The effect of pollinating insects on blackcurrant fruitset, yield and quality

Abstract: Bees are the primary pollinator of most fruit crops and improve not only crop yield,but also crop quality, e.g. fruit size in blackcurrant. Blackcurrant is susceptible to poor pollinationbecause it is early-flowering and, therefore, often subject to lower temperatures when pollinatinginsects are less active. The extent to which blackcurrant is self compatible and/or wind pollinatedis not clear. It is possible that enhancing the provision of pollinating insects will increase cropyield and quality and, potentially, reduce infection by Botrytis cinerea, particularly in seasonswhen blackcurrant flowering occurs over an extended period. Premature fruit drop occurs in moreself-pollinated compared to honey bee-pollinated blackcurrant.The decline of honey bees across Europe has been well documented, and it is reported thatthey are less effective at pollinating early season crops as the colonies are not up to full size andthe temperatures are generally too low for foraging. Alternative, more effective pollinating beespecies are practically available, but have not been fully tested in UK blackcurrant plantations.Bees of the genus Osmia Panzer (Hymenoptera: Megachilidae) are among the contenders tosupplement honey bees as fruit pollinators. Osmia rufa (species endemic to Europe) is,potentially, a commercial pollinator of spring-flowering fruit crops. Their foraging distance is~400m, hence, they are easier to confine to a plantation. They are active for 10-12 weeks inspring (bumblebees and honey bees 8-9 months), when native bumblebee and honey bee colonieshave not built up to full size. Osmia is also able to forage at lower temperatures than honey beesand can be active under strong wind or light rain. Bumblebees are commonly used in protectedcrops, but until recently have not been available commercially for outdoor fruit crops in the UK.A preliminary field trial was set up at East Malling Research, UK in 2010 in which beeswere contained in 12 x 1.5m insect mesh tunnels over cultivated blackcurrant bushes (Ben Hopeand Ben Gairn). Four treatments were evaluated: provision of 1) Bombus terrestris dalmatinus or2) Osmia rufa, 3) no pollinating insects and 4) open pollinated plots (no mesh tunnel). Theweather at the time of 100% open flower in Ben Gairn was dry and warm compared to Ben Hopewhere there was a period of rainfall, low solar radiation and low temperatures. As a result fruitset was higher in Bombus and open pollinated plots in Ben Gairn, and Bombus pollinated plots inBen Hope. Fruit size was effected by the treatments. Berries in the Ben Gairn and Ben Hope werelarger in the Bombus, and the Bombus and open treated plots, respectively. At harvest, the yieldfrom the Bombus treated plots was significantly higher than that of all other treatments includingthe open plots. This study highlights the vulnerability of blackcurrant to poor pollination if theweather is not adequate for insect activity at the time of flowering. Supplementing naturalpopulations of pollinating insects with bumblebees may ensure a high yield.