Categories
Species Profile

Species Profile: Introduced Pine Sawfly

Diprion similis

Diprion similis larva at Algonquin Provincial Park, September 2019.

Sawflies are a group of insects that many people haven’t even heard of. Part of the reason is because, in appearance and behaviour, they are like a hybrid between two major groups: their larval stages look like caterpillars (larvae of Butterflies and Moths ie. Lepidoptera), and their adult stages look like bees or wasps (Order Hymenoptera). Despite appearances and lifestyle, it is the latter category that they actually fall under: Hymenoptera which also includes the Bees, Wasps, and Ants. The major features that set sawflies apart from their relatives is that they eat plants, and they don’t have the constricted “wasp waist”. You might find this a little confusing, as Bees certainly don’t have an obviously thin waist, but they actually do have a constriction between their thorax and abdomen, it’s just more difficult to see than in many wasp species.

Like many insect Orders, the name Hymenoptera refers to a distinct aspect of the members’ wings (‘ptera’ is derived from the Greek for wing). Hymenoptera doesn’t have an easy translation though, like say Diptera for the True Flies (di = two, ptera = wings). The beginning part of the word is either from the word “hymen” which means membranous, or from the word “hymeno” which refers to the Greek God of Marriage. Hymenopteran wings are membranous, but they also have tiny hooks that link their fore- and hind-wings, meaning that they could be said to be “married” wings as well (Grissell, 2010). Whatever the case, the group is one that includes thousands of species of wasps, bees, ants, and of course, sawflies.

The common name “sawfly” is describing the way the female sawfly lays her eggs. Instead of a stinger or stinger-like ovipositor (egg-layer) at the end of her abdomen (like most of the other Hymenopterans), the female sawfly has a saw-like ovipositor, a cutting tool that she uses to open up plant tissue, and then inserts her eggs within.

This is what the Introduced Pine Sawfly (Diprion similis) does to pine needles. D. similis prefers White Pine (Pinus strobus) as its host plant (in North America), but will lay eggs and successfully grow to maturity on several other pine species. The female lays about 10 tiny eggs inside a pine needle (Cranshaw, 2004). After inserting the eggs, the female seals them in with a secretion that hardens for protection (Wagner and Raffa, 1993). The larvae that hatch from the eggs begin to feed on the pine needles. For the first part of their life, they will remain together but begin to disperse as they grow older. These larvae prefer to feed on needles that are at least 1 year old, probably because the younger needles are full of more toxins (Wagner and Raffa, 1993). As they consume needles, they grow, from 2.5 mm long upon first hatching to almost 3 cm before the larva is said to be “mature”. They don’t grow continuously, but rather have to molt and enter a new size class each time they’ve gained enough nutrients. For female larvae, they have six growth stages between molts and the males have five (CABI, 2020).

During this time, you would be forgiven for thinking they were caterpillars, because they look very similar. The way to tell caterpillars from sawflies is to count the number of legs. Their first set of legs will be six, and jointed for both groups, but they will also have a number of legs behind these called “prolegs”. If the larva you’re looking at has more than 5 pairs of prolegs, it’s a sawfly. Another giveaway is the distinct single eyes of sawfly larvae, as opposed to tiny ocelli (miniature eyes in clusters) in caterpillars.

Once they’ve reached their final larval stage, they spin a cocoon around themselves with silk, and transform within. Diprion similis larvae prefer to form their cocoons in the pine trees where they feed, rather than on the ground like many other sawflies.

In Europe and most of North America there are two generations per year, which means that what happens next depends on what time of the year it is. If the larvae have grown enough and created their cocoons in the summer, they will develop within in about 2 weeks into adults, but if they have reached this point near the end of fall, they will enter diapause (essentially insect hibernation) for several weeks before emerging in the spring (CABI, 2020). When they emerge, the adult sawflies are entirely different creatures, just as butterflies and moths are very distinct from their caterpillar young. The adults have wings, and with these they search for mates.

Adult Male D. similis, displaying the feathery antennae used to track down a female (Photo credit: Scott R. Gilmore.)

Males are attracted to females by pheromones (a chemical signal between members of the same species), as one would guess by the male’s elaborate antennae. The males can be attracted to a female across 61 m of open field, which is a great distance for an insect only a matter of centimeters long (Wagner and Raffa, 1993). Once mated, the female lays eggs in pine needles, and we are back at the beginning of their life history.

One note about mating: it isn’t necessary for the female to mate to be able to lay eggs. She shares with the other Hymenoptera a bizarre (to us) chromosome setup known as haplodiploidy. Females have one set of chromosomes (the mother’s) and males have two (mother and father). What this means in practice is that a female sawfly can lay an egg that will develop into a fully functional male offspring without ever going through the trouble of mating. This has implications for the spread of such organisms, as not all members of the population need to pair up to contribute to the next generation.

