The bees are stingless native honeybees, Tetragonula carbonaria (Trigona carbonaria).
Their hive has thrived, and each day a stream of busy little workers heads back to the hive, with healthy corbicular loads of pollen on their back legs. Although we hadn’t noticed it, some would also have been carrying resin, which they use to make their comb.
Its door usually looks like this:
(The pairs of yellow dots are the rear ends of bees scurrying into the hive laden with pollen, their little black bottoms invisible in the photo.)
Then a few days ago we noticed it looked like this:
Alas, what has happened is that my little native bees have found a cadarghi tree, Corymbia torreliana (Eucalyptus torelliana), whose sticky seeds have been abandoned all around the mouth of the hive.
Cadarghi is a north Queensland gumtree which doesn’t grow naturally south of Ingham. It was widely introduced into southern Queensland as a fast-growing shady garden plant, and has thrived, becoming an environmental weed which damages local ecosystems.
It is interesting botanically, because it has been discovered that native bees disperse its seeds, and this is the first known example of bees as dispersal agents.
The bees don't mean to bring the seeds home. They only want the resin out of the gumnuts, and (no doubt to their annoyance) the seeds stick on to their load of resin. Getting rid of them after cleaning, they are likely to disperse the seeds within 10m of my hive. Here is one of my little bees, carrying out a C. torreliana seed which has accidentally been brought into the hive.
Every 10 seconds or so in the working day, another bee brings out another seed, so hundreds of seeds are being dropped around my garden. Frankly, I could do without this! I already weed out privet, lantana, camphor laurel, jacaranda, Chinese celtis and lantana. Another environmental weed tree is something I do not need.
The bees try to leave their seeds outside before entering their home which is why some seeds are clustered around the door. This bee in the photo below seemed to have almost succeeded. The seed hung by a little sticky thread from the resin on its back leg. It gave up the attempt, and limped into the hive with the seed still attached.
The bees will probably not manage to clean out all the seeds in their hive. What they leave behind may clog passageways and prevent ventilation and free movement of bees. I have since scraped away the external seeds, which you can see (Photo 3) were beginning to block the doorway, reducing the flow of the air to interior. But I can do nothing about the seeds inside.
Meanwhile there is a risk that the C. torreliana resin which they are now using for interior construction, will start to melt on a hot day. (It has a lower melting point than the other resins they have been using.) We can just hope that the hive’s shaded position and its insulated container will keep it safe from internal collapse.
These native bees are essential pollinators of large numbers of native plant species, and are the only pollinators of some of them. There is a risk that the spread of this harmful tree species, and the resulting damage to the population of our local pollinators, will interfere with the fertilisation of seed of who know which species of native plants. Pollination of Australian plant species is still only very partially studied, so there is much still unknown about how many plants are pollinated. In the fragmented modern remnants of our ecosystems, people are noticing that some healthy trees don't seem to be producing a new generation of seedlings. Perhaps our unwitting damage to populations of pollinators is part of the problem.
To quote from a Biosecurity Queensland website on Weeds of Australia:
“Cadaghi (Corymbia torelliana) is regarded as an environmental weed in south-eastern Queensland and as a potential environmental weed in New South Wales. It is becoming a serious weed in Queensland, where native bees collect its resin-coated seeds and spread it into eucalypt forests all over south-eastern Queensland.
This species has a very dense canopy of large leaves that is well suited to the rainforests of northern Queensland. When growing in open woodlands, outside its native range, it creates a heavy shade over the native understorey plants and prevents them from growing. It therefore has a significant potential to modify the diversity and structure of the native forests in sub-tropical Australia....
... The sticky resin from the fruiting capsules is collected by native stingless bees (e.g. Trigona carbonaria) and can completely clog the honeycombs and sometimes also seal the bee hive entrance, killing all the bees inside. This tree often gets covered in a sooty fungus, which can dirty or discolour things that are underneath it (e.g. vehicles, pavers and outdoor furniture). The large horizontal limbs are also prone to snapping off and damaging property.”
A 2013 study on Australian stingless bees by Megan Halcroft, Robert Spooner-\Hart, and Anne Dollin adds this information:
“Corymbia torelliana seeds are mainly dispersed by gravity; 88% of seeds drop to the ground soon after the fruit opens. However, one or two seeds remain within the gum nut and all are dispersed by Trigona. Resin is produced in the gum nut... When the bee enters the nut to collect resin, the seeds attach to the sticky corbicular load. Seeds are dispersed by bee(s)...and may be spread during the flight back to the nest or transported to the nest itself. This may be up to 1 km away from the tree...
... Trigona are strongly attracted to the resin from C. torelliana and the colonies stop normal foraging activity to collect as much of this resource as possible. As foragers return to the nest some attempt to dislodge seeds on the nest exterior, while others transport seeds directly into the nest cavity. The colony removes some, but not all, of the introduced seeds and these are either disposed of, up to 10m outside the nest or adhere to the sticky surface of the nest entrance... reducing airflow within the nest...
... Resin from C. torelliana may have a lower melting point than many other plant resins. Collection of the resin and its seed occurs during the hottest months of the year in Australia — December to February... and as temperatures rise, the resin begins to soften. Reports of structural collapse due to seed weight and resin softening are not uncommon, particularly if ambient temperatures exceed 39°C. As a result, some beekeepers remove their hives from C. torelliana areas during resin flow to prevent colonies from collecting the resin and seed mixture. While many Australian beekeepers consider C. torelliana to be a major management problem, others consider it to be a useful source of pollen, nectar and resin.”
The site also remarks:
“Not declared or considered noxious by any state government authorities.”