Category Archives: ants

Western Thatching Ants (Formica obscuripes)

Posted on by Leave a comment
Formica obscuripes, Western Thatching Ant

I went for a walk yesterday on Three Corner Lake Road just to get out of the house. Springtime on San Juan Island has been cold and dreary, but sometimes there’s a moment or two when the sun peeks out, giving a bit of hope that the winter doldrums are coming to an end. Don’t get me wrong, I love the Pacific Northwest, but the Visitor Center’s claim that San Juan gets 247 days of sunshine a year is a stretch. I think they actually count the days when it is overcast and rainy as long as the sun peeks out for just a minute before the clouds darken the day again.

It was cold and I wasn’t expecting to see any insect activity, so I was surprised to find this long time ant colony with a mass of individuals near the top of the nest. The sun had been out intermittently, so I believe what was happening was the workers were at the surface to warm themselves (thermoregulating). There were a few straggling workers carrying thatch debris (conifer needles) to the nest from the forest. If the weather had been nicer, I would have enjoyed plopping myself into a chair to ant watch for an hour or so. They are quite pretty when you see them up close.

Western Thatching Ants (Formica obscuripes) are native to our region. They seem to like forest edge habitat, and you’ll find the nests situated in areas where there is some clearing to take advantage of the sun’s warmth. These ants are considered general omnivore-predators. They scavenge or prey upon insects and other arthropods for food. Foraging activity takes place both on the ground and on vegetation, including high in trees. Thatch ants also harvest honeydew from aphids and other homopterans as well as from extrafloral nectaries. You can read more about extrafloral nectaries or EFN’s (little nectar producing glands on plants) here https://bygl.osu.edu/node/1774 or check out one of my earlier blog posts about them here https://buggingyoufromsanjuanisland.com/2022/05/17/what-are-extra-floral-nectaries%EF%BF%BC/

Extrafloral nectary on Fruitless Cherry Tree

The ant workers are considered weakly polymorphic, which means they have only slight variations in size and shape within the worker caste as opposed to other species where workers may be monomorphic (all the same size and shape), or strongly polymorphic (a high degree of variability between worker sizes and shapes). You can read more about polymorphism in ants here https://www.antwiki.org/wiki/images/1/1c/Wheeler_1908f.pdf .

Given all the information you’ll find from pest control companies online about these ants and others, you might ask, why or if there is any benefit to having these ants around. The answer is YES! Western Thatching ants are beneficial in ecosystems because they not only aerate the soil with their burrowing practices in nest construction, but they also provide FREE pest control, helping regulate populations of other insects that defoliate forests, fruit trees, and even our vegetable gardens. They also help clean up the environment through scavenging dead animals. There is no need to eradicate these nests and if you get into ant watching, they might provide a bit of intriguing summer entertainment. Pull up your lawn chairs and sit and watch with your friends. As long as you aren’t poking the nest, they are not inclined to bother you at all.

Thanks for reading!

References:

Formica obsuripes. 2025. Bugguide.net. https://bugguide.net/node/view/31269

Formica obscuripes. 2025. AntWiki. https://www.antwiki.org/wiki/Formica_obscuripes

McIver, J.D., Torolf, T.R., Cimon, N.J. 1997. A supercolony of the thatch ant Formica obscuripes forel (Hymenoptera: Formicidae) from the Blue Mountains of Oregon. Northwest science., Vol.71(1), pp.18-29. https://rex.libraries.wsu.edu/esploro/outputs/journalArticle/A-supercolony-of-the-thatch-ant/99900502747101842

Bird notes 🎶

Posted on by Leave a comment

Something happened in the yard yesterday afternoon. We have a fruitless cherry tree in a cage that I didn’t plant. At least I think it’s a fruitless cherry tree. It sort of popped up on its own.

In the beginning, I didn’t really want it there, so I cut it down – a few times. It kept coming back, intent on sticking around. A couple of years ago, I just stopped trying to get rid of it. I even put a little cage around it. That was actually for the juncos that built a nest in the grass below that little tree though, to keep them safe from the mower.

Aside from me personally whacking it down a few times, the little tree has suffered other adversities. It has been attacked by cherry slugs (sawfly larvae). They ate its leaves.

Sawfly larva or Cherry Slug

It has had a few tent caterpillars munching too. One year that it was super dry, it lost its leaves and I thought it was dead. Only it came back again. I started to pay more attention to this little tree, even watering it in the summer. It’s not very tall, but this year it looks more healthy than ever before.

Last year, I noticed in the spring that it put out these cool little red bumps on the stem just above the leaf.

extrafloral nectaries
Formica ascerva ant at extrafloral nectaries

Those little bumps soon had little red and black ants visiting. I learned the red bumps are extrafloral nectaries. They produce nectar that attracts pollinators to the tree. I haven’t seen any flowers, so I’m not sure what would be pollinated. Maybe the tree isn’t old enough yet. I’m still learning. I sure need to figure out if I’m right that it’s a fruitless cherry.

Well, the other thing that is interesting is shortly after the little red bumps attract the red and black ants, there are little black aphids. I’ve seen this for 2 years in a row now. The ants tend those little aphid flocks like sheep. They are guarding them in exchange for the sugary honey dew the aphids produce.

The aphids and the ants seem to multiply. I have seen 2 or 3 ants per leaf with aphids. Not all the leaves have aphids, but maybe 1/3 of the tree does, and each guarded by the ants. The ants are Formica ascerva species.

