cabbage butterfly, Pieris brassicae, larvae was unaffected by wide variation (up to 200 ppb) of the neonicotinoid imidacloprid in their diet, but even the lowest dose (1 ppb) reduced adult wing size 34. The monarch butterfly, Danaus plexippus (Lepidoptera: Nymphalidae), is an iconic species whose population is declining in North America. Numerous causal factors have been implicated in their decline 35-37 , with disappearance of its larval host-plant, milkweed (Asclepias sp.), due to the introduction of the herbicide glyphosate being the leading hypothesis 38-43. Some have noted, however, that the decline temporally coincides with the introduction and growing popularity of neonicotinoids and thus speculate that insecticides or other agrochemicals could act as a contributing factor. To date, only a few studies have experimentally assessed the impact of agricultural pesticides on monarch larvae, providing some pieces of the puzzle. Yet, these studies suffer from methodological limitations that prevent us from extrapolating to the broader question of monarch decline. All used ornamental milkweeds that are planted in home gardens such as tropical milkweed A. curassavica 44,45 or swamp milkweed A. incarnata 46,47 , rather than the common milkweed A. syriaca, which is the most abundant and widely used larval host-plant for eastern migratory monarchs 48,49. This discrepancy is potentially important since milkweed species differ dramatically in defensive traits affecting host-plant suitability 50 (e.g., latex, trichomes); for instance, tropical and swamp milkweeds are notable for having extremely high and low levels of cardenolides, respectively 51. Similarly, experiments mostly employ brief 'pulsed' exposure periods with larvae consuming a known insecticide dose over a 24-48 h period 45,46. This approach is useful in defining acute toxicity and calculating LD 50 values but does not accurately portray natural exposure where larvae consume trace amounts over their entire developmental period. Additionally, all prior studies investigated insecticides; none have tested other pesticide types, e.g., fungicides, herbicides. Here, we evaluated the effects of six pesticides-the insecticide clothianidin; the herbicides atrazine and S-metolachlor; the fungicides azoxystrobin, pyraclostrobin and trifloxystrobin-and their combination on monarch development. These were among the most commonly detected chemicals on A. syriaca leaves in a 2-year field survey of milkweeds bordering cropland 52. Because we know their specific concentrations, we tested each at mean and maximum levels; thus, all data represent field-realistic pesticide types and doses. Importantly, we exposed monarchs to these treatments on A. syriaca leaves over their complete larval period, measuring both immediate (i.e., larval mortality) and delayed responses (i.e., changes in adult size and longevity). These data are ultimately used in risk assessment to estimate the likelihood of lethal or nonlethal effects and their potential to harm monarchs in the wild. Methods Pla...