Unfortunately, Sal has been the victim of heartwood borer, Hoplocerambyx spinicornis. Ever since it was noticed as a pest on sal in 1899, its 20 epidemic outbreaks have been recorded in India causing extensive damage to sal forests. During the latest 1996-2001 epidemic, the borer affected more than 3.5 million sal trees in an area of around 5 000 km². The emergence of beetles takes place immediately on the onset of monsoon. This pest is immune to all known forms of control measures as most of its year-long life cycle passes in the tree, except for 20-30 days of adult beetle stage. Natural regeneration and the establishment of sal have been the other major problems, which are compounded by the borer attacks.

Although there have been considerable studies on this pest, there has been no breakthrough as its outbreak in epidemic forms is governed by weather conditions. Despite cyclic outbreaks, the latest epidemic had to be confronted with the previously known silvicultural and mechanical measures only. Out of 3.5 million infested trees, 1.59 million trees had to be felled and removed from the worst affected Dindori and Mandla Forest Divisions alone. Though the forestry research organizations are trying to find a suitable solution to the problem, cyclic epidemics demand the need for immediate coordinated research efforts to evolve effective preventive and remedial measures. In order to assist natural regeneration and its establishment, site-specific regeneration plans are being executed with intensive protective measures to mitigate the damage caused and restore sal forest ecosystems.

Introduction.

Sal is a fairly large tree with a majestic shining foliage. It is widely distributed from Srilanka and India through Myanmar and other countries of South East Asia up to Philippines. It is considered to be native of North East India (Kulakarni 1956). Sal forests in India occupy about 105,790 km² area in two distinct regions, viz. northern and central region (Fig.1). The extent of sal forests in different States in furnished in Table 1. Sal forests in Undivided Madhya Pradesh (Central India) are distributed in eastern parts over an area of 27,800 km² in which cyclic epidemic area is shown in Fig. 2.

Fig. 1 Map showing distribution of sal in India

Fig. 2 Map showing borer epidemic area in Central India

Table. 1 Extent of Sal Forests in India

Source: Central Forestry Commission (1972)

Sal forests are well known to harbor maximum biodiversity and being semi-evergreen, they constitute an important ecosystem, which provides cool and calm environment. A wide range of Non Timber Forest Products found in these forests which apart from timber, are source of livelihood for millions of people living in and around forests. Timber, because of its excellent qualities, is used for different purposes. Calorific value of completely dried heartwood is as high as 5433 calories (Krishna & Ramaswamy 1932). Collection of Sal seed is an economical activity, which generates large-scale employment to the tune of 4.545 million persons for a period of 40 days in rural areas (Namdeo & Pant 1989). Unfortunately, this beautiful tree (Fig.3) has been the victim of notorious borer which causes extensive damage (Fig.4).

Fig. 3 Healthy sal tree

Fig. 4 Devastated sal forest by borer

History of Epidemics in Central India. (Note: Being based on practical experience on the subject, the presentation does not follow the ' Introduction-Materials and Methods-Results-Discussion' format)

The heartwood borer was first noticed as a pest on sal in 1899. The attack of the borer is considered epidemic when the sal trees affected are more than 1% of the total number of trees. In 1905, the first epidemic was observed in Balaghat district of Central India. Stebbing (1906) was the first to publish a note on the life history of Sal heartwood borer. In 1923-24, serious attack of sal heartwood borer was observed in Mandla and Dindori sal forests in which 7 million trees were affected. As per the advice of the then Forest Entomologist, Forest Research Institute, Dehradun, control operations were carried out. About 428,000 adult insects were killed and 387,500 trees felled and removed from affected sal forests. Beeson (1924, 1928, 1934,1941) and Beeson & Chatterjee (1925) described the distribution, economic importance and preliminary methods of borer control. In 1950-51, attack of sal borer was observed in Mandla and Dindori forests but was soon brought under control.

During 1950-55, about 100,000 adult insects were killed and 56,500 trees were felled. In 1959-62, the epidemic surfaced in Mandla and Dindori forests in which 2 million beetles were killed and 350,000 affected trees were felled. Studies by Chatterjee and Thapa (1964) had confirmed this pest to be endemic to Dindori-Mandla sal forests. During 1979-1982, borer attack was observed in sal forests of Hoshangabad in which about 50,000 tress were felled. Due to the repeated attacks, sal borer operations have been included in the Working Plan (Bajpayee1983). Till date, over 20 epidemics have been recorded in the country throughout the range of sal distribution.

