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Who’s really pollen their weight: Bees, or Flies?

Biology, Diptera, Environment, Publication 5 Responses »

Apr 222015

March flies (Bibionidae) pollinating both flowers and each other.

March flies (Bibionidae; Bibio albipennis) pollinating both flowers and one another.

When it comes to pollination ecology research, bees are their own knees. Along with butterflies, birds, and bats, bees reign supreme as the queens of pollinator studies, with huge amounts of money and time spent each year trying to understand everything about their biology, from how they choose which flowers to visit, to the structure of their societies, and of course, why some species seem to be in decline. While some flies (like flower flies — family Syrphidae) are beginning to break into the hive of pollination research, bees so dominate the pollination ecology landscape that suggesting alternative groups, like other flies, may also be important pollinators can result in quizzical looks, derisive scoffs, and even disbelief at results that run counter to popular thinking.

The latter is exactly what happened when Dr. Katy Orford submitted a paper from her PhD that showed flies play a major role in grasslands pollination; the editor rejected it due to a lack of literature supporting her Dipterous conclusions. So, Orford set out to do what no one had done to this point: show beyond a shadow of a doubt that flies are important, and overlooked, pollinators.

Crane fly hanging out among the flowers.

Crane fly (Tipulidae) hanging out among the flowers.

Orford began by gathering and assembling previously published datasets that looked at the connections between pollinators and plants across the UK, specifically datasets that looked at plant-pollinator-visitation networks (what insects visit which plants based on observations) and pollen-transport networks (how many grains of each kind of pollen was found on each insect’s body). Orford immediately found that few studies had actually looked at these metrics for entire insect communities rather than just targeted groups like bees, but she ended up with a dataset spanning both natural and agricultural ecosystems that included over 9,000 insect specimens, 520 pollinator species, and 261 species of plants.

With her dataset in hand, Orford had four questions she wanted answered: how specialized are flies with regards to the plants they pollinate; how prevalent are dipteran pollinators in agriculture and how much pollen are they carrying; and most importantly, how do flies stack up against bees, butterflies, and beetles when it comes to transporting pollen?

Flies, it turns out, aren’t overly picky about what flowers they’ll visit and feed from. While flower flies visited a broader spectrum of the floral smorgasbord available in the study plots, they were found to be no better at transporting specific pollen species than the other fly families. This isn’t to say that there aren’t any specialized relationships between plants and flies (cacao and biting midges in the genus Forcipomyia being the most famous example of flowers and flies being in league with one another, much to our enjoyment), only that in the particular environments Orford examined she found no evidence for specialization among the residents.

When Orford looked at the composition of fly visitors on farms, non-syrphids were not only more speciose than their flower fly cousins, averaging 7 species to 3, respectively, but they also outnumbered them 4 to 1 in the sheer number of individuals. In fact, Orford found that only 3 farms out of the 33 she had data for reported more flower flies than other flies. Not only were non-syrphids more diverse and more abundant, but they also carried more than twice the number of pollen grains on their bodies as flower flies did in agricultural fields. All of this suggests that the role of syrphids in pollination ecology, a topic that has received at least some study at this time, may only be the tip of the iceberg when considering the importance of flies in agricultural pollination.

Urophora affinis (Tephritidae)

This is all well and good when deciding which flies are better pollen bearers among themselves, but how do they stack up against the rest of the competition? Do bees really pull their weight in the great pollen wars, or have flies been shouldering the load without us realizing it?

Unsurprisingly, bees are really good at carrying pollen. Not counting the pollen trapped in their specialized storage structures (like the corbicula of Apis mellifera, or the scopa of Megachilidae leaf-cutter bees), Hymenoptera still beat out all the other insect groups when the number of pollen grains on each individual was counted, while flies, butterflies and beetles were all found to be roughly equal in their carrying capacity. This result shouldn’t really come as a surprise, as bees have specialized branched hairs all over their bodies that have evolved to efficiently trap pollen, which is then combed out of the hairs and into their pollen storage structures. So while flies are usually pretty hairy, they’re essentially catching pollen with a comb, rather than the hair net that bees are employing.

But, while each individual bee may carry more pollen than each individual fly, Diptera are much more abundant, at least in agricultural settings. In fact, Orford found that two-thirds of all pollinating insects recorded in her agricultural datasets were flies. That means that when we talk about agricultural pollination ecology, which is predominantly focused on bees currently, we’re a long ways from seeing the complete picture.

