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Species Page

African oil palm weevil

Elaeidobius kamerunicus

Distribution

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Host plants / species affected

Main hosts

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Elaeis guineensis (African oil palm)

List of symptoms / signs

Inflorescence - external feeding

Symptoms

The weevils chew anther filaments of opened male flowers. When they crawl or move about on the spikelets the pollen grains adhere to their body and during their subsequent visits to the female infloresences the pollen grains are deposited in the stigma of female flowers. When the weevils crawl over the male inflorescences a large amount of pollen grains are dispersed which are carried by wind (Ponnamma et al., 1986).

Prevention and control

Control methods are not appropriate for E. kamerunicus.

Impact

Chew (1982b) discussed the reasons why E. kamerunicus was considered the most suitable insect for introduction into Malaysia and other areas where pollination of oil palms is inadequate. Of the various species of this genus studied in Cameroon, E. kamerunicus was the most abundant in both wet and dry seasons in coastal areas of the country, it carried more pollen than other species and had a fairly good searching ability.

Yield increases following the introduction of E. kamerunicus into Malaysia were greater from the Tenera palm variety, than from the Dura variety. Weevil pollination greatly improved fruit set, which led to the production of larger, heavier and more compact bunches and an increase in the oil to bunch ratio from under 19 to 23-25%, mostly from trees 4-10 years old. A less efficient existing pollinator, Thrips hawaiiensis was not apparently displaced by E. kamerunicus. The presence of larvae appeared to attract rats. A mill survey revealed that the large compact bunches were difficult to process with the existing machinery, resulting in reduced extraction rates of oil. The advantages of the introduction of E. kamerunicus appeared to outweigh the disadvantages, but further research is required (Wahid et al., 1983).

Greathead (1983) stated that in Malaysia, especially Sabah, assisted pollination of oil palm is necessary and even then yields are lower than those in Africa. One explanation was that in selecting for more female flowers, plant breeders had produced varieties with insufficient male flowers or with less viable pollen. Elaeidobius species are attracted to male flowers only during anthesis when a strong aniseed-like scent is emitted. When the stigmas are receptive, the female flower emits short pulses of a similar but sharper and more penetrating odour which attracts Elaeidobius species away from the male flowers. On arrival in the female inflorescence, the beetles find no food and after wandering around, they soon leave and return to male flowers. Examination of Elaeidobius species showed that they become covered in pollen which is shed when they clamber about the female inflorescence. Syed (1979) estimated that during the period of receptivity, over 5000 beetles visit each female inflorescence and that each beetle carries up to 600 pollen grains. Oviposition takes place after anthesis so that the larvae develop on tissue which has served its purpose and no longer has any function for the palm and, therefore, Elaeidobius species cause no significant damage.

The weevils are extremely beneficial and have been used by man, through introduction to palm plantations, for the pollination of these palms. Previously, hand pollination was required in areas where palms and weevils were not native, at great expense to the growers. In these countries, the inadequacy of pollination has been a major problem. Besides the low rate of fruit setting, sometimes lack of adequate pollination also results in bunch failure.

Two liberations of E. kamerunicus were made in February 1981 on two estates in Malaysia. Within a year it had spread throughout the Peninsula and was extremely abundant in all plantations. Freed of competition with other species of Elaeidobius, it breeds in all parts of the male inflorescence and is equally common in all rainfall zones. Oil palm fruit set now regularly reaches the levels found in Cameroon, hand-pollination has been discontinued, and yields raised by about 20%. Calculations indicate that, with the money saved on hand pollination, the introduction of the pollinator will be worth some US $115 million per year to planters. However, palm oil factories in Malaysia were designed for the former lower yields and are unable to extract all of the extra oil now available, but this is being remedied by introducing the processing methods used in West Africa. The weevils were taken to Sabah in March, 1981, with even more impressive results. They have also been taken to Papua New guinea, the Solomon Islands, Sumatra and Thailand. Reports of successful establishment and enhanced yields are now being received from these countries (Greathead, 1983).

E. kamerunicus was successfully introduced and established in oil palm plantations in Malaysia by the middle of 1981. Over the first 7 months of 1982, bunch weight was 10% above the mean for the previous 5 years in Kluang and 35% above in Sabah. Total yield was 20% higher in Peninsular Malaysia and 53% higher in Sabah (Ponnamma et al., 1986).

