Insecticidal seed treatments are used commonly worldwide to protect seedlings against root feeding insects. Organophosphate insecticides that have been used for seed treatments are being phased out and replaced with neonicotinoid insecticides. Concerns about the environmental impact of neonicotinoids have prompted a search for alternatives. Microbial insecticides are a biological alternative for seed treatments to target root feeding insects. Six field trials with organophosphate granules (diazinon, chlorpyrifos), neonicotinoid seed treatment (clothianidin) and microbial (Serratia entomophila) seed treatment targeting grass grub, a New Zealand scarab pest, were conducted in wheat crops at several sites over 4 years (2012–2015). Sites were selected each year that had potentially damaging populations of grass grub present during the trials. Untreated seeds led to significant losses of plants and wheat yield due to lower seedling establishment and ongoing plant losses from grass grub damage. Insecticide and microbial treatments increased plant survival in all trials compared with untreated seeds. Better plant survival was associated with higher yields from the insecticide treatments in four out of six trials. Neonicotinoid seed treatment alone gave similar yield increases to combined neonicotinoid seed treatment and organophosphate granules. Microbial seed treatment with S. entomophila gave similar yield increases to insecticide treatments in two out of six trials. Seed treatment with S. entomophila is an alternative for grass grub control; however, development of a commercial product requires effective scale-up of production, further research to improve efficacy, and viability of the live bacteria needs to be maintained on coated seed.