What insecticide did you use for grub control? Imidacloprid (Merit) is a common active ingredient used for the control of grubs for over ten years and has shown no impact on earthworms. Copper fungicides are organic yet copper in the soil is death to earthworms. It all depends on what you use. Not all pesticides kill earthworms and not all organic or home remedies are safe for worms.
In the future, consider the use of nematodes that target white grubs. They are natural, safe (except for white grubs), and last for more than one season. Milky spore is also an effective natural control for Japanese Beetle grubs. Unfortunately milky spore will not affect other annual white grubs.
I know that Scotts does a good job of marketing the need for yearly grub control applications, but this is seldom the truth for the average homeowner. Good Integrated Pest Management teaches that you should scout first and only apply pesticides when economic or aesthetic thresholds are exceeded. For grubs that means that if you have a density of annual white grubs that exceed 10-12 per square foot, you should consider control. Below this number and your lawn grows faster than the damage created by grubs so no pesticide application is really needed.
"Earthworms are generally susceptible to carbamate compounds, which will significantly reduce their populations. Carbaryl, a carbamate pesticide often used for insect control, acts as a cholinesterase inhibitor, thereby producing long-lasting immobility and rigidity. Bendiocarb (Turcam) and propoxure (Baygon) are two other carbamate insecticides that cause paralysis in earthworms at normal dose rates. Carbofuran, another carbamate, is also very toxic to earthworms. Moreover, a sublethal response, characterized by weight loss, delayed clitellum development, and absence of cocoon production, has also been observed at recommended rates of carbofuran application.
Organophosphates are the most widely used class of turf insecticides. They have been successful in controlling white grubs, mole crickets, chinch bugs, and sod webworms, to name a few. Of the organophosphates, ethoprop is the most toxic to earthworms. In contrast, chlorpyrifos, isofenphos, and trichlorfon are considered non-toxic to earthworms when applied at normal dose rates."