Control Measures adopted.

In all the above epidemics, the following control measures as have been adopted.

(i) Catching and killing of insects through Trap-Tree method. In this Operation, one to two trees ha^-1 of 60-90 cm girth are felled, cut into 2-3 m long logs and the bark near cut ends is beaten up to provide shelter for the beetle. The beetles gets attracted to the smell of sap, gets intoxicated after consuming it and become sluggish, rendering their collection and killing easier. Beetles are collected from traps every morning and evening. Dey (1999) observed that the total catch during the night hours was over three times the total catch of insects during the day hours which is contrary to the observations of Beeson (1941). The heads of beetles are severed and counted for record of daily catches ( Fig. 5 & 6). Trap tree operation starts on the onset of monsoon and that continued till the day the insect catches are nil.

Fig. 5 Collection of Beetles from Trap tree

Fig. 6 Severed beetle heads,100 in a string

(ii) Felling and removal of affected trees away from the sal forests. Based on different intensities of infestation, the affected trees are classified into seven categories as under (Beeson 1941)

Infested trees of category I to V are to be felled and removed from forests. Stumps of category VI are burned before next rains and category VII trees are not felled.

(iii) Burning of debris and stumps after harvesting of trees.

(iv) Stacking of infested timber in depots five km away from sal forests in order to prevent the beetles from flying back to forests.

Life Cycle of Borer.

Sal heartwood borer (Hoplocerambyx spinicornis Newn.) is a beetle of the order- Coleoptera, family-Cerambycydae. The year long life cycle has four stages viz. Egg (3-7 days in June -July), Larva (July to April), Pupa( April to May) and Adult (June -July). The adult female and male beetles and life cycle are shown in Fig.7 & 8 respectively.

Fig. 7. Female and Male adult beetles

Fig. 8. Life cycle of sal borer

The beetles of sal borer emerge from infested trees each year as soon as the monsoon starts in the month of June, lasts till the end of July. The beetles are 3-7 cm. long, blackish to reddish brown in color. Pairing takes place immediately after emergence. Female begins to lay eggs 7 to 9 days after fertilization. Eggs are laid in cracks as deep as possible on fallen trees, on the trunk and branches of standing trees. During its life period, a beetle lays 100 to 300 eggs. The eggs hatch within 3 to 7 days of the laying and 80 to 90% of them hatch into larvae. After hatching from the eggs, the larvae enter the bark and then sapwood and finally bore into the heartwood. The progress of boring by the larvae can be judged from the heaps of wood dust, which accumulates at the base of trees

Before pupation, the larva bores a tunnel running horizontally from the sapwood and makes a pupal chamber where it pupates. The pupa turns into immature beetle in May, waiting to emerge when the monsoon sets in June. The longest recorded life is 9 days for the male and 38 days for the female beetles. Fig. 9 & 10 shows the external and internal views of damage caused by the borer to sal trees. The control measures need to be intensified during monsoon.

Fig. 9 Infested tree with wood dust

Fig. 10. Borer damaged log of sal

Latest Epidemic.

The latest epidemic during 1996-2001 was one of the worst in the series of outbreaks. The affected trees ha^-1 in Dindori Forest Divisions were as high as 50 and the larvae in some trees were counted upto 1500/tree (Dwivedi 1998). Sal forests of Dindori, Mandla, Shahdol, Balaghat, Bilaspur, Rajnandgaon and Surguja districts were affected by the borer in about 5,000 km² area. In the worst affected Dindori and Mandla forest of Central Circle, Jabalpur, borer attack was first noticed in December, 1995. Trap-tree operation was carried out in 1996 rains in which 2.16 million beetles were killed and 14,478 trees were felled as trap. Besides this, 40,343 affected trees of all categories were also felled removed from coupes due for working. In 1997 rains, trap-tree operation was again carried out in which 15.17 million beetles were trapped which indicates favorable climatic conditions for multiplication of insect beyond proportion.

