The Life of a Single Mom (Bee)

Most of what we read in the news about declines in bee populations focus on (non-native) honey bees.  Yes, those populations are suffering declines from the combined impacts diseases, habitat loss, pesticide use and other factors.  However, there are nearly 4,000 bee species in North America, and many of them are dealing with the same pressures and threats as honey bees.  In addition, honey bees are social insects, living in large collaborative colonies of workers and queens.  The vast majority of bees in North America, however, are not social, and they succeed or fail on the backs of single moms.

Solitary bee

At first glance, some solitary bees might appear similar to honey bees (I’ve certainly been guilty of making that mistake many times) but while their appearance might be somewhat similar, their life stories are very different.  This one is a solitary bee in the genus Svastra.  At least I think it is…

Solitary bees – bees that don’t live in colonies – are all around us, but they go largely unnoticed.  Many escape our attention because of their small size, but others are as big as or bigger than honey bees.  Solitary bees can vary greatly in their diet preferences.  Some are generalists, feeding on nectar and pollen from a wide variety of flower species.  Others have much more narrow diets, feeding only from sunflowers, for example, or other categories of flowering plants.

Most solitary bees in prairies live in underground burrows, though others live in hollow plant stems or similar spaces.  In colonies of social bees, the work of gathering food, maintaining and defending the home, and feeding and caring for the kids is split between hundreds or thousands of bees.  In the case of solitary bees, the single mom does everything.  In most cases, she finds a likely spot, digs a burrow and prepares it for eggs.  Then, she flies around the neighborhood in search of the kinds of flowers she can collect food from.  As she nears the flowers, she’s likely to encounter males of her species, who basically spend their entire lives buzzing from flower to flower, hoping to find females to mate with.

Male

A male solitary bee (Dufourea sp.) waits for females at a sunflower.

Assuming the single mom can find food nearby, she returns from foraging with a load of pollen and nectar, which she combines into a ball of sticky dough.  She places that in a cell within her burrow, lays an egg on or next to it, and seals up the cell.  Then, she takes off to repeat the process: find food, mix it together, lay an egg with it, seal up the cell.  Later, the eggs will hatch, and the larvae will stay in their cells and feed on the dough balls provided for them until they grow into adults and leave the nest.

As you might imagine, life isn’t easy for single mom bees.  They have to gather food for themselves and their kids, while fighting off overly-enthusiastic males with only one thing on their minds.  When they aren’t out finding food, they are building and provisioning baby rooms or sitting vigilantly at the entrance of the burrow, defending it from marauding wasps or other threats.  After mother bees have filled their burrow with eggs-in-cells, they seal up the whole nest and fly away, hoping for the best.

Single mom solitary bees have difficult lives, but there are ways we can help them.  First, we can help ensure the availability of nesting sites.  Some ground-nesting bees need areas of bare ground, and many others need at least access to the soil without having to fight through a dense layer of plant litter.  Similarly, stem nesters would appreciate it if you didn’t chop down all of last year’s plant skeletons, especially those of raspberry, sunflower, rose, leadplant, and other plants with hollow stems.  Providing this kind of nesting habitat is important in prairies and other natural areas, but also in backyard gardens and other urban areas.  Because solitary bees aren’t aggressive toward humans, there’s no downside to sharing your yard or garden with them (and, as pollinators, they’ll work for their housing).

A "long-horned bee" (Melissodes sp) on dotted gayfeather (Liatris punctata).

A “long-horned bee” (Melissodes sp.) on dotted gayfeather (Liatris punctata).

Perhaps more importantly than housing, what bees need most is food.  The key to supporting strong bee communities is plant diversity.  A prairie or garden with lots of different kinds of flowers will support lots of different kinds of bees.  Specialist bees will be able to find the particular flowers they need, and generalist bees won’t run out of food when one kind of flower stops blooming, gets eaten by insects, or is wiped out by disease.  Early spring can be a particularly difficult time for bees to find food because of the relative scarcity of flowers at that time of year.  Boosting the spring-time abundance of both native wildflowers and flowering shrubs in gardens and natural areas can be very helpful.

In prairies and other large-scale habitats, it’s important to think about the flight range of bees.  Honey bees can travel up to several miles to find food.  Most solitary bees are considerably smaller, however, and they may be limited to a range of a few hundred yards or less from their nest.  During their nesting season, bees will need to find everything they need to survive and supply their nests from that relatively small circle of habitat.  The availability of abundant flowers of many kinds within that circle helps ensure that bees can find food throughout the season.  If a large area surrounding a bee’s nest is mowed or grazed intensively, it is left stranded with a nest in the middle of a food desert.

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This tiny sweat bee fits perfectly into this puccoon flower (Lithospermum carolinense).  Its size gives it access to flowers larger bees (including honey bees) can’t get into, but that size also limits its ability to forage far from its nest.

