Agroecosytem Dynamics on the Interactions between Passion Fruit (Passiflora Edulis) and Insect Pollinators

The passion crop is one of the climbing plants that last longer and it has been a common fruit in Kenya since it was first introduced in the 1920’s. Passion fruit farming in Kenya does well in the upper midland to upper highland zones. The most suitable altitude for its production ranges from 1200-2000 meters above sea level and it requires an optimum temperature between 18 °C to 25 °C and well-distributed annual rainfall of 900mm to 2000mm. It grows on a variety of soil with a pH range of 5.5-6.5 and should be reasonably deep and fertile. Passion fruit plant is grown in Thika, Kisii, Nyeri, Kiambu, Nakuru, Machakos, Uasin Gishu and Trans- Nzoia Counties in Kenya.

Natural pollination is related to an increase in production yields in crops such as sunflower (Helianthus annuus) and coffee (Coffea arabica), [1,2]. Pollination of Capsicum annum flowers by some specific insects has been shown to have a positive effect by enhancing fruit quality and seed set [3]. A study carried out in Kakamega forest, in the western region of Kenya, to evaluate the effectiveness of the stingless bee Hypotrigona gribodoi on the pollination of green pepper showed an improvement in fruit quality [4]. Studies on bottle-gourd (Lagenaria siceraria) in Kenya have shown how important a diverse pollinator community is, to maintaining the extraordinarily diverse forms of gourds [5]. The richness of pollinator species increases fruit set [6] because of complementary pollination among species [7,8]. In a well-pollinated flower, a rapid development of ovary occurs, and the fecundated seeds produce plant growth hormones, leading to a good fruit development [9]. Reduced agricultural yields and deformed fruit often result from insufficient pollination. Fruit set is the proportion of a plant’s flowers that develop into mature fruits or seeds and is a key component of crop yield.

There has been a constant human population increase in both the arid and semi-arid regions of Machakos County which has fostered the rapid expansion of agriculture to help meet the population needs. The sensitive ecosystems such as the hilly areas of Mua hills location have been encroached for horticultural farming. This has led to habitat destruction, degradation, and fragmentation - resulting in a loss of foraging, mating and nesting sites of insect pollinators. These factors are likely to cause pollinator decline and this study aimed at evaluating the effect of land use on diversity and abundance of carpenter bee in relation to passion crop production.

Objectives

A. To determine the diversity and abundance of carpenter bee in the agro-ecosystem

B. To correlate passion fruit set with the diversity and abundance of insect pollinators.

Map of Machakos County locating Mua Hills sub-locations and farm owners where sampling was done.

This study used a sweep net with an eversible stick and a diameter of 38cm to increase the collection radius. A single back and forth sweep covering a 1500 to 1800arc was considered as a single sweep. The researcher walked through the habitat and swept with the hand net with ease over the vegetation. This caused the insects to fly up only to land inside the net as anticipated. The time taken was not more than five minutes at any single plant or flowering patch and the collector did not have to re-visit the same patch again. Two 30 minutes surveys a day were carried out. The insects caught with a hand net were carefully put into a killing jar with 70% alcohol to be later identified at National Museums of Kenya (Figure 1).

Figure 1:

At each site, 15 pan traps were put out in areas where pollinators were likely to be found, especially where the vegetation was open and the pans could be seen from some distance and where there were flowers which the pollinators might visit. The position of each pan trap in the arrangement was randomly chosen. The spacing between pan traps was 5cm. The three groups of pan traps containing one of each color served as replicates. This experiment was replicated through time by repeating the same experiment two seasons apart.

The passion crop blossoms on selected stems were counted. Shortly before harvest, when most of the fruits were ripe, a second count was done. Passion fruit set was calculated as the ratio of the number of harvested fruits to the number of flowers. Pearson correlation was used as a measure of the strength and significance of a relationship between two variables; flowers and fruits.

The results revealed that there was a positive and significant association between the abundance of the insects and the number of flowers (r=0.513, p=0.001) (Table 1). The results in Table 2 revealed that there was a positive and significant association between the abundance of the insects and the number of fruits (r=0.504, p=0.002).

Table 1: Correlation Analysis for Flowers recorded during the study.

**Correlation is significant at the 0.01 level (2-tailed).

Table 2: Correlation Analysis for Fruits recorded during the study.

**Correlation is significant at the 0.01 level (2-tailed).

Fruit set was recorded highest in the lowland (semi-arid) compared with the other parts of Mua hills location, although it had the least diversity and abundance of insect pollinators. This is the only agro-ecosystem which had the carpenter bee, Xylocopa nigrita, and Xylocopa flavorufa. Such evidence is similar to scientific reports in other parts of the world [10-13]. The findings from this study confirmed that carpenter bee (Xylocopa spp) is the effective pollinator of Passion fruit crop. However, its habitat in this study seemed to be restricted to the lowland generating the question about whether land fragmentation could be a factor affecting its ecology.  It was also worthy to noted that the lowlands of Mua hills has moderately high temperatures than its midland and this could possibly  be another factor influencing fruit set in purple passion fruit in confirmation with a research done by Utsunomiya [14] on effect of temperature on fruit growth and quality.

In the Midland of Mua hills, it was observed that insect pollinator for purple passion fruit was A. mellifera and A. cerena. This could have affected the fruit set since studies have shown that neither honey bees (Apis mellifera) nor social stingless bees are effective pollinators [15,16], due to their small size, they take nectar without achieving pollen transfer such bees are sometimes referred to as "Thieves". Solitary and facultative social bees of the genus Xylocopa, the carpenter bees, are the effective pollinators of this crop because they present appropriate size and foraging behavior. To minimize the effects of insufficient pollination, the findings from this study suggest that the presence of carpenter bees in the landscape should then be enhanced.

We are indebted to the National Museums of Kenya for technical assistance in the field and helping identify specimens collected during this work. We thank the National Commission on Science, Technology, and Innovation (NACOSTI) for financial support and making this research possible and successful. We express our gratitude to South Eastern Kenya University especially the Department of Environmental Science and Land Resources Management and sincerely thank The Ministry of Agriculture, Machakos Sub County Mua ward and, the group of farmers who allowed us to do the fieldwork and collect data in their farms and land.

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