A new pest causing decline of mangrove forests in Kenya

J.G. Mwangi - Associate Professsor, Moi University, P.O. Box 3900, Eldoret, Kenya

Abstract

The survey of Kenyan coast mangrove forests reveals a serious decline. A new pest Salagena discata has been found infesting Sonneratia alba mangrove species causing death and deformity. Aerial survey indicates that substantial portions of mangrove forest in Kiunga in Lamu District and Gazi in Kwale are dead while infestation continues along the whole coast covering Kenya and Somalia. The nature and pattern of attack by the insect is discussed. A review of records on mangrove pests from other parts of the world gives a detailed account of the damage caused by insect, crabs, fungi and diseases. This paper proposes that a programme be initiated for monitoring the health of mangroves on a regular basis. This will enable determination of stock losses to pest attack which is useful in making management decisions and in setting of research priorities.

Introduction

Mangroves which have flourished along the Kenyan coast without any significant bio-deterioration are now under pest infestation (Ferguson W., 1993; Kairu J., 1993). The infestation which is rapidly spreading through S. alba along Kenyan coast is caused by Salagena discata larvae. The damage was first noticed in Kiunga, Lamu in early 1993, where the pest caused massive death of natural stands of S. alba. Recently the attack has spread to Gazi and Tudor.

Importance of Mangroves

Evaluating S. alba cannot be done singularly without incorporating the other seven types of mangroves, namely, Avicennia marina, Rhizophora muronata, Bruguiera gymnorrhiza, Cerriops tagal, Xylocarpus granatum, Lumnitzera racemosa, and Heritiera littoralis. All of these cover an area of 62,380.9 hectares of which 46,229 are situated in Lamu. Uses and values of these mangroves centrally lie in wood fuel and charcoal production, building poles, boat-building materials, fodder, furniture-making, medicine, fisheries, wildlife (including birds) protection, honey production (beekeeping), tourism attraction, and soil erosion control.

Although the main commercial species are R. mucronata and C. tugal, especially in production of poles, all other types are equally important for they jointly contribute to the aforementioned array of utilization. Mangrove forests supporting these species are extremely sensitive and need ultimate care in order to sustain the limited resources which are renewable if managed wisely. Current and past management plans of mangroves in Kenya have neglected the aspect of the health of mangroves.

Health of Mangroves

The health of mangroves can be effectively protected if the following goals are set forth (United States Department of Agriculture, April 1993).

Planning:
The ecological significance of pests is considered in all forest resource management planning processes.

Suppression:
Pest suppression and funding continue to be available to meet resource management objectives.

Pesticides:
Environmentally acceptable pesticides are available to protect the resource (no chemical would be used in environmentally sensitive mangrove area).

Forest protection technology:
Effective, economical and environmentally acceptable protection technologies are available to meet forest management objectives.

Forest health monitoring on a regular basis:
This may be annually or frequently if additional problems are detected. Aerial sketchmapping, photography, satellite imagery or aerial videography could be used singularly or in combination. There is a need to develop baseline data on the health of the forest and maps of resources.

Damaging agents:
These need regular monitoring and may include insects, diseases, fungi, bacteria, viruses, parasitic plants, animals (including humans), oil spills, dumping of rubbish and other miscellaneous pollutants, crabs and other arthropods.

Literature Review of Mangrove Pests

Insects:
In Kenya, little has been documented on insect damage (because not one has investigated). Elsewhere in the world, insect damage information is scanty except for Ranong mangrove ecosystems southwest of Thailand (UNDP/UNESCO, RAS/86/120, 1991) and a few other places as shown on Tables 1.1, 1.2 and 1.3.

In Central Java, nearly 80% of newly planted (3-month-old) seedlings of Bruguiera spp have been killed by Chaetocnema sp. (CAB, 1990).

Crabs:
Serious crab damage has been reported in Malaysia, Brazil and Java.

In Malaysia, conversion of Avicennia forest has been proposed to increase the productive area of production forest in Perak, where yields have declined in second generation Rhizophora stands. Planting R. apiculata and R. mucronata propagules at 1.5 metre spacing under canopy resulted in 32% and 37% survival, respectively, after three months. Losses were mainly due to crab attack (often leading to total disappearance of plants). Damage by Scolytids was also significant. Caging to exclude crabs increased R. apiculata survival to 55%, losses being mainly due to Scolytids. Results suggested that large-scale planting in Avicennia forests is feasible (CAB, 1986).

In Brazil, a tree-climbing crab, Aratus pisonii, has been observed to consume mangroves.