If you’re a landowner or land manager, think about your property from the perspective of a single mom bee.  Pick a few spots on your land and visit them every few weeks to see what the abundance and diversity of flowers looks like.  If a bee was nesting where you stand, could she find what she needs for food within a short distance of that location?  Are there times of year when it’s hard to find abundant flowers?  If so, can you tweak your management or implement restoration strategies to make more flowers available?  Are there places where bees can find bare soil for nesting, or is there a layer of thatch covering the soil across your whole site?  Burning, intensively grazing, or haying portions of your land each year can help reduce thatchiness and help ensure bees’ access to soil.  However, creating patches of prairie habitat representing a full spectrum of vegetation structure types (tall/dense, short/sparse, mixed-height, etc.) will be of maximum benefit to both bees and other insect and wildlife species.

Single mom bees deserve our respect and admiration.  They build and prepare their nest, seek out and harvest food while dodging predators and lustful males, and provision their eggs with food and a safe place to grow up.  Oh, and along the way, they also pollinate and help ensure the survival of the majority of plants on earth.  It seems only fair that we should acknowledge their work and do what we can to help them out.

 

More information:

While the vast majority of native bees are solitary bees, some are social as well, including bumble bees, some sweat bees, and others.  Bumble bees, in particular, are very important pollinators because of their size and mobility as well as their willingness to visit many different kinds of flowers.  As opposed to honey bees, whose colonies can survive the winter intact, all bumblebee individuals except fertilized queens die at the end of the growing season.  Those fertilized queens overwinter and then become single moms in the spring.  Once the queen’s first brood matures, those bees take over the foraging work and take care of the queen.  You can learn much more about solitary bees and other native bees here.

Many thanks to Mike Arduser and Jennifer Hopwood for reviewing this post for accuracy.  Any remaining errors are mine, not theirs.

Regal Fritillary Butterflies in the Platte River Prairies – 2011

We completed our second season of regal fritillary butterfly data collection this summer.  Funded by State Wildlife Grant funds from the Nebraska Game and Parks Commission, our surveys are designed to help us better understand how this butterfly species is responding to our prairie restoration and management work.  The species is doing very well in Nebraska, and the central Platte River valley has some of the highest concentrations of regals in the country, but the species is of conservation concern because of it’s extreme rarity elsewhere in its historic range.  In addition to informing our work, we hope that studying the species where it’s common can help inform conservation efforts in those places where it’s rare.

Some people may be surprised to hear that regal fritillary butterfly populations are thriving in the central Platte River valley – a landscape dominated mainly by rowcrop agriculture and degraded prairie plant communities.  The neighborhood around our Platte River Prairies is about 30% grassland – a higher percentage of prairie than many eastern tallgrass landscapes, but certainly not a large unbroken prairie landscape.  Moreover, Platte River grasslands tend to have long histories of chronic overgrazing and broadcast herbicide use which has left them with limited plant diversity.  Even most of our annually-hayed prairies are missing many wildflower species that would be expected to be dominant in those prairie types.  The fact that regal fritillaries are thriving in this landscape seems counter to some of what is published about the species’ reliance on “high quality prairie”, but they’re here nonetheless! 

Regal fritillaries rely on violets as the sole food source for their larvae.  In our Platte River Prairies, the only perennial violet species we have is the common blue violet (Viola sororia).  In much of the eastern tallgrass prairie, prairie violets (V. pedatifida) or bird’s foot violets (V. pedata) are thought to be the preferred species, but we don’t get those species in our area.  Fortunately, the common blue violet thrives in pastures with long histories of annual intensive grazing, so the supply of larval food sources is not limited in our landscape.  It may be that the abundance of violets and the amount of grassland left in the valley are the two primary reasons for the regal fritillary’s continued success here.

Common blue violets (Viola sororia) are abundant in our degraded prairies - along with lots of Kentucky bluegrass. The species appears to support strong regal fritillary populations.

A brief report of our findings from 2011 can be found in this PDF, but I’ll lay out a few of the highlights here.  First, the seasonal patterns of butterfly abundance we’re seeing seems to match what we know of the life history of the species.  Males emerge first (late June), with highest numbers occurring in areas with relatively high amounts of thatch and lots of violets.  Those males tend to spend the next couple weeks flying around the same place they emerged from until the females also emerge and mating takes place.  That mating period is when we see the most fritillaries along our transects.  Following that, the number sightings drop as males begin to die off and females enter a period of “reproductive diapause”, during which they spend much of their time sitting around in the shade.  Finally, in late August, we see a little bump in our numbers again as females begin flying around and laying eggs.  (Those eggs then hatch, and the larvae – if they find enough violets to eat – will overwinter in the thatch.)

