The easiest way to keep them in check is to hand pick them and kill them. In general hornworms don't appear in huge numbers, although the damage one insect can do looks like you have more than there are in your garden. I've found that in the evening these pests have a tendency to climb to the tops of the plants where they are easy to hand pick and squash. I don't recommend spraying for them but if you want to, you can use Bt, Bacillus thuringiensis, which is a naturally occuring bacterium that will block the animals gastric activities and kill them. I'd start with hand picking and if that doesn't work go with the biological control Bt. You can buy it at most garden centers. I never recommend using a commercial insecticide on food crops."