Perhaps Wired’s editors were on to something here. If it looks like a bee, and carries pollen like a bee…

There was one other thing that Dr. Orford discovered, however. When she broke down her pollen-load data beyond just Hymenoptera and Diptera, and started looking at the pollen loads of bees and flies on a finer taxonomic scale, she found that, statistically speaking, flower flies carry just as much pollen on their bodies as European honey bees.

Does this mean flower flies are as effective pollinators as honey bees? It’s too early to say; honey bees may be better at transferring pollen from flower to flower and causing flowers to develop seeds; or they might not be. More research into the pollination efficiency of flies is clearly needed, but the potential implications of this pollen equality are staggering. Orford’s data shows that on farms, flower flies make up about 16% of all flower-visiting insects, while bees, butterflies and beetles together combine to make up only 33% of visitors. It’s very possible that we’ve been attributing a little too much success to those “busy” little bees.

Orford’s work presents another fly in the ointment, so to speak: if bee populations, including honey bees, are indeed declining as has been suggested by several recent papers and hyped by the media and special-interest groups like beekeeping societies, what’s happening with flies? Are they experiencing similar declines as social bees, or are they shielded from the effects of human-trafficked diseases and parasites, along with pesticide accumulation in hives by their solitary and undomesticated lifestyle? Are monocultural agriculture practices and denuded, degraded, and destroyed natural habitats reducing fly diversity in the same way that other pollinators appear to be experiencing? We just don’t know at this point.

And while bees become an increasingly popular talking point and agenda item for politicians, Diptera remain undiscussed. US President Barack Obama in particular has become a champion for bees, with a pollinator garden and bee hotels supposedly being built on the grounds of the White House. Why not monitor and speak up for all of the pollinators, two-winged or four, in President Obama’s backyard as Dr. Orford did?

Geron sp. (Bombyliidae)

Well, as she notes in the conclusions of her work, flies aren’t as easy to study as bees are. For one, flies don’t return to a predictable location such as a hive or nest like bees do, which makes observing and experimenting with them considerably more difficult. The other major issue, of course, is taxonomy. There are more than 6 times as many species of fly currently known than there are bees, and those flies are notoriously difficult to identify, even to the proper family in some instances, never mind trying to determine genus or species. While the flower flies have received a great deal of taxonomic attention in the past 50 years, and are generally more easily identified than most groups of flies, the same is not true for the top non-syrphid pollen carriers identified by Dr. Orford: Bombyliidae, Muscidae, and Calliphoridae, all of which pose significant identification and/or taxonomic challenges at the moment.

The solution? From Dr. Orford: “training in dipteran taxonomy should be more available to ecologists. Alternatively, specialist taxonomists should be included in research projects to prevent pollination biologists being deterred from recording Diptera due to identification difficulties”.

I couldn’t agree more.

Dipterists around the world are working hard to make the flies they’ve devoted their careers to more accessible, both through the publication of identification resources, and through the organization of workshops and other educational events. However, as has been shown by Dr. Orford’s work, we should expect a growing demand for keys and other identification tools, along with the people who create them, to usher in a new era of pollination ecology; an era defined by a greater understanding of pollinators of every ilk through collaboration and communication between Diptera taxonomists and pollination ecologists.

As for Dr. Orford, since successfully defending her PhD last fall, she’s taken a position working with government policy in the UK, providing an important voice for flies alongside those advocating for more “traditional” pollinators. As for her paper on grasslands pollination, whose initial rejection inspired this long-overdue look into the flowery lives of flies, now that she’s shown the pollination hivemind the importance of Diptera, she hopes her work will fly through the peer-review process.

Toxomerus marginatus (Syrphidae)

Orford K.A. & J. Memmott (2015). The forgotten flies: the importance of non-syrphid Diptera as pollinators, Proceedings of the Royal Society B: Biological Sciences, 282 (1805) 20142934-20142934. DOI: http://dx.doi.org/10.1098/rspb.2014.2934

The Fly Who Came In From The Cold

Biology, Cool Science, Insects 3 Responses »

Jan 072014

The extreme cold snap encompassing a large portion of continental North America (termed a Polar Vortex, which you can learn more about via NPR and Quartz) has made it dangerous to remain outside for long, even when bundled up in more layers than a Thanksgiving turducken. While we can rely on our technological ingenuity to find solutions to this chilling problem, what about our insect neighbours who have been left out in the cold?

Eurosta solidaginis has a warning for you.

