Tiny house, invertebrate style

by Brendan Keegan, Landscape Crew Gardener

January 8, 2018

Tiny house, invertebrate style

Hollow logs filled with Japanese knotweed reeds

Hollow logs filled with Japanese knotweed reeds are an attractive habitat for many invertebrate species.

Last spring, several colleagues and I spent half an hour putting together a “tiny house” for invertebrates. We filled hollow logs with dry Japanese knotweed segments and drilled holes around the border, with the goal of attracting solitary bees. Over the course of the summer, these houses were busy with life. Now, in the winter, they are host to overwintering species of bees, ants, spiders, and beetles. Below are a few of the noticeable organisms that made these habitats their winter home.

A leaf cutter bee created a nest in this cavity. It appears that a predator, probably a bird, may have gotten to the first cocoon.

Given their partially shaded and rather isolated location, the logs were surprisingly successful at attracting solitary bees. Solitary bees are native bees that do not live in colonies. Rather, individual females search for cavities (typically made by boring beetles) and use them to create nests to lay their eggs. The majority of the bees that visited the logs were leaf cutter bees, species in the genus Megachile.

During the spring, females flew back and forth from their tunnels with pollen and circular leaf cuttings. They laid eggs in their tunnels, left each one with a packet of pollen, and then created walls to separate them using the cut leaves.

A mason bee (species in the genus Osmia) built this nest. These solitary bees use mud instead of leaf cuttings to make compartments and seal their tunnels.

When the tunnel was filled, they used more leaves to create a cap. Over the summer, larvae hatched from the eggs, fed on the pollen, and matured into adult bees, ensconced in protective cocoons. They will emerge from the nests early next spring to start the process all over again.

However, bees were not the only invertebrates to use this habitat. Perhaps my favorite inhabitant is the “acorn ant”, Temnothorax curvispinosus. These are common ants in the forests of New England. However, they are so small (workers are only 2.4mm long) that they often go unnoticed, unless you are on a picnic and happen to drop a cookie. This is a pity, since their honey-colored bodies and inquisitive movements are fun to watch.

Nest caps keep intruders and elements out and trap moisture within. In the summer, acorn ant workers often perch in the entrance hole, perhaps guarding against other ants or maybe just warming up in the sunlight.

In the summer, acorn ants build tiny nests in hollow twigs, curled leaves, and nuts (hence their common name). If the entrance to the nest is too large, they make it smaller by building a thin cap of detritus. Acorn ants are polydomous, meaning that a single colony often consists of several satellites. This may be a beneficial risk management strategy, since it reduces the probability that the entire colony (rarely more than 100 ants) will be destroyed by an attacking species, runoff from rainfall, or the footstep of an unsuspecting jogger. In the winter, acorn ants gather back together at a single nest site.

A queen ant from a widely recognized, but much maligned, species was nestled at the bottom of another knotweed segment. This large, black ant is a carpenter ant, Camponotus pennsylvanicus. Carpenter ants typically establish colonies in tree cavities, where there is lots of moist, rotting wood. They are also polydomous, but their colonies number in the thousands of ants.

Although this queen ant looks dead, she is actually just in dormancy.

You can tell this is a queen ant by her distinctive physical characteristics. A queen is typically much larger than her workers, has a bulkier thorax (middle segment), and a longer gaster (bulbous rear segment). The thorax is bigger since it stores the wing muscles she used during her mating flight. The gaster stores the large ovary necessary for producing thousands of offspring.

Carpenter ants are hated by many Boston homeowners, often unfairly. This is due to their tendency to establish colonies inside houses and to the common misperception that these ants eat wood (they do not). Instead, they are experts at locating moist and rotting wood, which they merely excavate to build their nests. In forests ecosystems, this attribute makes them an extremely important organism, since they accelerate the decomposition of rotten logs. So, homeowners, take heart! Carpenter ants are not eating your house, they’re just an unfortunate indication that you may have issues with wood rot.

Although all of these organisms are common in New England, finding them in our small, constructed shelter illustrates the importance of urban habitat for invertebrates. At the Arboretum, we are preserving and increasing invertebrate habitat by letting much of our dead, herbaceous material stand during the winter months. Gardeners and homeowners can do the same by leaving some perennial stalks through the winter (instead of the customary fall cleanup) and by making insect hotels in the spring. Do this, then take out a magnifying glass and look for the results during a bitterly cold December afternoon. You’ll see that tiny houses can provide great opportunities for tiny organisms, one hollow stem at a time.

4 thoughts on “Tiny house, invertebrate style

  1. Enjoyed the article, very informative with good photos. I usually just put out native bee tubes but putting out for others is great. Letting people know the ants are not eating the wood just making a home in a moist area, is just letting them know of a problem. We need to learn to get along and live with all the life around us. Deb.

  2. Thank you for this article. It so very well reminded me of the intersection of our interest in trees and shrubs with the need to take interest in the other animals who share this world, their vital role, and the necessity of not always relying on our first judgment of who is “good” and who is “bad”. For one, I will take more steps to make my garden more invertebrate friendly. I urge all us to check out the resources of the Xerces Society for Invertebrate Conservation http://www.xerces.org. The current issue of their Wings journal takes several good looks at the role we urban/suburban dwellers can play.

  3. It’s so neat to hear about all of the different species that ended up colonizing those “insect hotels”! Fantastic piece, incredible writing, and great efforts underway (and such a clever use for knotweed!)

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