There are
two main reasons why the napa cabbage does not form a tight head – variety and
temperature. Some varieties will simply produce
a firmer head than others. The best approach is to plant several
varieties and see which is best for your area. In tropical locations, obviously plant the tropical varieties.
Going hand-in-hand with the proper variety is temperature. Napa cabbage prefers cooler temperatures. Some area may be too warm to produce a firm head, especially if they are grown during the warmer months. For warm climates, try different planting seasons - fall, winter and early spring.
Concerning insect pests, aphids are quite common on cabbage and other crucifers. A soap or oil spray should work well. If worms are eating holes in the leaves, use BT (Bacillus thuringiensis).