The giant scale Drosicha corpulenta: on uncovering its biology

Photo credit: Isabelle Vea
Photo credit: Isabelle Vea

May 4, 2014

Walking back from lunch and discussing with my boyfriend and a friend visiting from Kobe, I was looking down and spotted a large bug crawling on the ground. I first thought it was a pill bug with a very strange color, but half a second later, I kneeled and realized it was a giant scale insect, Drosicha corpulenta! What a sight, I have never seen this genus alive and was elated to be able to observe them walking. They were many on the ground, probably because it was very windy. We collected a couple of them.

May 7, 2014

Morning: I tried to look for the tree where they were feeding and found a few different tree species infested. I could not find any males.

After lunch: Next to my lab, I noticed a group of trees, and immediately was shocked to see the large infestation of the same scale insects. I pass by this group of trees everyday since I arrived in Nagoya University, how come I haven’t seen them before? Immediately, I try to evaluate the extant of the infestation. They are almost on every single tree. Trying to look for a male, I finally find one hanging on a tree trunk. End of afternoon: I go back to look for more males. I will find a total of 4 in the following days.

I kept a dozen females and the 4 males in a box for a few days. This video shows a male walking on a female.

The males died after 3-4 days of frantic copulation.

Photo of a male, dropping dead after his last female encounter. The endophallus (= penis) is out. This is the first time I see a scale insect penis (I have only seen slide preparations and fossilized ones). In a few scale insect families, the endophallus looks like this, and can be longer, with many spikes covering it. Other families such as the soft scales or mealybugs don’t have an endophallus. Notebook 4-8 May 10, 2014

With a colleague we bought a persimmon tree so we could collect and observe some females. Drosicha corpulenta feeds on many species and is considered a plant pest in Japan. Here is the list of plant species they have been found feeding on.

May 11, 2014

I collected a few females and put them in the soil of our persimmon tree. Half an hour later, I checked the tree and a few of them had climbed up the branches and settled to feed.

May 12, 2014

I checked the persimmon tree and the females are still feeding.

May 15, 2014

I went back to the infestation spot to collect some more individual for the persimmon tree. Most of the females are GONE. Where they were gathering by dozens, nothing is left. I was puzzled at first. What did happen? I thought that the gardeners of the university wiped out the branches of all trees, but this would be unlikely as even the oak higher branches don’t have scale insects anymore. Also, the branches really looked clean. So my alternative hypothesis is that the females marched down to the ground to lay eggs at the same time! Alternatively, on my persimmon tree, the females that were feeding on May 12 are also gone. I need to look at the soil and check my theory. According to Scalenet, the reference database for scale insects, there is an account on their biology in:

Kuwana, S.I. 1922. Studies on Japanese Monophlebinae. Contribution I: The genus WarajicoccusBulletin of Agriculture and Commerce, Imperial Plant Quarantine Station, Yokohama 1: 1-58. 

I went to the agriculture library of my building and could only find Contribution II of the journal…

May 20, 2014

The university  library managed to find a copy of the publication! This study from 1922 provides a biological account of the genus Drosicha (then Warajicoccus) additionally to morphological descriptions of all stages (juveniles, female and male). Beautiful hand drawn plates are also presented at the end of the booklet. Kuwana confirms that when the adult females are ready to lay eggs, they migrate to the ground and hide among tree leaves and other litter matter.

To be continued….

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