Beautiful buds beset bumblebees with bad bugs

Sexual liaisons can be difficult to achieve without some type of purposeful motion.  Flowering plants, which are rooted to the ground, are particularly challenged to bring the male close enough to the female to have sex.  One awesome adaptation is pollen, technically the male gametophyte –  or gamete (sperm)-generating plant. These tiny males get to females either by floating through the air, or by being transferred by animal pollinators such as bees. Plants can lure bees to their flowers by producing nectar – a sugar rich fluid – which bees lap up and use as a carbohydrate source.  While nectaring, bees also collect pollen, either intentionally or inadvertently, which provides them with essential proteins. When bees travel to the next flower, they may inadvertently drop some of their pollen load near the female gametophyte – in this case a tiny egg-generating plant (though tiny, the female gametophyte is considerably larger than is the male gametophyte).  We call this process of “tiny boy meets tiny girl” pollination. Once the two gametophytes meet, the pollen produces one or more sperm, which it uses to fertilize an egg within the female gametophyte.  There is more to it, but this will hopefully clarify the difference between pollination and fertilization.


Bumblebee forages on beebalm, Monarda didyma. Credit: Jonathan Giacomini.

All of this business takes place within the friendly confines of the flower.  The same flower may be visited by many different bees of many different species. While feeding, bees carry on other bodily functions, including defecation.  They are not careful about where they defecate; consequently a bee’s breakfast might also include feces from a previous bee visitor. Bumblebee (Bombus impatiens) feces carries many disease organisms, including the gut parasite Crithidia bombi, which can reduce learning, decrease colony reproduction and impair a queen’s ability to found new colonies. Because pollinators are so critical in ecosystems, Lynn Adler and her colleagues wondered whether certain types of flowers were better vectors for harboring and transmitting Crithidia bombi to other bumblebees.


Bumblebee forages on the snapdragon, Antirrhinum majus. Credit: Jonathan Giacomini.

The researchers chose 14 different flowering plant species, allowing uninfected bumblebees to forage on inflorescences (clusters of flowers) inoculated with a measured amount of Crithidia bombi parasites.  The bees were reared for seven days after exposure, and then were assessed for whether they had picked up the infection from their foraging experience, and if so, how intense the infection was. The researchers dissected each tested bee and counted the number of Crithidia cells within the gut.


Researcher conducts foraging trial with Lobelia siphilitica inflorescence. Credit: Jonathan Giacomini.

Adler and her colleagues discovered that some plant species caused a much higher pathogen count (mean number of infected cells in the bee gut) than did other plant species.  For example bees that foraged on Asclepias incarnata (ASC) had four times as many pathogens, on average, than did bees that foraged on Digitalis purpurea (DIG) (top graph below). Bees foraging on Asclepias were much more likely to get infected (had greater susceptibility) than bees that foraged on several other species, most notably Linaria vulgaris (LIN) and Eupatorium perfoliatum (EUP) (middle graph). Lastly, if we limit our consideration to infected bees, the mean intensity of the infection was much greater for bees foraging on some species, such as Asclepias and Monarda didyma (MON) than on others, such as Digitalis and Antirrhinum majus (ANT) (bottom graph).


(Top graph) Mean number of Crithidia (2 microliter gut sample) hosted by bees after foraging on one of 14 different flowering plant species. This graph includes both infected and uninfected bees. (Middle graph) Susceptibility – the proportion of bees infected – after foraging trials on different plant species. (Bottom graph) Intensity of infection – Mean number of Crithidia for infected bees only. The capital letters below the graph are the first three letters of the plant genus. Numbers in bars are sample size.  Error bars indicate 1 standard error.

It would be impossible to repeat this experiment on the 369,000 known species of flowering plants (with many more still to be identified).  So Adler and her colleagues really wanted to know whether there were some flower characteristics or traits associated with plant species that served as the best vectors of disease.  The researchers measured and counted variables associated with the flowers, such as the size and shape of the corolla, the number of open flowers and the number of reproductive structures (flowers, flower buds and fruits) per inflorescence.


Flower traits measured by Adler and colleagues (example for blue lobelia, Lobelia siphilitica). CL is corolla length. CW is corolla width. PL is petal length. PW is petal width. Credit: Melissa Ha.

The researchers also wanted to know whether any variables associated with the bees, such as bee size and bee behavior, would predict how likely it was that a bee would get infected.  Surprisingly, the number of reproductive structures per inflorescence stood out as the most important variable. In addition, smaller bees were somewhat more likely to get infected than larger bees, and bees that foraged for a longer time period were more prone to infection.


Mean susceptibility of bees to Crithidia infection after foraging on 14 different flowering plant species, in relation to the number of reproductive structures (flowers, buds and fruits) per inflorescence.

These findings are both surprising and exciting. Adler and her colleagues were surprised to find such big differences in the ability of plant species to transmit disease.  In addition, they were puzzled about the importance of number of reproductive structures per inflorescence.  At this point, they don’t have a favorite hypothesis for its overriding importance, speculating that some unmeasured aspect of floral architecture influencing disease transmission might be related to the number of reproductive structures per inflorescence.


Bumblebee forages on Penstemon digitalis. In addition to the open flowers, note the large number of unopened buds.  Each of these counted as a reproductive structure for the graph above. Credit: Jonathan Giacomini.

