Mission to the RSUP Pulau Burung coconut plantation (Sumatra/Indonesia) 3rd November to 14th December 1997: Research on the factors limiting the yields of the PB121 hybrid coconut variety

This joint mission by an entomologist and a physiologist specialized in the study of root systems, undertaken in November and December 1997, confirmed that attacks by Sufetula caterpillars are partly responsible for the low coconut yields at RSUP in Pulau Burung. Indeed, large excavations around the foot of high-yielding and poor-yielding 10-year-old palms revealed limited lengthwise growth of the roots: 1 to 3 m, rarely more, as opposed to 7 to 9 m for palms of the same age under non-limiting conditions. The root elongation rate seems to be very slow. The number of primary roots is abnormally high, by successive reiterations .. Moreover, it is surprising to see the existence of a dry zone stretching for a radius of 80 cm around the stem and up to 30 cm deep, whereas the water table is only 1 m down. It is precisely in that dry zone that most of the tertiary and quaternary roots, the most absorbent roots in the system, are found. Lastly, the dry zone has been observed just beneath the stem, down to a depth of 80 cm in some palms, within the plot. There is also severe soil erosion, following heavy rainfall, in a radius of 2 m around the stem. The erosion is combined with natural compaction of the peat, all of which goes to form a characteristic mound around the base of the coconut palms. This leads to exposure of mostly absorbent roots near the surface, which then dry out in the sun and are consequently more vulnerable to caterpillar attacks. Be that as it may, the caterpillars of this species are most probably not responsible for the substantial heterogeneity seen in coconut palm growth; it is mainly induced by other factors, such as the level of the water table. The heterogeneity is seen in the existence of very tall palms ( over 7 m tall), medium-sized palms (5 to 6 m) and smaller palms (2 to 3 m). All three of these categories are attacked to varying degrees by the caterpillars, which partly prevents a correlation being established between the caterpillar attack rates and the number of nuts. However, it was possible to show a good triangular relation between coconut palm height, the fresh weight of their roots and the number of nuts borne by the palms