Fieldwork fun: Eristalis tenax and pollinator diversity

The field season is on in earnest now. Yesterday I was surveying blooming plant species at my field site, taking photos of the blooming plants as informal vouchers for now (vouchers = samples to prove that I correctly ID'd the plant, often collected specimens deposited at an herbarium in my field), and I managed to snap this awesome shot:

Eristalis tenax on Achillea millefolium

This guy is rather interesting, and not just because close-up shots of insects are cool by default.

I am pretty sure this is Eristalis tenax (a.k.a drone fly), and positive that it is a syrphid fly (a.k.a. hoverflies or bee flies). Syrphid flies are a group of flies which are bee or wasp mimics, meaning that they have characteristics resembling those of bees or wasps, which in theory is an antipredator adaptation conferring the advantages of the mimicked species against particular predators. E. tenax, our awesome, rather big (13-15mm wingspan [1]) syrphid fly is native to Eurasia [1] and was introduced to North America [1] before 1874 [1]. It is now widespread in North America [1,2].

The larval stage of this species is rather unappealing (called a rat-tailed maggot) and can pose problems particularly at agricultural sites, where they can become overabundant in ponds and livestock areas [3]. There have been cases of accidental ingestion of the eggs/larvae and subsequent myiasis (infestation by flies) of humans, causing unpleasant illness etc., but apparently the myiasis is treatable [3].

E. tenax is a pollinator, as adults feed on nectar [3] and so can be pollen vectors. indeed, although we tend to think of bees when we talk about pollinators, there are many other types of pollinator: flies, syrphid flies, butterflies, moths, skippers, wasps, birds, bats... There is even one documented case (research article) I am aware of where a lizard was demonstrated to be a pollinator!

I went digging around in the literature about E. tenax and found a study which compared the efficiency (transfer of pollen per visit) and effectiveness (number of visits per unit time) of E. tenax (and several other non-managed species) at pollination of Brassica rapa var. chinensis (pak choi) [link to open-access article]. The researchers conclude that E. tenax is equally effective and efficient as A. mellifera (European honeybee, a managed pollinator of considerable economic importance which is used extensively globally as a crop pollinator) on an individual basis as a pollinator, but due to much lower numbers of individuals in the populations of this and other alternative pollinators, A. mellifera remained the most important effective pollinator.

Just for fun, here's another syrphid fly I've photographed before. I think it's Toxomerus marginatus, but I'm not positive on the I.D. I am fairly sure it's at least in the genus Toxomerus, but I may be wrong about the species.

Toxomerus marginatus (?) on Rudbeckia hirta

Syrphid Flies - Syrphidae

I have mentioned syrphid flies before on my blog, usually when mentioning the pollinators of a given plant. But what's a syrphid fly?

"Syrphid fly" refers to an entire family of insects, the Syrphidae, which are bee and wasp mimics. They resemble bees or wasps, which confers some of the natural defenses and safety that would be accorded those more dangerous species that they emulate.

Many syrphid flies are nectar and pollen eaters, and so will be attracted to flowers and provide pollination services. Many of these species are not nectar-robbers but pollinators, though we don't necessarily think first of flies when we think of pollinators.

This family is quite large and diverse. I am not, for the most part, able to identify particular species of syrphid fly. Sometimes it is obvious that two individuals belong to different species; other times, it takes close observation.

There were quite a lot of syrphid flies visiting the flowers of the insect gardens in the Montreal Botanical Gardens. They were mostly of the same size and shape, so probably the same genus. It wasn't until later when I was looking at my photographs that I realized that there were multiple species. Take this one here:

Syrphid fly

 This one has dark eyes, and a stripey abdomen which transitions from yellow to red at the tip.

Syrphid fly

This second one (I posted this photo yesterday for the discussion of Rudbeckia hirta), on the other hand, has red eyes, and a yellow patterned abdomen.

Likely these are two different species (I say likely because I am not certain of this; it is theoretically possible that there might be extreme polymorphism in a single species, but unlikely).

As I was looking through the photos another thing jumped out at me. In that first photo, the syrphid has a strange, bulgy yellow protuberance between its eyes. I wondered what it was: mouth parts? A sensory organ of some sort?

So I trotted down to reddit and asked the helpful entomologists on /r/whatsthisbug about it. I was told that that yellow protuberance is actually the antennae, which are frequently heavily modified in syrphid flies. So. Syrphids can have very strange-looking antennae indeed.