DW: Bee populations are on the decline and in some parts of the world they’ve been wiped out altogether. What's behind this decline?

Dave Goulson: It's a combination of there not being much food, they've got diseases and they're exposed to lots of pesticides. It's these three things combined that are causing bees to decline. Intensive farming doesn't leave many flowers for the bees. Apart from the crop itself, which only flowers for two or three weeks, for the rest of the year there's nothing for them to eat. Basically the natural habitat that bees used to live in has mostly gone, and that's affected bees around the world.

We've also accidentally spread bee parasites and diseases around the world. So, now bees are often infected with parasites they don't have any resistance to. The best example of this is the varroa mite, which came from Asia, but we've spread it all over the world. European honeybees are now attacked by this mite that spreads diseases and sucks their blood.

Bees are vital pollinators, but in some areas where there are no longer any bees, locals have turned to hand pollination. Can you explain how this method works?

Normally, people would use a paintbrush and a little jar or pot, so you basically go from flower to flower, brushing the pollen into your pot, which is very slow work. And then you go around brushing the pollen onto other flowers, trying to get it onto what's called the stigma - the female part of the flower. And if you've done that correctly, then hopefully your flower will turn into a fruit. Bees are very good at this, and people are relatively speaking rather rubbish. Bees have been practicing for 120 million years so they've got a head start.

Which countries are currently using hand pollination and why have they resorted to that method?

In parts of China they now hand pollinate their orchards. It's mainly apple and pear orchards in Szechuan in the south west of China. I'm told that this is also often done in parts of Brazil for passion fruit. The reason they're doing this is there isn't enough pollination going on naturally - they haven't got enough bees. In China in particular they use terrifying amounts of pesticides, and farm in a very intensive way. There's very little wildlife left at all in some areas. In these apple and pear orchards there aren't any bees, and hence they have no choice. They either get no crop or they pollinate it themselves.

Given the decline in bee populations around the world, could hand pollination be the main pollination method in the future?

I hope not. For most crops it just isn't possible. In China, these orchards are quite a valuable crop and labor is cheap compared to Europe, so it's really hard to imagine farmers in Europe or North America hand pollinating their crops. Farmers would stop growing insect pollinated crops if they didn't have bees - it just wouldn't be economic to do it in most parts of the world. All we'd be left with are things that are wind pollinated, so that's basically wheat, barley, rice and corn. So we'd have to live off bread and porridge and rice if we lost our bees, and we really don't want to go down that road.

Are there other more efficient ways to pollinate crops without bees?

There are rumors of people developing robot bees that will fly around and do it for us, but I think that's a completely bonkers idea. I would be absolutely amazed if any technique we could devise would be anywhere near as efficient as a bee. Bees have evolved to collect pollen and nectar from flowers, and they're really good at it, so it seems crazy to try and replace them.

Is there anything that can be done to reverse the decline of the world's bee populations?

There are two things we can easily do: try to reduce their exposure to pesticides, and leave a little bit of room for flowers. We don't have to farm 100 percent of the land. We can leave corners here and there for wildflowers, and if we don't we'll end up losing a huge proportion of our crops through the absence of pollination.

There are plenty of good examples in Europe and North America now of farmers doing this. It's still a minority of farmers, but some are leaving strips of flowers along the edges of their fields, and it's a really effective way of conserving bees. And it's not just about farming - one of the nice things about looking after bees is that it's something everyone can join in. Plant some bee friendly flowers in your garden and you'll see bees turning up and happily feeding.

Professor Dave Goulson is a professor of biology at the University of Sussex in the UK, specializing in the ecology and conservation of bumblebees. He is the author of "Bumblebees; their behavior, ecology and conservation" (2010) and the popular science book about bumblebees, "A Sting in the Tail" (2013).