Hornets are the most important pollinators for some plants

Ko Mochizuki, a researcher from the University of Tokyo, has made a significant discovery challenging the common belief about the pollination of the genus Angelica, traditionally seen as having a broad range of pollinators. The findings reveal that two species within this genus, Angelica decursiva and Angelica hakonensis, are primarily pollinated by hornets, a rarity in the plant world where hornets are seldom seen as main pollinators.

Flowers of the Apiaceae family

The typical flowers of the Apiaceae family, to which Angelica belongs, are white, small, open, and produce nectar and pollen that attract diverse insect species. It’s not uncommon for over 100 different insect species to visit the flowers of some plants in this family, categorizing them as generalists. 

However, Mochizuki’s observations in Nagano Prefecture in 2015 and later in 2018 at the Nikko Botanical Garden showed a different story for these two specific Angelica species. 

“I observed a fierce visitation by hornets for the first time in 2015 in Nagano Prefecture and then again in 2018 in the Nikko Botanical Garden where they were feeding on the nectar of the inflorescences of A. decursiva and A. hakonensis,” he reported.

Apiaceae species and hornet pollinators 

Mochizuki embarked on a detailed study. He was motivated to investigate whether these species are predominantly pollinated by hornets, a belief contrary to the established view that Apiaceae species are generalists.

The research method involved quantifying the number and type of “visitors” to the flowers, capturing some to check the amount of pollen on their bodies, and conducting experiments with different mesh covers to isolate the influence of hornets and other insects on pollination.

Importance of hornets as pollinators 

The results from all three approaches confirmed that hornets were the most frequent visitors. Interestingly, Mochizuki found that excluding hornets from the flowers resulted in higher than expected seed production. 

“Excluding the hornets allowed other visitors to gather on the flowers. Especially considering that I had previously observed hornets hunting and killing other insect visitors when they encountered each other on the inflorescence,” he said.

The findings highlight the need to reconsider the ecological roles of hornets, often viewed only as pests or invasive species. 

“Hornets are generally considered pests in their native range and problematic invasive species in some areas such as North America. Nevertheless, this study underscores the importance of hornets as pollinators, opening new avenues for research and conservation,” Mochizuki concluded.

Hornets as pollinators

Hornets, though often associated with their stinging ability and predatory habits, also play a role in pollination. Unlike bees, which are primary pollinators, hornets are considered secondary pollinators. Here’s how they contribute to pollination:

Incidental pollination

While hornets primarily feed on other insects, they also visit flowers for nectar. In the process of feeding on nectar, pollen can stick to their bodies and get transferred from one flower to another, facilitating pollination.

Flower visits

Certain species of hornets are more likely to visit flowers than others. For instance, the Asian giant hornet (Vespa mandarinia) has been observed visiting flowers, though it is less common compared to smaller species.

Limited role

Compared to bees, butterflies, and other dedicated pollinators, hornets’ overall contribution to pollination is relatively minor. Their body structure is not as well adapted for carrying pollen as that of bees, and they do not have behaviors specifically aimed at pollination.

Ecological impact

Hornets can impact ecosystems through their predatory behavior on other insects, including some pollinators. This can indirectly affect pollination dynamics, especially if they reduce the numbers of primary pollinators in an area.

More about Apiaceae species

The Apiaceae family, commonly known as the carrot or parsley family, is a diverse and significant group of plants, primarily recognized for its aromatic members that are often used in cooking and traditional medicine. 

The Apiaceae family contains about 3,700 species spread across 434 genera. These plants are widely distributed across the globe, particularly in temperate regions. They thrive in a variety of habitats, from mountainous areas to flat plains.

Plants in this family are usually aromatic herbs with hollow stems. The leaves are often compound and the flowers are typically small, clustered in umbels, which is a defining characteristic of the family. These umbels present as a series of short flower stalks spreading from a common point, somewhat like the ribs of an umbrella, hence the name.

Many Apiaceae species are cultivated for their edible roots, seeds, or leaves. Familiar examples include carrots (Daucus carota), celery (Apium graveolens), parsley (Petroselinum crispum), fennel (Foeniculum vulgare), and dill (Anethum graveolens). Several species also have medicinal properties and have been used in traditional remedies for a variety of ailments.

The study is published in the journal Ecology.

Image Credit: Mochizuki 2024

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