On our walk on Monday at the Nichols Arboretum in Ann Arbor, we noticed the milkweed plants (Asclepias spp.) along the edges of the prairie were looking a little ragged. The leaves were chewed up with caterpillar-munched holes, and there were also hordes of little orange yellow aphids having a party drinking the milkweed sap.
We tentatively identified the aphids as Aphis nerii, the oleander aphid. A scatter of ants inspected the lines of aphids clustered along the leaf veins, which reminded me of tiny cowboys surveying a herd of grazing cattle. Presumably, the ant ranchers were milking the aphids for their honeydew, a sticky sweet substance that aphids secrete from their butts.
However, something troubled me about this bucolic scenario. These aphids from the Mediterranean feed on poisonous plants such as milkweed (Asclepias spp.) and other members of the dogbane Family (Apocynaceae), so they can store the toxic cardenolides (aka cardiac glycosides) that deter predators.
The aphids’ bright orange-yellow aposematic color serves as “Don’t Eat Me or You’ll be Sorry” warning. (One famous example of this is the notoriously noxious-tasting Monarch Butterfly.) The aphids can also secrete the cardenolides from their cornicles (organs on their butts)to get predators to back off.
So why the heck would ants be cultivating these toxic little critters as a honeydew source?
I did some research.
Ant/aphid relationships can get complicated. The aphid species A. nerii and A. asclepiadis both coexist and feed on milkweed, but according to Smith et al. 2008, only A. asclepiadis establish a mutualism with ant caretakers.
In contrast, other studies have observed the Argentine Ant (Linepithema humile) tending A. nerii aphids under field conditions in California (Bristow 1991, cited in Pringle et al. 2014.)
So what’s going on in my photo?
A few possibilities:
1) The ants aren’t actually herding the aphids for honeydew production, but are there for some other reason.
2)The aphids that I identified as A. nerii are really another species, such as A. asclepiadis. Here is a good photo showing the color contrast between the two species. Fun fact: A. nerii orange aposematic coloring is an ancient gene transfer from a fungus!
3) I haven’t identified the ant species. They could be Argentine ants (or another ant species that herds A. nerii for honeydew. However, as far as I can tell, L. humile doesn’t live in Michigan (it’s too cold!)
4) I also haven’t identified the milkweed species of the leaf. Aphids feeding on different species of milkweed metabolize the cardenolides differently. Pringle’s experiment suggested that ants may be prefer aphid colonies that produce honeydew with lower cardenolide content.
Maybe this particular milkweed produces a mellow-flavored honeydew?
At any rate, I still don’t know what these ants are doing. But this little project has provided me with hours of entertainment and a new appreciation for tritrophic interactions!