Most insects seek shelter in the fall before temperatures begin to dip, either laying their eggs in sheltered locations, or hiding out as larvae, pupae or adults in the comparative warmths of the leaf litter, deep within trees, or even taking advantage of our warm hospitality and rooming with us in the nooks & crannies of our homes. But what about species like the Goldenrod Gall Fly (Eurosta solidaginis) which are literally left hanging out in the middle of nowhere and completely at the mercy of Jack Frost?

Polar Vortex vs. Goldenrod Gall Fly. Polar Vortex map courtesy of RightWeather.com, Eurosta solidaginis range map from Foote et al. 1993

If you live in eastern North America, you’re probably familiar with the Goldenrod Gall Fly, even if you don’t realize it. This fruit fly — the ripe fruit kind (family Tephritidae), not the rotting banana kind (family Drosophilidae) — is one of the more ubiquitous insects, and is found pretty well anywhere goldenrod grows, including in urban environments like parks & abandoned lots. Adults are weak fliers and aren’t often seen unless you’re actively looking for them, but in this case, it’s the larvae that you’ve likely seen a hundred times — rather, you’ve likely seen their makeshift homes a hundred times. The larvae of this species live within the stem of goldenrod plants (Solidago spp.), and trick the plant into growing a big spherical nursery for the fly maggot to live & feed in (technically called a ‘gall’), and which stands out like the New Year’s Eve ball in Times Square, albeit without the mirrors and spotlights of course.

Goldenrod Gall Fly galls in Guelph, Ontario

While these galls provide a modicum of protection from predators and parasitoids (although some still find a way), they don’t provide much, if any, insulation from the elements, meaning that the larvae must be able to survive the same air and windchill temperatures that we do. To do so, Goldenrod Gall Fly larvae are not only able to safely freeze without their cells being torn apart by tiny ice daggers by partially drying themselves out, but they also change the temperature their tissues freeze at by manufacturing anti-freeze-like chemicals. Together, these cold-tolerance strategies allow the maggots to survive temperatures as low as -50°C (-58°F)! Just take a moment to consider what it would feel like to stand outside almost anywhere in central North America on a day like today wrapped in only a few layers of tissue paper; BRRRRRRR!

All that stands between a Goldenrod Gall Fly maggot & the extreme cold is a few centimeters of dried plant tissue. (The maggot is the little ball of goo in the bottom half of the gall)

For us, the multiple warm layers of clothing we bundle up in on days like today allow us to survive and eventually have children, thus passing our genes along, despite living in a habitat that is occasionally unfit for human life. It would stand to reason then that other organisms would also enjoy the same benefits and evolutionary advantage from thermal insulation, but, for the Goldenrod Gall Fly at least, the complete opposite is true! Goldenrod isn’t exactly the most robust structure, and it doesn’t take much effort from the wind, passing animals like people or dogs, or other not-so-freak phenomena to knock goldenrod stems over, allowing galls to be buried in snow and protected from the harshest temperatures (snow is an excellent insulator, and temperatures in the snowbank generally hover around 0°C (32°F)). This would intuitively seem like a good place to be if you were fly maggot, out of the daily temperature fluctuations and extreme cold and in a more stable environment. However it turns out that individuals that mature in galls on the ground and covered with snow are at a significant disadvantage evolutionarily speaking, with grounded females producing 18% fewer eggs than females who grew up fully exposed to the elements (Irwin & Lee, 2003)!

This Goldenrod Gall Fly, while warm(er), will likely produce fewer offspring when it emerges (assuming it’s a female).

Why might that be? Well, let’s think about it for a moment. If you’re a fly maggot hanging out above the snow when it’s -20°C, you’re likely going to be frozen solid and in a cold-induced stasis, not doing much of anything, even at the cellular level. But, if you’re as snug as a ‘bug’ under the snow at ~0°C, your body won’t be frozen, and thus you’ll be forced to carry on with day-to-day maintenance & cellular functions like breathing, waste removal, etc, even if only minimally. When you live in a closed system like a hollowed-out stem gall on a dead plant without any food, any energy you spend on daily functions as a “teenager” putting in time under the snow all winter long means you’ll have less energy you can put towards making eggs as an adult. If you’re a Goldenrod Gall Fly maggot, it pays to be left out in the cold!

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Foote, R.H, Blanc, F.L., Norrbom, A.L. (1993). Handbook of the Fruit Flies (Diptera: Tephritidae) of America North of Mexico. Comstock Publishing Associates, Ithaca NY. 571pp.