In Central Java, Indonesia, seedlings of R. mucronata have been severely attacked by crabs called Sesarma sp. Which cut off the shoots of young seedlings immediately after planting (CAB, 1990).

Fungi:
Two common terrestrial pathogenic fungi have been found to cause butt and heart rot on old growth stand of R. apiculata. These are Phellinus pachyphloeus and Phellinus rimosus. A foliage disease on young leaves was found to be sooty mould caused by Meliola sp. Brown spot disease was caused by Pestalotiopsis sp.

Game damage:
In India, the spotted deer (Axix axis) and barking deer (Muntias muntyak) have been found to dig out the roots of and ultimately destroy pure stands (Naskas and Bakshi, 1987). The deer also graze on the leaves. Mucaca mulatta (monkey) has been seen feeding on leaves and fruits of mangroves in India.

Pest damage on Sonneratia alba in Kenya

A preliminary survey by the staff of Forest Health Management Centre for pest of S. alba in Gazi, Vanga and Ramisi included collection of mummies of different pests, leaf miners, twig notchers (girdlers), molluscs, and Plannococcus spp. The results obtained indicated that the greatest damage was caused by the Salagena discata.

Damage by the Salagena discata:

The larva notches away the bark of the mangrove, concentrating on between one and three inches. It then identifies one spot in this clear region and burrows into the wood (usually a branch). The frass it generates while burrowing is ploughed outward to close the burrowed tunnel. Consequently, the overflowing frass is concentrated round the notched, debarked zone of the tree to give a muddy brown colour. Several borers may colonize a single branch. For example, four larvae may infest a single branch of four feet in length. A consequent dieback of the branch ensues. Recurrent or concurrent infestation on all branches of the tree may cause the death of the mangrove tree. The holes made by the insect may provide entrance for numerous disease-causing organisms such as bacteria, fungi, etc. Possibly, the larva may also introduce such agents.

Identification:
After rearing the larvae to adult stage, the specimens were taken to 11E, UK for examination.

Dr. J. Hollaway identified the pest as Salagena sp. (Cossidea: metarbelidae) which is native to Kenya/East Africa. Kenya National Museums further identified this as Salagena discata. Literature search reveals that the species may differ but the genus was found on cashew in Zambia.

The metalselid Salagena sp. was reported for the first time as a pest of cashew. The larvae feed on the bark and make galleries in the wood, covering the point of attack with a typical mass of frass and silk.

Aerial survey:
The detection of Salagena discata pointed to the need for aerial survey as the S. alba forests’ affected trees were dying from the top.

The mangrove forests stretch from the furthest end of the south coast from Vanga on Kenya - Tanzania border to the northern-most part of the Kenya - Somalia border at Kiunga. The stretch of forest is not continuous; from the Gazi Islands to Mombasa to Kilifi, there are few or no mangroves. At Mida Creek from Kilifi Town the Forest emerges and it’s managed by Gede Forest Station. The forest begins at Mamburui from Malindi Town stretching considerably and ending near salt mines. The forest reappears at the mouth of River Tana spreading northwards to Kiunga. Only about 40% of the coastal strip is covered by mangroves.

The observations made indicated isolated clusters of damaged portions with individual dead trees. Photography was carried out which served to determine the rate of mortality by visual observation of several photographs produced. These showed substantial damage and would be updated by a second aerial sketchmapping.

In conclusion, the mangrove aerial survey was a reconnaissance aerial survey and was qualitative rather than quantitative. No specific areas, volume, losses or extent of damage is given; only general information on the status of the infestation.

Proposed immediate action

A complete programme for monitoring and evaluation (not only for the insect damage but any other agents affecting health of the mangrove forests) of the health of the forest on a regular or periodic basis should be instituted.

In collaboration with forest manager, recommendations should be made for a course of action for management decisions and possibly setting of research priorities.

Table 1.1: Insect and Crab attacking living mangrove species Avicennia alba at Ranong

Table 1.2: Insect and Crab attacking living mangrove species Avicennia officinalis at Ranong

Table 1.3: Insects and Crabs attacking living mangrove species Bruguiera parviflora at Ranong

References

CAB of Forestry Abstracts (1986), 047-03967.

CAB of Forestry Abstracts (1990), 041-08074.

Ferguson, Wankja (1993). A Land (scope) Ecological Survey of the Resource of Kenya (a draft report).

Kairu, J. (1993). Personal communication.

UNDP/UNESCO RAS/86/120 (1991). Integrated Multi-disciplinary Survey and Research Programme of Ranong Mangrove Ecosystem.

United States Department of Agriculture (1993). Healthy Forest for American Future: A Strategic Plan, MP 1513.