Insect pollinators of oil palm in Kerala
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Date
2010
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Department of Agriculture, College of Horticulture, Vellanikkara
Abstract
Oil palm, Elaeis guineensis (Jacq.) (Arecaceae) is one of the most commonly available and extensively cultivated perennial trees in many parts of the world. Previously pollination was considered as a serious constraint, so manual pollination was often carried out, to sustain the yield. Wind and honey bees play an important role in pollination of many crops. But they alone can not give adequate level of pollination in all circumstances. In nature, pollination by specific insect species is vital for many crops. In this context, the present investigation on the “Insect pollinators of oil palm in Kerala” was carried out in the Plantation Corporation of Kerala, Athirappilly and Laboratory trails were conducted in the Department of Agricultural Entomology, College of Horticulture, Vellanikkara during 2009 to 2010.
Fifteen different insect species were observed to visit on the inflorescence. Insect species such as, African oil palm weevil, Elaeidobius kamerunicus (Coleoptera: Curculionidae); an earwig, Forficula sp.? (Dermaptera); a moth, Pyroderces sp.? (Lepidoptera: Cosmopterigidae) and Indian honey bees, Apis cerana indica (Hymenoptera: Apidae) were main species from the collection on male and female inflorescences.
Population of different insect visitors on anthesising female inflorescence was highly varied and E. kamerunicus was found to be more, among all insect species. Weevils were found to be visiting the female flowers between 10.00 a.m. to 2.00 p.m. and the peak weevil population was observed on the 1st and 2nd day of anthesis, around 12.00 noon. But, on male inflorescence weevil population was less during 10.00 to 2.00 p.m. Thereafter, it was gradually increased and attained the maximum at 4.00 p.m. Highly significant positive (r = 0.712**) correlation was observed between population of E. kamerunicus on 1st day of anthesis of female flowers and per cent pollination whereas, it was not significant (r = 0.199) on the 2nd day of anthesis which indicates that maximum amount of pollination occurred in the 1st day of anthesis.
Weevils of E. kamerunicus were congregating on the male inflorescence during the entire period of anthesis and their statistical median weevil population was 48, 76, 80 and 35, 56 and 72 on top, middle and bottom portions of male inflorescence during rainy and summer seasons respectively.
Adult weevils of E. kamerunicus feed on anthers of male inflorescence and multiply. Everyday, during occasional visits to female inflorescence, they would shed the pollen grains and get back to its original place of host which would cause adequate level of pollination of female flowers. The anise seed like odour (estragole 5, 4 allylanisole) attract the weevils towards both male and female inflorescences.
The biology of E. kamerunicus was carried out in both rainy and summer seasons under wire cage. The female weevil laid eggs singly on the spent male inflorescence of oil palm. The fecundity ranged from 32 to 33 eggs (median = 32) in rainy and 34 to 35 eggs (median = 35) in summer seasons with an ovipositional period of 1 to 2 days during both seasons.
The eggs hatched into grubs within 1 to 2 days. The grub period lasted for 8 to 10 days in rainy and 7 to 9 days in summer seasons respectively with three larval instars. The grubs pupated for 3 to 4 days in rainy and summer seasons and the adult weevils emerged out. The total life cycle from egg to adult was completed within 14 to 16 and 11 to 14 days in rainy and summer seasons respectively. The longevity of adult weevils varied with the sex and males lived longer than females. The life span of male weevils was 5 to 7 and 4 to 6 days during both rainy and summer seasons respectively. Female weevils lived for 4 to 6 days in rainy and 4 to 5 days in summer seasons.
Among the duration of different life stages viz., egg, larva, pupa and adult of E. kamerunicus, the grub period was found to be longest (7 to 9 days) followed by adult longevity (male = 5 to 6 days; female = 5 days), pupa (3 to 4 days) and egg (1 to 2 days). The number of female weevils in the field was more, when compared to the male with the sex ratio (♂ : ♀) of 1 : 1.87 in rainy and 1 : 1.96 in summer seasons respectively.
