Dubious helpers

By Athayde Tonhasca

Eikyu Matsuyama, an apple grower from Aomori (the northernmost prefecture on Japan’s main island), noticed mason bees building nests inside holes in utility poles and wooden walls near his orchard. As this species of bee (Osmia cornifrons) was a keen visitor of apple flowers, Mr Matsuyama pondered whether he could attract more bees to his orchard by supplying them with pieces of reed as extra nesting sites. This was the 1930s, when apple pollination in Japan was mostly done by hand – a labour-intensive, costly operation. Sometimes the military was recruited to help pollinate apple flowers during the two-week bloom period. Aomori was – and still is – the leading apple-producing region in Japan, accounting for nearly half of the country’s harvest. So any extra help from bees would be much appreciated by the growers.

An apple blossom in the Aomori prefecture with Mount Iwaki in the background © 岩浪陸, Wikimedia Commons.

Mr. Matsuyama’s experiment was a huge success; soon the numbers of mame-ko bachi (the bee’s Japanese name) expanded dramatically, with a corresponding improvement in apple production. Other growers quickly followed suit, and Mr Matsuyama went on to lecture about mason bees at several Japanese universities (Mader et al., 2010). 

In the 1960s, apple growers in the Aomori Prefecture began to use the European honey bee (Apis mellifera) as an alternative pollinator, while researchers improved propagation and management methods for the mame-ko bachi, named the Japanese hornfaced bee in English-speaking countries. Nowadays Aomori growers of apple, pears, peaches and plums are likely to use commercially available Japanese hornfaced bees instead of honey bees because the former is considerably more efficient. It visits about 15 flowers per minute (4050/day) and carries ~267,000 grains of pollen on its scopa (pollen-carrying bristles), while the honey bee visits about 6 flowers per minute (720/day) and transports ~100,000 grains of pollen. Equally important, the Japanese hornfaced bee, like other species in the group, is a bit sloppy in transferring pollen to her nest; about 10% of it remains on her body (Matsumoto et al., 2009). This residual pollen, viable for up to 12 days, has a good chance of ending up on a receptive apple flower and pollinating it.

A Japanese hornfaced bee leaving its nest © Beatriz Moisset, Wikimedia Commons.

The Aomori Prefecture achievements with the Japanese hornfaced bee didn’t go unnoticed. In the 1960s, the American Agricultural Research Service (Department of Agriculture) introduced this species to America to improve pollination of fruit crops such as apple and blueberry. Since then, the Japanese hornfaced bee has spread through the eastern and mid-west United States. And in 2002, another Asian mason bee was found doing its bit for pollination in America: the taurus mason bee (Osmia taurus). Nobody knows how or when this species entered the country: it probably was introduced accidentally at the time of Japanese hornfaced bee importation. These two species look alike superficially, so an unintentional introduction is plausible.

Female Japanese hornfaced bee (L) © Chelsey Ritner, and taurus mason bee © Chelsey Ritner, Exotic Bee ID.

Bees are overwhelmingly valued by the public, so the addition of two new species to the local fauna, even if inadvertently in one case, must be a good thing. Indeed, introduced bees may increase the local pollinating force. But these newcomers may also be harmful to native species and their habitat, most likely because of competition. This happens when two of more species need a common resource – food, water, a place to live – that is in short supply.

Assessing competition: Diet analyses from carbon (δ13C) and nitrogen (δ15N) isotopes indicate a considerable overlap of food taken by the invasive American mink (Neovison vison) and the native, critically endangered European mink (Mustela lutreola) in Spain; these results suggest a substantial competitive pressure imposed by the American mink on the European mink © García et al., 2020.

In the case of bees, the most likely problems from introduced species is competition for food (nectar and pollen) or nesting sites. These interlopers may cause less evident but no less serious problems; they may transmit novel pests and diseases to the native fauna, or they may alter the habitat by pollinating invasive plants and therefore helping them spread. These concerns are more than speculation; the European honey bee and the buff-tailed bumble bee (Bombus terrestris) are notorious for causing trouble in places where they have arrived. We know less about the other 80 or so bees introduced accidentally or deliberately outside their native range, but problems have been documented (e.g., Russo et al., 2020).

The yellow star-thistle (Centaurea solstitialis), a Mediterranean native, has been spreading through 41 of the 48 contiguous U.S. states thanks in part to pollination by the non-native European honey bee © J.smith, Wikimedia Commons.

The Japanese hornfaced bee and the taurus mason bee found America to be very much to their liking; their populations have increased dramatically since their arrival – reaching growth rates of three to five times per year, when conditions are favourable. This is not good news for the blue orchard bee (Osmia lignaria), a reliable orchard pollinator and a native species that has much in common with the intruders: they all emerge more or less at the same time, take pollen from similar plants, and have similar nesting habits. This overlapping of resources inevitably raises the possibility that the blue orchard bee would get the short end of the stick in these species interactions.  

A female blue orchard bee © Chelsey Ritner, Exotic Bee ID.

LeCroy et al. (2020) set out to assess just that. They examined trap catches of the two introduced bees, the blue orchard bee and other five native mason bees from four eastern American states from 2003 to 2017 (5,901 records). Their results: all native species declined from 76 to 91% since 2003, while the exotic species were doing just fine; populations of Japanese hornfaced bee were stable, while taurus mason bee numbers increased by 800% since 2003. This latecomer represented 22% of captures in the years 2003–2009, raising to over 43% of all captures from 2010 to 2017, making it the most common mason bee species in the region.

We don’t know the consequences of such dramatic population shifts. The two exotic species may take up the pollination work from the bees displaced by them, although a complete, seamless substitution is improbable: the dynamics between plants and pollinators are bound to become different, although we can’t say how. Native and alien bees may adapt and co-exist; in the worst case scenario, native species may become threatened. Many Americans would not be happy to watch their native bees pushed aside by intruders, no matter how benign or useful they may be. But there’s little they can do about it. Once alien species become established, it’s extremely difficult and expensive to get rid of them. 

Here in the UK, gardeners, allotment holders, schools and assorted nature lovers have shown increased interest in buying mason bees – and many suppliers, domestic and from abroad, are ready to oblige. But it’s impossible to tell whether these newcomers are healthy, or whether they will take food and shelter away from the local bee fauna. The case of the Japanese hornfaced bee and the taurus mason bee in the US is a cautionary tale about the uncertainties of species introductions: you are never sure how the story will end.

Pandora trying unsuccessfully to make amends for releasing humanity’s evils. Art by Frederick S. Church (1842–1924), Wikimedia Commons.