Aculeata Agonistes: Ants, Aphids & Aspen

The Home Bug Garden doesn’t look like the picture above, that is of our Moose Pasture about 55 km to the East as the raven flies, but it may once have. Both are located in the Aspen Parkland biome, named for its dominant tree, Quaking Aspen (aka White Poplar) Populus tremuloides, and its park-like mixture of open grasslands and patches of trees. In the Prairie Provinces of Canada, Aspen Parkland is rich in postglacial water bodies. And, as my late father-in-law pointed out repeatedly, he used to going boating in what is now our neighbourhood when he was a boy.

The prairie parklands occur across a broad stretch of central Canada, but aspen parkland also extends well to the south in the Rockies. South Park in Colorado, made famous by the potty-mouthed cartoon, is just one of many such mixtures of aspen and meadow. Interestingly, parkland is considered a transitional biome between prairie and boreal forest. It’s as if it isn’t really a natural system, but somehow aberrantly refuses to become its destiny: conifer forest if moisture remains high enough and fire frequency low enough or prairie if the opposites occur.

We love aspen, but whenever I find a seedling growing in the HBG, I grub it out. This is an entirely practical behaviour: aspen roots have a reputation for seeking out and clogging water and sewage pipes. Unfortunately, that means we miss out on a lovely tree and all but a passing acquaintance with many of the insects associated with aspen. Not all though, there appears to be at least one aspen-associated aculeate hymenopteron that finds both the Moose Pasture and our yard agreeable: the Black Mound Ant Formica podzolica Francoeur, 1973.

In zoological nomenclature, the year that comes after the author of a species name refers to when the diagnosis of a new species was first published. In this case, I haven’t inverted the 9 and 7 – this species name wasn’t properly sorted out until 1973. Before that it travelled under several names including the widely distributed Formica fusca. Insects tend to be interesting in direct proportion to how much one can learn about them and unless one can first learn their real name, such information as to make them interesting neighbours remains elusive. Ant species are particularly difficult to identify – species level keys are technical, and, well, many ant species tend to look alike. This seems to be especially true of Formica species. In this case, we are indebted to James Glasier for identifying the black ants that caught our attention with their herds of speckled aphids on young aspen as Formica podzolica (misattributions in any other images are mine).

Although the scientific name may be relatively new, people in my neighbourhood have been at war with black mound ants since soon after the first expanses of green lawns rose out of the former slough. Formica podzolica excavates and piles up largish mounds of black clay wherever it feels at home here and no one seems to find lumpy lawns attractive or interesting. In the bush, though, the mounds are an interesting feature and the black ants seem much less aggressive than their red and black neighbours Formica aserva.

I’m less than excited by ants, but no lover of lawns, so generally I leave my ants alone as long as they leave me alone. Unfortunately, ants are infamous for guarding patches of sap-sucking homopterons (aphids, hoppers, and the like). They protect these annoying insects from ‘natural enemies’ such as ladybird beetles, lacewings, and small parasitic wasps. ‘Natural enemy’ is another of those bizarre concepts that has somehow become commonplace. Are there unnatural enemies? Well, I suppose Count Dracula’s cockroach-munching Igor would do. In any case, usually ‘natural enemy’ refers to an arthropod that eats or parasitizes an arthropod that eats something we value. Ants can be natural enemies, but not when they are guarding aphids. By chasing off or killing things that want to eat aphids, ants allow plant lice (= aphids) numbers to rise, the amount of sap they drink to increase, more honeydew to spatter, and sooty molds to cover leaf surfaces. None of this would seem to be good for the plant, but the ants benefit by ‘milking’ the aphids for their honeydew and the aphids are assumed to do better with the guards.

Two things especially intrigued us about the black ants and their aphids on the young aspens, and the first was that the aspen leaves have a pair of nectaries (red spots where the petiole joins the leaf under the leg of the top ant). That plants can produce nectar in flowers to attract pollinators is well known, but nectar-producing glands are not limited to flowers. As a group (and giving an indication that scientists were once surprised to find this out themselves) nectaries not in flowers are called ‘extrafloral’. Ants are known to be attracted to extrafloral nectaries, and one can imagine that having aggressive black ants scuttling around one’s peony buds could be a good thing when some hungry caterpillar was on the prowl for a meal. But why would aspen want ants on their leaves if they end up tending aphids? This particular aphid is a well known pest of aspen – the Poplar Leaf Aphid (aka Speckled Poplar Aphid) Chaitophorus populicola (thanks to Bryan Brunet for the id).

