Threat to Citrus in a Global Pollinator Decline Scenario: Current Understanding of Its Pollination Requirements and Future Directions

Pollinators are vital for world biodiversity and their contribution to agricultural productivity is immense. Pollinators are globally declining with reports such as colony collapse being documented. Citrus exhibits a varying degree of pollination requirements due to its vast cultivars being developed all the time. The article intends to understand the breeding system of a few commercially important Citrus groups and discern its dependency on pollination services. The threat related to pollinator decline to the Citrus industry is measured not only by its reliance on pollinators but also the requirement of the consumers and manufacturers who mostly seek seedless varieties. Therefore, the threat can be tackled by developing high-quality seedless varieties where pollination requirement is absent. Although the importance of pollinators on several self-incompatible varieties cannot be negated, the impact of pollinator decline on its production will entirely depend upon the demand of the market.

Ongoing decline in insect-pollinated plants across Danish grasslands

Loss of habitat, eutrophication and reduced grazing intensity are known drivers of landscape-level changes in plant species composition; however, consequences of the massive decline in insect abundance are still to be understood. Pollinator decline can reduce seed set in plants relying on insects for successful reproduction. This may result in a reduced recruitment of insect-pollinated plant species with associated changes in species composition. So far, large-scale studies addressing this issue have relied on few data points—typically consisting of ‘historic’ records of numbers of insect-pollinated plants compared to present-day records. Such comparisons can provide information as to whether the diversity of insect-pollinated plants has changed, but not whether the process is still ongoing. Here, we use nationwide monitoring data of plant species richness in Danish grasslands from the period 2004–2014, covering 244 grassland sites and encompassing more than 790 flowering plant species. We show an ongoing decrease in insect-pollinated, but not wind-pollinated, plant species across different habitat types. In both dry calcareous and Nardus grasslands, loss of insect-pollinated plants was greatest at sites with low grazing intensity. However, insect-pollinated plants also declined from sites with higher grazing intensity, and plants requiring more specialized insect pollination tended to decline most. In addition to changes in plant diversity driven by land-use intensification, loss of pollinators may also play a role in reducing the richness of insect-pollinated plants. Ongoing reduction in floral richness could further increase the threat to insects relying on these plants as a food source.

“Farming with Alternative Pollinators” approach increases pollinator abundance and diversity in faba bean fields

Pollinators are threatened worldwide and strategies and measures to support their conservation are proliferating. Among them, the approach “Farming with Alternative Pollinators” (FAP) aims to support pollinators by seeding strips of pollinator-attracting cultivated plants surrounding the crops, and simultaneously providing income to the farmer. In this study we assessed whether this approach supports pollinator diversity in agro-ecosystems and increases flower visitor diversity and abundance in faba bean fields in north-west Morocco. We tested the impact of FAP using a variety of marketable habitat enhancement plants (MHEP): flax, coriander, arugula, chia and canola. A total of 62 pollinator species were recorded, among which almost half of them are new records for the region. Most wild pollinators recorded in faba bean were digger bees (genus Anthophora) and long-horn bees (genus Eucera). MHEP shared diverse flower visitors with faba bean and hosted diverse pollinator groups that did not meet their food requirements from the main crop. The FAP approach highly increased flower visitor abundance and diversity in the whole FAP fields, however it did not generate significant pollinator spillover towards the main crop. Implications for insect conservation: our results show that the FAP approach is an effective approach to mitigate pollinator decline in agro-ecosystems.

Putting pesticides on the map for pollinator research and conservation

Wild and managed pollinators are essential to food production and the function of natural ecosystems; however, their populations are threatened by multiple stressors including pesticide use. Because pollinator species can travel hundreds to thousands of meters to forage, recent research has stressed the importance of evaluating pollinator decline at the landscape scale. However, scientists' and conservationists' ability to do this has been limited by a lack of accessible data on pesticide use at relevant spatial scales and in toxicological units meaningful to pollinators. Here, we synthesize information from several large, publicly available datasets on pesticide use patterns, land use, and toxicity to generate novel datasets describing pesticide use by active ingredient (kg, 1997-2017) and aggregate insecticide load (kg and honey bee lethal doses, 1997-2014) for state-crop combinations in the contiguous U.S. Furthermore, by linking pesticide datasets with land-use data in the contiguous United States, we describe a method to map pesticide indicators at spatial scales relevant to pollinator research and conservation.

Farming with Alternative Pollinators benefits pollinators, natural enemies, and yields, and offers transformative change to agriculture

Low- and middle-income countries cannot afford reward-based land sparing for wildflower strips to combat pollinator decline. Two small-grant projects assessed, if an opportunity-cost saving land-sharing approach, Farming with Alternative Pollinators, can provide a method-inherent incentive to motivate farmers to protect pollinators without external rewards. The first large-scale Farming-with-Alternative-Pollinators project used seven main field crops in 233 farmer fields of four agro-ecosystems (adequate rainfall, semi-arid, mountainous and oasis) in Morocco. Here we show results: higher diversity and abundance of wild pollinators and lower pest abundance in enhanced fields than in monocultural control fields; the average net-income increase per surface is 121%. The higher income is a performance-related incentive to enhance habitats. The income increase for farmers is significant and the increase in food production is substantial. Higher productivity per surface can reduce pressure on (semi)-natural landscapes which are increasingly used for agriculture. Land-use change additionally endangers biodiversity and pollinators, whereas this new pollinator-protection approach has potential for transformative change in agriculture.

