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Sawfly Larvae And A Parasitic Wasp

September 14, 2014

There have been some new visitors to my garden recently. When I wasn’t paying attention a weed grew quite tall and once I realized it was there I discovered that it was absolutely covered in what appeared to be tiny caterpillars.

The larvae assumed a very odd posture by clinging to the leaves with their front legs and holding their rear end up in a big S shape. Whenever a breeze blew they would thrash their back ends quite aggressively.

Since it’s autumn I have no desire to raise any more caterpillars but I have been observing them outside for about a week now. Last night as I was watching them and taking some photos we received a visitor from our old friend the parasitic wasp!

Below I’ve included a bit more information, a few photos and a video. 

sawfly larvae

A two minute google search helped me to identify these larvae as those of the sawfly. They resemble moth caterpillars however there are differences, the most obvious of which is the number of prolegs. Prolegs are the stubby back legs that caterpillars have down the shaft of their bodies. Caterpillars only tend to have 5 or fewer pairs of these legs but sawfly larvae have more.

Their front (true) legs are long, segmented and quite strong. They use them to grip onto leaves and twigs. The larvae in the photo above was accidentally knocked from the leaf it was on. I was trying to make it flail by touching the leaf it was attached to but it just dropped off. That made me think that the caterpillar was either unwell or it was a different sort of defense mechanism.


I gently transferred it back onto the plant and it quite lethargically climbed its way back into position.

The thing that attracted me to these bugs the most was the way that they posture and flail their back ends. They line up all over the plant and outright draw attention to themselves as if to say… PLEASE eat me, I’m plump, tender and delicious!


There appear to be at least two batches of them on the plant now judging by their sizes and they certainly caught my eye.  Luckily for the larvae, I wasn’t hungry enough to eat them but as I observed them I saw that they caught the attention of someone else as well.

The video above is 30 seconds of one of the larvae demonstrating its waggle.


I noticed a delicate tiny wasp with beautiful iridescent wings was flitting all over the plant. After my experience with the exploding cabbage white caterpillars I knew that if I sat there for long enough I would get a chance to see her do what she came to do and sure enough, she did it.


The large clusters of larvae seemed to ward her off quite effectively by swatting their back ends around whenever they sensed her. The larvae at the edges of the clusters or by themselves didn’t fare so well.


To give you an idea of perspective, the largest of the caterpillars was only about 1.5 cm long. I wish that I was able to get better photos but even when manually focusing my camera I wasn’t quite good enough to catch the details. A very expensive macro lens would be so much fun to play with. (If you’d like to buy me one, I promise to send you bug photos)


So now that I know they were targeted by a parasitic wasp I do wonder if the “caterpillars” which were looking overly plump were in fact well fed or if they were due to burst very soon. (my money is on burst)

Just as I was getting the photos of the wasp injecting her eggs into the caterpillars my husband informed me that our little girl was ready for bedtime and as much as I love observing the bugs in my garden, story time with my little fluff a nutter will always win.

It was great to see the actual wasp (as opposed to just the grubs) and yes, it’s sad that those larvae are now doomed but it’s all completely natural and that little wasp is the one saving my garden from all those little beasties breeding on my plants so it only makes sense to me to let her do what she was born to do.


From → Garden, Scotland

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  1. Raising Cabbage White Caterpillars | StuwahaCreations

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