Q: What is this on my tomato leaf?
A: It’s the trail left by a tomato leaf miner. Leaf miner adults are small, black and yellow flies. Eggs are inserted in leaves and the resulting larvae feed between leaf surfaces, creating a meandering “mine.”
Mature larvae then leave the mines and drop to the ground to pupate. The life cycle takes only 2 weeks in warm weather; there are seven to ten generations a year.
My recommendation is not to spray insecticide immediately. Several species of parasitic wasps attack leaf miner larvae; if left undisturbed, beneficial insects often keep leaf miners numbers below economic injury levels.