Opinion: bumblebees as managed avocado pollinators?
By Plant & Food Research pollination scientist David Pattemore
Around one third of the food we consume relies to some degree on pollination by the humble honey bee. They are the pollination workhorse of the insect world, but with Varroa mites having eliminated feral bee populations in New Zealand and driving sharp increases in the cost of maintaining managed colonies, we are starting to shift our attention to other insect pollinators. Bumblebees are excellent pollinators, yet they are not managed for pollination other than for the commercial production of temporary colonies to pollinate glasshouse tomatoes.
It has been demonstrated that, for some crops, bumblebees are about 50 times more effective as pollinators than honeybees. They are larger, so they pick up more pollen on their bodies and they also need to visit more flowers. As a consequence they are often better at moving between the pollenizer cultivars and the main cultivar, thus increasing cross-pollination.
Our pollination team at Plant & Food Research is currently investigating bumblebees to better understand how they might be effectively used for pollination services in orchards. The aim is not to replace honey bees but to provide growers with a second managed pollinator species to optimize pollination. The key challenge with bumblebee colonies is that they do not survive over winter; only the new queens over winter who then emerge in spring to find a nest site in which to build a new colony.
Our goal is to determine how to make the most attractive artificial nest sites so that queens prefer to establish their colonies in our nest boxes that can then be easily monitored and managed by growers.
Our research comprises two key components; studying wild bumblebee queens to determine how they choose nest sites; and trialling artificial hives and different nest materials in avocado orchards to assess which designs are most attractive to the queen bumblebees.
Tackling alternate bearing
Our pollination team started working with the complex world of avocado pollination as part of a wider effort to address the issue of biennial bearing. Our data so far suggests that fruit set in avocado can be limited by pollination, and this led us to ask whether better pollination could increase fruit set in low flowering years. If this is possible, it could be one way to reduce the extreme variability in yield between years that is a continuing problem for growers.
A number of complicating factors seem to contribute to problems with avocado pollination. Flowers open first as female, then close, and then open on a subsequent day as male. The timing of this sequence is altered by temperature and humidity. The cold spring weather encountered by avocado growers in New Zealand means that female Hass avocado flowers are often open in the evening or overnight instead of the typical morning opening supposedly characteristic of the Hass cultivar.
While our initial study of flowering focused solely on Hass trees, we are now also looking at pollenizer cultivars such as Zutano and Etinger, to determine when the male pollenizer phase coincides with Hass' receptive female phase; what I call the "pollination window". The next step then is to identify which pollinators are active during these pollination windows.
Our research with avocado pollination suggests that bumblebees could play an important role in improving pollination if we could increase and maintain colonies within orchards. Bumblebees are active in a wider range of temperatures and weather conditions than honeybees. In the evening and on cooler days while honeybees stay in the hive to keep warm and snug, bumblebees are still out foraging.
As Hass flowers often appear to be open and receptive in the evening on cold days, bumblebees may contribute significantly to pollination. Bumblebees have several other advantages over honeybees as pollinators of avocado as mentioned earlier. Research in Israel has demonstrated that bumblebees can increase fruit set in low flowering years on trees that are farther away from pollenizers. This could help to decrease variability in fruit set within an orchard between low and high flowering years.
The biggest limitation with bumblebees currently is the difficulty getting sufficient numbers of foragers to have a significant effect on pollination. Small, commercially-produced colonies can be purchased, but the cost of each colony makes it prohibitive to purchase enough colonies for pollination in an open orchard environment. Our team’s approach is quite different, by seeking to develop easy and cheap techniques to attract wild bumblebee queens to artificial nests that can then be monitored and managed.
Numerous attempts worldwide have been made to use artificial nests to attract wild queens, with very limited success. However, one study conducted in New Zealand by Nelson Pomeroy in the 1980s reported very high nest establishment rates (over 90% for one design). The artificial nests we have designed incorporate elements from the two most successful of Pomeroy’s designs, in an attempt to replicate his findings. The unusually high success rate of the New Zealand study has been suggested by other authors as evidence that bumblebee populations in New Zealand are limited by the availability of nest sites.
We are now assessing whether the provision of these artificial nests can encourage colony establishment by queen bumblebees using artificial nests in five avocado orchards in the Bay of Plenty. The results of this initial study will be used to refine future nest designs, both for attracting colonies and for supporting their survival throughout the year.
This project, joint funded by Plant & Food Research and New Zealand’s Avocado Industry Council (AIC), is the first step of a wider programme to determine how to manage bumblebee colonies in orchards.
The other part of our bumblebee program is to understand how bumblebee queens choose nest sites by following individual wild queens. We can use this information to improve our nest designs in subsequent years. We plan to follow the queens by using miniaturised radio transmitters that are glued to their backs.
Our initial trials have shown that the queens continue to forage and search for nests successful when fitted with these radio transmitters. We are now seeking funding to conduct this research on a larger scale in collaboration with researchers from the University of Aarhus in Denmark so that we can study nest site selection in two springs each year.