By Athayde Tonhasca
In 1993, two German entomologists came across some unusual specimens of plasterer bees – their epithet comes from their use of saliva to smooth over and strengthen the walls of their underground nests: taxonomically, they belong to the genus Colletes. Those specimens, collected in Germany and Croatia, didn’t seem to fit in with any known plasterer bee. It turns out the entomologists were dealing with a new species. Because the bee was found collecting pollen from ivy (Hedera helix), they named it Colletes hederae. Discovering a new European species of bee is unusual, as the fauna is well known and recorded. The ivy plasterer bee or just ivy bee escaped detection for so long because it takes microscopic examination to tell it apart from similar species.
Few bee species have experienced such a meteoric career. Unknown until 1993, the ivy bee has been dispersing rapidly since then, at rates of 6- to 7-fold between 2001 and 2010, with records from all over Europe. It arrived in Dorset in 2001, reaching Wales and northern England in 2016. It is still moving northwards, so may soon be recorded in Scotland. The UK Bees, Wasps and Ants Recording Society (BWARS) is plotting and monitoring the spread of the ivy bee in Britain with their online recording platform for sightings submitted by the public.
The ivy bee is not only spreading out fast, but its numbers are increasing rapidly too. They nest close to each other in soft, crumbly banks and cliffs, and aggregations of thousands of bees have been reported in southern England. This rate of expansion is a bit unusual, since most bees are phylopatric, that is, they generally remain in or return to the same nesting site across generations (Philopatry, from the Greek philo, ‘liking, loving’ and patra, ‘fatherland’).
We don’t know the reasons for such a dramatic population growth, but abundance of food must have contributed considerably.
The ivy bee is the last to emerge among all British solitary bees, and the only autumn species. It is active from late August to late October, occasionally stretching to the beginning of November, when most other species are no longer collecting nectar or pollen. August to November is ivy´s main flowering period, so the bee has no problem finding food. The ivy bee takes pollen from other plants, but over 80% of it comes from ivy. Being a pollen specialist can be tricky because food may be locally scarce. This is not the case when the host plant is all over the place, and may even become more widespread thanks to an increase in summer temperatures.
The absence of natural enemies in the newly occupied areas is another possible contributor to the ivy bee’s success. In continental Europe, the variegated cuckoo bee Epeolus fallax and the ivy bee blister beetle (Stenoria analis) invade ivy bees’ nest to eat their eggs and stored food, so their host may not do so well if these nest parasites were around. Unfortunately for the ivy bee, they are likely to follow their host into Britain.
Introduced species may be highly disruptive by outcompeting the native fauna, or by harming it or the environment some other way. These are cases of invasive species. We usually don’t see bees as belonging to this category of undesirables, but they might: the buff-tailed bumble bee (Bombus terrestris) and the honey bee (Apis mellifera) have caused environmental damage when introduced outside their native ranges. But most introductions are neutral: species move to new places or are moved naturally all the time (by the wind or currents, carried by birds, etc.), with no significant consequences. The ivy bee is likely to expand its distribution in Britain and eventually settle in, its limits imposed by food, weather, natural enemies, or a combination of factors. But it is here to stay: our bee fauna has gained one more species.