Inch worms (also called cabbage loopers, measuring worms or cankerworms) received their name because of the way they inch along, arching their bodies up tree trunks or vegetable stems, according to Cornell University Extension. Normally birds and beetles feed on these 1-inch caterpillars, which vary in color from reddish-brown to green, and parasitic wasps gradually weaken them, keeping the population in check. Inch worms are unlikely to attack tomatoes if their preferred silking ears of corn are nearby.
Apply a pesticide to the tomato plants when inch worms are still small, as soon as they appear, in the last three weeks of May in most parts of the country. Both state extension services and organic gardening websites recommend either spinosad (Monterey Garden Insect Spray; Ferti-lome Borer, Bagworm, Leafminer & Tent Caterpillar Spray; and Bulls-Eye Bioinsecticide) or Bacillus thuringiensis or bT (Thuricide). Spinosad and bT are derivative of naturally occurring bacteria and cause the inch worm to stop feeding and die.
Alternate between treatments of bT and spinosad so that inch worms do not develop resistance, recommends the Cornell University College of Agriculture and Life Sciences.
Shake the pesticide well. Add 2 to 4 tsp. of bT per gallon of water in a three-quarter-filled spray tank after double-checking label directions for application for tomatoes. Apply to thoroughly soak the tomato plant's leaves. Repeat a week after applying spinosad.
Add 4 tbsp. of spinosad per gallon of water after double-checking label directions for application for tomatoes, filling the tank and soaking the plant leaves as above. Apply spinosad four to seven days after applying bT, a maximum of four times in a season.