It is leafminers damage. One of the common insect pest on vegetable crops. There are several species of leafminers like vegetable leafminer (Liriomyza sativae), serpentine leafminer (Liriomyza trifolii), and the pea leafminer (Liriomyza langei). The adult flies insert eggs into leaves. The hatched maggot feeds on internal content of leaves. This results in appearing of twisting trails (or mines) on leaves. The mature maggots fall out of leaves and pupate in soil.
The insects are very active during warm weather. It take just two weeks to complete a life cycle and can produce 5 to 10 generations/year.

Generally leafminers are not threatening to plants, but often necessary to manage the highly visible tunnels in leaves that can reduce crop value.

Management:
1. Squeeze the leaf at the tunnel between two fingers to crush any larvae
2. If few leaves are infected remove and destroy them
3. Use yellow sticky traps to catch adults
4. Maintain proper health of plants by applying organic fertilizers, soil amendments and proper watering to allow plants to outgrow and tolerate pest damage.
5. Cover soil with plastic mulches to prevent larvae from reaching the ground and pupating.
6. Spray neem oil to kill/repel the insects.