Solitary bees make up the vast majority of the world’s roughly 20,500 known bee species. Unlike honey bees, they live without colonies, and many nest in hollow cavities, sealing brood cells with resin, cut leaves or cellophane-like glandular secretions. These bees are often equally effective or better pollinators than honey bees for many crops, yet they are acutely vulnerable when landscapes become simplified and flower-poor. Despite their ecological and economic importance, the specific plants that different solitary bee species rely on have remained largely unknown, particularly in tropical and subtropical regions where most resource-use research has been conspicuously absent.

To close this gap, researchers installed timber trap nests at three orchards and four forests in south-east Queensland, each unit containing drilled cavities lined with removable paper tubes. The sites, ranging across latitudes from Bundaberg in the north to the Sunshine Coast in the south, were classified as forest or orchard depending on canopy cover within a 500-meter radius. Over two years, from 2017 to 2018, the team recovered 590 occupied nests, dissected them, and collected samples of nest caps, pollen bread, larvae and frass. Emerging adult bees were identified to species using taxonomic literature and the Queensland Museum collections, allowing every nest to be linked to a known bee species.

The molecular workhorse of the study was DNA metabarcoding. The researchers amplified two plant DNA markers, rbcL and ITS2, using carefully controlled polymerase chain reactions, then sequenced the amplicons on an Illumina MiSeq platform. Reads were quality-filtered, denoised into amplicon sequence variants, and matched against a custom-built local reference database covering 93 percent of the plant species recorded in vegetation surveys at the study sites. Unmatched sequences were classified against broader Viridiplantae databases, and every assignment was cross-checked against the Atlas of Living Australia to confirm the taxon could plausibly occur in the region. The dual-marker approach was essential, because each barcode region carries different strengths in resolving plant identities.

The results were striking. Across all seven bee species, the team identified 118 plant taxa spanning 22 orders, 34 families and 67 genera. Yet the diet was far less diverse than that number suggests: just 20 plant species accounted for more than 70 percent of all resources used. Bees nesting in forests used roughly 5 to 13 plant species per nest, while those in orchards used only about 4 to 7, indicating that agricultural simplification narrows the botanical menu even for generalist foragers. Statistical analysis confirmed that bee species identity and land use, but not season, significantly shaped the composition of plants detected in nests.

Perhaps the most surprising discovery concerned plant form. Trees, not the wildflowers and herbaceous strips typically planted for pollinators, were the most frequently provisioned pollen source for every bee species studied, with 48 tree taxa detected. Metabarcoding allowed the researchers to quantify bee use of 43 tree species, half of which grow taller than 20 meters, including giants such as Eucalyptus pilularis, which can reach 70 meters, and Syncarpia glomulifera. Traditional observation of bee visits to tall canopies requires climbing ropes or flight-intercept traps, which is why this resource channel has been so badly underestimated. The study adds to emerging molecular evidence that a disproportionate share of bee-collected pollen comes from trees.

The nest materials told an equally unexpected story. Leafcutter bees, which line their brood cells with cut leaf pieces, sourced materials mostly from herbaceous plants, including the introduced annuals Modiola caroliniana and Passiflora foetida. Cellophane bees, which seal nests with glandular secretions, used vines and trees, and the detection of plant DNA in their nest caps suggests these bees mix their own secretions with plant-derived material, something never reported before for hylaeine bees. Most remarkably, resin bees relied heavily on parasitic plants, the mistletoes of the order Santalales, particularly species of Amyema. Mistletoe DNA appeared in roughly 87 percent median abundance in resin bee nest caps compared with only about 5 percent in pollen bread, indicating the collection was deliberate rather than accidental contamination. The researchers speculate that the glue-like secretions of mistletoe haustorial hairs or their waxy cuticles may make ideal nest-building materials.

Indigeneity mattered too. Introduced plants appeared in the resources of all seven bee species, even those nesting in natural forests, with the proportion ranging from as low as 1 to 16 percent for one resin bee species to as high as 39 to 86 percent for the cellophane bee Hyleoides concinna. This is the first record of exotic plant use by these species. Yet native plants still dominated, accounting for more than half of relative read abundance for six of the seven bee species, with the most heavily used native plants all belonging to the myrtle family, including Eucalyptus racemosa, Lophostemon confertus and Lophostemon suaveolens. The researchers suggest that fast-growing introduced annuals may fill temporal gaps in flowering when native plants are scarce, acting as ecological safety nets rather than preferred forage.

Several plants emerged as true multi-purpose resources. Modiola caroliniana and Passiflora foetida were detected in brood provisions and every type of nest material, while the myrtaceous trees Lophostemon suaveolens, Lophostemon confertus and Eucalyptus racemosa appeared in pollen, resin and cellophane samples alike. Notably, the plants used by bee species present in both forests and orchards were all tree and vine species, reinforcing the idea that landscapes retaining patches of structurally complex habitat sustain bee populations better than simplified farmland. The team also demonstrated that larvae and frass can serve as reliable proxies for pollen bread in metabarcoding diet studies, opening the door to analyzing nests even when pollen stores have been consumed.

