This is how strawberries turn out when the insects haven't been at work long enough

You have probably seen a nice red strawberry that has a slightly knotty and bumpy centre. This is the result of insects responsible for fertilisation have left a half-finished job.

And then the fertilisation has gone slightly awry. This happens if the pollinating insects visiting the strawberry blossom have not been poking around enough and have not been everywhere.

Plant scientist Siv Fagertun Remberg, who researches fruit and berries at the Norwegian University of Life Sciences (NMBU), is passionate about what happens in the flower.

Flowers and bees

Discussing the strawberry flower and the role of bees in fertilisation, Remberg says:

"The strawberry flower is made up of five petals. In the centre is a bushy part with a whole bunch of griffins with stigmas, and then then there are pollen carriers at the outer edge, which are the female and male sex parts of the flower, respectively. The stigmas must have pollen on them for fertilisation to occur. At the bottom of each style is a seed with an egg, which will eventually be fertilised if pollination is successful".

Jordbærblomst — Den mørkere ringen ytterst i blomsten er pollenbærere med pollenknapper. I midten er det grifler med arr på toppen. **Photo**: Janne Karin Brodin

"After fertilisation, the seed develops. It eventually forms mature seeds, which are the dots you see on the outside of the strawberry. Because a strawberry is actually a swollen flower base, and not a real berry," says Remberg.

Siv Fagertun Remberg ser på jordbær — I jordbærtunnelen på NMBU har planteforsker Siv Fagertun Remberg mange sorter med remonterende jordbær. Det vil si sorter som blomstrer og produserer bær hele sesongen. **Photo**: Janne Karin Brodin

Fertilisation goes wrong

But for the strawberry, parts of the fertilisation process can go slightly awry, resulting in an imperfect berry. If some of the seeds in the strawberry flower are not fertilised, the base of the flower will not develop in the same way as in areas with fertilised seeds.

This leads to an underdeveloped part of the berry, with sunken fields where the surface appears knotty and where the unfertilised seeds are close together.
Admittedly, there may be other things besides poor pollination that can cause such knotty berries. Insect bites, for example.

"Heteroptera are one example. They suck on the unripe berry. The plant will then try to repair the damage, with an imperfect result in terms of appearance.

Hairy, clumsy insects are super pollinators

Of course, there may well be insects other than bumblebees that pollinate, insects that are looking for food, such as wasps and honeybees. But the bumblebee is particularly well-suited to this job. It visits many flowers, has a large body and is slightly clumsy, which makes it a great pollinator.

"Bumblebees rummage around in the flower. Because of their furry body, they will carry pollen that is stuck in the hairs all over their body around the flower and then carry pollen onto the stigmas of other flowers," says Remberg.

All insects that land in the flower contribute to pollination to some extent. How effective they depend on the type of insect, how long they stay in the flower and whether they visit just one flower or several.

"Some insect’s bodies are a little hairier than others. Having hair on your body is good, because the pollen grains stick to the fur and are transported further," says Remberg.

Bie i jordbærblomst — Superpollinator på jobb **Photo**: Kari Grønnerød

Insects need to be out and about at the right time

The weather is changing faster now than before. Cold and wet and hot and dry in succession. Hot and then cold days, with heavy rain and sun create new situations for pollination. The climate is changing, and as a result flowering is happening earlier than before.

If the weather is suddenly warm and the flowering period is short, insects need to be present to ensure good pollination.

"Such an abruptly warm period will shorten the effective pollination period. This is the time left from when the pollen tube with the pollen grain reaches down via the stylet to the egg cell and fertilises it before the egg cell dies. That time is far shorter than the actual flowering," says Remberg.

Fewer insects at work

The insect population has declined in recent years.

"At low temperatures, we just have to hope that there are some pollinating insects present to do the job," says Remberg.

If conditions are unfavourable for insects pollinators, there are things we humans can do to help with pollination, for example by using our finger. A tip from the researcher is to let your thumb barely touch the inside of the flower. Very lightly, otherwise it will be destroyed. Just tickle it maybe a couple of times.

Remberg explains:

"You can also use a small brush or paintbrush, but you have your finger with you all the time. Then you can just "tap" it a little, from flower to flower - or between trees, if you need to cross-pollinate between different varieties and the tree isn't too big so you can reach all the flowers."

Support insect populations with food, water and a place to stay

"Insect hotels are great to have in the garden. What's more, insects tend to overwinter in slightly messy environments, so if you have a very well-manicured environment and monotonous planting, the insects will look elsewhere".

Insects need food and water just like other animals and humans. If you remove their source of food, the insects will look for food elsewhere. That's why you should have flowers that bloom at slightly different times to maximise the amount of time the insects spend in your garden.

"In order to produce the plant food we need, we depend on our skilful and efficient pollinators. They are absolutely vital for this," says Siv Fagertun Remberg.