Loopers are light green caterpillars that completely or partially eat leaves of vegetable plants. These caterpillars are the larvae of variuos moths; the most common is the cabbage looper.
Bt works when it is eaten by the caterpillars, so should be applied to leaf surfaces where you see caterpillars feeding. Bt is not effective against eggs or adults, so it should be used as soon as you see small caterpillars, and then reapplied in a few days to treat the newly hatched.
Pick off and destroy these infested leaves. Read more about leafminers in the vegetable garden.
Poor seed germination
If your spinach seeds do not germinate well, the most likely cause is that the soil temperatures are too high. Spinach is a cool-season vegetable and prefers soil temperatures to be between 50 and 60 degrees F.
If the spinach leaves become faded yellow, the plant could need more nitrogen. Fertilize with a nitrogen-containing fertilizer. Overwatering can also cause yellowing of leaves.
Be sure and plant resistant varieties. Remove all plant debris when you harvest or remove plants. Rotate your crops, especially if you are having fungal problems.