Neonicotinoids are the most widely used insecticides in North America. Many studies have documented neonicotinoids’ negative effects on bees, and there is evidence that neonicotinoids correlate with declines in monarch butterflies (Danaus plexippus). We examined how monarch development, survival, reproduction, and flight were affected by neonicotinoids, and how these effects depended on milkweed host plant species. Larval ingestion of low neonicotinoid doses did not affect monarch fitness traits. At the highest dose, neonicotinoids affected monarch pupation and survival for caterpillars that fed on the least toxic milkweed species; with differing effects on other species of milkweed. Adult ingestion of low and moderate neonicotinoid doses reduced reproductive activity only. At high doses, adult monarchs showed reduced flight performance and survival. Overall, monarchs tolerate low and moderate neonicotinoid doses, but experience detrimental effects at higher doses. These findings indicate that neonicotinoids are unlikely to cause widespread declines of monarchs at field-relevant levels.