Gardening Harmful insects Seasons Shrubs

The Azalea Sawfly: My Exception to The Rule

I share many horticultural ideologies with the Laidback Gardener. One of them is to avoid putting any effort into plants that grow poorly or are overtaken by insects or diseases. If a plant doesn’t grow well under my conditions, I get rid of it and plant something else.

That said, like any self-respecting passionate gardener, there are still a few plants that I love and for which I am ready to do ANYTHING! Even accept the worst! And that’s the case with rhododendrons (Rhododendron spp.).

A Passion for Rhododendrons

Rhododendrons form a large group of shrubs and in gardening circles, are usually subdivided into three categories. Small-leaf evergreen rhododendrons, large-leaf evergreen rhododendrons, and deciduous rhododendrons, the latter commonly known as azaleas.

It’s in this last group, the azaleas, that small green larvae appeared in my garden a few years ago. They quickly devour the leaves and then attack the flowers. In less than a week, an azalea plant can be completely defoliated. This voracious insect is the azalea sawfly.

The Azalea Sawfly: Two Pests In One

In truth, there are two species of azalea sawfly, Amauronematus azalea and Nematus lipovskyi, both native to North America. However, they are quite similar and cause the same damage.

Fingers holding an azalea leaf being eaten by several azalea sawflies.
Azalea sawflies are small, greenish and blend with the foliage. They’re hard to spot if they aren’t moving. Photo: Julie Boudreau, horticulturist

You don’t need to be able to tell them apart, but, for the form, here are the differences. In Amauronematus, the circumference of the eye orbits, the base of the antennae, the neck and the legs are whitish, while its sister is more orange.

In both cases, the adult is a flying insect with an elongated body, long erect antennae, beige legs and wings longer than the body. It’s called a sawfly, because its ovipostor is like a small handsaw, with sharp-toothed edges, allowing it to make incisions in plant tissues to lay its eggs.

An Early Bird Insect

The adults emerge from the ground as the leaves begin to emerge, in April where I live, and the females immediately begin to lay eggs. They deposit their eggs in the midrib of young leaves. Seven to ten days later, the tiny larvae get to work and start chewing the leaves, usually leaving the midrib intact.

Since the caterpillarlike larvae are small (often less than ½ inches/1 cm long) and they are about the same color as the foliage, they are difficult to detect and it is often too late to act once you discover their presence.

Fighting With Soap and Water

Azalea sawfly larva sitting a the bottom of a bowl of soapy water.
Azalea sawfly larva sinking to the bottom of a bowl of soapy water. Photo: Julie Boudreau, horticulturist

The beginning of the attack coincides perfectly with the flowering of early azaleas. The first flower therefore sounds the alarm. That’s when I fill a bowl with soapy water. I mix bout 2 tsp (10 ml) of soft soap in 2 cups (500 ml) of water… although, I actually never measure! Then I go off … on a bug hunt!

Yes, you heard me. I treat ecology very seriously, so I don’t use insecticides. Not even those approved for organic farming. I don’t need them. If a plant is infested with insects, I toss it into the compost pile without even flinching.

However, for my beloved rhododendrons… well, it’s just not the same thing! So, I hand pick the larvae and drop them into my bowl of soapy water. When they feel threatened, the hind end of sawfly larvae rears up, while it continues to hold onto the plant with their mouth and legs, taking on the shape of a question mark. In 30 minutes I can make a careful round of all my deciduous rhododendrons (azleas) and harvest about one hundred. Then I go back two or three days later, because eggs hatch over about 2 to 3 weeks.

Any well-fed larvae that have escaped my hawk eye drop to the ground to metamorphose into pupae. This is how they will spend the winter. Luckily, there is only one generation per year.

Worthwhile Effort

An azalea sawfly eating an azalea flower.
Ultimate insult: an azalea sawfly eating an azalea flower! This is just unacceptable! Photo Julie Boudreau, horticulturist

This rigorous hunt is very essential in order to preserve a maximum of foliage. I have seen azaleas completely defoliated within a few weeks. And for rhododendrons and azaleas … well, let’s just say the foliage doesn’t regrow as quickly as it does in other shrubs, say willows or spireas. With azaleas, every leaf counts. The effort is therefore well worth it and the resulting bloom all the more spectacular.

Spring is my favorite season, and my spring would be nothing without my twenty azaleas covered with flowers… and leaves!

Julie Boudreau is a horticulturist, a graduate of ITA in Saint-Hyacinthe. She has been working in the horticulture field for over 25 years. She has published a dozen books and participated in numerous television and radio programs. She is a teacher at the Center de formation horticole de Laval. Passionate about her profession, Julie Boudreau is dedicated to promoting gardening, garden design, botany and ecology, in all its forms. Julie is an epicurious with a strong penchant for everything that is pronounced in Latin.

4 comments on “The Azalea Sawfly: My Exception to The Rule

  1. Judy Villeneuve

    Are there no nematodes that would deal with the problem like pack attack that does kill larvae of saw fly, that I have uses to get rid of them striping foliage on columbines with great success.

