Winter Ants

On a hike along the Little Sac River with the the WOLF School, the students photographed this finding on the rain dampened rocks.  On closer inspection it was a former earthworm being recycled by an army of ants. I couldn't tell much about them from this single photo but with a trained eye, myrmecologist (ant specialist) James Trager gave me the answer.

"This is a fairly common sight for those who go out with their eyes to the ground in these cooler months. The ant is the winter ant - Prenolepis imparis."

Looking closer, this turns out to be a very interesting ant species.  P. imparis (PI) is mainly active in cooler weather when it isn't in competition with other ants.  They are most active at temperatures between 45° and 60°F and can occasionally be found foraging at near freezing temperatures.

Tschinkel and PI nest

During warm weather they seal their nests entrances and stay deep underground.  Myrmecologist Walter Tschinkel used molten aluminum to create casts of their nests.  While this PI cast to the right is from Florida, nests in Missouri and Ohio are "only" one third that depth.*  This is attributed to the softer Florida sands and I can only imagine our PI using picks and dynamite to excavate the Ozark rocky clay.

Winter Ant - Antwiki

Their colonies usually contain a few thousand workers and some of the colonies last 7-9 years.  Young adult workers eclose from their pupae and spend 10 months inside the colony.  Older adults forage outside the nest, returning to the nest to pass food to the younger workers by mouth-to-mouth regurgitation (word of the day - stomodeal trophallaxis).

"Young workers have softer, more flexible cuticles and their gasters can swell and distend to accommodate large amounts of liquid food. " This is stored as lipids in hypertrophied fat bodies. These 'corpulents' store more than twice their body weight in fats and nutrients. Once a corpulent worker has exhausted its food stores it becomes a forager during the next active season. This indicates that workers may live for up to two years, serving as a corpulent for the first and a forager during the second." **

Argentine Ant - Alex Wild, myrmecos.net

In addition to cold tolerance, these are tough little soldiers when faced with competitors.  Once they find a food source they aggressively defend it.  The invasive Argentine ant, Linepithema humile, tends to spread out in large colonies, out competing and displacing many other ants.  Our little PI is not only aggressive to the Argentine invader but it produces abdominal secretions that are lethal to it.

All this, and more to follow, came from a "little" 4.5 mile hike with the WOLFs.

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There are more details at *Antwiki and **University of Florida Entomology.

Thanks for the patience and knowledge of James Trager.