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Comparative activity of four Elaeidobius spp. oil palm pollinators visiting oil palm inflorescences in Central Africa

Beaudoin-Ollivier L., Frérot B., Ripoll J., Morin D., Meyobeme H., Flori A.. 2010. In : Rival Alain (ed.). Palms 2010, Biology of the palm family (Abstracts books) : International Symposium 5-7 May 2010, Montpellier, France. s.l. : s.n., p. 63-63. International Symposium on the Biology of the Palm Family, 2010-05-05/2010-05-07, Montpellier (France).

Pollinating insects on oil palm, Elaeis guineensis Jacq., directly influence the fruit set rate, hence the production of well-formed fruits in bunches. Four small Curculionidae of the genus Elaeidobius are the main pollinators in Cameroon. They feed off pollen and complete their entire cycle on male inflorescences, depositing pollen when they visit female inflorescences. We assessed insect frequencies at different stages of anthesis, depending on climatic conditions over 6 weeks, using sticky traps installed near 9 male and 12 female inflorescences. Observations were carried out in March- April 2009 in a 118- ha plot of 6-year-old hybrid oil palms near Kribi in Cameroon. Elaeidobius specimens were also taken from male inflorescences in anthesis. Specimens of each species and both sexes were prepared and photographed; the male genitalia were dissected. Captures revealed four species: E. kamerunicus, E. subvittatus, E. plagiatus and E. singularis. A photo-plate and simple identification key are given to distinguish between the main three species which have similar external traits. Elaedobius subvittatus was the most abundant in sunny conditions, averaging around a hundred individuals per trap, as opposed to 30 individuals for E. kamerunicus and E. plagiatus. In heavy rainfall, insect activity decreased around male inflorescences (27 E. subvittatus, 37 E. kamerunicus and 8 E. plagiatus on average), with even fewer around female inflorescences (1.5 E. subvittatus, 0.7 E. kamerunicus and 0 E. plagiatus on average). The male inflorescence stage, time of day and weather conditions had a significant effect (P<0.001) on the distribution of the 3 species. The results did not include E. singularis which was too scarce in the field. Species-stage interaction was significant (P<0.01). E. subvittatus was more abundant on the second and third days of anthesis, while E. kamerunicus and E. plagiatus were abundant on the first and second days of anthesis. Daytime screening every 1½ hours confirmed the insects were active around inflorescences between 8 am and 11.30 am, peaking around 10.00 am. A rainfall effect (P<0.001) led to a sharp drop in insect numbers on traps. As for male inflorescences, the female inflorescence stage had a significant effect (P<0.005) with the 3 species predominantly present on the first day of anthesis. A species effect was found (P<0.0001), with E. kamerunicus and E. subvittatus apparently attracted to female inflorescences (averaging 3.9 and 2.4 respectively, against 0.7 for E. plagiatus). The orientation of male or female inflorescences had no significant effect, but the trap face had a significant effect for female inflorescences (P<0.0001), unlike male inflorescences, with an average of 1.2 individuals on the outer face and 2.8 on the inner face. Lastly, distribution of the three species differed depending on the phenological stage of the inflorescences and their sex. Insects were more numerous around inflorescences on which they fed and completed their development cycle. This preliminary study suggests that fluctuations in Elaeidobius spp. populations should be studied over a longer time scale, to determine the current situation in central Africa and compare the evolution of a native species and an introduced species playing a decisive role in oil palm pollination efficiency....

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