Irwin J.T. & Lee, Jr R.E. (2003). Cold winter microenvironments conserve energy and improve overwintering survival and potential fecundity of the goldenrod gall fly, Eurosta solidaginis, Oikos, 100 (1) 71-78. DOI: 10.1034/j.1600-0706.2003.11738.x

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Some additional thoughts: You’d think that a nearly 20% difference in egg production would create significant evolutionary pressure for Goldenrod Gall Fly females to select the strongest, least-likely-to-break-and-fall-over goldenrod stems. It’s possible that the randomness of goldenrod stem breakage negates any evolution of host plant selection, but I would tend to doubt it. I did a quick Google Scholar search to check whether anyone had examined this in greater detail, but I didn’t see anything. Perhaps an avenue of future study for an evolutionary biology lab out there?

How do Cicadas Keep Track of Time?

Biology, Cool Science, Entomology, Pop Culture 5 Responses »

May 072013

The east coast is about to get a little more crowded, and whole lot louder, as Brood II of the 17-year cicada (which is actually a synchronized cohort of three different species: Magicicada septendecim, Magicicada cassini, & Magicicada septendecula) prepares to make its first appearance since 1996.

Conceived, laid and hatched while the Macarena was sweeping the globe, Brood II has since been biding it’s time underground in nymphal form, feeding off sap stolen from the roots of trees and counting down the years until it was time to make their grand appearance. But how DO they count down the years? 17 years is an incredibly long time, especially when you live more than a foot underground, insulated from traditional stimuli like photoperiod and temperature.

Richard Karban, who wrote that he’s dreamed of tricking periodical cicadas into emerging early for most of his adult life, had an idea, and designed an elegant experiment to see if he could confuse his cicadas by accelerating the life cycle of the trees they were dependent on.

Rather than making a poor graduate student sit and wait 17 years for a cicada to emerge, Karban dug up and transplanted 15-year old Brood V nymphs from Pennsylvania onto potted peach trees in his University of California, Davis lab, a difficult procedure that involves potatoes and a cross-country road trip with some unusual company, and which had failed the 3 previous times it was attempted. This time however, Karban successfully managed to transplant 13 nymphs, with 11 surviving on his accelerated-cycle trees which underwent 2 flowering cycles per year (bud-> leaf-> flower-> leaf drop-> dormancy-> bud-> leaf-> flower-> fruit-> leaf drop), and 2 surviving on his control trees which only underwent a single cycle per year (bud-> leaf-> flower-> fruit-> leaf drop-> dormancy).

Back in the wilds of Pennsylvania and on the control trees, Brood V adults were expected to emerge in the spring of 1999, which is exactly what they did. However, the ones who were feeding on the accelerated-cycle trees got the party started a full year early, with 8 of the 11 individuals emerging right when Karban hypothesized they would: spring 1998!

Karban realized his dream, having successfully fooled a few periodical cicadas into emerging early, and in the process showed that cicadas are able to count the seasonal cycles (or phenology) of their host trees to keep track of time rather than relying on other direct stimuli. The exact mechanism by which cicadas keep track of how many cycles have passed is still not well understood, although it’s probably safe to assume that the cyclic availability of tree sap & nutrients influences the development of the nymphs in some way. The fact that there are still such large pieces of the phenomenon still waiting to be understood is just as exciting as the prospect of millions of brightly coloured bugs emerging en masse to serenade you this summer.

So, if you happen to find yourself on the East Coast in the coming weeks, stop and take the opportunity to listen to a symphony 17 years in the making. And if you notice a subtle-but-catchy Latin beat to the buzz of periodical cicadas, just be glad it’ll only last a couple of weeks; those poor cicadas have been humming the Macarena to themselves for the past 17 years!

Photograph by C. Simon. doi:10.1371/journal.pone.0000892.g003. Creative Commons Attribution 2.5 License.

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Karban R., Black C.A. & Weinbaum S.A. (2000). How 17-year cicadas keep track of time, Ecology Letters, 3 (4) 253-256. DOI: 10.1046/j.1461-0248.2000.00164.x

Fly Cankles and Literary Evolution

Cool Science, Entomology, Publication 7 Responses »

Jan 272011

Flies are cool. You might even say they’re phat (I know, I’m pretty fly for a white guy). But a newly described dance fly species (Empididae) from Japan takes the term “phat” to an all new level. Meet Empis jaschhoforum:

Empis jaschhoforum male morphs from Daugeron et al. 2011

Figures 1a-c from Daugeron et al., 2011