Gastridiota adoxima

On the 2nd of April 2020, I found a caterpillar. It was an incidental find, but it captured my attention. I nicknamed it the capture-pillar because it was so difficult to photograph (it would not sit still). As my first observation of this caterpillar occurred in the grass - I didn't know the host plant. I could guess it was one of the trees nearby, which narrowed it down to Silky Oak, Wattle, Camphor laurel, and Moreton Bay Fig.

Armed with my photos, I started searching but could not find anything that looked like it. Asking around, it was suspected it may be a Hawk Moth species due to the tail of the capture-pillar, congruent with the tails seen on many Hawk Moth caterpillars. I'd only just started my naturalist (i.e. bug finding with purpose) journey. I joined an Australian Moth group which eventually lead me to join iNaturalist. Thus, I waited and monitored the trees. Keeping an eye out online for any caterpillar that may resemble my capture-pillar. And finding other insects.

In 2020 I found a further two caterpillars, on the 12th and 19th of April, located on one of the two Moreton Bay Figs. These had the same distinctive features, however, slight colour variation. The first one I found was greyer, with the subsequent finds being browner and greener. I particularly like the 4 'tentacles' that are mint green with black specks and electric blue and orange at the base. As well as the pink tail (sometimes with a noticeable yellow tip). Its body language was intriguing; when disturbed or unrelated, it would shift it's 'tentacles' from resting against its body to extended in the air and / or curl the tip of its tail downward in a spiral fashion.

On the 19th of April, I found a seemingly unrelated egg sac/cocoon on the fig leaf's underside. It was metallic green with an intricate construction of a brown anchor to the stem. It was another mystery, more on this later.

Fast-forwarding to 2021, I began checking the fig tree more frequently as we were approaching April. Hoping to find more, but wondering if I would see one at all this year. I found an old egg sac/cocoon in my searches, and as it had a hole, I pulled it apart. It was strongly constructed; the outer green silk layer especially, inside the silk, was a dark brown, thin layer that crumbled relatively easily.

On the 5th of March 2021, I found a caterpillar. I brought it inside on the leaf it was eating and placed it in a safe container. I provided it with other leaves. It ate the rest of its leaf and another before pupating early on the morning of the 7th of March 2021. Initially, the cocoon was a light brown, caramel or straw colour; however, I noticed the construction was similar to the metallic green cocoons I found previously on the tree. I'd been entertaining the idea of bringing one of the egg sacs/cocoons inside, and this similarity lead me to find one. I selected one I'd noticed on the tree recently and detached it and the leaf (combined) from the tree, and placed it in a separate container inside on the 7th of March 2021. It emerged from the cocoon the same day. I released the moth on the evening of the 7th of March 2021 after taking photos of it.

Initially, the abdomen shape guided my search to Silk Moth species (Family Bombycoidea), and the wing pattern/colouration lead me to Gastridiota adoxima (http://lepidoptera.butterflyhouse.com.au/bomb/adoxima.html). Silk Moths are categorised in the same superfamily as Hawk Moths, which explains some of the caterpillar's features. Meanwhile, the cocoon that formed on Sunday morning was developing a green tinge, and I began to suspect further the caterpillar was linked to the cocoon and newly emerged moth. I've since found other records of this species here on iNaturalist, which support my finding.

The cocoon is even greener today (8th March 2021), not yet as green as the one that the moth emerged from or the others on the tree. I anticipate it will continue to become greener.

Currently, the only remaining life cycle question I'm currently curious about is a) what the earlier instars of the caterpillar look like (is it completely different?), and b) if I've not seen one, where do they live on the tree? As all of the caterpillars I've seen that I've linked to this species so far are around 7-8 cm long and 4 mm thick - which seems to be just before they pupate.

Posted on March 8, 2021 10:40 AM by matilda_c