E. kamerunicus, imported into South-East Asia in 1981, has increased fruit set in oil palm, for example, from 50 to 70-80% in Peninsular Malaysia, and populations still appear to be adequate with assisted pollination no longer needed. The main ecological side effect of weevil introduction is the feeding of three rat species on the larvae which develop in the male flowers. High fresh fruit bunch yields in 1982 resulted in the production of a low crop in 1983, numbers of bunches being lower, although bunch size was still high. The higher yields expected where pollination was previously poor and the change in spacing optima now that access for assisted pollination is not needed may affect breeding requirements and fertilizer needs. Improved set has caused the production of more but smaller fruitlets, but has affected oil yield per bunch very little. The change in numbers of fruit set affects the harvesting standards used for optimizing oil production, as these are based on numbers of loose fruits. Harvesting all bunches in which fruitlet detachment had commenced every 10-15 days gave optimum oil production (Wood, 1985).

Wahid and Hassan (1985) reported the results of a survey covering 433 Malaysian oil palm estates to assess the increase in rat populations since the introduction of E. kamerunicus in 1981. Rats in these estates have developed a liking for the larvae of E. kamerunicus found in the post-anthesis male inflorescences. Rattus tiomanicus, R. argentiventer and R. rattus diardii were reported in 64.6, 42.3 and 11.5% of estates, respectively. The annual cost of rat control was estimated at M$ 17.9 million, compared with M$ 11.8 million prior to E. kamerunicus introduction (Wahid and Hassan, 1985).

The effects of the curculionid, E. kamerunicus on rat control programmes in oil palm estates was investigated in Peninsular Malaysia in 1983-84. The main rat species in the oil palm estates, R. tiomanicus and R. argentiventer, feed on larvae of Elaeidobius. Since the introduction of E. kamerunicus in late 1981, it was found that there was a general increase in the rat population in areas of poor rat control, which was attributed to the sudden and abundant source of protein, and rat damage to unripe fruits increased. There was also a steady build-up of R. rattus diardii, an urban species, previously found only occasionally in oil palms and an increase in both the quantity of rat bait/ha per year and the cost of application compared with areas before the introduction of the curculionid (Basri and Halim, 1985).

Ablation or disbudding of oil palm at the immature stage is commonly practised in Malaysia on the assumption that more nutrient will be channelled for vegetative growth, and in order to prevent rotting of the poorly pollinated early bunches. Since the introduction of the pollinating weevil, E. kamerunicus, however, fruit set has improved and early bunches no longer tend to rot (Nazeeb et al., 1988).

Millions of dollars have been saved in Malaysia, following the introduction of E. kamerunicus, and growers have begun investigations into the biological control of oil palm pests in order to reduce use of insecticides, to enhance the development of these important weevil pollinators (Anon., 1981, 1982).

The pollinating weevil, E. kamerunicus was well established 6 months after its introduction into oil palm plantations. As a result of this introduction, average fruit set increased from 38.7 to 63.8%. An increase in rat damage, particularly to rotting male flowers, was noted following weevil introduction (Hutauruk and Ps, 1984).

The population density of E. kamerunicus, 10 months after its introduction into Indonesian oil palm plantations was studied. More weevils were found on mature palms (98-132/spikelet) than on young palms. Increases in fruit set of 30.3 and 38.0% in mature and younger palms, respectively, and increases in bunch quality are reported following weevil introduction (Hutauruk et al., 1984).

In Indonesia, the introduction of E. kamerunicus led to an increase in mean bunch weight, fruits/bunch, and fruit set. There was no significant effect on mesocarp, shell and kernel, but fruits tended to become smaller and bunches more compact. Fruit detachment from the bunch was delayed as a result of pollinating weevil introduction (Chairani and Taniputra, 1985).

In field studies at the Bukit Sentang Estate in Sumatra, Indonesia, the yield of fresh fruit bunches increased in the first year after the release of E. kamerunicus, but decreased in the second year. The increase in yield was due to an increase in the average bunch weight, although the bunch number decreased. During the second year production fell because the bunch number was still decreasing, in spite of a slight increase in the average weight. Increased fertilizer application could not prevent the decrease in yield during the second year. It is estimated that the yield at Bukit Sentang Estate will return to the level of the pre-weevil period (Taniputra, 1985).

In Africa, the curculionids E. kamerunicus, E. subvittatus, E. plagiatus and E. singularis ensure the major part of pollination of the oil palm, with E. kamerunicus the most important. In Indonesia, the low pollinating capacity of Thrips hawaiiensis, led to the introduction of E. kamerunicus in 1983 (Mariau and Genty, 1988).

Four-year observations after the release of E. kamerunicus in an oil palm plantation at Bukit Sentang, Sumatra, Indonesia, showed a marked increase in average bunch weight (88.8%), a minor increase in fresh fruit bunch yield/ha (1.7%) and a decrease in bunch number/palm (44.4%). Extra fertilizer application of 500 g urea and 500 g rock phosphate/palm above the normal application during the 4 years did not stop the decrease in bunch number. Leaf nutrient content of plots with and without extra fertilizer showed no significant difference. It appeared that the weevil did not change the monthly yield pattern. One third of the yield was produced during the first semester and two thirds during the second, with the peak in September-October (Taniputra and Muluk, 1987).

