Beetles: Tiny Titans of Pollination

Pollination often brings to mind bees and butterflies and long-beaked sunbirds buzzing about fragrant flowers, hungry for nectar. But a largely unrecognised fact is the role of lesser glamorous beetles in pollination. A recent review paper sheds light on their importance, revealing that beetles are globally recognised as crucial insect pollinators, with a profound impact on ecosystems.

According to a 1999 study cited in the review, beetles emerge as major pollinators of more than 184 species across 34 different plant families, with 17 families of beetles specialising in this role. Quoting multiple sources, the review reports that crop pollination is an important ecosystem service attributed to beetles as they are considered key pollinators to economically important plants including magnolia, palms, nutmeg, sugar and custard apples.

An incredibly diverse group of animals, beetles constitute the largest order in the animal kingdom, known as Coleoptera. Despite the name sounding somewhat like the fabled Egyptian princess, these creatures do not enjoy royal status in scientific studies. They are markedly one of the least studied across the globe, a fact often attributed to their staggering diversity — with over 400,000 identified species, they represent 40 per cent of the entire insect population. Literature on beetles’ role as pollinators is scant, with credit for pollination typically attributed to bees, butterflies and birds.

Unbeatable floral visitors

Beetles are recognised as among the earliest pollinators of basal angiosperms, a group of primitive flowering plants including Magnoliids that are almost fully dependent on beetles for pollination. Over time, a co-evolutionary relationship emerged between beetles and angiosperms, leading to the diversification of flowering plants and the development of intricate floral characteristics, often termed as pollination syndromes, aimed at attracting beetle pollinators. These syndromes, observed among beetle-dependent plant species, feature highly specialised floral structures and reduced pollen delivery.

Scientist Soubadra Devy, who has extensively researched pollination, notes that unlike bees and butterflies, beetles do not seek nectar as a pollination reward. Consequently, flowers pollinated by beetles often feature soft, fleshy petals for beetles to gnaw and linger upon, and may even provide a nuptial pad for beetle pairs to spend some intimate moments for the sake of their stock. Devy’s study of pollinators in 2003 in Western Ghats also underscored beetles’ role as pollinators in evergreen forests.

For the plants, however, humouring beetles also means putting up with a lot of crap, literally. Beetles are categorised as “mess-and-soil” pollinators because they defecate on the flowers and deface them by chewing on the petals during the pollination process. Devy points to Magnolia champaca or Champaka flowers as a prime example of soft-petaled blooms enticing beetles for pollination.

Stop and smell the faeces

The review states that flowering plants send out olfactory signals, apart from visual ones, to attract pollinators. Beetles also rely on odour to locate flowers even in darkness and different plant families produce different odours and floral scents that suit their target beetle pollinators (specialists). For instance, Amorphophallus johnsonii, better known as “corpse flower” produces a carrion and dung odour to attract the beetle, Phaeochrous amplus which is a key pollinator of the plant species, according to a study. At the same time, the plant species in Annonaceae or custard apple attract its target beetle pollinators by releasing a fruity smell.

At times, certain beetles even exhibit a form of “martyrdom” while executing this crucial role. Senior fellow at ATREE, Priyadarsanan Dharma Rajan recalls observing a yam flower Amorphophallus commutatus in the Western Ghats emitting a stench reminiscent of dog or cat faeces, specifically to attract various species of Onthophagus. This beetle, while visiting the flowers for pollination, inadvertently trip on the warts or glands on the flowers, fall back into them and die. The review highlights that most of the beetle-pollinated flowers exhibit a diurnal-nocturnal floral rhythm during which the pollinators are trapped in the flower overnight, “where they feed on exudates, pollen, petals, and mate before being released to other flowers with pollen stuck on their bodies”.

At a time when a global decline in insect pollination has been reported due to climate change, habitat degradation, pesticide use and others, there is a corresponding decline in pollinators, too. The review points to the significance of beetles as pollinators in this scenario since fossil evidence shows that they have survived more environmental and climatic changes than the other evolutionary younger pollinator groups.

They also provide many ecosystem services such as dung removal and nutrient recycling. Dharma Rajan highlights that the dung recycling done by beetles contribute significantly to mitigating greenhouse gas emissions through cattle dung. The voracious herbivores that they are, their involvement in nutrient recycling too is noteworthy. Beetles are also natural pest-control agents that farmers can utilise for sustainable agriculture. Devy says that the conservation status of the animal is largely unknown since very little study has been done on them in India. In the absence of even a baseline study, beetles remain strangely mysterious despite their wide presence.

The paper argues for a need to research on the impact and economic valuation of beetle pollination on the edible and economically important plants such as the palms. As a future direction, the paper identifies research gaps in beetle studies and suggests solutions such as exploring the possibility of conducting chemical assays to determine the compounds that attract beetle pollinators; the role of heat change on beetle pollinators in and out of the floral chamber; and the magnitude and value of beetle pollination in different regions through regular data surveys and constant beetle diversity monitoring, among others.

This story was first published on Mongabay India

Photo sources: cover image, golden-bloomed grey longhorn beetle, beetle in a rose plant.