Pollinering med norske humler for økt fruktsetting og avling hos søtkirsebær under tunneldekke

Plastic tunnels are increasingly used for the growing of sweet cherries (Prunus avium), which include advantages such as less frost damage, plant diseases and less cracking of fruits. But in addition to optimal growing conditions it is also crucial to obtain optimal pollination by insects. In Norway, pollination of sweet cherries and other fruit trees has traditionally been done by using domesticated honey bees (Apis mellifera), due to short flowering periods and demand of effective pollination. One of the problems being completely dependent on honey bees is low temperatures, since honey bees are known to show very little activity under 12 °C, while other pollinators such as bumble bees are active also at low temperatures. The main problems with plastic tunnels though, is that honey bees get disorientated by changes of the light reflection and therefore have problems to orientate back to the colony. An alternative to honey bees is to use domestic bumble bees for pollination of fruit trees. In Norway, bumble bees (Bombus terrestris) have mainly been used in greenhouses and to some degree in strawberry fields. In this project the Norwegian Institute of Bioeconomy (NIBIO) together with the Norwegian Agricultural Advisor Agency investigated the effect of using bumble bees together with honey bees, and comparing this by only using honey bees. The results showed that use of bumble bees in general increased the fruit set, but this varied from field to field and from year to year. In both 2014 and 2016 there was low precipitation and relatively high temperatures during large parts of the flowering period, thus optimal conditions for honey bees. This may partly explain why the fruit set was generally high and that bumble bees only slightly increased the fruit set or did not improve the fruit set at all. The experiments in 2015, however, resulted in poor fruit set, but the fruit set was twice as high with bumble bees compared with only honey bees. This result was expected as bumble bees are less dependent of high temperatures and sunny weather to be active. We found large differences in fruit set of the different cultivars of sweet cherries. The self-fertile cultivar Lapins had on average the highest fruit set, followed by Sweetheart, which is also self-fertile. The self-sterile cultivar Van, however, generally showed a much lower fruit set. In the last year of the project, 2017, a controlled field experiment was carried out using a fine meshed net to separate rows of cherry trees. Bumble bees were used inside the nets while the rest of the field was open for honey bees. In addition, nets were wrapped around single branches to exclude all potential pollinators. The fruit set and harvest with bumble bees was significantly higher than was obtained with honey bees. However, no significant difference was found in size of berries. Our study has shown that both honey bees and bumble bees together or separately may result in an optimal fruit set, but this will vary according to the weather during flowering. Especially in years with a cold and wet spring one may expect improved pollination with bumble bees. We also showed that self-fertile cultivars are also dependent of an optimal pollination, although this is not as crucial as for self-sterile cultivars. In parallel with the pollination experiments, developmental work on breeding bumble bees have been carried out. This work has been done by the project owner Bombus AS and involves optimization in all parts of the breeding from overwintering of queens, reducing diseases, and optimal timing of different activities. It has been necessary to make some changes in the breeding of bumble bees. Breeding of bumble bees has earlier been unpredictable to some extent regarding success rate and mortality of the bumble bees. This has not been easy to explain. After a longer period of work the company now seems to partly be able to explain this, which has increased the production as well as obtaining a more stable quality of the bumble bee hives. This has led to major changes in breeding methods and has led to lower mortality and considerably higher rate of success. The changes have been significant for the company´s capability to deliver bumble bee hives, and Bombus has for the first been able to deliver bumble bees all year round. The capacity in the company today exceeds the Norwegian demand. This could not have been possible with old technology and without new knowledge.

Dyrking av søtkirsebær i Norge er en spesialisert produksjon lokalisert til de klimatisk beste dyrkingsområdene i landet; Telemark, Hardanger, Agder og Sogn. All kommersiell produksjon blir dyrket under tak for å unngå at fruktene sprekker. En økende del av produksjonen blir nå dyrket i plasttunneler, selv om dette innebærer ekstra utfordringer når det gjelder pollinering. Hovedmålet med dette FoU-prosjektet er å øke verdiskapingen av søtkirsebær ved at fruktdyrkere, rådgivingstjenester og birøktingen får nye kunnskaper om hvordan man kan oppnå optimal fruktsetting, avling og fruktkvalitet ved rett pollinering. Honningbier har så langt blitt brukt for pollinering selv om biene har problemer med å orientere seg under plastdekke. Humler kan være en annen lø sning siden humler orienterer seg på en annen måte og allerede blir brukt med stor suksess i veksthus. Det foreslåtte prosjektet har som mål å finne hvilken løsning som gir den mest optimale pollineringen av søtkirsebær. Forsøk vil bli utført der resultat et av pollineringen vil bli målt i form av fruktsetting av avling. Honningbier og humler vil bli testet ut hver for seg og i kombinasjon. Optimal humlebol størrelse og plassering vil også bli testet for å finne den mest optimale pollineringen av søtkirseb ær. Klimavariabler som temperatur, nedbør og finværsdager antas å påvirke resultatet og dette vil derfor bli tatt høyde for. Gode resultater fra dette prosjektet kan på lengre sikt utvide markedet for røkting av norske humler til pollinering av frukt og b ærproduksjon under plasttunneler. Dette kan igjen skape større verdiskaping i produksjon av frukt og bær i Norge i årene fremover.