Weeds Enhance Pollinator Diversity in Mango: Comparison
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Please note this is a comparison between Version 2 by Nora Tang and Version 1 by Suzanne Koptur.

Weeds increase the diversity of pollinating insects on mango flowers, and mangos with weeds growing below produced more fruit than those without weeds. Weeds can provide ground cover and flowers that can support insect pollinators, natural enemies, and increase biodiversity at the field and landscape level. 

  • weeds
  • insects
  • mango
  • pollinators
  • sustainable agriculture

1. Introduction

Cultivated crops are often subject to insect–plant interactions for high yield. There has been a growing interest in environmentally and ecologically sound agriculture using beneficial insects rather than pesticides to produce food and fiber without harmful chemicals in produce and the environment [1,2][1][2]. Ecological intensification is the use of biological regulation to manage agroecosystems at various scales [3]. Natural ecosystems can inspire cropping system designs [4], and these approaches may have greatest impact in high-input farming systems [5,6][5][6]. Ecological replacement, substituting biodiversity for synthetic inputs, can enhance ecosystem services with similar crop output [6]. The presence of non-crop plants in planted floral strips may be useful in this approach, as a recent meta-analysis has shown [7]. Weeds may also provide resources that attract and maintain populations of beneficial insects, such as pollinators. Weeds—wild plants growing where they are not wanted—are seen as detrimental to crop production in agriculture by pulling resources away from the crop. This lack of weeds diminishes beneficial insects through the loss of floral and prey resources [8]. The benefits of using insectary plants in farms is well known [9,10][9][10]; however, using weeds as such in tropical fruit production dependent on pollination is relatively unexplored. Previous work has shown increased success of beneficial insects in the presence of weeds, as these insects use nectar or pollen during their adult life stage to increase life span and fecundity [11]. Pollinator populations may be bolstered in the presence of weeds, and some have been shown to be dependent on them [12,13][12][13]. This study examined how leaving, rather than removing, weeds in a mango farm affected pollinators and fruit set of this popular tropical fruit cultivated in southern Florida.
Pollinators are especially important in crops that require pollination by insects [14[14][15],15], such as mango, which is known to benefit from the presence of a diversity of weeds. Many tropical crops may be most susceptible to pollination failure from habitat loss [16,17][16][17]. Almost 35% of crops depend on pollinators globally, with pollination of at least 63 crops vulnerable to negative effects of agricultural intensification, which may reduce the diversity and abundance of pollinators [18]. The global annual economic value of insect pollination is upward of USD $173 billion worldwide [19]. Pollination by bees and other animals increases the size, quality, and stability of yields for 70% of leading economically important crops around the world [16], including mango [20]. Because native species pollinate many of these crops effectively, conserving habitats for wild pollinators within agricultural landscapes can help promote pollination services for most of the world’s crops [21].
There is a pollinator crisis in areas of intensive human land use and landscape simplification, including farmlands [22]. Insects have shown marked population declines over the past 30 years, with the average decline of terrestrial insect abundance at about 9% per decade [23]. Decline in pollinators is intertwined with habitat simplification through the expansion of monoculture practices [24] and increased applications of pesticides and fertilizers [25,26][25][26]. Pollinator abundance increases with flower abundance, vegetation height, and floral diversity [27]. The conservation of plant diversity safeguards native pollinator diversity as well as overall biodiversity and ecosystem services [28]. Mass flowering plants can act as “pollinator hogs”, which can reduce the pollination success of adjacent co-flowering neighbors by drawing pollinators from these plants [29,30][29][30]. However, mass-flowering plants can also act as magnets, producing pollination “spillover effects” through increased pollinator movements to adjacent co-flowering taxa, potentially either increasing pollination [31] or impacting it through the transfer of mixed-species pollen [32]. The presence of flower diversity before and during crop flowering facilitates pollination of the hyperabundant crop flower resource [20,33][20][33]. For a pollinator-dependent crop, creation of flowering areas can be profitable, improving production within existing areas and reducing the need for agricultural expansion, contributing to the conservation of biodiversity within a region, and increasing the habitat and resources for insects within farms [34].

2. Weeds Enhance the Biodiversity

Weeds have the potential to increase biodiversity of native pollinators by providing alternative floral resources for beneficial insects and encouraging them to remain in an area between crop flowering events [14,35,36][14][35][36]. Pollinators can use weeds as alternative resources before, during, and after the bloom of a crop, and increase crop yields if given these resources [37]. Weeds are an essential pollen and nectar resource for insects because of their continuous flowering phenology and their high species richness, which contributes directly to the pollen diversity dietary needs of insects [28]. Pollinators are healthiest, with at least 15 flowering species providing a season-long food supply [38], and weeds can provide this floral diversity. Arable weeds have specific functional traits that make them tolerant to farmlands, such as soil disturbances and fertilization, making a large overlap in the weedy potential of non-weed species.
Effective insect pollination is essential for good fruit set and yield in mango (Mangifera indica L., Anacardiaceae) [39]. Mango flowers are unspecialized, enabling pollination by most insects that are critical for fruit yield [40]. Nectar production for the attraction of insects indicates entomophilous pollination, and mango does not show adaptations for wind pollination [41]. Managed pollinators are unsuitable [42] or insufficient when acting alone [20,43,44][20][43][44]; honeybees are generally not attracted to mango flowers [45[45][46],46], and hand pollination is not economically viable [47]. Pollination is highly dependent on a diverse assemblage of flying visitors, which is strongly negatively affected by distance to natural habitat [46]. Small patches of native flora, planted in nonproductive margins of large mango orchards, enhance the abundance and diversity of mango flower visitors in South Africa, ameliorating the negative effects of isolation from natural habitat and pesticide use [34]. These increases were associated with significantly higher mango production, including the Keitt variety. Two co-flowering species can compete with or facilitate each other for flower visitors, although some studies suggest that facilitation is more likely between plants with unequal flower abundance [29] or that attractive species may facilitate less attractive species, as is the case with mango [20,33][20][33].
A field experiment was conducted to investigate the importance of weeds in promoting pollination of mango in an orchard in Homestead, Florida, USA. The insect orders Diptera, Hymenoptera, Lepidoptera, and Coleoptera are the most common insect visitors to mango flowers and carry mango pollen [39,41][39][41]. Most pollinators of mango belong to order Hymenoptera, but Diptera (flies) have been suggested as the dominant pollinators in Israel [39]. Dipteran species are good pollinators of more than 550 species of flowering plants, and the family Calliphoridae (blowflies) are the suspected main pollinators of mango in India [48]. Additionally, Chrysomya, Lucilia, and Musca sp. (Diptera) were reported as mango pollinators because of their visiting frequency and abundance [48]. Flower flies (Syrphidae) are also good pollinators of mango [49], as they transport pollen for long distances and reproduce rapidly [50]. More visitors to mango flowers in this study resulted in much higher yield of mango trees surrounded by weeds vs. mango trees with weeds cleared around them [39]. Common blow flies may also serve as effective mango pollinators and are less likely than honeybees to abandon the mango orchard for more attractive blooms, in Israel as well as in the United States [39]. Blow flies visited mango flowers more frequently and ubiquitously than any other pollinator when the mangoes were in bloom, and were increased in mango flowers when the weeds were present.

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