Answer: I assume you mean tomato hornworms, which are fairly large, fat, green caterpillars that chew foliage. They have diagonal white stripes and a tiny "horn" sticking up at the rear. They are most easily handpicked. Another control method is to sprinkle both sides of the leaves with Bt (Bacillus thuringiensis), which is a natural occurring biological insecticide. After consuming treated foliage, a caterpillar will stop feeding within a few hours and then die in a few days. It is only effective for caterpillars! It will not work for other types of insects. Without knowing exactly what you mean by "white gnats" it is difficult to recommend a treatment. I think you might be referring to whiteflies, tiny little insects that fly up in a cloud if the plant is brushed against.
I always start with the simplest method first, and if that isn't successful, move on from there. A strong blast of water from the hose often works for whiteflies. Spray underneath leaves where they hang out. Do this daily if you notice insects.
Whiteflies are attracted to the color yellow. You can purchase or make yellow "sticky" traps from yellow cardboard smeared with petroleum jelly. They fly to it and get stuck.
Soapy water sprays are another possibility. Use 1 teaspoon to 2 tablespoons of liquid detergent soap per gallon of water. Use regular, not concentrated soap. Don't use soaps with lemon, as the citric acid can burn plants. Start with the lower amount and work up as needed. Spray as often as needed. As with any spray you might wish to test it on a few leaves first before you treat all your plants. Spray early in the morning before the sun heats up. Next on my list would be an insecticidal soap spray.
The insecticidal soaps are made from plant-derived fatty acids and target soft-bodied insects. There's really no way you can target the bad guys without fallout on the good guys. If you can regularly monitor and tolerate some damage to your plants, over time Mother Nature strikes a balance, with the beneficials coming in to control the bad guys. Healthy vigorous plants will withstand insect attacks best, and it's really the best thing you can do to prevent insect problems. Hummingbirds and other birds rely on insects and spraying will kill beneficial pollinators (which may explain poor fruit set) so start with simplest method first.
Unless you have a cold frame, starting seeds indoors is the way to go. New Mexico State University offers a downloadable planting schedule for vegetables: http://cahe.nmsu.edu/pubs/_h/ Scroll down the list to Circular 457 and 457-B. Good luck with your vegetable garden this year!
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