Managed honeybees (Apis mellifera L.) increase onion (Alliun cepa) seed yield and quality

Gebreamlak Bezabih and Kiros Gebretsadikan

Tigray Agricultural Research Institute, Mekelle Agricultural Research Center,
P.O. Box 1132, Mekelle, Tigray, Ethiopia
gbtesfay@gmail.com

Abstract

Nearly 75% of the world’s flowering plants are dependent on insects for pollination, with honeybees being well known for their importance for several crops. The effect of managed honeybee pollination on onion seed yield and quality was investigated through pollinator exclusion and pollinator surveys on onion field plots at Mekelle Agricultural Research Center experimental farm. The treatments were: plots accessible to all flower visitors ( CTL); plots not accessible to any insects – the plots were covered with an insect proof mesh cage before the ray florets started opening (NI);  plots accessible only to honeybees – the plots were covered with an insect proof mesh cage and a honeybee colony with four frames was placed inside the cage during the flowering peak (HB). Insect proof mesh cages (5m x 3m and 2.5m high) were made of wood covered with 20% shade cloth. All insects were removed from all the cages before blooming, to exclude unwanted pollinators. Honeybee colonies used in this experiment received supplementary feeding (dissolved sugar) and water before and after they  were placed in the cages.

Open pollination treatments especially with honeybees increased onion seed quantity and quality.

Key words: germination, pollination, seed quality, seed yield

Introduction

Nowadays the natural habitat is disturbed for many reasons and the vegetation cover is declining worldwide (Kearns et al 1998). Agriculture plays a role in declining native pollinators through the modification and elimination of pollinator habitats and the use of agricultural chemicals (pesticides, herbicides and fertilizers) (Donaldson 2002). Free (1993) stated that clean and intensive cultivation of land may affect wild insect pollinators. He mentioned practices such as destruction of hedgerows and rough verges which destroyed many natural food sources and nesting sites of wild pollinating insects. Generally it has been concluded that habitat degradation, pesticide misuse, diseases and intensive cultivation of lands may be the causes of decline in managed honeybees and wild pollinators (Collette 2008; Davila and Wardle 2008; Dewenter et al2005; Gallai et al 2009; Gross 2001; Morandin and Winston 2005). When many hectares are occupied by a single crop and certain localities are selected for growing particular cultivars there may be too few insect pollinators due to the factors mentioned above and it may be necessary to enhance pollinators in that area (Du Toit 1988).

Honeybee pollinators are estimated tobe involved in producing up to 30 % of the human food supply directly or indirectly; farmers rely on managed honeybees throughout the world to provide these services (Greenleaf and Kremen 2006; McGregor 1976). In the United States, the annual value of increased agricultural production in yield and quality that is attributed to honeybee pollination varied from US$9.3 billion in 1989 to US$14.6 billion in 2000 (Morse and Calderone 2000). In Western Cape (South Africa) the deciduous fruit industry which is entirely dependent on honeybees as pollinators generates R1 billion per year and creates job opportunities for 80,000 people (Picker et al 2004). Honeybees are responsible for 70-80% of insect pollination (Johannsmeier and Mostert 2001). The contribution of managed honeybee pollination to crop production and quality has been estimated to be more than the value of honey and wax production (Shrestha 2004).

At present, the need for onion seed production is highly demanding and nationally it becomes an important development component since the release of Adama Red Cultivar (Lema and Shimeles 2003). Onion is an important condiment and vegetable crop in Ethiopia. It is a cash crop and serves as a spice for flavoring local dishes and hence it is a highly valuable crop throughout the country. It fetches a very high price during rituals and holidays.

Inadequate pollination of the onion plant may result in deformed, smaller seeds which have low germination capacity (McGregor 1976). Insufficient pollination caused difficulties in onion hybrid seed production because of low quality seed (Free 1993). This is because the onion pollen usually sheds before the female part is respective (protandry) (Lema 1998). Several pollination factors could be taken into consideration for agricultural production such as wind, hand pollination, some pollen dispenser methods and insects, but wind has little effect on onion pollination because of its sticky pollen (McGregor 1976). McGregor (1976) reported that honeybees were effective pollinators on onion because both pollen and nectar are available from the plant.

Onion seed is imported from abroad with hard foreign currency. Buyers of the seed are facing the problem of germination and imported seeds are susceptible to disease (Lemma 1988). The productivity of the crop is very low and the low seed yield of self-pollinated onion has been reported from small scale producers and state farms everywhere in the world (Yucel and Duman 2005).

This study was designed to examine the role of managed honeybee pollinators in increasing seed yield and germination percentage of the onon plant and to identify insect visitors other than honeybees. In addition, the research described in this project aimed to improve the understanding of the use of managed honeybee colonies in cultivated crop pollination. The findings of this project will therefore contribute to the definition of general guidelines to maintain or improve onion crop pollination.