The conservation implications are concrete. Current agri-environment schemes often focus on flower-strip plantings between crop rows, but this study suggests such approaches miss the most important pollen sources for solitary bees. The authors recommend retaining or planting patches of complex habitat with trees, shrubs and vines, and managing herbaceous weeds and mistletoes in ways that reduce their spread while preserving the forage and nest materials they provide during resource-scarce periods. Species such as Modiola caroliniana and Crotalaria lanceolata, both annual herbs that colonize disturbed sites, pose little invasion risk to undisturbed areas while offering valuable forage. As pollinator declines accelerate worldwide, this research makes clear that saving bees requires looking up into the canopy as well as down at the flower strip.

Subject of Research: Plant resource use by cavity-nesting solitary bees in Australia identified through DNA metabarcoding of pollen and nest materials

Article Title: Tall trees and exotic herbs identified in pollen and nest materials of seven cavity-nesting solitary bee species in Australia

Article References: Wilson, R. S., Keller, A., Leonhardt, S. D., Smith, T. J., Burwell, C. J., Fuller, C., Shapcott, A., Kaluza, B. F., & Wallace, H. M. (2026). Tall trees and exotic herbs identified in pollen and nest materials of seven cavity-nesting solitary bee species in Australia. Frontiers in Bee Science, 4, Article 1744662. https://doi.org/10.3389/frbee.2026.1744662

Image Credits: AI Generated

DOI: 10.3389/frbee.2026.1744662

Keywords: solitary bees, DNA metabarcoding, pollen diet, nest materials, mistletoe, eucalyptus, pollinator conservation, agri-environment schemes, Australia, Megachile, Hylaeus, tropical ecology

Mistletoes are among the most widespread canopy-dwelling parasitic plants in the world. Unlike fully parasitic species, hemiparasites such as T. bangwensis retain the ability to photosynthesise, but they rely on haustorial connections to tap into the vascular tissues of their hosts, drawing water, dissolved minerals and carbon compounds through these physiological lifelines. Because mistletoes transpire at exceptionally high rates, they maintain powerful water potential gradients that pull xylem sap preferentially through their own tissues rather than through the leaves of the host branch. Over time, this can turn the parasite into a mineral nutrient trap, accumulating elements at concentrations equal to or greater than those found in the foliage it feeds upon.

In cocoa, this interaction matters enormously. The crop depends on a carefully balanced supply of nitrogen, phosphorus, potassium, calcium, magnesium and a suite of micronutrients to sustain canopy growth, energy metabolism, stomatal regulation and the transport of assimilates that ultimately fill cocoa pods. Conventional nutrient management in cocoa systems rests on two pillars: soil fertility assessment and foliar nutrient diagnosis. Soil analyses indicate what is available for uptake, while leaf analyses provide an integrated snapshot of the tree’s actual nutritional status. The implicit assumption is that leaf chemistry largely mirrors soil supply. The new study shows that a parasite can break that assumption.

The scale of the mistletoe problem in West African cocoa is substantial. Surveys in Cameroon have reported infestation incidences exceeding 40 percent in some cocoa-growing communities, with the mistletoe Phragmanthera lapathifolia among the most common offenders. In Ghana, previous work has estimated that roughly 14 percent of all cocoa trees carry mistletoe, and more than 70 percent of those infestations involve T. bangwensis. Farmers readily recognise the visible symptoms, including branch weakening and dieback, but the nutritional consequences of infestation have remained poorly quantified, particularly in perennial tropical crops where nutrient cycling is complex and slow to respond to intervention.

To address this gap, a research team led by Michael Ansong of Kwame Nkrumah University of Science and Technology sampled fifteen smallholder cocoa farms across five communities in the Ahafo Ano North and Sefwi Wiawso districts of Ghana between May and July 2024. The farms, which averaged 0.90 hectares, sit within the humid cocoa belt on predominantly Ferralsols and Acrisols, receiving between roughly 1,300 and 1,750 millimetres of rain annually. On each farm, the researchers selected a pair of mature cocoa trees: one visibly infested with at least one active T. bangwensis plant attached to a main or secondary branch, and one confirmed uninfested after careful canopy inspection. The paired trees were matched for stem diameter, height, productive stage, canopy condition and shade environment, minimising confounding variation between the two members of each comparison.

