Have you seen worms so far? Are you growing sweetcorn? Are you growing in a home garden or a farm? Which State are you in?
If you choose to use Bt (as Charlie B. recommended) you have the choice of powder or liquid. The powder is best applied before the silks dry up and the liquid when they have dried up. see here https://www.youtube.com/watch?feature...
If you grow sweet corn at it is harvested before mid-August they you usually avoid most of the damage. But this has been a cold year and root worm is delayed (also because of the rains and water logging). There are reports of adults coming into pheromone traps, so they are out there. Check with PestWatch at Penn State for your area for when adults are around. Seems to be very little activity now http://www.pestwatch.psu.edu/sweetcor...
If you use a synthetic insecticide (like Sevin) you have to know how many moths are out there (based on trapping data, like PestWatch). There is resistance to these chemicals meaning you could spray and have no effect because of resistance. Again, important to know where in the country you are. See this piece http://www.omafra.gov.on.ca/english/c...
The following document http://glvwg.ag.ohio-state.edu/docume...
Provides excellent data on the distribution and discusses efficacy of different synthestics. It says Sevin is a poor control
Here is the list
Asana – Pyrethroid
Ambush/Pounce – Pyrethroids
Baythroid – Pyrethroid
Capture – Pyrethroid
Furadan – ECB only
Intrepid – ECB only
Lannate – ECB only (some possible value) Larvin – Possibility
ustang MAX – Pyrethroid
Penncap-M – ECB only
Sevin – Poor/fair control
SpinTor / Entrust – Expensive; fair/good control
Warrior – Pyrethroid
The cold, wet weather delays the normal cycles of insects. This is pronounced for soil dwelling insects like the root worm but also applies to flying insects. Now that the rain is stopping in PA and we have had more warm weather except egg laying by Corn Earworm. Below is a table from PestWatch by county.
I cannot find definative statements on resistance to Sevin or its efficacy. On indeed the efficacy of Bt. One PSU document is 2001 and I asked the author to come here and provide an update. But for the moment you will find it informative http://ento.psu.edu/extension/factshe...
I look more into resistance and as far as I can tell we do not know if resistance to pesticides is documented county by county or even state by state. There is a database from Michigan State (http://www.pesticideresistance.org) and for your pest Corn Earworm, Helicoverpa zea there are are number of known resistance issues (attached images). The active ingredient of Sevin is Carbyl and there is one publication on that which is 10 years old now.
Regarding the local densities. I had this from Andrew Frankenfield at Penn State Extension
"Ear Worm Numbers have been low. One to two sprays would be adequate for a commercial grower a home owner wouldn't likely need to spray. Spray at first silk until silk browning, usually a 2 week period."
What he is saying is that you do not need to spray since you have such a small amount. I would say it depends on whether you are surrounded by corn fields. Are you?
In response to Bill's question what to use: SEVIN or Bt.
SEVIn (Carbyl) is an insecticide that by design kills insects. So, it will kills the moths but also kills bees and other beneficials (source: http://extension.oregonstate.edu/cata...). It was discussed back in 1982 as there was a big debate about its effect on people. The EPA said that there was a slight risk and correct spraying was needed and contact with pregnant women was to be avoided. Here is the report that by Penn State Extension that will jump out as a pdf http://tinyurl.com/kzmnf59
Bt is a living organism (a virus) and so does not have negative effects on non-target organisms and certianly not bees which are evolutionarily distant from Bees (Orders Lepdioptera verse Hymneoptera) and very very distant from us (Insects vs mammals). All this would suggest to me that Bt is a better choice. I think Andrew Frankenfield's comment is great. What is the Corn for? Can you accept some damage? Just cut off the damages parts?
The record for resistance to Carbyl (Sevin)