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Every year around this time I take my basil outside. After a couple of days maybe a week, transparent spots begin appearing in otherwise perfectly good leaves. Last year I had to throw it all away and start over inside.

This year I'm hoping to treat it early and save him, but I have no idea about what it could be. Here's a photo of the first spots appearing. They tend to be large single spots rather than diffuse transparent dots. No sign of webbing for what I can see, no visible insect anywhere.

Any idea?

symptom

UPDATE: I found this guy under one leaf, what is it? it's about 2mm long. Can I spray something to prevent further invasion?

larva

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Answering my own question in case anybody else stumbles here with a similar problem.

It's a larva of some kind of moth. I believe cabbage moth but no way to tell for sure. The youngest ones leave those transparent spots but they soon eat the full leaf as soon as they grow a bit. The infestation wasn't limited to basil, all leafy herbs got seriously damaged.

I finally beat them treating the plants with BT (bacillus thuringiensis kurstaki), a bacterium whose spores are lethal for these little guys. A couple of days after the treatment you find black caterpillars hanging dead from the leaves.

I will probably have to repeat the treatment regularly as it's doing nothing against the adults.

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