The Hawkesbury Institute for the Environment pollination team at Western Sydney University spent the 2021 mango flowering period in and around the Darwin and Katherine mango growing regions as part of a Hort Innovation project studying how wild stingless bees are contributing to crop pollination services.

In the previous 2019 survey we found that Darwin and Katherine mango farms are frequently visited by wild stingless bees, hover flies, blow flies and several species of native bees. In both regions stingless bees were the dominant flower-visiting group, comprising almost half of the total insect visitation, and travelling up to 300 metres into the crop.

On this latest trip we wanted to understand:

1. What stingless bee species are present around Darwin and Katherine farms?
2. Where are these stingless bees nesting, and how far are they travelling to visit the crop?
3. Are they significantly impacting mango yield?

Species present

In 2021 we found a total of 75 wild stingless bee colonies from two species in bushland adjacent to mango farms in the Darwin area. Most of the colonies found were that of Tetragonula mellipes (96%), while three colonies (4%) were of a currently undescribed species commonly called ‘NT hockingsi’ by local beekeepers.

Tetragonula mellipes workers can be easily distinguished from ‘NT hockingsi’ by looking at their eyes; T. mellipes has bluish-grey eyes (see image below), while ‘NT hockingsi’ has black eyes (see image in headline).

We also collected stingless bees from the NT side of the border near the Kununurra mango growing region, and in the Katherine region. We are currently conducting DNA analysis to determine what species they are, but it appears that we are seeing a new, undescribed stingless bee species near Kununurra.

Nesting trees and mango pollen

Stingless bee colonies were found up to 460 metres from the crop in bushland adjacent to mango orchards. Of 50 colonies located next to a mango farm in Lambells Lagoon, 37 (74%) were found nesting in Ironwood (Erythrophleum chlorostachys), seven (14%) in Darwin woollybutt (Eucalyptus miniata), and the remaining six (12%) in a range of other tree species. Mango pollen was found in the entrances of all but four of the colonies, up to the furthest distance of 460 metres from the crop. This demonstrates that stingless bees were travelling nearly 500 metres to visit mango flowers!

Are stingless bees impacting mango yield?

To test this question, we performed bagging experiments on Kensington Pride mango trees at four farms near Darwin. We bagged mango panicles with fine mesh bags (0.42mm) to exclude all insects and coarse mesh bags (5mm) to exclude all but small insects like stingless bees. We then compared the resulting fruit set with fruit set from unbagged open panicles.

We are still analysing this data, but preliminary results show that the number of fruits on unbagged open panicles and coarse mesh bagged panicles were similar, but significantly higher than on panicles bagged with fine mesh. This suggests that small insects less than five millimetres in size, such as stingless bees, are playing an important role pollinating mangoes in the farms we studied. Our results suggest that stingless bees can efficiently pollinate mango flowers in farms where wild colonies are abundant in the native vegetation adjacent to the crop.

Future work

Our results demonstrate that maintaining native vegetation around the crop margins can provide natural habitat for local stingless bee species, resulting in a positive impact on crop yield. Our findings so far suggest that every top end mango growing region appears to have a unique stingless bee species. These local species are likely the best adapted species for their growing region, which could be threatened by the introduction of stingless bee colonies from other regions.

In the upcoming flowering season, we want to compare the different stingless bee species in terms of their pollination efficiency; looking at questions such as how much body pollen they carry when visiting flowers, how large are the forager population colonies of these different species, and whether they have potential as managed pollinator species that could be deployed onto farms.

We also want to see whether cross-pollination between co-flowering mango varieties such as Kensington Pride and R2E2 might have an impact on fruit yield and quality in both varieties, and if stingless bees can assist in this cross-pollination.

We plan to return to the Darwin, Katherine and Kununurra mango growing regions this year, and hope to work with some of you on your farms. If you would like to get involved, please contact j.makinson@westernsydney.edu.au or 0491 124 016.

Written by James Makinson and Gaurav Singh, Western Sydney University