We have had calls and messages from around Iowa the past 2 weeks asking about large numbers of yellow or yellowish-orange butterflies resting on gravel roads or flying along roadsides. Most specimens and photos viewed were the orange sulphur butterfly (Colias eurytheme). This species is also called the alfalfa sulphur or alfalfa butterfly (and yes, "sulphur" is the preferred spelling, though "sulfur" is also used).
It is possible there have also been large numbers of the clouded sulphur butterfly (Colias philodice). Both are common and occasionally abundant. If you are interested in seeing the subtle difference between these two closely related species, see the photo near the bottom of the BugGuide page on the orange sulphur.
Most butterflies, but especially the yellow alfalfa butterflies are frequently observed on gravel roads and at mud puddles where they use their coiled proboscis to sip moisture from the ground to obtain salts and minerals that they can't get from flower nectar. This behavior is called puddling and gives you a wonderful vantage point for butterfly watching. See About.Com:Insects for more information on puddling.
The Colias butterflies develop from caterpillars that fed on alfalfa, clover and other legumes. It's not clear where the current "outbreak" of butterflies developed. It could be that the caterpillars were in forage fields, feeding on alfalfa and clover, though this caterpillar can occasionally be found defoliating soybean leaves. In the BugGuide link above, click on the "Images" tab in the upper center of the page, then click on "caterpillars" to see what the larvae look like.
It's also not clear why there are so many yellow butterflies this year. The over-simplification is to say that the weather earlier in the year was conducive to reproduction and survival of the caterpillars, but that doesn't really explain what was different this spring and early summer.
The caterpillars are not economically important most of the time, and if control had been warranted we would have sprayed to control the caterpillar stage weeks ago. Treatment of the adult stage is not warranted. Enjoy the show while it lasts!