Pest damages on the root system of oil palms planted on peat soil.

In the province of Riau (Sumatra - Indonesia) palms planted on peat soils show unusual low rate of growth and development, as well as very poor yield levels. Many palms present a stunted appearance, and a high number of dry fronds in the lower part of the canopy. These symptoms indicate nutritional disorders for both water and mineral nutrition (specially nitrogen) despite a permanent water table relatively close to soil surface, and increased rates of fertilizer. Soil profile observations clearly reveal heavy damages consecutive to pest attacks on the roots system of the palms. Three different types of attacks can be described, all of them affecting the tip of respective roots: - galleries inside primary and secondary rots. - damages at the periphery of primary roots. - entire depletion of the inner part of primary rots. We observe lots of reiterations on attackted roots, being attacked themselves soon after emerging. This reiterations certainly require a significant amount of carbohydrate, which are therefore not available anymore for bunches development. Three insects have been identified as the origin of these respective damages: one belongs to the Crambidae family (sub-family Spilomelinae) genus Sufetula palmivora, while 2 others one belong to the Elateridae family (order Coleoptera). The genus of the last two one has not been identified yet as only larvae and pupae were found. A complete description of the three pests is presented. Two methods have been used in order to assess the impact of these attacks on root development. Both extensive observations on voluminous but pre-defined hole, and measurements along several complete root length from the base of the palm to the root tip provide pertinent indicators. The results of these measurements are well related to the performance of the palms. Advantages and constraints of the two techniques are discussed. A policy for monitoring is proposed. (Resume d'auteur)