Specialized Metabolites in Floral Resources: Effects and Detection in Buff-Tailed Bumblebees

The selection of appropriate food resources by bees is a critical aspect for the maintenance of their populations, especially in the current context of global change and pollinator decline. Wild bees have a sophisticated ability to forage selectively on specific resources, and can assess the quality of pollen using contact chemosensory perception (taste). While numerous studies have investigated the detection of pollen macronutrients in bees and their impact on bee health and reproductive success, only a few studies have described the gustatory responses of bees toward specialized metabolites. In addition, these studies mostly focused on the response to nectar and neglected pollen, which is the main food resource for both bee imagines and larvae. Whether bees have the ability to detect specialized toxic metabolites in pollen and then rapidly adapt their foraging behavior to avoid them is very little studied. In this study, we tested whether pollen specialized metabolites affect bumblebees at both the micro-colony and individual levels (i.e., bioassays using supplemented pollen), and whether foragers detect these specialized metabolites and potentially display an avoidance behavior (i.e., preference tests using supplemented syrup). Bumblebees were fed with either amygdalin-, scopolamine- or sinigrin-supplemented pollen diets in ratios that mimic 50%, 100%, and 200% of naturally occurring concentrations. We found no effect of these specialized metabolites on resource collection, reproductive success and stress response at the micro-colony level. At the individual level, bumblebees fed on 50%-amygdalin or 50%-scopolamine diets displayed the highest scores for damage to their digestive systems. Interestingly, during the preference tests, the solution with 50%-scopolamine displayed a phagostimulatory activity, whereas solution with 50%-amygdalin had a deterrent effect and could trigger an active avoidance behavior in bumblebees, with a faster proboscis retraction. Our results suggest that regulation of toxin intake is not as well-established and effective as the regulation of nutrient intake in bees. Bees are therefore not equally adapted to all specialized pollen metabolites that they can come into contact with.

Combined Effects of Pesticides and Electromagnetic-Fields on Honeybees: Multi-Stress Exposure

Honeybee and general pollinator decline is extensively reported in many countries, adding new concern to the general biodiversity loss. Many studies were addressed to assess the causes of pollinator decline, concluding that in most cases multi-stress effects were the most probable ones. In this research, the combined effects of two possible stress sources for bees, pesticides and electromagnetic fields (multi-stress conditions), were analyzed in the field. Three experimental sites were chosen: a control one far from direct anthropogenic stress sources, a pesticide-stress site and multi-stress one, adding to the same exposure to pesticides the presence of an electromagnetic field, coming from a high-voltage electric line. Experimental apiaries were monitored weekly for one year (from April 2017 to April 2018) by means of colony survival, queen activity, storage and brood amount, parasites and pathogens, and several biomarkers in young workers and pupae. Both exposure and effect biomarkers were analysed: among the first, acetylcholinesterase (AChE), catalase (CAT), glutathione S-transferase (GST) and alkaline phosphatase (ALP) and Reactive Oxygen Species (ROS); and among the last, DNA fragmentation (DNAFRAGM) and lipid peroxidation (LPO). Results showed that bee health conditions were the worst in the multi-stress site with only one colony alive out of the four ones present at the beginning. In this site, a complex picture of adverse effects was observed, such as disease appearance (American foulbrood), higher mortality in the underbaskets (common to pesticide-stress site), behavioral alterations (queen changes, excess of honey storage) and biochemical anomalies (higher ALP activity at the end of the season). The overall results clearly indicate that the multi-stress conditions were able to induce biochemical, physiological and behavioral alterations which severely threatened bee colony survival.

Socioecological Factors and Farmer Perceptions Impacting Pesticide Use and Pollinator Conservation on Cucurbit Farms

While research suggests that pollinator decline is linked with agricultural practices, it is unclear whether farmers share this view and adapt management to promote pollinators based on their understanding of these threats. To address these issues, we surveyed farmers of pollinator-dependent cucurbit crops across four states in the Midwest, USA. We grouped farmers by their perceptions of pollinator declines and routes of pesticide exposure and used statistical models to evaluate if farmers manage pests and pollinators based on these perceptions. Out of 93 completed surveys, 39% of farmers believed pollinators were in decline. When grouped, 17% of farmers were classified as proponents, ranking (on a 1–5 Likert scale) the factors mediating pesticide exposure and pollinator declines as important or highly important. For comparison, 44 and 39% of farmers were classified as neutral or skeptical, respectively, of these same factors. Compared to the neutral and skeptic groups, proponents were on average younger, had fewer years farming but more years in family farming, and were more dependent on income from outside the farming system. Proponents also on average reported smaller farms, higher pest richness, more land in cucurbit production, and greater richness of crops that are not pollinator dependent, when compared to the neutrals and skeptics. We did not find pest and pollinator management to be related to farmer perceptions of pollinator decline or routes of pesticide exposure, but farmers classified as pollinator “proponents” were more likely to indicate participation in future pollinator habitat restoration programs. Rather, management strategies were better explained by on-farm environmental conditions (e.g., pest richness, farm size, number of pollinator dependent crops) and economic factors (e.g., sources of income). Generally, our research shows that farmers who perceive pollinator threats may not be using pollinator supportive practices. Thus, while some farmers believe in pollinator declines, there remains a need to connect this knowledge with on-farm practices.