My guess is Leaf Miners.

ZM

zenman, I'm going with your guess too, the leaf miners. Had no idea those little pests could do this much damage! I've removed all the damaged leaves I could find, even found a little larvae on one of them.

Are there any effective preventive ideas for leaf miners? Right now I'm Googling but if someone knows a method with fair success, pleease feel free to suggest. I'm learning so much from you guys! =)

SH,

Perhaps they are seasonal, and now that you have removed the leaves containing them, they may not be a problem until next year. The only problem I ever had with marigolds was spider mites and, in Fort Worth's heat many years ago, they won despite my spraying insecticides labeled for spider mites. I thought it was ironic, since I had planted marigolds to repel insects. Well, technically, spider mites aren't insects.

We may need to wait until someone with specific experience on leaf miners and marigolds responds here or until your Google search pays off. I have seen occasional leaf miner damage, but never so extensive as yours, and I was always able to just remove a few affected leaves and ignore the problem. If leaf miners became a serious problem in my zinnias, I would try a systemic insecticide containing imidacloprid and/or acephate.

ZM

Systemic insecticides can contaminate the nectar and pollen of flowering plants, making the use of such chemicals a problem for the animals (insects and birds, usually) doing the pollinating.

Best recourse is to, as has been said, keep a close watch on your plant so that you can remove infested leaves before the population gets too high. Also, it's been reported that Neem oil, when applied in a timely manner, will act as a preventative against miners. Neem is an anti-feedant.