They are interesting in their own right because aside from guarding the “sheep,” they also are fierce warriors, driving away (and killing) the large Camponotus sp. carpenter ants that try to invade every spring.

This colony of Formica ants lives under a bedroom of our home. They are welcome to stay since they don’t eat wood.

Today, when I looked out the window though, I saw something challenging the fearsome Formica ants. There were birds inside the tree caging and they were hungrily pecking and even tearing at the leaves with aphids. I grabbed and clumsily set up my camera to video the scene.

Birds attacking aphids farmed by Formica ascerva ants

I saw finches first, then there was a chickadee, and even a hummingbird. I thought I saw bumble bees, but they wouldn’t have been eating the aphids. Perhaps they were visiting the extrafloral nectaries or maybe even sipping honeydew produced by the aphids, but I wasn’t close enough to be certain.

Afterwards, I went out to survey the damage. I saw one leaf with lots of shreds. Aphid parts scattered about. Ants wandering somewhat aimlessly. Another leaf had an ant that died courageously in battle protecting his little flock.

I wanted to share my observations with you because a lot of folks really dislike aphids. Others also dislike ants and don’t understand their role in nature’s ecosystems. Maybe this will help others to understand there are a lot of hungry birds that eat aphids (and ants). Without them, the birds might not find enough food to feed their nestlings. It’s all part of the cycle of life.

If you see aphids your garden that you are worried about, you can mitigate some of their feeding damage by hanging a hummingbird feeder nearby. The hummingbirds have to have protein along with all that carbohydrate sugar water and they will most certainly find your aphids delicious. In fact, insects and spiders make up about 85 % of the diet of hummingbirds. Birds need bugs. And not all ants are pests. Some are extremely beneficial. I challenge you to take up bird and bug watching. You will not be disappointed!

Thanks for reading!

Below are more images of the birds feeding on the aphids inside the caged tree.

References and further reading

Bentley, B. L. (1977). Extrafloral Nectaries and Protection by Pugnacious Bodyguards. Annual Review of Ecology and Systematics, 8, 407–427. http://www.jstor.org/stable/2096735

Bugguide. 2024. Formica Ascerva. https://bugguide.net/node/view/497956

What are extrafloral nectaries?

Posted on by Leave a comment

Ants (Lasius sp.) on Big Leaf Maple Extra-Floral Nectary – May 17, 2022, San Juan Island, WA

When you study insects, or even birds for that matter, you start to understand you have to get to know plants a bit too.  It’s all connected.  

Plants (including trees and shrubs) provide food and shelter for many different species of animals.  Admittedly, I just don’t know a lot about the parts of plants, beyond things like a tree trunk, bark, limbs, branches, leaves, or stems or flowers, nuts, fruit.  The obvious parts.  

There are some not so obvious parts.  Like these extrafloral nectaries.  Huh? Sounds weird.  Keep reading. 

Extrafloral nectaries (EFN’s) are glands occurring on more than 2000 plant species in 64 families.  Extrafloral literally means outside of the flower. When we think of nectar, we usually think of little bees and hummingbirds flying around, visiting pretty flowers to sip nectar and in the process, pollinate all of our plants.  It’s just that plants are a bit more complex.  These glands are located in various places on plants (including trees and shrubs), and may be found on the laminae of leaves, petioles, rachids, bracts, stipules, pedices, fruit, etc.  (Mizell, 2019).  

These glandular secretions are a fascinating part of how plants attract and sustain a diverse, ecological community, providing sustenance for a multitude of species, including both pests and predators.  You can find ants, aphids, beetles (including ladybugs), bees, wasps, and possibly even birds utilizing this excretory faucet to sip what consists of mostly carbohydrate-rich sugar, but also comprised of a wide array of amino acids and other nutrients.  

Why are these important? Well, scientists are still trying to fully understand all of the diverse relationships around extra-floral nectaries. It is thought perhaps, beyond attracting organisms to a food source, they play a role in orchestrating a plant’s defense strategy against predators. They also are believed to provide a source of food and/or beneficial nutrients for various organisms during the off-season – when flowering and pollen sources are not available. They may also reduce conflict between ants and other pollinators by partitioning resources (Villamil & Stone, 2019).

Lasius sp. Ants at Extra-floral nectaries on Big Leaf Maple, 05.17.2022, San Juan Island, WA

Ant and aphid hanging out on extrafloral nectaries on Cherry Tree, San Juan Island, 05.17.2022

References

Bentley, B. L. (1977). Extrafloral nectaries and protection by pugnacious bodyguards. Annual Review of Ecology and Systematics, 8(1), 407-427.

Holopainen JK, Blande JD, Sorvari J. Functional Role of Extrafloral Nectar in Boreal Forest Ecosystems under Climate Change. Forests. 2020; 11(1):67. https://doi.org/10.3390/f11010067

Mizell, R. 2019. MANY PLANTS HAVE EXTRAFLORAL NECTARIES HELPFUL TO BENEFICIALS.  UF IFAS Extension Bulletin. https://edis.ifas.ufl.edu/publication/IN175

Villamil, N., Boege, K., & Stone, G. N. (2019). Testing the Distraction Hypothesis: Do extrafloral nectaries reduce ant-pollinator conflict?. The Journal of ecology, 107(3), 1377–1391. https://doi.org/10.1111/1365-2745.13135