Nice image showing how peacefully it is resting. Around 30 years ago, we used to try and get the lowest ISO film - ISO 64. ISO 100 was normal. I still remember a group photo that I had mistakenly clicked using a ISO 400 film in 1999 and while blowing up the grains were like golf balls. These days the high ISO ability of the cameras have improved tremendously. So you can safely use higher ISOs at least ISO 400 in most cameras. The newer cameras have better high ISO ability. When I first shifted to digital, with the EOS 10D, I always used to shoot in ISO 100. Then slowly moved to ISO 200 etc.

You will find photographers telling you to use ISO 100 because that is what they have heard from other photographers. Beginners with small lenses crop heavily because their subject is far off and appears tiny in the frame. When they crop they find lot of noise. So they shoot ISO 100. Most of these fellows don't know that they can also set their cameras to an even lower ISO - ISO 50. :-)

When you crop the grains become more visible. If you are framing your subject well and are not cropping or only moderately cropping then you can safely use higher ISOs. Also, another variable is how you expose your image.

If you expose image properly then no issues. However, if you underexpose then the dark areas will have less image information and also more noise. Generally if you expose to the right without blowing the highlights, then the image will have more information and less grains.

Mrudul and myself, we generally don't crop. So we often use higher ISOs to ensure that the shutter speed is adequate for getting a sharp image.

You are using macro lens. Even in macro photography ensuring that the subject appears large in the screen is a challenge. So if you want to crop a lot, then use a lower ISO. Else increase the ISO and you will be fine.