PAGE 14 –– OCTOBER 2017 THE GROWER
FOCUS: GREENHOUSE INNOVATION
Shedding light on how to improve bumblebee pollination
AAFC researcher Rose Labbe is pictured with a bumblebee hive in a commercial greenhouse. KAREN DAVIDSON Lights! Action! Let’s roll! Not so fast. As greenhouse growers move to year-round production with supplemental lighting, there’s one important aspect of the ecosystem to remember: bumblebee pollination. Insects navigate on light cues and if growers don’t finetune their lighting regimes, they may not maximize pollinator activity. Commercial bumblebee hives are a common sight in greenhouses to pollinate tomatoes, cucumbers and peppers. However, as LightEmitting Diode (LED) lights are increasingly used to supplement High-Pressure Sodium lights (HPS), the effects
A bumblebee pollinates a strawberry plant in a commercial greenhouse.
on insect pollination must be factored into the equation. “Dusk is a biological cue to help bees navigate back to the hive,� explains Rose Labbe, greenhouse entomologist at Agriculture and Agri-Food Canada’s (AAFC) Harrow Research and Development Centre (HRDC). “Growers must turn off supplemental lights before dusk.� In 2015-16, the application of supplemental light for increasing greenhouse production in Canada really began to take off. Thanks to HRDC research at the southern-most tip of Canada and in many European institutions, it’s been demonstrated that effective light spectra and combinations are required to optimize year-round vegetable production.
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For many years now, research on the interaction of light and greenhouse arthropods has been conducted at the HRDC. Most recently, Dr. Labbe has examined the impact of lights on biological control agents used to suppress thrips and whiteflies in the greenhouse environment. This work has been funded in part by the Ontario Greenhouse Vegetable Growers. Earlier research by Harrow’s greenhouse entomology group showed that HPS supplemental lighting on tomato crops could impact bumblebee pollination. A key finding is that extending the photoperiod through the application of HPS lighting could contribute to increasing daily mean bumblebee activity. Research also showed that bees
remained active for the duration of the natural plus extended photoperiod. That means light supplementation has the potential to improve overall pollination in short-season crops and to contribute to yield increase. Since the start of research in this field, some interesting facts about bumblebees are now known that could help develop best management practices for the use of bumblebees on crops lit by LED lights. As with many insects, bumblebees do not have a red photoreceptor. This means they perceive red light as darkness. This differs from plants for which red light is readily absorbed and often used as a source of light energy that allows for improved plant growth and productivity in wintertime in Canada. So, using red LED light alone in a greenhouse is unlikely to improve pollinator activity. Light waves such as blue or UV, however, can stimulate bumblebee activity and could be useful in achieving good crop pollination. Secondly, UV light, which is abundant in natural sunlight, has an important range of wavelengths that bumblebees use for orientation. Since HPS lights are frequently used in greenhouses to supplement day length, and this source of light is low in UV irradiation, the bumblebees may not respond as effectively to HPS as a light source. Thirdly, bumblebees are diurnal so are most active in the presence of light in the day, and spend the night in their hives. They rely on a gradual decrease of light intensity, which naturally occurs at dusk, to signal the onset of nighttime and which serves as a cue for them to find their way back to their hives before dark. In complete darkness, bees will not fly, but can crawl if needed. If supplemental lights are switched off abruptly leading to complete darkness, it is unlikely that bumblebees will find their way back to their hives. This is
an important consideration as it may constrain when supplemental lighting is applied to the crop. Current practices suggest that supplemental illumination of greenhouse vegetable crops is optimal when plants have the chance to “go to bed� along with the natural dusk. In the same sense, during a typical Canadian winter, supplemental greenhouse lighting can be started early in the morning, can be turned off midday when light intensity is sufficiently high, then turned on again until the onset of natural dusk to achieve an optimal photoperiod for crop production. This natural dusk also suits pollinator biology. This strategy works well in places such as southern Ontario, where such a photoperiod is also sufficiently long for greenhouse workers to achieve their daily plant maintenance routine. However, for winters in some Nordic countries such as Finland, and at higher latitudes in Canada, this strategy may not work. Longer periods of darkness in winter mean that more supplemental lighting is used overall, both at the start and at the end of the day to complete routine plant maintenance. In such a situation, natural dusk is not perceived by bumblebees. To get around this, commercial hives are now available in which bumblebees will enter and exit from separate holes, with the exit hole blocked at a particular point in the evening so that bees may enter but not exit. This way bees are retained inside nightly, with no significant loss due to abrupt light changes. “We hope to keep this area of research going as the greenhouse industry continues to change and adopt new lighting technologies,� says Labbe. Dr. Rose Labbe will be speaking on October 5 at the Canadian Greenhouse Conference (www.canadiangreenhouseconference.com). For her talk titled “Pollination and Artificial Lights� go to Ballroom D at 9:30 am.