Plant corn very close together, this helps the pollen fall onto the corn silk. Each strand of silk is attached to a potential kernel of corn. When pollen from the tassels connects with the silk strands, they become fertilized and kernels of corn are formed. If you plant only one row of corn or just a few plants, the potential of all the kernels getting fertilized is not as likely and you end up with an ear of corn with empty spaces.
Corn earworms
Corn earworm moths, Heliothis Zea / Helicoverapa Zea, lay their eggs on the corn silk. When they hatch, the worms go into the ear and feed on the kernels of corn.
A good non-systemic pesticide that we have used for years is "Sevin", it works well and can be applied to edible crops.
Always read the package directions before spraying.
When to pick sweet corn
Once the silk starts to turn a little brown, it is a good bet the corn is ready to pick. Peel back the top of the husk to reveal the kernels, they should be well filled out. Also it is a good idea to do this before the corn is completely ready, so you can pick out any corn ear worms before they eat too much.