Floral attraction in Glen Affric

July is one of my favourite months of the year, as it is the peak time for a lot of activity in nature – the blossoming of  many summer flowers, their pollination by a host of insects and the dispersal of young birds and mammals from their parents, to name some of the main events. In the middle of the month I spent a day in Glen Affric, visiting various areas of forest there, but the highlight of this trip was the time I spent with a few hogweed plants (Heracleum sphondylium) that were flowering beside the road, between Badger Falls and Dog Falls.

They caught my eye as I was passing by, so I stopped to have a look at them and was rewarded almost immediately with the sight of one of our largest beetles, the black-spotted longhorn beetle (Rhagium mordax), feeding on the flowers of one of the plants. Reaching 23 mm in length, this is a readily-distinguished species that commonly feeds on flowering plants in the umbellifer family, such as hogweed. It was moving slowly across the blossoms, feeding on nectar and pollen.

It is quite a hairy beetle, and this facilitates its role as a pollinator, as pollen that gets stuck on the hairs is transferred from one plant to the next when it moves. This species is widely distributed throughout the UK, and the female lays eggs in the rotting wood of trees, where the larvae feed and develop. Adults emerge in April or May each year and can be seen until July. The common name of longhorn for this group of beetles is derived from the relatively long antennae that they have on their heads.

While I was looking at this hogweed plant a smaller, reddish-brown beetle caught my eye, as it stood out conspicuously against the creamy-white flowers.

 

 

 

 

 

 

 

I didn’t recognise this beetle so I collected the specimen and sent it to Sholto Holdsworth, who helps me with beetle identifications, and he confirmed it was a species called Cychramus luteus. This is apparently quite a common beetle in Europe, and the adults feed on pollen, so it was no surprise to have seen it on the hogweed. After a couple of minutes it moved alongside the longhorn beetle, and I was able to take this photograph of the two together, which shows the remarkable variation in size between them.

As I continued to look at this cluster of hogweed flowers, I noticed that on one of the umbels (as the umbrella-shaped radial pattern of flowers on an individual stem are called) there were a number of small, narrow-bodied black beetles. These all looked like they were same species and Sholto was later able to confirm their identify as being a species of false flower beetle (Anaspis rufilabris). This is another common beetle, and like the longhorn, its larvae develop in dead wood and the adults feed on the flowers of umbellifers.

They were crawling all over the blossoms on several of the hogweed umbels and by waiting patiently I was able to get a few photographs of individual beetles feeding on some of the flowers.

 

 

 

 

 

 

Because of their small size, these beetles have a large area to clamber over on each flower head of the hogweed plants. While the floral topography may seem simple to us humans, for a tiny beetle it is complex and highly fractal in nature, with a huge surface area on every umbel. While I was looking at these beetles, I saw a different insect on one of the blossoms. This was a small moth with some distinctive pale orange spots on its wings, and was a species I’d not seen before. However, by consulting later that day with Roy Leverton, a specialist in moths, I learned that it was the orange-spot piercer (Pammene aurana).

This is apparently a common species in the UK and hogweed is its food plant. The larvae sew the seeds of hogweed together, to provide protection from predators and then feed on the seeds, out of sight.

 

 

 

 

 

 

I knew from previous experience that hogweed umbels often have aphids feeding on them, so I began looking at the top of the main stems, where the individual flower stems radiate out in a rosette pattern, as that’s where they usually occur. It didn’t take long to find some, as they were on the third or fourth plant that I looked at. I knew these were aphids in the genus Cavariella, and when I sent some of my photos to Bob Dransfield, an aphid specialist, he replied that there were two species feeding together – Cavariella theobaldi and Cavariella pastinacae.

 

 

 

 

 

 

 

It’s not easy to tell these two species apart, especially from photographs rather than actual specimens, so Bob cautioned that his identifications were not absolutely certain.

However, for some, like the individual aphids shown here on the right, he was able to be more specific. These are the willow-parsnip aphid (Cavariella theobaldi), and the common name is derived from the fact that the species alternates between two different hosts – willows and then umbellifers such as hogweed and wild parsnip (Pastinaca sativa). The other closely related species is called the willow-umbellifer aphid (Cavariella pastinacae), and it also alternates between the same two groups of host plants – willows and umbellifers.

As I continued to look closely at the aphid colony I spotted a winged adult on one of the hogweed stems, and then made another particularly pleasing discovery – I saw one of the willow-parsnip aphids (Cavariella theobaldi) giving birth!

 

 

 

 

Aphids are different to most insects in that the females can give birth to live young at a certain stage of their life cycle – usually in the spring and early summer. Later in the year, a different generation of female aphids lay the eggs that will overwinter and give rise to new aphids the next spring. I’d seen aphids giving birth before, but it’s not easy to take good photographs of this phenomenon, so I was especially pleased to get this image now.

Unlike the beetles and moth I’d photographed earlier, the aphids are not attracted to the hogweed directly by the flowers themselves. As sap-sucking insects, they go to parts of plants where the sap is flowing, and in the case of a plant in flower like this hogweed, the sap is necessary to sustain the growth and development of the flowers, and their production of nectar and pollen. So, the aphids were drawn to the hogweed by the flowers as well, but in a more indirect fashion.

I realised that I too had experienced the floral attraction of the hogweed, although in my case it was obviously not because I wanted to feed on the nectar or pollen, but rather by the knowledge that there were likely to be a variety of insects on the plants. This had been borne out quite spectacularly, and I finished the day feeling very well satisfied by the couple of hours I’d spent looking at all the insect activity on the hogweed umbels.