DOI: 10.5061/dryad.dncjsxm9k
title: Larval fitness of a generalist butterfly under experimental warming and spring drought depends on host species and timing

description: 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.
description: # Host choice and larval fitness of a generalist butterfly under drought
 stress depends on host species and timing Dataset DOI:
 [10.5061/dryad.dncjsxm9k](10.5061/dryad.dncjsxm9k) ## Description of the
 data and file structure The data in the file Hairstreak_data.csv were
 collected during an experiment on the green hairstreak butterfly at two
 sites at a boreal forest in Western Norway. Small field cages were erected
 in the field offering adult butterflies a choice of bilberry or
 lingonberry. Eggs laid were removed and returned to the lab until
 hatching. When larvae were 5-7 days old they were transported back to the
 field and randomly assigned to a block, treatment (Warming or Ambient) and
 host species (bilberry or lingonberry). Healthy host plant ramets between
 12 and 25 cm in height were then selected as close to the edge of the
 plots as possible in order to prevent perturbation of plots being used for
 other studies. Larvae were placed on their selected host and prevented
 from escaping using mesh bags. They were checked regularly and moved to a
 new host of the same species in the same location twice during the season.
 Larvae were weighed when they reached the 4th instar stage (to assess
 relative growth rate) using a field scale (RS PRO Bench Weighing scale,
 10g), and again when they pupated as an additional measure of fitness.
 Pupae were also weighed after three weeks of the pupation date to measure
 the amount of weight lost during the pupation stage. We repeated the above
 process so that we had two cohorts of 24 larvae at each site (hereafter
 termed “Cohort 1” and “Cohort 2”). The two cohorts consisted of larvae
 that hatched from eggs laid 14-20 days apart.  In addition, 24 extra
 larvae were also used in larval “choice” bioassays. These were designed to
 compare performance between larvae fed only bilberry and lingonberry in
 ambient conditions, with those fed a choice of both species together.
 Larvae were treated and measured in the same way as described above, but a
 ramet of each species was included in the mesh bags. Survival was recorded
 as a binary variable (1 = survived to pupation, 0 = died in larval stage),
 and for those larvae surviving we recorded Time to Pupation as the number
 of days spent in the larval stage. We used the 4th instar weights to
 calculate relative growth rate using the formula RGR = ln(weight at 4th
 instar) – ln(hatchling weight) / number of days between measurements. For
 initial hatchling weight, single individuals were too light to accurately
 weigh, so we used 24 hatchling individuals from the 2023 pilot experiment
 to estimate mean weight at 7 days old, which was 0.001 g. The initial
 pupal weight was also used as a measure of total weight gain, and
 proportion of weight lost during pupation was calculated using the
 equation 1 - [pupal mass/4th instar mass]. ### Files and variables ####
 File: Hairstreak_data.csv **Description:** CSV file of all data collected.
 ##### Variables Variables are defined in the file Hairstreak_metadata.csv
 Missing data code: NA