The world is losing pollinators at a rapid rate, and there are concerns that if present trends continue, there may not be enough pollinators to pollinate flowers of some of our most important food crops. Disease is implicated in many of these declines, so it behooves us to understand how plants can serve as vectors of diseases that affect pollinators. Identifying floral traits that influence disease transmission could guide the creation of pollinator-friendly habitats within plant communities, and help to maintain diverse pollinator communities within the world’s ecosystems.

note: the paper that describes this research is from the journal Ecology. The reference is Adler, L. S., Michaud, K. M., Ellner, S. P., McArt, S. H., Stevenson, P. C. and Irwin, R. E. (2018), Disease where you dine: plant species and floral traits associated with pathogen transmission in bumble bees. Ecology, 99: 2535-2545. doi:10.1002/ecy.2503. Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2018 by the Ecological Society of America. All rights reserved.

Sweltering ants seek salt

Like humans, ants need salt and sugar.  Salt is critical for a functioning nervous system and for maintaining muscle activity, while sugar is a ready energy source. In ectotherms such as ants, body temperature is influenced primarily by the external environment, with higher environmental temperatures leading to higher body temperatures.  When ants get hot their metabolic rates rise, so they can go out and do energetically demanding activities such as foraging for essential resources like salt and sugar. On the down side, hot ants excrete more salt and burn up more sugar.  In addition, like humans, very high body temperature can be lethal, so ants are forced to seek shelter during extreme heat.   As a beginning graduate student, Rebecca Prather wanted to know whether ants adjust their foraging rates on salt and sugar in response to the conflicting demands of elevated temperatures on ants’ physiological systems.

Prather at field site

Rebecca Prather at her field site in Oklahoma, USA. Credit: Rebecca Prather.

Prather and her colleagues studied two different field sites: Centennial Prairie is home to 16 ant species, while Pigtail Alley Prairie has nine species.  For their first experiment, the researchers established three transects with 100 stations baited with vials containing cotton balls and either 0.5% salt (NaCl) or 1% sucrose.  The bait stations were 1 meter apart.  After 1 hour, they collected the vials (with or without ants), and counted and identified each ant in each vial.  The researchers measured soil temperature at the surface and at a depth of 10 cm. The researchers repeated these experiments at 9 AM, 1 PM and 5 PM, April – October, 4 times each month.


Ants recruited to vials with 0.5% salt solution.  Credit: Rebecca Prather.

Sugar is easily stored in the body, so while sugar consumption increases with temperature, due to increased ant metabolic rate, sugar excretion is relatively stable with temperature.  In contrast, salt cannot be stored effectively, so salt excretion increases at high body temperature.  Consequently, Prather and her colleagues expected that ant salt-demand would increase with temperature more rapidly than would ant sugar-demand.


Ant behavior in response to vials with 0.5% salt (dark circles) and 1% sucrose (white circles) at varying soil temperatures at 9AM, 1 PM (13:00) and 5PM (17:00). The three left graphs show the number of vials discovered (containing at least one ant), while the three right graphs show the number of ants recruited per vial.  The Q10 value  = the rate of discovery or recruitment at 30 deg. C divided by the rate of discovery or recruitment at 20 deg. C. * indicates that the two curves have statistically significantly different slopes.

The researchers discovered that ants foraged more at high temperatures. However, when surface temperatures were too high (most commonly at 1 PM during summer months), ants could not forage and remained in their nests.  At all three times of day, ants discovered more salt vials at higher soil temperatures. Ants also discovered more sugar vials at higher temperatures in the morning and evening, but not during the 1 PM surveys. Most interesting, the slope of the curve was much steeper for salt discovery than it was for sugar discovery, indicating that higher temperature increased salt discovery rate more than it increased sugar discovery rate (three graphs on left).

When ants discover a high quality resource, they will recruit other nestmates to the resource to help with the harvest.  Ant recruitment rates increased with temperature to salt, but not sugar, indicating that ant demand for 0.5% salt increased more rapidly than ant demand for 1% sugar (three graphs above on right).

The researchers were concerned that the sugar concentrations were too low to excite much recruitment, so they replicated the experiments the following year using four different sugar concentrations.  Ant recruitment was substantially greater to higher sugar concentrations, but was still two to three times lower than it was to 0.5% salt.


Ant recruitment (y-axis) to different sugar concentrations at a range of soil temperatures (X-axis). Q10 values are to the left of each line of best fit.

Three of the four most common ant species showed the salt and sugar preferences that we described above, but the other common species, Formica pallidefulva, actually decreased foraging at higher temperatures.  The researchers suggest that this species is outcompeted by the other more dominant species at high temperatures, and are forced to forage at lower temperatures when fewer competitors are present.

In a warming world, ant performance will increase as temperatures increase up to ants’ thermal maximum, at which point ant performance will crash.  Ants are critical to ecosystems, playing important roles as consumers and as seed dispersers. Thus many ecosystems in which ants are common (and there are many such ecosystems!) may function more or less efficiently depending on how changing temperatures influence ants’ abilities to consume and conserve essential nutrients such as salt.

note: the paper that describes this research is from the journal Ecology. The reference is Prather, R. M., Roeder, K. A., Sanders, N. J. and Kaspari, M. (2018), Using metabolic and thermal ecology to predict temperature dependent ecosystem activity: a test with prairie ants. Ecology, 99: 2113-2121. doi:10.1002/ecy.2445Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2018 by the Ecological Society of America. All rights reserved.