Continuous availability of food resources, such as pollen, is vital for many insects that provide pollination and pest control services to agriculture. However, there is a lack of knowledge about the ...shared or complementary use of floral resources by such species, which hampers more effective landscape management to simultaneously promote them in agroecosystems.
Here, we simultaneously quantified pollen use by a bumblebee (Bombus terrestris) and a mason bee (Osmia bicornis), two bee species recognized as important crop pollinators, as well as a lacewing (Chrysoperla carnea) and a ladybeetle species (Harmonia axyridis), both common predators of crop aphids, throughout the season in 23 agricultural landscapes in Germany and Switzerland.
Pollen diets were more diverse and similar among C. carnea and H. axyridis compared to the two bee species, but all four species shared key pollen types early in the season such as Acer, Quercus, Salix and Prunus. All species exhibited a pronounced shift in pollen sources from primarily woody plants (mainly trees) in spring to primarily herbaceous plants in summer. The majority of pollen (overall ≥64%) came from non‐agricultural plants even in crop‐dominated landscapes.
Synthesis and applications. Our results highlight the importance of trees as pollen sources for many insect species, particularly early in the season. Our findings support incentives that promote heterogeneous agricultural landscapes including both woody and herbaceous semi‐natural habitats, ensuring phenological complementarity of floral resources for insect species that can provide pollination and pest control services to agriculture. The identified key plant species can help to design and optimize agri‐environment schemes to promote these functionally important insects.
•Reproduction of two Osmia species increased with landscape-wide pollen availability.•Woody semi-natural habitats contributed disproportionately to the Osmia diet.•Benefits of forest proximity to ...Osmia indicated functions beyond pollen supply.•Resource and land cover maps help to understand the spatial ecology of wild bees.
Floral resources are crucial for wild pollinators. Identifying the spatio-temporal floral resource use of wild pollinators and effects of resource distribution on their development might help to promote them and their pollination services to crops in agricultural landscapes.
We established populations of Osmia cornuta and Osmia bicornis, two solitary wild bees, in 24 agricultural landscapes with varying floral resource availability. Based on their pollen use, we mapped the landscape-scale distribution of the visited plants, estimated pooled specific floral resource availabilities and measured its effects on reproductive output.
Woody semi-natural habitats such as hedgerows provided the majority of pollen sources for both Osmia species. Pollen use differed strongly between the two species. The offspring of both Osmia increased with availability of pooled specific pollen resources. In accordance with their preferred pollen types, offspring of O. cornuta increased with increasing cover of trees and shrubs of the Rosaceae family, and that of O. bicornis with increasing cover of Papaver rhoeas, Ranunculus acris and Quercus spp. as well as with the proximity to oilseed rape. In spite of their specific responses to pollen resources, the offspring of both species decreased with the distance to forest. The floral resource availability did not significantly affect the proportion of adult females and the weight of the offspring. As forest does not appear to be a main foraging habitat for both species, the benefit of forest proximity indicates an additional role of forest in addition to food availability.
Specific flowering plants and forests should thus be conserved and enhanced to maintain and support O. cornuta, O. bicornis and likely other wild bee populations in agricultural landscapes. The combined information of land cover and detailed floral resource availability gives a deeper understanding into population processes in agricultural landscapes.
Context
Predatory insects contribute to the natural control of agricultural pests, but also use plant pollen or nectar as supplementary food resources. Resource maps have been proposed as an ...alternative to land cover maps for prediction of beneficial insects.
Objectives
We aimed at predicting the abundance of crop pest predating insects and the pest control service they provide with both, detailed flower resource maps and land cover maps.
Methods
We selected 19 landscapes of 500 m radius and mapped them with both approaches. In the centres of the landscapes, aphid predators – hoverflies (Diptera: Syrphidae), ladybeetles (Coleoptera: Coccinellidae) and lacewings (Neuroptera: Chrysopidae) – were surveyed in experimentally established faba bean phytometers (
Vicia faba
L. Var. Sutton Dwarf) and their control of introduced black bean aphids (
Aphis fabae
Scop.) was recorded.
Results
Landscapes with higher proportions of forest edge as derived from land cover maps supported higher abundance of aphid predators, and high densities of aphid predators reduced aphid infestation on faba bean. Floral resource maps did not significantly predict predator abundance or aphid control services.