The effects of prescribed fire on regal fritillaries and other butterflies is a frequent topic of discussion among ecologists.  At our sites, we definitely see the highest numbers of regals emerging in June and July from prairies with high numbers of violets and with a build up of thatch.  Prairies that were burned in the spring have very few fritillaries during that peak period of emergence and mating – the larvae almost surely perish in spring fires.  However, during the summer of 2011, two of our top three sites for nectaring activity had been burned in the spring of that year.  It may be that in landscapes of relatively abundant habitat, a mixture of burned and unburned areas can complement each other by providing high quality larval habitat (unburned) and nectaring habitat (burned). 

In addition to prescribed fire, our other major management tool is cattle grazing – and the two are often combined within our various patch-burn grazing systems.  The burned areas mentioned above where nectaring activity was high were also being grazed.  Under our light stocking rates, cattle focus primarily on grasses, leaving most wildflowers to bloom – making them available to pollinators, including regal fritillaries.  Under higher stocking rates, fewer wildflowers escape grazing and regal fritillary are lower.  However, even areas that are burned and intensively grazed recover quickly, and within a year or two (as cattle focus on more recently burned areas) become hot spots for regal fritillaries again.  As stated earlier, it appears that a mixture of management treatments across a landscape with fairly abundant habitat is compatible with successful regal fritillary populations.

After our 2010 butterfly surveys, I posted a report on the nectar use by regal fritillaries and other butterfly species.  Those 2010 surveys included a slightly different mixture of prairies from our 2011 surveys, and included some transects where we counted all butterfly species.  Within that context, the clear favorite nectar plant was hoary vervain (Verbena stricta).  In 2011, vervain was still among the top nectar species for regal fritillaries – particularly in degraded remnants where it can be very abundant – but it was not quite as dominant as in last year’s surveys.  I think this was mainly because we included more restored prairies in our surveys this year, and the diversity of flowers was higher in those sites.  Even so, hoary vervain tied for first place in nectar species use with the combination of common and showy milkweeds (Asclepias syriaca and A. speciosa), followed closely by bee balm (Monarda fistulosa) and various thistle species (Carduus nutans and Cirsium spp).  It’s important to point out, however, that we only had 27 sightings of nectaring regal fritillary butterflies in 2011 (out of 352 total sightings), so our sample size was fairly limited.

Regal fritillaries in the central Platte River valley really like hoary vervain (Verbena stricta) as a nectar plant. Vervain survives intensive grazing very well, so it can be abundant in some of our more degraded pastures, as well as in our restored prairies.

Among those 27 sightings of regal fritillaries nectaring, all but four of them were in restored prairies (former crop fields).  At least based on the data we’ve collected so far, it appears that our regal fritillaries are tied to degraded remnants (lots of violets) for larval production but move into our restored prairies (lots of nectar plants) later in the season.  A quick and dirty correlation analysis showed that after the mating season was over, butterfly sightings were most likely to occur in portions of prairie with high abundances of their favorite nectar plants – but there was no such correlation prior to or during the mating season.  Again, a mixture of habitat types (this time, restored and remnant) appears to provide complementary resources for the butterflies in the Platte River Prairies. 

Clearly, regal fritillaries managed to survive just fine along the Platte River before our restored prairies were available, so it’s not that we’ve saved the species through our restoration work.  Intensively grazed pastures tend to have high numbers of hoary vervain flowers for nectaring, and some hayed prairies in the valley contain good numbers of milkweeds and other nectar species as well.  However, watching the behavior of fritillaries when areas of high nectar plant abundance (e.g., our restored prairies) become available may tell us about what their preferred habitat conditions are. 

Thistles, including this musk thistle, are among the favorite nectar plants for regal fritillaries - along with hoary vervain, bee balm, and common milkweed. At least for now, musk thistle is surviving just fine in the Platte Valley. (hooray?)

I feel pretty confident that regal fritillaries are managing to survive within the context of our current prairie restoration and management work.  However, we’re still not able to figure out where they’re laying their eggs.  According to the literature, regal fritillary females don’t lay their eggs on violets (why??) and no one is even sure if they select locations near violets.  I sure hope our regal fritillary females return to remnant prairies (where violets are abundant) to lay eggs, rather than laying them in our restored prairies.  If our restored prairies – with their abundant nectar plants – are luring those females away from prime egg-laying habitat, that’s not a good thing.  To date, we just can’t get enough sightings during that late summer egg-laying period to figure out what’s going on.  We even tried following females around to see if they were laying eggs but that turned out to be pretty difficult.  I guess since the population seems to be strong I’m not going to worry about it too much, but I’d still like to know what they’re doing. 

Because of the limited time we can devote to this project, it’s necessarily more of a pilot project than a full-blown research effort.  If there are scientists out there who would be interested in following up on our initial work, I’d be more than happy to work with you. 

In the meantime, you can see read the report from our 2011 surveys here, including lots of fun graphs and tables.