Cherry flower, Haygrove Farm organics 2010

Even as early as December English strawberry growers will be planting cold stored plants that have received enough chilling and glasshouse salad crops will be taking delivery of new plants for early, heated crops. Both these will be have consider pollination very carefully and will be using bumble bees.

Some important points to remember if you are planning to seal up your tunnel or glasshouse for maximum earliness:

• If the structure is sealed to retain heat (or to exclude insects) pollinating insects will need to be placed on the inside or flowers pollinated by hand
• Bumble bees are best for pollinating early crops. They are not aggressive and fly at relatively low temperatures.
• Don’t forget that, if the sun comes out, temperatures in a sealed structure will rocket upwards. That will cause bees to retreat to their hive, on fanning duty, to protect their brood and will also mean that your plants are use more energy respiring than they make photosynthesising. For most crops its not a good idea to let the air temperature rise above 25 degrees C.
• For small structures its usually best just to leave the door open and let local bees and other nectar feeding insects do the pollinating. For tomatoes many growers appear to find that their own business and some air movement provides pollination.

Some of the most frequently asked questions about growing of crops under tunnels are concerned with bees and pollination. These questions have become even more pertinent as legislation governing the use of imported bumble bees has tightened in many countries.

There are a few basic things to remember:

Bees cannot ‘see’ the polythene films used for tunnels

Honeybees waiting in hives outside don’t know what the temperature is inside tunnels

UV light is an important steering aid for bees and other insects. Some films block more UV than others.

Bees aren’t the only pollinating insects

Cherry blossom above a commercial bumblebee hive inside tunnels

During the summer there is normally a surplus of pollinating insects, including a great many different nectar feeding insects. It is rare to hear of fruit setting problems under tunnels that are adequately vented during the summer months. For most berry crops the best temperatures for bees coincide with those for good yields – mostly less than 25^oc. Above that temperature both bees and photosysnthesis slow down significantly.

IN THE SUMMER VENTING IS IMPORTANT FOR GOOD YIELDS, POLLINATION AND FRUIT QUALITY

The danger time is in the Spring, especially when crops are being ‘forced’ to flower early. To gain temperature tunnels are kept closed. Bees kept outdoors may be shivering in their hives unaware that the tunnelled crop is warm and in open flower. In closed, permanent tunnels strawberry growers used to introduce blow flies (as pupae close to emergence) as pollinators to overcome this problem but many countries now have the luxury of bumble bee hives from commercial companies like Koppert http://www.koppert.com/pollination/. Bumble bees are active at lower temperatures than honeybees, pollinate using a ‘buzz’ method that has technical advantages for crops such as tomato and the commercially available species are less aggressive towards crop workers.

A large wild bumble bee in Argentina.

Honeybees are still used under glass for various crops – often in combination with bumblebees. The large size of hives makes this less practical for small tunnels but for large, field scale crops honeybees are worth looking at. Hives used in these situations will tend to lose bees as they waste energy attempting to fly through the sides and roof or are lost to the outdoors. Where bumble bees are not available, tunnel growers are working with honeybee hives however at the time of writing we don’t have sufficient experience to provide reliable advice on this option.

A honeybee hive inside a blueberry tunnel

IN THE SPRING DO NOT KEEP TUNNELS CLOSED DURING FLOWERING UNLESS A DAMAGING FROST IS DUE OR YOU ARE CERTAIN THAT THE NUMBER OF POLLINATING INSECTS INSIDE THE TUNNELS IS SUFFICIENT.

REMEMBER THAT, EVEN IN THE EARLY SPRING, SUNNY CONDITIONS CAN CAUSE FLOWERS TO MOVE FROM WHITE BUD TO PETAL FALL VERY QUICKLY. DON’T BEE LATE!

Bee Steering

UVb = Light in the ultra violet spectrum between the wavelengths of 315nm to 400nm

Bees do use other cues for navigation and experiments have demonstrated that bumble bees can learn to identify flowers in the absence of UV light but in general terms the impact of bees and other pollinating insects is strongly influenced by it.

When purchasing polythene films for tunnels it is worth asking for clear information about the transmission of light in the UV range. Standard Visqueen films supplied in the UK allow sufficient UVa through to facilitate the work of bees but some films, especially those formulated for use in countries with very high light levels, contain a lot more inhibitors. When these polythenes are used in dull weather or in other countries there may be problems with fruit set.

Bee Information

www.bumblebee.org

www.ibra.org.uk

Mason bee shelter at a Biodynamic farm in Poland: What potential for tunnel use?