It can make a big difference, if you are very careful about making sure you're not actually enclosing one of the worms or larvae in the sleeve you've made. Look carefully around top of fruits/stem for larvae, as that's where eggs are generally laid. Twist the top of that hosiery tightly and secure with a bread tie.

With what you're describing, sounds like you've got quite an infestation, so you definitely need to treat as well. First, try and pick off as many as you possibly can. Then treat. If you use Bt, reapply at least twice a week as sun degrades it over time and rain washes it off. Be aware that the Bt only works if the worms actually ingest it, not on contact.

Sevin (carbaryl) would be highly effective and will kill the little buggers deader than dead. Your call if you want to use it. Just be sure to pay attention to pre-harvest application guidelines if you do so.

See attached link for writeup on cornworm (fruitworm).

Also, from the FAQ:
"Tomato Fruitworms - are also referred to as the Corn Earworm. They are green, brown, or pink with light stripes along sides and on back; up to 1 and 3/4 inches long and are the larvae of various moths. The moths lay eggs near the stem of the green fruit and within 1 week the hatched larvae bores into the fruit where it feeds. It may move from fruit to fruit until it is ready to fall to the ground where it pupates till the following spring. Discard portions of fruit that have earworms in them. Disk or rototill plants immediately after harvest. Organic/Biological Control:
Parasitic Trichogramma sp. wasps parasitize tomato fruitworm eggs. Bt, (Bacillus thuringiensis) can be used for control of tomato fruitworm. Birds may feed on larvae. Moles and tilling destroy the pupae. Chemical Control:
Sevin 5% dust. Treat every 5-7 days when fruit begins to set as once inside the fruit the pesticide has little or no effect. Continue as long as fruit is present, if needed."

Here is a link that might be useful: Helicoverpa (Heliothis) zea

Use Bt. Blossom bags are also good.