I'm not familiar with that pest on lettuce.

Suggest you contact your county's Extension Service office to ask. Locate that office with this clickable map from the USDA
http://www.csrees.usda.gov/Extension/

Here is a link that might be useful: locate your county's Extension Service office

Spinosad is a good Thrip killer and is considered safe for vegetables. Don't over use it though, or the Thrip will develop resistance.

Thrips are a major pest of lettuce but can be controlled with an Integrated Pest Management program. Floating Row Covers are a very good method for keeping the wee buggers away, especially since lettuces do not need the attention of pollinators. Use any pesticide with care since those that do control thrips are fairly broad spectrum and will also kill off the predators of thrips.

Here is a link that might be useful: thrips