Sampling was deliberately precise. From each infested tree, the team collected four mature, fully expanded cocoa leaves from the branch bearing the mistletoe, taken close to the attachment point and free of disease, herbivory or senescence. Four comparable leaves were taken from the same canopy positions on the uninfested partner tree, and four mature mistletoe leaves were harvested from the same infested branch. Each sample type was composited at the tree level. A single composite soil sample per farm, drawn from the 0 to 20 centimetre depth at five representative positions, characterised background farm fertility. All plant and soil samples were analysed at the university’s Soil Science Laboratory using standard methods: Kjeldahl digestion for nitrogen, molybdenum-blue colorimetry for phosphorus, EDTA titration for calcium and magnesium, turbidimetry for sulphur, flame photometry for potassium, and atomic absorption spectroscopy for iron, copper, zinc and manganese.

The results revealed a strikingly selective pattern of nutrient redistribution rather than a blanket depletion. Infested cocoa leaves contained significantly lower concentrations of nitrogen, phosphorus, potassium and copper than leaves from their uninfested partners. These are nutrients with high physiological stakes: nitrogen and phosphorus drive canopy development and energy metabolism, potassium governs stomatal regulation and assimilate transport, and copper, though needed only in trace amounts, is essential for enzyme activity, photosynthetic electron transport and oxidative stress defences. In contrast, infested leaves showed significantly higher concentrations of calcium and magnesium, while sulphur, iron, zinc and manganese showed no significant differences. The parasite, in other words, does not simply starve its host; it reorganises the host’s internal nutrient economy.

The elevated calcium and magnesium in infested leaves point to a plausible mechanism rooted in water relations. Because mistletoes transpire so vigorously, water arriving through the host branch is diverted through parasite tissues, reducing the flow that would otherwise deliver dissolved minerals to host leaves. Calcium and magnesium are relatively immobile once deposited in leaf tissues, so under this altered flow regime they can accumulate in older foliage rather than being redistributed to actively growing regions of the canopy. The mistletoe leaves themselves told a complementary story: they contained significantly higher concentrations of phosphorus, potassium, calcium, magnesium, sulphur and manganese than the host leaves on the same branch, confirming their role as nutrient-enriched sinks. Differences in nitrogen, iron, copper and zinc between parasite and host were not statistically significant.

Crucially, the team went beyond simple paired comparisons. Using nutrient-specific linear mixed-effects models, with infestation status and the corresponding soil nutrient as fixed effects and farm identity as a random intercept, they tested whether the infestation signal persisted once measured farm-level soil fertility was taken into account. It did. Soil phosphorus, calcium, magnesium, copper and zinc were significant predictors of the corresponding leaf nutrients, confirming that background fertility matters, but infestation remained significantly associated with lower leaf nitrogen, phosphorus, potassium and copper and higher leaf calcium and magnesium even after soil covariates were included. Soil sulphur, iron and manganese were not significant predictors of their leaf counterparts. Measured soil nutrients explained part, but not all, of the variation in cocoa leaf chemistry, leaving parasite-mediated redistribution as an independent and influential factor.

The practical implications are considerable. If low nutrient concentrations in infested cocoa leaves were caused purely by poor soil, fertiliser would correct them. But because the parasite acts as a sink that diverts nutrients after the host has already absorbed them, fertiliser alone is unlikely to restore balance while active mistletoe remains attached. The authors argue that foliar diagnosis in infested trees should always be interpreted alongside infestation status, and that soil fertility management will be most effective when combined with early detection, timely pruning and complete removal of active parasite tissues. For smallholder farmers, where fertiliser access is limited and mistletoe control demands labour-intensive manual pruning, that combined strategy may determine whether cocoa canopies, and the beans they produce, reach their full potential. The study also cautions that its farm-level soil sampling, one composite per farm, cannot capture tree-specific nutrient supply, and that future work with tree-level soil data, wider chemical characterisation and yield measurements would strengthen the soil-leaf-parasite picture. What is already clear, however, is that a parasite long treated as a visible nuisance is also an invisible accountant, quietly rewriting the nutrient ledgers of the trees it inhabits.

Subject of Research: Nutrient allocation between hemiparasitic mistletoe and its cocoa host trees

Article Title: Hemiparasitic mistletoe, Tapinanthus bangwensis, alters nutrient allocation in cocoa host-parasite systems

Article References: Ansong, M., Musah, S. A., Owusu, P., & Boadu, K. B. (2026). Hemiparasitic mistletoe, Tapinanthus bangwensis, alters nutrient allocation in cocoa host-parasite systems. Discover Plants, 3(1), Article 446. https://doi.org/10.1007/s44372-026-00936-y

Image Credits: AI Generated

DOI: 10.1007/s44372-026-00936-y

Keywords: mistletoe, Tapinanthus bangwensis, cocoa, hemiparasitic plants, nutrient allocation, foliar diagnosis, soil fertility, Ghana, host-parasite interaction, plant physiology, smallholder farms, mineral nutrition