Morphometric study of different life stages of E. kamerunicus showed that the diameter of egg measured 0.258 mm. The freshly laid eggs were creamy white in colour. The first instar grub was pinkish yellow in colour with brown head capsule and other instars are yellow coloured. The average length of I, II and III instar grubs were 1.59, 2.73 and 3.84 mm. Similarly mean body width of corresponding instars was 0.52, 0.86 and 1.62 mm. The pupa also showed yellow colour with the length of 3.53 mm and width of 1.63 mm. The newly emerged adult weevil was light brown in colour and later on changed to dark brown. The male and female weevils could be differentiated based on their size and additional structures on their body. Males (length = 4.13 mm; width = 1.54 mm)) were bigger than female (length = 3.81 mm; width = 1.28 mm). Males also had setae on the peripheral side of elytra with some dorsal spines. Female has smaller body and absence of above mentioned structures over its body.
Both male and female inflorescences were produced separately in successive whorls. Production of male inflorescence ranged from 0 to 2 from 10 different marked palm trees and at least one male inflorescence (mode = 1) could be seen in each tree from July ‘09 to Jan ‘10 except during Sep ‘09 (mode = 2). Female inflorescence production ranged from 0 to 1 during July ‘09 to Feb ‘10 except during Aug ‘09 (1 to 6) and Oct ‘09 (0 to 3). The mode value of female inflorescence production was 1 during July ‘09 and Aug ‘09 and it was zero from Sep ‘09 to Feb ‘10. Anthesis period of male inflorescence accounted for 10 to 14 days with more weevil population between 4th to 7th days which coincided with the maximum odour emission. Stigma receptivity lasted for 2 to 3 days but, insect visit was observed for only 2 days. So, assisted pollination can be done only up to 2 days of anthesis of female flowers. The bunch set was not found to be uniform in each month. The mode value of bunch set was varied in different months like 0, 1, 5, 5, 4, 6 and 2 from July ‘09 to Feb ‘10.
Production pattern of male and female inflorescence with regard to different climatic factors was recorded from July ‘09 to Feb ‘10. An increasing trend of male inflorescence production was observed from July ‘09 to Sep ‘09 (mode = 2) with the maximum and minimum temperature of 29.91 and 24.28 oC, relative humidity of 94.37%, rain fall of 12.67 mm and sun shine hours of 0.52 hrs. Later on uniform production was recorded until Jan ‘10 and no inflorescence was seen during Feb ‘10. Similarly female inflorescence production was found to be uniform during July ‘09 and Aug ‘09 (mode = 1) which coincided with monthly maximum (28.4 and 29.94 oC), minimum temperature (23.45 and 24.08 oC), relative humidity (98.38 and 93.52%), rain fall (19.72 and 12.31 mm) and sun shine hours of 2.48 and 3.05 hrs. There after, it was very much reduced (mode = 0). Bunch set was low in July ‘09 which, gradually increased and reached the maximum during Jan ‘10 (mode = 6) with the weather parameters such as, maximum (32.24 oC), minimum temperature (21.7 oC), relative humidity (85.23 %), rain fall (0.18 mm) and sun shine hours (8.96 hrs).
Correlation study of inflorescence production and various weather parameters showed that there was a significant positive correlation with relative humidity and rain fall and significant negative correlation with maximum temperature and sunshine hours. On the contrary, monthly bunch set showed a significant negative correlation with relative humidity. Wind could be offer only less contribution towards pollination and fruit set and, it was also not influenced by birds.
Totally 336 to 800 flowers were counted from female inflorescences (mean = 596.4) and the range of fruit count was from 186 to 583 (mean = 393) with 27 to 132 partially developed fruits (mean = 73.8) per bunch. Per cent pollination was from 60.36 to 88.37 (mean = 77.13 %). It took around 145 to 161 days for the complete development of fruits (mean = 153.3) and the bunch weight was 5 to 29 kg (mean = 17.13) with a mean fruit weight of 32.52 g per bunch.
Observations on period of stigma receptivity of female inflorescence and time of weevil visit to female flowers may be helpful to the farmers when they would need assisted pollination either manually or through artificial means by using E. kamerunicus. The present study showed that, they can restrict the pollination just for 2 days after anthesis which will ultimately save labour, money and time spent on this.
Artificial culturing of weevils can be done with kairomones to maintain their population during summer season since, very poor inflorescence production was noticed during summer. It is also necessary to retain the male inflorescence at least for one month after anthesis. This will help for multiplication of weevils. During rainy months, the spent inflorescence must be removed within 40 to 45 days to avoid disease occurrence
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