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

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
spring, and under artificially warmed conditions. At both sites, 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 performed poorly on the early
season host bilberry, particularly under the most stressful conditions at
the lowest elevation during an unusually warm spring. 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 drought
conditions. These results suggest that the 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.