HMC Bee Lab: Rise and fall of an empire: reflections on culturing harvester ants

What do fruit flies, nematodes, hydra, zebrafish, brewer’s yeast, mice, and our beloved honeybee have in common? If one were to look at the biological literature, I bet you could find a surprising amount of information comparing the biology of various pairings of this odd menagerie. That’s because these animals are historically well-researched lab subjects. The lab mouse, the fruit fly, and the honeybee especially are inarguably iconic, cultural symbols often linked to science as a profession or practice. Why is this? A common thread, relevant to our very own lab, is that the care requirements of the classic “model” organisms are easily reproducible, and are well documented by some of the thousands of labs working on them.

To house bees, we have many frames and hive boxes, all of a standard dimension consistent with the most common honeybee hive design, called the Langstroth. With the standardization of the Langstroth comes some important economic advantages such as interchangeable parts, scalability, and portability.

[1] The Langstroth hive design

My project this summer was to compile a care protocol for the California harvester ant and carry it out (Part of the protocol I followed for capturing the queens is included in an earlier post, Incubating an Empire). To pluck an animal from its habitat and cultivate it in isolation in the lab requires some knowledge about the ecological cradle that it relies on for survival in the wild, in hopes of sufficiently recreating the most important conditions in the lab. There is no Langstroth for a harvester ant colony, much less a housing specifically designed for the California harvester. All of the materials to care for and maintain the ant colonies in the lab had to be assembled, all of the founding chambers that would house the nuclear colonies and the larger observation frames to house larger colonies had to be made from raw material.

This gave me an exciting amount of freedom to research and design the optimal setup to encourage colony growth. Plenty of literature exists studying ants of this genus reared in the lab (See Further Reading in my last update), but I found a true wealth of information on the open web, in forums devoted to hobbyists who capture and raise ants for fun.

It was only through internet trawling that I found information that referred specifically to the California harvester (specifically at http://www.formiculture.com and http://antfarm.yuku.com/ ). A few hobbyists from Southern california partially documented their experience with Pogonomyrmex queens that they self identified as californicus.

Hobbyists seem to experience very high mortality among queens of this species. One poster caught 15 queens only to have one survive to produce workers. This had me thinking that the California harvester must be fairly particular about the conditions they need to properly found a colony. To ensure comfortable queens, I used an electric incubator with a water basin to maintain a hot, humid environment. This appeared to work great for the first few weeks, as the queens were noticeably more active when kept in the warmth of the incubator.

To fill you in on the project status after my first post:

Three weeks in, all the queens were still alive and burrowing. The first casualties were seen in the fourth week when most queens had tiny but visible clutches of eggs. Six weeks in, half of the queens were still alive. This was very promising, as larvae and some pupae became visible suggesting decently healthy queens.

Two queens made it to week eight, and both had multiple mobile workers. The most prevalent cause of death for the queens appears to be mold. It is an eerie sight to see a queen curled in stiff repose, sprouting stalks of lime green fuzz. It is quite disappointing to care for the queens, seeing them produce viable workers, only to have them succumb to the same problem. I have a few thoughts on why mold was such a problem, and a few suggestions to abate the problem in the future.

[2] Five queens in their chambers, newly fed with cracked barley (the bag on the bottom right)

For feeding I originally used cracked barley, which is easily picked up and eaten by even the first small workers. But I think the choice of food may be part of the problem: that the cracked seed may be growing mold because the seed’s natural casing is destroyed, exposing the seed’s carbohydrates to drifting, opportunistic mold spores. If you're a mold, the high heat and ample moisture of the incubator coupled with the harvester ant’s propensity for hoarding seed underground make for cozy quarters. The ants simply couldn't keep up with the mold.

Although we do not currently have a captive colony, this first try is promising for a few reasons: all twelve queens burrowed, all laid eggs, and two lived long enough to produce workers. To me, this suggests that the conditions were close and we can approach next year’s nuptial flight with some critical experience and promising revisions. To keep the mold at bay, I think being more sparing with food and humidity is a good place to start.

As we look to future experiments using the California harvester, there are a few interesting directions we could go. The Bernard Field Station allows access to a wild population of colonies, and feasible lab culturing means observations could be made in the field and in the lab. But what would we watch? What questions would we ask?

One reason to study Pogonomyrmex is the amount of interesting research already associated with the behavior and ecology of harvester ants. Research focused on the foraging activity of Pogonomymex barbatus performed by Deborah Gordon revealed that the ants didn’t lay pheromone trails to guide fellow ants to seed caches, nor did they seem to utilize spatial information at all in recruiting foragers to leave the nest. Interestingly, all of the foraging decisions can be made right at the nest entrance, where incoming foragers are received by idle workers within the colony. If successful foragers are returning often, the workers at the entrance will encounter them frequently. These idle workers are sensitive to these interactions such that if enough successful foragers return, the individuals will be compelled to forage themselves.

During foraging, harvester ant workers face a high risk of dying from desiccation. In the low-moisture habitats that harvesters occupy, this positive feedback mechanism serves to limit foraging during hot days (in which the first foragers will be more likely to die and not return) and encourages foraging when conditions are suitable for successful foraging (expressed by the number of successful returning foragers). This mechanism operates independently of spatial information gathered by foragers outside the nest, and appears to be the most effective strategy when resources are distributed and foraging is risky.

[3] A harvester ant colony in a typical dry, hot ecosystem.

For food, harvester ants rely on seeds that vary greatly in their distribution from very clumpy and concentrated to nearly randomly distributed in space. For clumpy resources, it makes sense that a forager would be more successful if efforts were focused on areas associated with prior success, as seeds are likely to be close to each other. For situations like this, Pogos can utilize pheromone trails and a strategy called site fidelity, meaning a forager returns directly to the place that she last found food. A paper by Tatiana Flanagan and colleagues focuses on how three different species of Pogo. respond to different resource distributions.

In our lab, we could consider the existing body of research in the specific context of California harvester ants at the BFS; exploring what foraging strategies they employ, and what this may reveal about the spatial distribution of seed resources at the BFS. If they appear to employ pheromone trails often, this may suggest a patchy distribution. If foragers seem to search randomly, the seeds may be relatively diffuse. With a captive colony, foraging experiments could be run using an arena, where conditions like seed placement, distribution, and abundance can be carefully observed, measured, and manipulated.

With an improved protocol, I await next year’s nuptial flight with great hope; encouraged by notable research already being done and excited to enter the fray with our own colonies.

HMC Bee Lab

Pages

Tuesday, August 4, 2015

Rise and fall of an empire: reflections on culturing harvester ants

No comments:

Post a Comment