Fossils in the Florissant beds in Colorado demonstrate that members of the genus Populus have had nectaries and been associated with ants and other predatory insects for at least 35 million years (Pemberton 1992. American Journal of Botany 79:1242-1246). Just why aspen have extrafloral nectaries, however, isn’t very clear since they do not seem to influence natural enemy abundance (see Wooly et al. 2007. Annals of Botany 100: 1337–1346). Of course, just calling an insect a natural enemy doesn’t make it useful to a plant (or a gardener). Ambush bugs and crab spiders, for example, make their livings hiding in flowers and eating pollinators.

So perhaps we have good natural enemies that help the plant and get their reward at extrafloral nectaries and other bludgers that just take a sip and go about their not very helpful lives. Life, however, is usually not that simple. For example, the usefulness of extrafloral nectaries may vary with age (younger aspen have proportionately more nectaries than older aspen) and time of year (many nectaries are known to dry up after young leaves have hardened off in early summer). So, ants may be useful for keeping caterpillars away in the spring or on young trees (which have fewer leaves they can afford to lose). Perhaps the ants don’t start farming aphids until after the trees cut off their supply of nectar. Perhaps later in the summer, when leaves will soon be dropping anyway, aspen find it less expensive to let the ants get their sugary rewards second hand from the plant lice than to feed them directly from extrafloral nectaries. There are enough ‘perhapses’ in this simple system to keep a host of scientists busy for years.

Speaking of which, Kailen Mooney & coworkers have found out something interesting about our black ant on pine trees in Colorado. There Formica podzolica herds large aphids in the genus Cinara. I once had a job dissecting Cinara aphids and looking for parasites – 10,000 aphids yielded only 3 parasites, so perhaps our black ant is a good Cinara shepherd (or I was a really bad aphid vivisectionist). However, Mooney found that birds were able to eat both ants and aphids, reducing the latter by 91% (those not eaten dropped to the ground) and causing the ant to look for less dangerous meals. That was good for the pine trees, so birds can be elevated to ‘natural enemies’ here. Black Mound Ants were good for the aphids only when birds weren’t around. But as usual, the story is even more complicated. As well as Black Mound Ants, another species in the same genus, Formica planipilis, also guarded pine aphids. This ant was a better aphid husbander even when birds were around (see Mooney & Mandel 2010. Oikos 119: 874–882 and references therein). I’m starting to feel a little bit sorry for the aphids that Black Mound Ants decide to protect.

This feeling is reinforced by a closer look at our erstwhile aphid farmer: several predatory hoverfly larvae are eating aphids right under the noses of the ants! Apparently the ants are oblivious to the foxes in the hen coop, or perhaps too nervous that a Yellow Warbler may swoop down and eat them to notice. Also, they may be being fooled by the hoverfly larva into thinking the maggots are aphids.  Syrphus ribesii uses its cuticular hydrocarbons to smell/taste like the wooly alder aphid and protect themselves from guarding ants (Lohman et al. 2006. Ecological Entomology 31: 41–51) and this may very well be happening here.

If you can think of being tended by ants as a characteristic of an aphid species, then you could ask the question, we’ll I wonder how many times it evolved in Chaitophorus aphids feeding on poplars and willows? Shingleton & Stern (2003. Molecular Molecular phylogenetic evidence for multiple gains or losses of ant mutualism within the aphid genus Chaitophorus. Molecular Phylogenetics and Evolution 26: 26–35) asked this very question and found that it appears to either have been acquired by several aphids independently or to be a fairly unstable character that has been lost multiple times. Considering this particular association and the results of Mooney and her coworkers, one wonders just how beneficial it is for an aphid to be associated with the Black Mound Ant?