Conclusions
Land cover maps allowed to relate landscape composition with predator abundance, showing positive effects of forest edges. Floral resource maps may have failed to better predict predators because other resources such as overwintering sites or alternative prey potentially play a more important role than floral resources. More research is needed to further improve our understanding of resource requirements beyond floral resource estimations and our understanding of their role for aphid predators at the landscape scale.
Semi‐natural habitats (SNH) are considered essential for pest‐suppressive landscapes, but their influence on crop pests and natural enemies can be highly variable. Instead of SNH per se, the ...availability of resources, such as pollen and nectar, may be more relevant for supporting pest control.
Here, we assessed the spatiotemporal variation of multiple insect pests (cereal leaf beetles and aphids) and natural enemies (predators and aphid parasitoids) in wheat fields and their responses to landscape context and flower availability. We combined detailed information on pollen use by natural enemies with the specific distribution of pollen‐providing plants across a gradient of landscape composition and configuration.
The abundance of wheat pests was tightly linked to wheat development stage. Syrphids colonised the fields early in the season, while the abundance of other enemies increased later in the season. The responses of pests to landscape structure were variable and, while some pests had low abundances in landscapes with high edge density and SNH cover, Sitobion avenae abundance was positively associated with SNH cover. Lacewings, syrphids and cereal leaf beetles were abundant in landscapes with diverse and abundant flower resources, whereas the abundance of parasitoids and Nabis sp. was driven by aphid abundance. We detected no significant indirect effects of landscape on pests via natural enemies.
Synthesis and applications. Our findings highlight the need for conservation biological control to go beyond ‘one size fits all’ and consider the specific ecology of the involved organisms, even for a single crop type. Landscapes with high edge density and flowering woody plants may support natural enemies, in particular syrphids, which colonised the fields early in the season. Incentives for pest‐suppressive landscapes should focus on tailored strategies that disfavour dominant cereal pests and simultaneously enhance natural enemies according to their ecological requirements.
Resumen
Los hábitats seminaturales (SNH) son considerados esenciales para fomentar paisajes supresores de plagas, pero su influencia sobre plagas y enemigos naturales en cultivos puede ser muy variable. En lugar de los SNH per se, la disponibilidad de recursos específicos, como polen y néctar, puede ser más relevante para apoyar el control de plagas.
En este trabajo evaluamos la variación espacio‐temporal de insectos plaga (escarabajos de la hoja de los cereales y pulgones) y enemigos naturales (predadores y parasitoides de pulgones) en campos de trigo y sus respuestas al contexto del paisaje y la disponibilidad de flores. Para esto combinamos información detallada sobre el uso de polen por parte de enemigos naturales con mapeos de la distribución específica de plantas proveedoras de polen en un gradiente de composición y configuración del paisaje.
La abundancia de plagas del trigo estuvo estrechamente relacionada con la etapa de desarrollo del trigo. Los sírfidos colonizaron los campos a principios de la temporada durante la floración del trigo, mientras que la abundancia de otros enemigos aumentó más tarde. Las respuestas de las plagas a la estructura del paisaje fueron variables y, mientras que algunas plagas tuvieron abundancias bajas en paisajes con alta densidad de bordes y alta cobertura de SNH, la abundancia de Sitobion avenae se relacionó positivamente con la cobertura de SNH. Las crisopas, los sírfidos y los escarabajos de la hoja de los cereales fueron abundantes en paisajes con recursos florales diversos y abundantes, mientras que la abundancia de parasitoides y Nabis sp. aumentaron junto con la abundancia de pulgones. No detectamos efectos indirectos significativos del paisaje sobre las plagas a través de enemigos naturales.
Síntesis y aplicaciones. Nuestros resultados destacan la necesidad de que el control biológico por conservación vaya más allá de estrategias generalizadas y considere la ecología específica de los organismos involucrados. Paisajes con alta densidad de bordes y plantas leñosas con flores pueden beneficiar a los enemigos naturales, en particular a los sírfidos, que colonizaron los campos a principios de la temporada. Los incentivos para promover paisajes supresores de plagas deberían centrarse en estrategias específicas que desfavorezcan a las plagas dominantes de cereales y, al mismo tiempo, beneficien a los enemigos naturales de acuerdo con sus requisitos ecológicos.
Our findings highlight the need for conservation biological control to go beyond ‘one size fits all’ and consider the specific ecology of the involved organisms, even for a single crop type. Landscapes with high edge density and flowering woody plants may support natural enemies, in particular syrphids, which colonised the fields early in the season. Incentives for pest‐suppressive landscapes should focus on tailored strategies that disfavour dominant cereal pests and simultaneously enhance natural enemies according to their ecological requirements.
