Larval fitness of a generalist butterfly under experimental warming and spring drought depends on host species and timing

Abstract Climate change is expected to lead to faster development times in herbivorous insects, but precise predictions are challenging because excessive dry weather may offset benefits and host plants may respond in different ways. Conflicting evidence from many laboratory studies suggest that water stress may enhance or lower plant nutritional quality and palatability, but few studies expose both trophic levels to stressful conditions. We investigated the egg‐laying preferences and larval performance of the green hairstreak butterfly ( Callophrys rubi ) at two elevations of a boreal forest in Western Norway during an unusually warm and dry spring, and under artificially warmed conditions. Observations of egg‐laying in field cages suggested that adults initially preferred to oviposit on the early flowering bilberry ( Vaccinium myrtillus ) and switched to lingonberry ( V. vitis‐idaea ) when it began to flower later in the season. However, larvae restricted to plant species by mesh bags performed poorly on the early season host bilberry, particularly under the most stressful conditions at the lowest elevation during a moderate drought. Conversely, larvae performed consistently better on the later host lingonberry throughout the season and at both elevations. Under artificially elevated temperatures, larvae performed poorly at the low elevation site generally, suggesting that the benefits of warming to herbivores are limited under moderate drought conditions. These results suggest that an adult preference for bilberry early in the season may result in high mortality and poor fitness in warm and dry years, but host‐switching behaviour later in the season is likely to enable persistence. We discuss the implications of these findings for future warming conditions and suggest that more frequent drought conditions may select for later emergence and a preference for lingonberry.