Bumblebees see the (UV) light

Bumblebees are a preferred pollinator for many greenhouse operators since their large bodies help carry pollen to other plants. However, changes in UV light can confuse them and reduce their productivity. ~ Pixabay photo

By Luke Edwards

Greenhouse growers installing new coverings should be careful of unintended consequences, a greenhouse vegetable specialist for the Ontario Ministry of Agriculture, Food and Agribusiness is warning.

While new coverings with enhanced ultra violet (UV) light blocking technology may increase their lifespan, they could also confuse bumblebees that are used for pollination within the greenhouse. At least that’s what research by Fadi Al-Daoud suggests.

“Low UV reduces bumblebee activity,” he told attendees at last month’s Growers Day event in St. Catharines.

Al-Daoud said he was recently contacted by a greenhouse tomato grower who noticed his bees weren’t pollinating as well after he installed a new triple poly panel system at his greenhouse.

Since UV light can damage greenhouse coverings, companies will apply UV blocking agents to them to increase the lifespan. However, much like humans detect red, green and blue wavelengths in the visible light spectrum, bumblebees detect UV light, which attracts them.

And since controlled greenhouses don’t have the wind or insect populations to provide pollination, many growers bring in bumblebees to act as pollinators.

Al-Daoud’s study used sensors to detect light infiltration at the top, middle and bottom of the canopy in a greenhouse. To start, he considered light quantity, essentially the overall amount of light coming into the greenhouse. The experiment included different kinds of coverings, both old and new, and double and triple poly. The idea is that as coverings age, their UV blocking ability diminishes.

However, overall light quantity didn’t seem to have a correlation.

“So we decided we’re going to start looking at light quality,” he said.

When he broke it down to different wavelengths, it became clear that less UV light was associated with newer, triple poly coverings. Al-Daoud used the ratio of UV light to  photosynthetically active radiation (essentially the visible light spectrum) to demonstrate this difference.

While this showed an association between less UV light infiltration and lower pollination, Al-Daoud said it doesn’t necessarily prove causation.

However, it does offer evidence that growers should be mindful when they decide to install new coverings. They may want to look at the material they’re using, or perhaps they’ll need to bring in more bees for pollination.

In addition to the connection between UV light and pollination, Al-Daoud also said growers should pay attention when opening vents, as that becomes a source of UV light, which attracts the bumble bees and could lead to them escaping.

Overall, it’s a difficult balance to maintain. Blocking UV light can help the plant, Al-Daoud said, by increasing photosynthesis and transpiration, which leads to bigger plants and greater yields.

However, in addition to inhibiting the bumblebees, UV blocking can also reduce phenolic compounds, flavonoids and carotenoids, which reduces fruit quality. Likewise, it has a mostly negative effect on all bugs and insects, including pests and beneficial insects.