How to find caterpillars of the Scarce Tortoiseshell

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In summer 2014 one of my primary targets was to find caterpillars of the Scarce Tortoiseshell, also known as Yellow Legged Tortoiseshell (Nymphalis xanthomelas). My aim was to get a closer view on this species rearing caterpillars to adult butterflies.

The Scarce Tortoiseshell is a rather occasional visitor in Finland. However, in 2013 the species mass migrated from East towards Finland. As a result, early spring 2014 this butterfly was one of the most sighted species in southern Finland. A great baseline to try tracking down caterpillars early summer.

Please note that all my knowledge and data has been collected during one summer and within one region only. Behavior may vary depending on location, and in a “normal” year finding caterpillars will most likely be difficult if not impossible (due to the lack of the species’ presence). Nevertheless, I’ve been browsing through the greenery nearby. Here’s my wrap up from the experience I collected.

Location 1: Caterpillar colony

This location relates to one of my season highlights last summer, finding the actual caterpillars. On a cold and windy day I got lucky. On June 19th, the caterpillars appeared late that summer which must have been a direct response to poor weather conditions.

I knew in advance this location would be good as I continuously spotted Scarce Tortoiseshells flying early spring. Since late May I started monitoring willow in this area. Though I had little knowledge what kind of breeding behavior this species would have.

The caterpillars were found on willow, Salix phylicifolia to be precise. They were feeding above a small ditch. I’ve once read that Mourning Cloaks (Nymphalis antiopa) prefer similar conditions. The ditch keeps the growth doing well while providing some level of humidity to the caterpillars above. This particular location has continuous and direct sunlight.

Location 2: Abandoned colony

This sight was located just about 50 meters across a field from location 1 (see photo in the conclusion). It is well possible this colony was laid by the same female as the caterpillars found nearby. In this case (actually my first finding on the search), there were no caterpillars left. Clearly visible from a distance were the feeding traces. A closer look also provided details such as twigs covered by web and moulted skins.

Compared to location 1 this colony must have been further developed already. Caterpillars on location 1 were just moulting to the last instar. Location 2 looked like moulting took place already some time ago. Perhaps the twig was abandoned as the adult caterpillars separated (which I believe is a common behavior for this species at this stage).

Here the facts about the location. Again, caterpillars were feeding on Salix phylicifolia, a common type of willow in Scandinavia. The differences to location 1 were that no ditch was nearby. The caterpillars fed on a twig hanging low and outside of the growth, which in this case was much older already. The sight has direct sunlight from noon until evening.

Location 3: Abandoned colony

Later on in summer 2014, around mid July, I accidentally sighted a third location where caterpillars had been feeding. This location took me by surprise as it was just around the corner where we live. I did spot adult butterflies flying in spring though, but I didn’t expect caterpillars this close.

This time it must have been already weeks since the caterpillars pupated, most likely the butterflies had already hatched as well. The habitat had very much in common with location 1. The caterpillars were, yet again, feeding on the same type of willow (Salix phylicifolia). The growth was located in a ditch with the twigs hanging over water. The location has direct sunlight throughout the day. This spot is just between a small road and a walking path. Just 50 meters away there’s apartment buildings and a much frequented street.

Conclusion

Based on my findings in 2014 the common conditions for all sightings were following: The caterpillars were feeding on Salix phylicifolia and they were located on twigs clearly separated from other growth of the plant. The “colonies” were located in a height of 20-50cm above ground. As in location 1 and 3 they fed on twigs growing in the ditch, which causes them to be located very low.

Direct sunlight and open terrain were other conditions matched at all sights. Nevertheless, location 2 was next to higher growth. Tracking down a twig with caterpillars (or traces left by them) was pretty easy, assuming the location was right. Caterpillars feed the twigs completely “naked”. Should there be caterpillars left they color the twig black as they feed in large numbers. Other traces are skins after moulting and a web covering the twig.

Due to the poor weather conditions in 2014 it was difficult to get hint on the exact time when caterpillars could be found. As a result, I had to accept finding abandoned sights as well. Despite of the butterfly flying in large quantities in spring the species almost disappeared in summer when the next generation was expected. Spring 2015 will show if there’s any individuals left, or if Finland has to wait for the next mass migration to enjoy the beauty of the Scarce Tortoiseshell again.

I’m definitely hoping to get another chance to raise these caterpillars. By the way, approximately 90% of the caterpillars had parasites. I was able to observe the larvae of some Tachinidae species leaving its host before building a snow-white cocoon, usually hanging at the tail of the dead caterpillar. Therefor, young caterpillars or eggs would be yet a better finding.

2 thoughts on “How to find caterpillars of the Scarce Tortoiseshell

    • Thanks, I really appreciate that. I’m always a bit short in finding the time to write and document what I’ve spotted. Good luck, every once in a while there’s a rainy day and a short moment to get some writing done 😉

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