The release of E. kamerunicus as a pollinator of oil palms (Elaeis guineensis) in North Sumatra, Indonesia, in 1983 has changed the pattern and results of pollination. For the first 4 years, yields oscillated, declines being coupled with lowering of bunch numbers and increases with increased bunch numbers. The average bunch weight doubled compared with that before release of the weevil, and now remains steady, with a tendency to increase further. Applying fertilizers did not reduce bunch numbers. As the yield at Bukit Sentang estate has now returned to the level of the pre-weevil period, it is concluded that it takes 5 years for the oil palm to adjust to the change in pollination system (Taniputra and Muluk, 1989).

Bunch analysis in oil palms in Indonesia was carried out for 5 years after the introduction of E. kamerunicus as a pollinator. Average bunch weight increased 19.84-67.25%, fruit to bunch 8.55-12.93% and fruit set 17.71-41.77% compared with the pre-weevil period. The percentage of kernel to bunch and oil to bunch were 5-6 and 20.84-25.16%, respectively. The other components were more or less constant. The inflorescences which reached anthesis were 6.14-7.31/tree per year and about 97.8% of the inflorescences could be harvested as normal bunches (Chairani et al., 1989).

In the Solomon Islands, the introduced pollinating weevil E. kamerunicus has increased fruit-set in oil palm from 47-62 to 66-78% (Macfarlane, 1985).

In Papua New Guinea, components determining the oil and kernel extraction showed no response to density when assisted pollination was used, but with the successful introduction of the pollinator, E. kamerunicus, oil and kernel extraction increased with planting density as a result of an increase in fruit per bunch (Breure et al., 1990).

Introduction of E. kamerunicus into India increased the fruit set of E. guineensis from 36.9 to 56.1%, resulting in a 40% increase in bunch weight and an 11% increase in the fruit: bunch ratio. Weevil pollination also decreased parthenocarpy from 34.3 to 24.3%, resulting in a 28.4% reduction in parthenocarpic fruit: weight ratio. Although wind, plus assisted pollination was more efficient than wind plus weevil pollination (60.5 vs 56.1%), the latter produced compact bunches owing to pollination of spikelets at the bottom of the inflorescences by the weevils (Dhileepan and Nampoothiri, 1989).

The pollen carrying capacity and pollen load of adults of the curculionid E. kamerunicus were estimated by collecting weevils from male and female inflorescences of oil palms, respectively, in Kerala, India during January, February, June and September 1988 and 1989 and May-June 1991. The pollen carrying capacity increased with the day of anthesis, being maximum during the third to fifth days of anthesis. The viability of pollen carried by the weevils congregating in the male inflorescences and by those leaving the male inflorescences were 83.1 and 81.5%, respectively. The pollen carrying capacity was inversely proportional to the weevil population per spikelet, too high weevil populations resulted in reduced pollinating efficiency, presumably due to intraspecific competition. The pollen carrying capacity of males was significantly greater than that of females, which was partially due to the greater surface area of males and especially the presence of pleural setae. The average pollen load of weevils visiting female inflorescences ranged from 610-2620, and represented only 32.6-48.9% of the carrying capacity. The average viability of the pollen load was 77.1%. Only 54-83% of the pollen load was transferred by each weevil in the female inflorescence (Dhileepan, 1992).

Pollination studies initiated at Pamol, Sabah in December 1993, indicated that there may be a seasonal trend in the population of E. kamerunicus, the pollinating weevil, which is closely related to male inflorescence density. High rainfall may also affect pollination by reducing weevil activity and overall viability of available pollen. Assisted pollination gives good fruit set during the months when natural pollination is poor. Severe pruning to induce male inflorescence production is being tested as a less labour intensive alternative to improve fruit set (Donough et al., 1996).

Elaeidobius spp. weevils were released to carry out pollination in the Palmas del Espino SA plantation, Peru in 1988-89, when the palms were 60 months old. However, during their first few years, young oil palms produce primarily female flowers, only producing sufficient male flowers for the establishment and effective activity of pollination insects from the age of 65-70 months. Therefore, before this age, artificial pollination is required. This is normally carried out by hand. As an alternative, trials were carried out, initially on an area of 25 ha, increasing to 600 ha, on the use of hives, each hive, a 21 cm high cylinder, containing 5000 weevils and 2 g of fresh, pure pollen. E. kamerunicus was the most abundant and suitable species. This technique achieved much the same percentage normal fruit set as hand pollination (60-70%), but required only one third of the labour of hand pollination (Vera, 1996).