Which brings me to my final discussion of this species: they are commonly referred to as the Introduced Pine Sawfly because they were accidentally introduced into North America from Europe, likely in plant nursery stock imported in 1914. They have become well established in North America since then. Thankfully, they only very rarely reach a high enough population density to be considered an “outbreak” invasive species, and though they feed on tree leaves (needles), many predators and parasitoids feed on them (Wagner and Raffa, 1993).

The last time we were camping at Algonquin Provincial Park, I encountered quite a few of their larvae likely because they were in the fairly mobile phase before finding a spot to spin a cocoon (it was the end of September, the beginning of October). They may be an introduced species, and they may feed on White Pines, defoliating some of the branches, but as with any organism, they have a story all their own, and I think it’s worth telling.

Diprion similis larva hanging onto the end of a pine needle in Algonquin Provincial Park.

References:

Wagner, Michael R. and Raffa, Kenneth F. Sawfly Life History Adaptations to Woody Plants, 1993.

Cranshaw, Whitney. Garden Insects of North America. 2004.

Marshall, Stephen. Insects: Their Natural History and Diversity. 2006.

Grissell, Eric. Bees, Wasps, and Ants. 2010.

CABI, 2020. Diprion similis. In: Invasive Species Compendium. Wallingford, UK: CAB International. www.cabi.org/isc.

Categories
book review

Book Review: Flora of Middle-Earth

Despite paying a lot of attention to the little invertebrates that scurry about in the undergrowth, I tend not to pay too much attention to plants. This isn’t on purpose, or because I dislike them for any particular reason, but I think it’s difficult for us to look at them in the same way that we look at animals because they don’t move about (on the same timescale or in the same ways) and they don’t seem to display varied behaviours. If we can move past these false ideas about plants we may realize that life doesn’t flourish against a green background, but rather, the drama of life plays out amid the foliage and thorns and seeds and roots just as much as it does amid the fur and feathers, claws and teeth.

One book that has opened my eyes to this world of green, growing things is Flora of Middle-Earth by Walter S. Judd and Graham A. Judd. 

I have been an avid fan of Tolkien’s mythos and the science of living things for most of my life, so when I found this book, I felt that it had been written specifically for me. What really drew me to the book was that it was written by a world-renowned plant scientist and this knowledge shows through details of plant biology and ecology within its pages. The book succeeds on both a scientific and a literary level, as the author and illustrator understand plants and Middle-Earth extensively. They even draw on the History of Middle-Earth series which are unfinished manuscripts by J. R. R. Tolkien put together by Christopher Tolkien, to fill out the botanical landscape of this fictional, yet powerful world. The first chapters give an overview of plant biology, evolution and ecology, as well as outlining the biogeography of plant ecosystems in Middle-Earth throughout its known history (First to Third Ages). After this, there is a chapter that contains a key to identifying the plants that are detailed through the majority of the book so that if you encountered a plant in our world you could follow the descriptions to its genus or species. This section isn’t particularly useful in my opinion because although Tolkien mentions over a hundred plant groups within his legendarium there are many more plants that exist, so that you could very easily follow the keys to a dead end. Next is a chapter devoted to the two most important plants in Middle-Earth: Telperion and Laurelin, the Two Trees of Valinor. This was interesting, seeing where Tolkien might have drawn his inspiration for the fictional Trees from plants that exist in our time and place. 

Following this is the largest section of the book which runs through over 140 plant types mentioned in Tolkien’s writings. For each plant, there is a quote containing a mention of it, a general overview of its place within Middle-Earth ecology and culture, a study of the plants names (how Tolkien can you get?), a description of the plant’s ecology and biology in our world, a mention of its place in human culture and a botanical description of its form. Alongside this impressively detailed treatment, many of the plants receive a woodcut-style illustration which shows them in the context of Tolkien’s stories and world. 
The final chapter is a Note from the Illustrator, so if you’re interested in this book primarily for the artwork, there is a description at the end of how and why he created the illustrations the way he did.

Part of what enhanced my appreciation of the book was reading it while in Algonquin Provincial Park, a place filled with memory and meaning for myself, just as Middle-Earth is. Reading about a plant’s place in Tolkien’s writings while encountering some of the same plants in Algonquin Park was an experience that is a microcosm of what is so special about this book and the Tolkien legendarium as well. Tolkien’s writings shed a new light on the world around us, just as this book sheds light on a piece of that world (Middle-Earth) and our own world.

The greatest accolade I can give this book is that I learned a lot about Tolkien’s Middle-Earth and the ecology of plants in our own world. If you’re interested in either of those things, I would highly recommend this excellent, beautiful book.

Algonquin Park in Autumn.