Data from: Extreme heat reduces host and parasite performance in a butterfly-parasite interaction

Environmental temperature fundamentally shapes insect physiology, fitness,
and interactions with parasites. Differential climate warming effects on
host versus parasite biology could exacerbate or inhibit parasite
transmission, with far-reaching implications for pollination services,
biocontrol, and human health. Here, we experimentally test how controlled
temperatures influence multiple components of host and parasite fitness in
monarch butterflies (Danaus plexippus) and their protozoan parasites
Ophryocystis elektroscirrha. Using five constant temperature treatments
spanning 18-34°C, we measured monarch development, survival, size, immune
function, and parasite infection status and intensity. Monarch size and
survival declined sharply at 34°C, as did infection probability,
suggesting that hot temperatures decrease both host and parasite
performance. The lack of infection at 34°C was not due to greater host
immunity or faster larval development but could instead reflect the
thermal limits of parasite invasion and within-host replication. In the
context of ongoing climate change, our experiment suggests that
temperature increases above the upper thermal range will reduce the
fitness of both monarchs and their parasites, with lower infection rates
potentially mitigating the impact of extreme heat on future monarch
abundance and distribution.