The study, published in the journal NeoBiota, examined honey, hive-stored pollen and honeycomb from two apiaries positioned near widespread ragwort populations in Prague. The plant, which reaches 30 to 100 centimetres in height and flowers from June to November in Central Europe, arrived in Europe in the late nineteenth and early twentieth centuries through wool-processing facilities. Since then it has colonised warm, sandy and gravelly disturbed soils along transport corridors across the continent, and climate change with mild winters and warm, dry growing seasons may facilitate its further spread. The concern driving the research is that the species contains nine pyrrolizidine alkaloids, including integerrimine, retrorsine, senecionine, seneciphylline, senecivernine and usaramine, with the highest concentrations found in young shoots and inflorescences.

Pyrrolizidine alkaloids themselves are not directly toxic to vertebrates. The danger emerges after ingestion, when the liver metabolises them into reactive pyrrole derivatives that bind to DNA and proteins, causing liver cell death, progressive cirrhosis and eventually liver failure. Some of the compounds are mutagenic and genotoxic, and significant effects on the metamorphosis of Hymenoptera have also been demonstrated. The toxins enter the human food chain most often through contamination of agricultural products by weeds, or through food supplements and traditional medicinal products such as certain teas. Honey is a well-documented route, with quantities sometimes reaching tens to hundreds or even thousands of micrograms per kilogram. According to the German Federal Institute for Risk Assessment, daily doses should not exceed 0.007 micrograms per kilogram of body weight, a threshold that underscores how little contamination is needed to raise concern.

The research team collected five honey samples spanning the entire growing season, along with one sample of hive-stored pollen and one honeycomb sample, from hives at two Prague localities where ragwort is widespread. A reference sample of ragwort inflorescences was also gathered to establish the pollen grain profile of the species. All samples underwent pollen analysis in December 2019 using a standard acetylation method, chosen for its robustness against contamination by non-pollen particles, whereas newer approaches such as metabarcoding could produce false positives and noise. Because ragwort pollen grains are not morphologically distinct from those of other Senecio species or similar members of the daisy family, the researchers classified them under a general Aster/Senecio pollen type.

The pollen analysis revealed a strikingly low presence of the Aster/Senecio type across all seven samples. The pollen type appeared only in one sample, the hive-stored pollen from hive 1 collected in August 2019, and even there in small quantities. Based on the pollen spectrum, the researchers concluded that bees preferred honeydew from flowering woody plants as their main food source, largely bypassing the ragwort stands despite their proximity. The honey samples themselves contained only low concentrations of pollen grains, with crystallised sugar visible, further indicating that the bees were not heavily exploiting the invasive herb.

Chemical analysis provided the second line of evidence. Using liquid chromatography coupled with mass spectrometry in January 2020, the team screened for 29 different pyrrolizidine alkaloids or their N-oxides across four honey samples. The analysis detected no such alkaloids originating from ragwort. The only compound found above the limit of detection was echimidin, a pyrrolizidine alkaloid traced in three honey samples at low concentrations ranging between 1 and 7 micrograms per kilogram. The likely source was not the invasive ragwort at all, but Echium vulgare, the common viper’s bugloss, a plant known as one of the major causes of pyrrolizidine alkaloid contamination in honey and found to be abundant in the vicinity of the studied sites. Pollen analysis of one honey sample also revealed the presence of Echium pollen type, corroborating the chemical finding. Nectar, the researchers noted, appears to be a more significant source of the alkaloids in honey than pollen in the case of viper’s bugloss.

The results align with the only comparable study available, research on S. jacobaea, which found minimal Senecio-type pollen in honey at 0 to 6.3 percent, consistent with the Prague findings. The low contamination likely reflects the current flowering phenology of narrow-leaved ragwort. Over the past twenty years the species has begun to bloom in June or July, a period when bees have abundant alternative forage. The researchers caution, however, that this timing is shifting. In recent years ragwort has already been observed blooming in May, and in warmer parts of Central Europe still-flowering overwintering stems have been recorded as early as February. Should the species start flowering earlier in the season, the risk of contamination would increase significantly, because in early spring other sources of honeydew and pollen are scarce and bees may turn to ragwort as an alternative resource.

Autumn flowering, by contrast, may not pose as significant a risk, since many beekeepers leave the honey from the last flow with the bees to help them overwinter, keeping it out of circulation. The geometry of foraging also matters. Bees typically use resources from up to about 3 kilometres from the hive, but if forage is dense enough they usually travel shorter distances. This behaviour suggests that hives for studying alkaloid content in honey and pollen should be placed in the immediate vicinity of the studied plant population, so that the species is offered to the bees as the most easily accessible forage. The Prague hives satisfied this condition, strengthening the conclusion that ragwort currently contributes minimally to honey contamination.

The pilot study’s overall verdict is that honey from hives near large ragwort populations contained negligible pollen and no ragwort-derived pyrrolizidine alkaloids, posing no health risk to consumers under current conditions. The researchers emphasise that the current flowering phenology of the invasive species minimises its potential negative impact on human health, but they stress that phenology is slowly changing. As the plant blooms earlier in the year, the alkaloids and pollen in honey need to be monitored to assess future risks, making continued surveillance of this alien invader an essential part of protecting both beekeeping and public health.

Subject of Research: Pilot study on pyrrolizidine alkaloid contamination in honey from hives near invasive Senecio inaequidens populations in Prague

Article Title: Toxin-free honey despite the alien Senecio inaequidens DC. invasion – a pilot study

Article References: Karlík, P., Kutlvašr, J., Kušková, K., Petr, L., Vachová, P., & Pergl, J. (2026). Toxin-free honey despite the alien Senecio inaequidens DC. invasion – a pilot study. NeoBiota, 106, 357-367. https://doi.org/10.3897/neobiota.106.175238

Image Credits: AI Generated

DOI: 10.3897/neobiota.106.175238

Keywords: Senecio inaequidens, invasive species, pyrrolizidine alkaloids, honey safety, beekeeping, pollen analysis, LC-MS, NeoBiota, Prague, Echium vulgare, food safety, flowering phenology