Objectives of the study

The immediate objective of the study was to compare onion seed production and seed viability in fields with and without managed honeybees. Moreover, it was also important to assess the contribution of other pollinators in the natural habitats to the cultivated onion crops.

Methodology

Description of the study area

This study was conducted at the experimental site of Mekelle Agricultural Research Centre, Illala during the 2010-2011 cropping season. Mekelle Agricultural Research Center Experimental Site, Illala (Figure 1) resides at longitude 13^o 5’N, 39^o 6’E and altitude 1970 m above sea level. The annual average rainfall is 548 mm and mean maximum and minimum temperatures are 26.5 ^oC and 11.8 ^oC, respectively.

Figure 1: Map of Mekelle Agricultural Research Center Experimental Site, Illala, Tigray Region, Northern Ethiopia.

Experimental set up

Three treatments were replicated three times in a Randomized Complete Block Design (RCBD). The Adama Red variety of onion (Alliun cepa)was used for the purpose. The bulb was raised during the growing season and transplanted into 5*3 m (15m²) seedling beds and recommended agronomic practices applicable to the crop were used. The treatments were: (CTL)  plots accessible to all flower visitors - the plots were left open for natural pollination as control; (NI) plots not accessible to any insects – the plots were covered with an insect proof mesh cage before the ray florets started opening;  (HB) Plots accessible only to honeybees – the plots were covered with an insect proof mesh cage and a honeybee colony with four frames was placed inside the cage during the flowering peak (50% florets open) time. Insect proof mesh cages (5m x 3m and 2.5m high) were made of wood covered with 20% shade cloth. All insects were removed from all the cages before blooming, to exclude unwanted pollinators. Honeybee colonies used in this experiment received supplementary feeding (dissolved sugar) and water before and after they  were placed in the cages.

Flower visitation surveys

For ten consecutive days flower visitor surveys were done in each of the CTL plots, to assess which and how many insect species were visiting the onion crop;  and in the HB plots accessible only to honeybees to count the number of honeybee pollinators. Fifteen minute surveys were done every hour from 6h00 to 18h00. Whenever identification of flower visitor species was not possible in situ, specimens were captured for later identification. Visiting insects were collected and identified by the entomologist at Mekelle Agricultural Research Center.

Seed collection and laboratory work

After the onion crop reached physiological maturity (upon senescence of flowering), all honeybee colonies and pollinator exclusion materials were removed to ensure uniformity of post pollination treatment. The effect of insect pollination on onion seed yield and quality was measured by comparing the yield of the three treatments based on total seed yield/plot, mass of 1000 seeds, and seed germination percentage as follows.

Mass of 1000 seeds, total seed yield (g) per 15 m² and germination percentage

An increase in yield and quality of onion seeds due to managed honeybee pollination was calculated using the formula as follows.

A germination success study was conducted by considering the principle of maximum percentage germination, following the necessary steps used by the International Rules for Seed Testing (ISTA 2009).

Statistics

Analyses of variance (ANOVA) were done using the statistical package SAS (2003).  Significant differences between the means of treatments were further analysed using Duncan's Multiple Range Test (SAS 2003).

Results

Flower visitors

Totals of 1748 insect visitors in the open pollination (CTL) and 1548 honeybee visitors in the caged treatment (HB) were recorded (Table 1). The onion visitor community was diverse, including insects from four orders. Hymenopteran visitors belonged to the families Apidae, Sphecidea and Halictidae, while Dipteran visitors were identified as Tabanidae, Lepidoptera as Nymphalidae and Coleoptera as Scarabaeidae and Meloidae. Hymenoptera constituted the highest percentage of insects, while Coleoptera and Lepidoptera were the least abundant orders in the open pollination.

Mass of 1000 seeds, total seed yield (g) per 15 m² and germination percentage

 The seed yield was increased by 41.2%, the mass of 1000 seeds by 25%  and germination percentage by 68%  by open pollination especiallywith honeybees (Table 2).

Discussion

The reason why we found large numbers of wild honeybees and other insect visitors during this study might be that the field was 200-400 m from apiary sites and the experiment was surrounded by natural habitat. Steffan-Dewenter and Tscharntke (1999) found that isolation from natural habitats diminishes abundance and species richness of bees, which are the most important flower-visiting insects. Honeybees were by far the most frequently recorded insects on onion flowers. The high proportion of honeybees compared to other insects visiting the flowers indicated that honeybees were the major pollinators of the onion crop at our field site, with both honeybee abundance and seed yield and quality increasing proportionally.

 In agreement with Yücel and Duman (2005), this study shows that pollination improves production and size of seeds of the onion plant. The increase in germination rate in the openplots is a result of a superior pollinating efficiency of honeybees. Simiraly Yücel and Duman (2005) reported that the germination rate was greater on average by 12% in onion with honeybee activity.