  2. Worked with Azaleas for 54 years & never seen this worm. I spent the first 12 years in a nursery rooting, weeding, repotting & planting Azaleas. So I am confused about this insect, I worked in three counties with Azaleas, including the wild swamp Azaleas & never seen a Azalea with there leaves being striped off.

    • Carol de Se?vigne?

      Thank you Marian for your comment. It was very eye opening for me. I had some idea of the impact
      of non native species causing havoc but not to the extent you describe. This summer is the first time I have ever used a ‘chemical’ to combat the defoliation of a much loved Aruncus. Picking the sawflies off was not an option. I couldn’t even see them! I will think twice before I throw an infested plant on the compost.

  3. marianwhit

    I agree that if it does not grow well get rid of it…IF it is horticultural stock (ie grown by humans for humans, especially human entertainment). I do the same thing with plants that grow TOO well, such as invasive species.

    But I wonder, is there anything that would be worth using insecticide on? How about saving the hemlock, or chestnuts, or elm, white pine, or beech, or alternate leafed dogwoods, or viburnum? Do we just stand by (with a “ban everything” attitude), and let a myriad introduced insect pests such as Asian Longhorn Beetle (which eats and kills some 40 species) just simply knock out parts of our ecology instead? Do we know how to build houses without wood? How long will it be before houses are made of plastic (etc.)?

    And what of plants introduced by people that overgrow, displacing native plants from their home ground, and outcompeting them for their evolved pollinators, should we just allow them to disrupt ecological function such as out-compete plants that make up the food supply for insects that feed birds (or our farmed food supply)?

    Human introduced problem plants and insects that can cause great harm to other living things seems like the time to break out the tools we have to fight them. I wish we had better tools…such as insecticides that only kill a particular introduced pest, or herbicides that would only kill the targeted species of plant. We have known now, for 100 years that moving living things around the planet can have serious, serious consequences that we do not understand much less are able to respond to….with a bowl of soapy water. Most people can’t even identify plants any more, much less care how they grow or where they come from.

    I am now seeing places where there are NONE of the plants that evolved in that space because so many have been introduced…and the few that are left are assaulted by opportunistic introduced pests. Is it any wonder in Canada that we have about one in four plants not native to here, and we also have bird losses of 30%? How does that equate to the number of plants declining because of insect introductions?

    How much of our forests are we going to lose while we stand back and take no responsibility for the things we have done? The “holes” in our ecology are rather large already. Scary large. Hemlock woolly adelgid is too small to hand pick with soapy water. In fact, people can easily carry the pest (from Japan that the trees have no defense for) from grove to grove without even knowing it.

    Also, I would not put insect infested material in the compost bin. The point is to eliminate the pest (as in kill them all) if they kill plants to the ground (especially plants we need for food). If a plant is surviving the impact and able to make seeds for future generations, natural selection (and future generations of that plant over a few centuries or millennia) MAY mean the species’ survival (if not hit with successive invasive species in a short period of time like the beech have had three in less than 100 years…we are in the process of losing them now). Things like the Viburnum Leaf Beetle and Golden Canker (both understood to be introduced via movement of nursery stock) are basically wiping out the species where I live…now migrating birds are missing a key part of their nutrition needed to make the trip to the tropics (they can’t just jump on a plane…and cause more ecological harm). The same is happening with all of our native ash trees from the Emerald Ash Borer, and this is NOT a complete list of the problems…not by any means.

    Fire ants (also found in nursery pots moved from place to place) kill baby birds in their nests…jumping worms destroy soil structure and make gardening with anything but co-evolved (Asian) plants very difficult.

    When will we have enough evidence that we need to be growing (preferably native) plants from seed, and stop jumbling up the ecologies world-wide so effectively? When will we start understanding the above, which is happening at the ground level too in some of the smallest but most essential plants, such as mosses and grasses? Is it worth trying to protecting the future of our forests and lands with the carefully considered and judicious uses of herbicides and insecticides? Do we have a choice? When the general public adopts a knee-jerk “all or nothing” rejection of “chemicals”, those on the front line of some of the above problems assume “the masses” are incapable of making intelligent (hard) decisions. So they work to take the power to make decisions away from us. I will take a carefully targeted one to two shot application of a chemical to give a species of plants a chance at survival ANY day rather than let a living, growing biological organism imported without its own natural population checks eliminate large swaths of plant life that help support our species survive. Folks, THIS is the Anthropocene, and there really are no simple answers.

    Honestly? I never in the world thought I would be supporting the use of “chemicals”. But then, I did not understand the scope the ecological “Pandora’s Box” that was opened with rapid world trade in living things. Under public pressure to not use “chemicals”, what are people doing? Importing more biological organisms. Great. I own an acre of fairly native ecology that functions pretty well, but am watching it function less well each year, and defense of it (90% without chemicals) is a full time job. We need to take a FAR more nuanced view of the problems we face and the few defenses we have to protect the unique planetary ecologies and resources we have. Because every animal here, including us needs them and does not adapt quickly to being thrown into a biological melting pot. We have, recently, with COVID, been given a warning by the forces of nature that these biological entities can also target us directly and then be rapidly transported world-wide. We will need all of the weapons we can gather and learn as much as we can…now.

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