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Why Do Leafcutter Bees Cut Leafs?

Because pollen sucks.

It does! But if you prefer the smart word: it’s hygroscopic. It attracts water.

And where is water, there is rot.
Which is a problem when you are an insect that relies on pollen to feed its brood and therefore needs to store pollen for weeks, if not months. You may be more or less okay when in a desert or at least able to avoid the rainy season – but in our friendly fertile temperate zones, you’re not.

Still, they are here. Plenty of them. Mason bees and leafcutter bees and carder bees and many more.

ResearchBlogging.org

In their new paper, Christophe Praz and his colleagues suggest a scenario for just how this could have happened:

Before the bees began to feed pollen to their brood (i.e. before they actually became bees) they were something similar to today’s apoid wasps (Grabwespen). They were hunting other insects, paralysed their victims and dragged them into the broodnest where their prey would stay alive for several weeks before being consumed by the larvae.
You may find this disgusting or not, but keeping your food alive until consumption is definitely a good way to keep it fresh.
And there is nothing „primitive“ or old-fashioned about it. There are still plenty of wasps around who do exactly this. But this method does have its costs. Hunting takes time, it’s not without risks, chances to find prey are limited and so on. So when the flowering plants arrived and offered pollen as an alternative source of protein, the bees’ ancestors skipped their carnivorous habits and became all out vegetarians. Which – as we all know – turned out to be a smart move.

But before the flowers and the bees could become one of the biggest success stories on the planet, there was one more innovation needed.

From gene sequencing data and the analysis of diversification rates and biogeography, Praz and his colleagues conclude that for a long time bees had been restricted to arid evironments and that it was only after the „invention“ of traits to impregnate the broodchamber that they were able to achieve their impressive diversity (3900 species today) and worldwide expansion.

They also argue that nest-lining behaviour with foreign material was „invented“ only once within the megachilid bees, some 90 to 100 million years ago (as marked by the green star in the figure below).

[Click picture to enlarge]

So flowering plants and pollen were very important for bees to evolve. But if it hadn’t been for new behavioural traits that allowed to keep the pollen safe from spoilage through water and fungi, the megachilid bees would probably never have been able to leave the deserts.
The cell linings they produce can vary widely. Some bees use mud or chewed leaf paste, others coat the nest with cut out pieces of leaf, and some cement together little pieces of gravel. Whatever the material, all these linings seem to have water-repellent and anti-microbial properties.

Photography: Neil Bromhall

There also are a few megachilid species that never got into cell-lining at all.
But they are still in the deserts.

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Litman JR, Danforth BN, Eardley CD, & Praz CJ (2011). Why do leafcutter bees cut leaves? New insights into the early evolution of bees. Proceedings. Biological sciences / The Royal Society PMID: 21490010