Caterpillars in the Greenhouse

[plantmark]

For the last couple of years caterpillars have ruined my tomato plants in the greenhouse. If I cover the windows and door with fine mesh now to keep out butterflies would this resolve it, or might there be pupae in there already?

I have considered Nematodes but treatment too expensive.

[DavidT]

I`ve not heard of caterpillars attacking tomatoes. What do these critters look like?