First record of the tortoise beetle Cassida seladonia GYLLENHAAL, 1827
in Hanover (Lower Saxony) and observations on its biology

A warm day in early summer, a long foray through nature. Back at home, we find a caterpillar casually strolling over our clothes; a stowaway. Which nature lover hasn't experienced this? Sometimes we are also amazed by the miracle of the appearance of an insect that was unintentionally introduced with collected substrate. Sometimes such guests are unwelcome, for example when they blatantly start feeding on the introduced substrate, or on the insects we want to rear with it. We usually hurry to detain the intruder, tear it off a strip and then compliment it out.

Fig. 1: Adolescent larva of Cassida seladonia, introduced into the garden with cudweed, observed on June 18, 2019.

But sometimes such a chance find is just the beginning of something bigger. In the spring of 2019, I equipped a flowerbed in the garden with native plants that I gathered from the surrounding area. Among them was a slender cudweed (Filago minima): a very delicate, silvery plant with inconspicuous flowers that I had fetched from an oligotrophic grassland in the middle of Hanover (30 km away) – a very valuable biotope by local standards, but unfortunately abused as the city's largest dog toilet.

Fig. 2: First discovery of an adult Cassida seladonia on June 19, 2019 in Hanover, on strongly damaged slender cudweed (Filago minima).

On the evening of June 18, 2019, I discovered a beetle larva on it (see Fig. 1). Its typical "tail fork" (consisting of two long spinous appendages) laden with old skin remains and excrement, laying protectively over its body, I recognized it as a shield beetle (Cassida sp.). The subsequent research brought the astonishing outcome, that it had to be Cassida seladonia, the cudweed tortoise beetle. An narrowly oligophagous species, especially of cudweeds, less of Helichrysum and Gnaphalium, which has so far only been found in very few places in Germany and is classified in category 1 on the Red List, i.e. is "threatened with extinction" [RHEIN­HEIMER & HASSLER, 2018].

Of course, I immediately set out for Hanover the next day, and promptly discovered an adult beetle in the inflorescence of a cudweed (see Fig. 2)!

The seeds of enthusiasm were sown. As a result of my efforts, more adults and larvae appeared on the scene (the latter partly brought in with other plants picked up for breeding), and I threw myself eagerly into the observation of the animals.

But first I would like to describe the location in more detail. The Kugelfangtrift – including the adjoining glider flying area, which is somewhat more nutritious – is a landscape protection area of 60 hectares in Hanover-Vahrenheide. Acidophilous grasslands prevail (Nardus grasslands and sand grasslands), with embedded, sometimes water-bearing small bodies of water, which were probably created by target practice. From 1890 until shortly before the turn of the millennium, the area was subject to military use.

Fig. 3: Feeding traces of Cassida seladonia on Filago minima. Visible larvae are marked with an arrow.

Fig. 4: Remarkable camouflage of young larvae of Cassida seladonia, which stay at the base of the host plant in resting phases.

The extreme lack of nutrients and lime, and the rapid drying out of the soil, which greatly slows down the scrub encroachment process, can also be attributed to the fact that there has never been any agriculture in the past. Podsol is the predominant soil type. Due to the temporarily high level of groundwater, which is rich in iron and aluminum, the formation of bog iron ore takes place below the surface – in contrast to the otherwise typical form of Podsol [LANDESHAUPTSTADT HANNOVER, 2001]. Specific plant species of the area include, for example, silver birch (Betula pendula), creeping willow (Salix repens), common heather (Calluna vulgaris), matgrass (Nardus stricta), spike hairgrass (Aira praecox), red sorrel (Rumex acetosella), heath dog-violet (Viola canina), maiden pink (Dianthus deltoides), sea pink (Armeria maritima ssp. elongata), heath bedstraw (Galium saxatile), petty whin (Genista anglica), dwarf everlast (Helichrysum arenarium), Morison's spurry (Spergula morisonii), and the slender cudweed (Filago minima), the host plant of Cassida seladonia.

The feeding on the food plant took place as follows: the larvae burrowed under the thick layer of hair and pushed it along like a bow wave when grazing a leaf. What remained was a downy fluff. Some of the leaves were completely eaten; even flowers were not spurned as soon as they were present (see Fig. 3). I discovered this fluff, which was lifted from the plant, quite often when looking for larvae in the biotope, but hardly any were ever to be found on the same stem. The animals seemed to change their location frequently, which also makes sense, since many cudweed individuals only remain single-stemmed and then don't provide much food. With the density of their host plants in the biotope, vagabonding shouldn't be a problem either.

The young larvae always kept the fecal shield over their bodies to protect them (see Fig. 4 & 5). It consisted roughly in equal parts of their own excrement and the discarded skins of past stages. This is typical of the subfamily Cassidinae and, according to RHEINHEIMER & HASSLER (2018), serves as camouflage as well as to deter predators. Full-grown larvae, on the other hand, often did not cover themselves, and even if they were slightly disturbed they usually remained relaxed. The younger stages were yellow-brown in color, only towards the end of the development period, presumably only in the last instar did the beetle larvae turn green (see Fig. 5). As the animals prowled for food, they vibrated their tail fork at swelling, merging intervals.

Fig. 5: Cassida seladonia larvae with increasing age. (a): Larva in the first stage (L1), recognizable by the fact that it hasn't yet had any stripped skin on its tail fork. (e): Due to the almost complete absence of feces, the stacking of the exuviae is exceptionally well visible in this specimen. It is therefore to be classified as L5 – freshly hatched, due to the still disproportionately large head. (f): Full-grown, now green larva in the last instar.

