When do harvester ants gather food before winter? (Javier, Illinois)


Hi AntAsk,

What specific dates (time frame) do harvester ants have to gather their food before winter strikes?

Best,
Javier


Dear Javier,

Thank you very much for your great question. We have contacted Christina Kwapich, who has done some great work on harvester ants, to address your query. Here is what she had to say:

"While there are numerous genera and species of seed harvesting ants, none is more charming (or easier to exhume) than the Florida harvester ant (Pogonomyrmex badius). Populations of P. badius dapple the coastal plain, east of the Mississippi river and favor spots with well drained, sandy soil. Between March and November of each year foragers seek out seeds, insect protein and even freshly cut slivers of fungus to feed crops of hungry larvae. Seeds are the only physical products that colonies store over winter and some colonies really know how to fill the pantry! Walter Tschinkel noted that one large P. badius colony housed over one pound of seeds (Tschinkel 1999). While we don't yet know how long the oldest seeds remain in P. badius seed caches, Preliminary data from the Tschinkel lab show that fresh seeds are frequently processed within days of collection, while some apparently 'uncrackable' seeds linger indefinitely. This means that the old "first in first out" adage may not apply when it comes to stocking the shelves in an ant nest.

Although numerous chambers brim with seeds all winter long, adults don't appear to ingest a morsel of the spoils. It might be tempting to exalt them for their willpower, but the truth is that the narrow waists of adult ants prevent them from ingesting solid food. In fact, seeds and other solid food items are collected expressly for consumption by hungry larvae; which only appear from April to November of each year. In the absence of a liquid diet, adult P. badius workers rely on ample fat reserves to tide them over the winter months. The age of adults entering the over-wintering period ranges from 10 to 60 days, but whether or not these individuals consume a special diet in preparation for the long stretch is unknown (Kwapich and Tschinkel, unpublished). We do know that when winter begins, workers enter a period of reduced activity, abandoning their posts across many meters of vertical nest space to occupy only the bottom-most chambers (a cozy 24 degrees Celsius, all year round, i.e., 75 degrees Fahrenheit).

The presence of winter seed caches may lead one to believe that seeds play a key role in springtime alate (those are the winged sexuals in ants that form the next generation) and worker production. Surprisingly, Chris Smith (2007) demonstrated that when prevented from foraging, colonies did not tap into their ample seed reserves and use them as a buffer against starvation. Instead, it appears that spring-time worker fat-reserves (garnered in autumn) are most important to the production of alates and new workers following winter. So, to answer the original question, it appears that P. badius workers spend nine months adding and subtracting from a large seed cache which may be of no critical importance at all! This should not be entirely befuddling, as Willard and Crowell (1965) noted that another seed harvester, Pogonomyrmex owyheei, successfully enters and exits winter without stored products of any kind."

All the best,
Christina Kwapich (guest expert), Steffi Kautz & the AntAsk Team

P. badius chamber.jpg

This photo shows a half-filled Pogonomyrmex badius seed chamber. Photo by Christina Kwapich.



Works cited:

Smith, C. R. (2007). Energy use and allocation in the Florida harvester ant, Pogonomyrmex badius: are stored seeds a buffer? Behavioral Ecology and Sociobiology 61:1479-1487.

Tschinkel, W. R. (1999). Sociometry and sociogenesis of colony-level attributes of the Florida harvester ant (Hymenoptera: Formicidae). Annals of the Entomological Society of America 92: 80-89.

Willard, J. R. & H. H. Crowell (1965). Biological Activities of the Harvester Ant, Pogonomyrmex owyheei, in Central Oregon. Journal of Economic Entomology 58:3.

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