As modern agricultural production has come to rely on large mono-cropping farms (for example in our region the case of Raya-Azebo onion farms), the dependence on wild insects living in the surrounding area for pollination has become less feasible. This is because of the decline in the availability of wild pollinators, likely due to disturbance of nesting habitats and food sources with the introduction of modern agricultural practices. Our results demonstrate the great importance of insect pollinators, essentially honeybees, on seed yield and quality, since caged onion crop produced lower quality seeds than the open ones. It is recommended that moving honeybee colonies to onion seed production areas during the flowering period is essential for maximum seed production and improved quality.

If it is assumed that wild honeybee populations and other natural pollinators are invariably not adequate for onion pollination, bringing in honeybee colonies to onion farms may be an easy and simple way of producing high yield and good quality seeds.

Conclusions

• Insect pollinators, essentially honeybees, increased onion seed yield and quality.

Acknowledgments

We thank Mr. Desta Hadera, Mr. Haftom Gebremedihin, Mr. Tetemke Beyene and Mr. Haftom Miglas from the Apiculture and Sericulture Case Team, Mekelle Agricultural Research Center for their continuous follow up during the research implementation. We thank Mr. Esayas Meresa, a GIS expert, for help during map preparation. We are very grateful to all who helped us during the research implementation and laboratory work. Finally, we owe our sincere gratitude to the staff of the Tigray Agricultural Research Institute for continual support during our survey work.

References

Collette L 2008 A contribution to the international initiative for the conservation and sustainable use of pollinators. FAO, Rome Italy.

Davila Y C and Wardle G M 2008 Variation in native pollinators in the absence of honeybees: implications for reproductive success of an Australian generalist pollinated herb Trachymene incia (Apiaceae). BotanicalJournal of the Linnean Society 156 (2009): 479-490.

Dewnter I S, Potts S G and Packer L 2005 Pollinator diversity and crop pollination services are at risk. Trends in Ecology and Evolution 20 (12): 651- 652.

Donaldson J S 2002 Pollination in Agricultural landscapes, a South African perspective. In: Kevan P. and Imperatriz Fonseca VL (eds) Pollinating Bees The Conservation Link between Agriculture and Nature Ministry of Environment/Brasilia Pp 97-104.

du Toit A P 1988 Pollination ecology of commercial onion (Helianthus annuus L.) in South Africa with special reference to the honeybee (Apis mellifera L.).  MSc Thesis, University of Pretoria, South Africa.

Free J B 1993 Insect Pollination of Crops (2^nd ed.). San Diego, CA: Academic Press

Gallai N, Salles J M, Settele J and Vaissiere B E 2009 Economic valuation of t he vulnerability of world agriculture confronted with pollinator decline.  Ecological Economics 68 (2009): 810-821.

Greenleaf S S and Kremen C 2006 Wild bees enhance honeybees’ pollination of hybrid onion. Proceedings of the National Academy of Sciences of the USA103: 13890-13895.

Gross G 2001 The effects of introduced honeybees on native visitation and fruit set in Dillwynia juniperina (Fabaceae) in a fragmented ecosystem. Conservation Biology 102 (2001): 89-95.

Johannsmeier M F and Mostert J N 2001 Crop pollination. In: Johannsmeier, M. F. (Ed.), Beekeeping in South Africa, 3rd edition, revised, Plant Protection Research Institute handbook 14. Agricultural Research Council of South Africa, Pretoria, South Africa, pp 235-245.

Kearns C A, Inouye D W and Waser N M 1998 Endangered mutualisms: the conservation of plant-pollinator interactions. Annual Review of Ecology and Systematics 28 (1998): 83-112.

Lemma  D 1998  Seed production guideline for tomatoes, onion and hot pepper. IAR, Addis Ababa

Lemma D and Simeles A 2003 Research experiences in onion production. Research report No 55.

McGregor S E 1976 Insect pollination of cultivated crop plants. Agricultural handbook  496. Agricultural Research Service, U.S. Department of Agriculture.Washington, pp 411.

Morandin L and Winston M 2005 Wild bee abundance and seed production in conventional, organic and genetically modified canola. Ecological Applications 15 (3): 871-881.

Morse R A and Calderone N W 2000 The value of honeybees as pollinators of U.S. crops in 2000. Bee Culture 128: 1-15.

Picker M,Griffiths C and Weaving A 2004 Field guide to insects of South Africa. Struik Publishers: South Africa.

Shrestha J B 2004 Honeybees and Environment. Agriculture and Environment. Gender  Equity and Environment Division. Ministry of Agriculture and Cooperatives,  HMG,Nepal, pp 1-8.

Staffen-Dewenter I and Tscharntke T 1999 Effects of habitat isolation on pollinator communities and seed set. Oecologia 121 (1999): 432- 440.

Yücel B and Duman I 2005 Effects of foraging activity of honeybees (Apis mellifera L.) on onion (Allium cepa) seed production and quality. Pakistan Journal of Biological Sciences 8 (1) 123-126.

Received 14 June 2013; Accepted 23 December 2013; Published 1 January 2014

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