Intensive agricultural landscapes can be hostile for bees due to a lack of floral and nesting resources, and due to management-related stress such as pesticide use and soil tillage. This threatens ...the pollination services that bees deliver to insect-pollinated crops. We studied the effects of farming intensity (organic vs. conventional, number of insecticide applications) and availability of semi-natural habitats at the field and landscape scale on pollinator visits and pollen delivery to pumpkin in Germany. We found that wild bumble bees were the key pollinators of pumpkin in terms of pollen delivery, despite fivefold higher visitation frequency of honey bees. Critically, we observed that the area of cropland had stronger effects on bees' pollen deposition than the area of seminatural habitats. Specifically, a 10% increase of the proportion of cropland reduced pollen delivery by 7%. Pumpkin provides a striking example for a key role of wild pollinators in crop pollination even at high numerical dominance of honey bees. In addition, our findings suggest that habitat conversion to agricultural land is a driver of deteriorating pollination. This underlines the importance to maintain sufficient areas of non-crop habitats in agricultural landscapes.
Context
Flowering plants can enhance wild insect populations and their pollination services to crops in agricultural landscapes, especially when they flower before the focal crop. However, ...characterizing the temporal availability of specific floral resources is a challenge.
Objectives
Developing an index for the availability of floral resources at the landscape scale according to the specific use by a pollinator. Investigating whether detailed and temporally-resolved floral resource maps predict pollination success of broad bean better than land cover maps.
Methods
We mapped plant species used as pollen source by bumblebees in 24 agricultural landscapes and developed an index of floral resource availability for different times of the flowering season. To measure pollination success, patches of broad bean (
Vicia faba
), a plant typically pollinated by bumblebees, were exposed in the center of selected landscapes.
Results
Higher floral resource availability before bean flowering led to enhanced seed set. Floral resource availability synchronous to broad bean flowering had no effect. Seed set was somewhat better explained by land cover maps than by floral resource availability, increasing with urban area and declining with the cover of arable land.
Conclusions
The timing of alternative floral resource availability is important for crop pollination. The higher explanation of pollination success by land cover maps than by floral resource availability indicates that additional factors such as habitat disturbance and nesting sites play a role in pollination. Enhancing non-crop woody plants in agricultural landscapes as pollen sources may ensure higher levels of crop pollination by wild pollinators such as bumblebees.
The yield of animal-pollinated crops is threatened by bee declines, but its precise sensitivity is poorly known. We therefore determined the yield dependence of Hokkaido pumpkin in Germany on insect ...pollination by quantifying: (i) the relationship between pollen receipt and fruit set and (ii) the cumulative pollen deposition of each pollinator group. We found that approximately 2500 pollen grains per flower were needed to maximize fruit set. At the measured rates of flower visitation, we estimated that bumblebees (21 visits/flower lifetime, 864 grains/visit) or honeybees (123 visits, 260 grains) could individually achieve maximum crop yield, whereas halictid bees are ineffective (11 visits, 16 grains). The pollinator fauna was capable of delivering 20 times the necessary amount of pollen. We therefore estimate that pumpkin yield was not pollination-limited in our study region and that it is currently fairly resilient to single declines of honeybees or wild bumblebees.
Wild bumblebees are key pollinators of crops and wild plants that rely on the continuous availability of floral resources. A better understanding of the spatio-temporal availability and use of floral ...food resources may help to promote bumblebees and their pollination services in agricultural landscapes. We placed colonies of Bombus terrestris L. in 24 agricultural landscapes with various degrees of floral resource availability and assessed different parameters of colony growth and fitness. We estimated pollen availability during different periods of colony development based on detailed information of the bumblebee pollen diet and the spatial distribution of the visited plant species. Total pollen availability did not significantly explain colony growth or fitness. However, when using habitat maps, the weight gain of colonies, the number of queen cells, and colony survival decreased with increasing distance from the forest. The better explanation of bumblebee performance by forest proximity than by (plant-inferred) pollen availability indicates that other functions of forests than pollen provision were important. The conservation of forests next to agricultural land might help to sustain high populations of these important wild pollinators and enhance their crop pollination services. Combining different mapping approaches might help to further disentangle complex relationships between B. terrestris and their environment in agricultural landscapes.
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