It’s complicated

By Athayde Tonhasca

Himalayan balsam (Impatiens glandulifera) was brought to the British Isles in 1839 as an addition to Kew Gardens’ collection of ornamental plants. As usually happens with introduced species, Himalayan balsam escaped into the wild, causing consternation ever since. It has spread throughout damp woodlands and along rivers, flourishing in thick stands up to 2 metres high that overshadow the local vegetation. This plant does well in a variety of climatic conditions and soil types, and has a tremendous capacity to spread.

So nobody likes Himalayan balsam. Nobody but pollinators.

Himalayan balsam © MurielBendel, Wikipedia Creative Commons
Himalayan balsam © MurielBendel, Wikipedia Creative Commons

This invasive is a nectar factory. Each flower produces about 0.5 mg of sugar per hour, a rate far higher than any European plant; flowers of most species yield less than 0.1 mg/h. And because the plant flowers late in the season, nectar it available at a time when other sources start to become scarce. So naturally, bumble bees, honey bees and wasps go for it with gusto. And there is something in store for hoverflies as well; they feed on the copious amounts of pollen produced by these flowers. Predictably, the number of bumble bees and other insects increase in areas invaded by Himalayan balsam. 

This abundance of food could have undesirable side effects. Many bees get the proteins, carbohydrates, lipids and amino acids they need from a variety of pollen sources. But thanks to the plentiful and readily available pollen from Himalayan balsam, bees stick to this easy option: in some situations, up to 90% of the pollen collected by honey bees comes from this plant, with unknown consequences to bees’ development and health. The profusion of pollen and nectar could also indirectly harm other plants: if native species receive fewer visitors, their pollination could be compromised. But the evidence for such outcomes is contradictory. Some studies suggest that Himalayan balsam reduces flower visitation and seed production of native plants; others have demonstrated no differences, or a ‘magnet effect’: Himalayan balsam attracted pollinators to itself and to plants nearby. 

A marmalade hoverfly (Episyrphus balteatus) and a common carder bee (Bombus pascuorum), two Himalayan balsam beneficiaries © Charles James Sharp (L) and André Karwath, Wikipedia Creative Commons
A marmalade hoverfly (Episyrphus balteatus) and a common carder bee (Bombus pascuorum), two Himalayan balsam beneficiaries © Charles James Sharp (L) and André Karwath, Wikipedia Creative Commons

Alien species are a hot and controversial topic among conservationists. Some highlight the damage caused by introduced species to the native fauna and flora, habitats, the economy and even human health. But other conservationists point out that alien species may have neutral or positive impacts: that is, they are alien but not necessarily invasive. The invasiveness of Himalayan balsam has been well documented, but there are mitigating factors in its favour: in some situations, this plant had no effect on local species composition, or at worst it only replaced a few ruderal species (plants that colonise areas that have been disturbed). And its presence may check the spread of harmful alternatives such as the giant hogweed (Heracleum mantegazzianum).

Assessing the impact of alien species is important because a great deal of money and resources have been spent on controlling or eradicating them, quite often unsuccessfully. It is usually assumed that invasive plants are bad for pollinators, but there isn’t much evidence to support this assumption. Like many aspects of species’ ecology, data are scarce, results are often contradictory, and generalisations are risky. In summary: it’s complicated.

Nectar network

The Irvine to Girvan nectar network boasts many projects. One interesting task centres around Belleisle and Seafield golf courses, particularly on making the rough more pollinator-friendly.  The courses have been surveyed recently by the Scottish Wildlife Trust working in partnership with South Ayrshire Council’s Golf Service, and the findings make for an interesting read.

Extensive scarification work was undertaken on the roughs in September 2018 .  Many of these areas were sown with wildflower in October 2018. With any project it is always a telling experience to revisit the site to see what has worked and what has struggled. And hence the golf course areas which were sown in 2018 were revisited in July 2019 to look for successes and challenges.

Red clover

Red clover

The surveys at the two golf courses was to prove valuable and given that there was also an intention to note the presence of any pollinating insects there would be a good insight gained into habitat and species.

Yellow rattle

Yellow rattle

At Seafield golf course several patches were inspected, and 50 plant species were noted in 9 distinct sites. It was interesting to note that of 17 species which were present in the seed mix that was used, 12 occurred on the ‘inspection visit’ and three of the 12 were widely evident (pignut, wild carrot and yellow rattle).  Red clover, bird’s-foot trefoil and lesser knapweed. fared relatively well but there was no sign of the devil’s-bit scabious or vetches that had been planted.

A few items which were flourishing at Seafield hadn’t actually formed part of the mix – including white clover, yarrow and harebell.  12 species of pollinators were noted whilst visiting Seafield, including three species of butterfly, three of bee and two types of hoverfly.  The digger wasp was one of the star finds.

Digger wasp

A digger wasp

When the team moved on to Belleisle they found rough that was more densely vegetated, and isolating the ‘sown’ areas was a deal trickier. Again three species in the mix that was sown thrived — lesser knapweed, yellow rattle and pignut. Five elements know to have been sown did not show, including red campion, devil’s-bit scabious and common vetch. Lesser stitchwort, which did not form part of the mix, had done well as had ribwort plantain and creeping buttercup. Four different species of butterfly and 3 kinds of bee were noted during the survey – others may have been present.

One of the Seafield plots that was analysed

One of the Seafield plots that was analysed

The surveys at the two golf courses were useful for gaining a picture on the success, or otherwise, of the mix sown. There was a sense that it might have been more economic, and more productive, to have narrowed the list in the mix. Given that plenty of non-sown items were evident it might be that simply sticking to the highly beneficial yellow rattle would be a good approach, particularly on light, well-drained soils.

The team have also concluded that more scarification would be a good thing — allowing the build up of grass to be cleared and helping move to a one-cut per year scenario. But overall their findings are fascinating and help prove that you never quite know beforehand what will thrive and what will disappear when sowing seeds.

Further reading:

Fnd out more about the Irvine to Girvan nectar network on the Scottish Wildlife Trust website.