When two larvae were ready for pupation, they attached themselves head-down to the flower stem or to the inflorescence and got rid of their fecal shield (see Fig. 6). The first one pupated on June 24, 2019, and the beetle emerged from the pupal shell on June 30, 2019 (see Fig. 7). This corresponds very well with the statement by RHEINHEIMER & HASSLER (2018), according to which the new generation should hatch from July / August onwards.
Fresh adults were almost completely green (except for the black eyes, mouthparts and antennae tips) and looked very ethereal. Only after a few hours did other areas of the body darken (see Fig. 8).

Fig. 6: The last steps to the imago. (a): Full-grown larva; it no longer holds its fecal shield permanently over it in a protective manner. (b): Prepupa fixed upside down for pupation; it has already got rid of its fecal shield. (c): Pupa resting head-down on the plant stem. The last larval skin that was pushed together at the rear end during molting, differs in color from the similarly spiky lateral outgrowths of the pupa. (d): Pupal exuvia after the emergence of the beetle.

Fig. 7: Hatching of the adult beetle (the last few seconds). The elytra are still gaping at the tip, but only six minutes later they had assumed their final shape (see Fig. 8b1).

Fig. 8: The maturation process. Pictures (a1) and (b1) show a newly hatched (immature) Cassida seladonia. The fully colored, mature imago can be seen in (a2) and (b2).

When keeping the animals, an attempt was made to imitate the strong sun and heat exposure of their habitat as well as possible. I planted cudweed on sand in a transparent plastic cylinder that was twice as high as it was wide, covered it with coarse gauze to allow for a good exchange of air and thus exposed it to the elements almost unprotected.

Since the summer- or winter-annual cudweed already takes its last breath in midsummer, there is not much left for the new generation of beetles to do. They became inactive at the end of July / beginning of August and withdrew to the bottom of the potted plant that was almost dead.

Up to this point in time, my three retained specimens, which I wanted to try to hibernate, were still green. I was very curious to see when they would turn "bright red", as RHEINHEIMER & HASSLER (2018) report – according to this work, this should happen "after a while".

Fig. 9: In the prime of life. (a1) and (a2) show a flawless free-range find from June 21, 2019 (Hanover), (b) shows an equally beautiful breeding animal from June 30, 2019.

After observing the inactivity of the beetles for a while, I decided to let them hibernate in the refrigerator – the local winter, which is more influenced by Atlantic climate, with its strong temperature fluctuations, especially on the southern side of the house, was too unsteady for me. To do this, I put the animals in a plastic Petri dish with a slightly moistened kitchen towel. They regularly changed their position in the vessel, but were immobile at every control. There was still no red coloration to be seen. Only their green color got a little warmer and reddish spots appeared on the shoulders; the edges of the elytra and pronotum also browned somewhat (see Fig. 10).

Fig. 10: This "old beetle", photographed on May 31, 2020 (a) or June 17, 2020 (b), had survived a whole year as a beetle: I saw it alive for the last time on July 7, 2020.

In mid-April 2020 I found out that two of the three beetles had not survived hibernation; I immediately put the third beetle outside, next to recently potted cudweed. The survivor – who became active again after a week in the fresh air – still didn't think in the slightest about changing his green dress to bright red throughout. It stayed that way until its death; on July 7th, 2020 I photographed it alive last time. Impressively, the little one lived a full year, which was perhaps helped by the fact that it couldn't wear itself out in the strenuous reproductive business, as it did not get the opportunity due to the lack of company after winter.

Since my intensive search for the larvae in early summer 2019 was not really successful – I only found three by eye, three I unnoticedly dragged in with plants – a new attempt was started on May 26, 2020. After an unsuccessful initial phase, I stumbled across a real kindergarten by chance: in an area of only 20 square meters, I discovered 27 larvae of Cassida seladonia within 1.5 hours (extensive photography included)! One animal was in the first stage, one was fully grown, and all the others were adolescent (see Fig. 5). The location can be described as a gently curved bump about 40 cm above ground level (see Fig. 11).

Fig. 11: No less than 27 larvae of Cassida seladonia were found at this point on the Kugelfangtrift (Hanover) on May 26, 2020.

Most of the larvae sat at the base of the plants, which were at most 5 cm high. At the time of the find (6–7 p.m.) some were busy eating. Most of them rested at the bottom of the cudweed stem and could easily be found through the feeding marks described above – directly on the damaged plant or nearby.

With this large number of individuals, I was also given the opportunity to observe their interaction with the ants on site. The beetle enthusiast will be happy to hear that these hymenoptera do not seem to pose a threat, at least there. Although they are relatively numerous on the Kugelfangtrift (unfortunately I cannot say anything about the species spectrum), they did not seem to notice the larvae or to classify them as edible: according to my observation they only touched the fecal shield briefly and then turned away.

The larvae of Cassida seladonia could possibly even benefit twice: they are probably not only spurned by the ants, but are also indirectly safer from other predatory arthropods through their mere presence. Besides, ants have a deterrent effect even on harmless photographers.

Acknowledgements

I would like to thank especially Dr. MICHAEL STERN from the University of Veterinary Medicine Hanover; he confirmed my initial identification on a specimen and gave me his time for a nice excursion into the biotope. Moreover, thanks are given to the rest of the kerbtier.de team for carefully confirming my identification of the first surprising larval find that was mentioned at the beginning. Last but not least, warm thanks are given to Dr. CHRISTOPH BENISCH, the webmaster of the extraordinary website kerbtier.de, for most of the translation of this article from German into English.

Literature

1. LANDESHAUPTSTADT HANNOVER – Umweltdezernat (2001): Landschaftsschutzgebiete der Stadt Hannover. Schriftenreihe kommunaler Umweltschutz, Heft Nr. 34. 103 pp.
   
   
2. RHEINHEIMER, J. & M. HASSLER (2018): Die Blattkäfer Baden-Württembergs. 928 S. Karlsruhe (Kleinsteuber Books).