Close Encounters

Honey bees are bad news for some plants in search of pollination

Some people fixate on honey bees as essential for pollination. Reality is more complicated. For one species, honey bee visits actively harm its chances of pollinating a partner.

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It’s easy to see why bees have such a good reputation as pollinators. Their buzzing is the sound of summer as they scoot from flower to flower. But other insects pollinate as well. For Gnetum luofuense, it’s nocturnal moths that pollinate the plant. G. luofuense is a gymnosperm, and they evolved before bees. It’s no surprise that the plant has no use for them. However, they still produce pollen, a food that bees like to eat. A recent study by Min Yang and colleagues has found that while G. luofuense makes no effort to attract bees, they visit anyway, taking away the plant’s pollen with them.

Gnetum luofuense. After Portioid / Wikimedia Commons.

When pollination is working well for G. luofuense, moths arrive at the male strobilus. A strobilus would be their pine cone, if they were pines. Here they get a reward of some sugary fluid and pick up pollen. They then flit to a female strobilus, deposit pollen and get another reward.

A problem for G. luofuense is that its pollen is a meal for the Asian honey bee, Apis cerana. It too can land on a male strobilus and pick up a lot of pollen. But, unlike the moths, they don’t then head for a female strobilus because they’re after pollen to eat, and the females don’t produce that. So, for the plant, the pollen is lost.

Yang and colleagues thought this loss of pollen could cause a problem for G. luofuense, so they decided to watch how the bees interacted with the plants. They examined bee and pollinator visits during late April and early May when the bees were around and pollinator visits outside this period.

“Under natural conditions, G. luofuense produced 7,428 ± 80 (n = 105) pollen grains per anther,” write Yang and colleagues. “After the first round of honey bee visits at dusk, the pollen remaining in each anther significantly decreased to 5594 ± 158 grains (n = 60), an average of 22.3% pollen loss after accounting for pollen dispersal by abiotic agents, which was insignificant during this period…. As expected, nocturnal pollination also significantly affected pollen remaining in each anther…; however, an average of only 2.1% of pollen production was removed by pollinators at night. In contrast, the second round of honey bee visits at dawn reduced pollen remaining in each anther to 2,638 ± 147 grains …, an average loss of another 25.4% of pollen production.”

The moths that the team caught carried pollen, whether they were caught on male or female strobili. The presence of pollen in both cases showed that the moth was an effective pollinator.

A problem for G. luofuense is that the moths are nocturnal. That means that the bees get to the anthers with fresh pollen before the moths arrive. The freshness of the pollen might be important. The team found that after a night, the anthers of G. luofuense wilt. The authors suggest that, for the plant, it’s better to make new anthers than maintain older anthers.

One possible reason for this turnover of anthers that Yang and colleagues raise is that it’s a way for the plant to foil some pollen thieves. If the anthers were more accessible during the day, then they’d be more vulnerable to bees. They get hit by some bees by opening at dusk but still have some pollen for their pollinators. Keeping the pollen around for longer would simply offer more pollen to unwanted visitors.

They conclude that bees visit many gymnosperms for pollen, whether they’re wind-pollinated, specialist insect-pollinated or generalist insect-pollinated. Just as bees can be pollen thieves of flowering plants, Yang and colleagues conclude that they might also be common pollen thieves of gymnosperms.

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