During October 1917-January 1998, 783,720 affected trees of category I to VI have been removed. Widespread reports on the epidemic have drawn the attention of public at large. Opinions against the large-scale felling were expressed from different quarters. Public Interest Litigation was filed in the High Court at Jabalpur on this matter. Consequently, the Ministry of Environment and Forests (MoE.F), Government of India constituted a Committee in December 1997 consisting of senior Forest Officers and Forest Entomologists from Central and State Governments and research organizations under the leadership of the Director General, Indian Council of Forestry Research and Education, Dehra Dun to find the causes of epidemic, extent of damage and to suggest remedial measures. The committee, having inspected the affected area, submitted its report to MoE.F suggesting the removal of category I to V trees and approving the measures taken by the State Forest Department.

The suggestion of the committee that borer affected trees of category I to V should be felled was again questioned by certain quarters. In order to deal with the situation more effectively as also to remove the public apprehension, the MoE.F has constituted a Task Force and a Steering Committee. One of the important terms of reference was to examine the impact and the necessity or otherwise of felling of borer affected sal trees with respect to management plan prescriptions. The steering committee was to suggest appropriate guidelines on ecological and management aspects to prevent and control such epidemics.

The Task Force submitted its interim report in January, 1998 in which majority of the 14 member committee recommended in favor felling and removal of all trees falling in category I to V. However, the minority view was that only trees of category I and II should be felled. The MoE.F after considering the recommendations of the Task Force, permitted felling of trees of category I, II, III and VI. On February 23^rd, 1998 the Supreme Court of India stayed all felling operations. After hearing, the Supreme Court of India ordered category wise marking of affected trees and constituted a committee under the leadership of the Director, Tropical Forests Research Institute, Jabalpur. Then the Supreme Court permitted felling of affected trees of categories II VI and I only. Out of total 2.56 million affected trees in Mandla and Dindori Forest Divisions, year wise number of trees felled and beetles killed are given in Table 2.

Table 2. Infested trees felled and beetles killed in Mandla and Dindori Divisions

Source: Office of the Conservator of Forests, Central Circle, Jabalpur, India

Difficult regeneration.

A large amount of seed during mid May to end of June is removed by way of seed collection for the use in industries (Tewari 1995). Exceptionally short period of seed viability and removal of seed from forest floor and lack a synchronization of seed fall with the advent of rainy season are primarily responsible for the regeneration problem. Sal seedling dieback occurs only in natural forests and the period of dieback or stagnation varies considerably (Maithani et al. 1989). Mishra, sharma and Wadera (2000) while studying the impact assessment of borer epidemic on the regeneration status of sal forests concluded that borer affected sal forests exhibited highly insufficient established regeneration while the fresh regeneration is found in abundance that is yet to be established.

Control measures adopted in latest epidemic.

Apart from the above earlier known silvicultural and mechanical measures, the Forest Research Organizations in collaboration with pesticide companies including bio pesticides conducted experiments with their products in the affected area. A known insect predator Alaus sordidus could not keep pace in number with the exploding population of the pest. But the tried measures did not yield any results. To assist natural regeneration and facilitate its establishment in the borer attacked areas, site specific regeneration plans are being executed. Intensive protection against fire and grazing, prohibition of sal seed collection, seed sowing in deficit areas, eradication of Lantana camera and other weeds, stump dressing and cut back operations to encourage coppice growth are the major operations included in the regeneration plans.

Conclusions.

Despite of repeated outbreaks of sal borer, there was no breakthrough as far as effective preventive and remedial measures are concerned. The invincibility of this notorious pest along with its infallible catastrophic affect has often baffled the minds of Entomologists and foresters. What is the normal tolerance level of the pest population in the non-epidemic years, what are those environmental factors that initially enhance the reproductive potential of the beetle, exact chemical constituents of the sal sap which attracts the beetle, the possibilities of additional sources of pest attractants, the flight behavior - dispersal of the pest and its biological control are still remain unexplored. These issues have to be immediately addressed by forestry research organizations in collaboration with other suitable agencies. Otherwise, with the reoccurrence of epidemics coupled with regeneration problems, valuable sal forests, which are source of livelihood for the indigenous people, can not be sustained.

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