Increased ladybird predation and metabolism do not counterbalance increased field aphid population growth under experimental warming

Climate change may have diverse and complex impacts on species
interactions, destabilizing food webs and ecosystem services. The effects
of warming on the top-down biological control of crop pests have been
considerably less studied than bottom-up effects through crop
physiological changes. We studied the effect of a 2 °C warming in the
laboratory and in wheat fields on the predation and metabolism of Harmonia
axyridis on wheat aphids using molecular gut content analysis. We also
measured the effects of warming on the predation rate and functional
response of H. axyridis on each aphid species in the laboratory, as well
as on DNA degradation rate. Field densities of Sitobion avenae and
Rhopalosiphum padi, the two most abundant wheat aphid species, were
increased by 2 and 2.5 times, respectively, under experimental warming,
but densities of H. axyridis were not. Field predation rate of H. axyridis
on these two aphids was found to be about 25% lower under elevated
temperature. This could have been due to faster prey digestion, since
degradation of the preferred aphid species, Sitobion avenae, was 1.5 times
faster under elevated temperature. However, the functional response of H.
axyridis larvae on these two species was 1.5 times higher under warming
over the range of prey densities tested (50 to 250 over 24 h). The total
predation rate of H. axyridis larvae on a mixture of S. avenae, R. padi
and Schizaphis graminum aphid prey was also increased by 1.4 times, but
consumption of R. padi aphids was increased while that of S. graminum was
decreased under warming. Overall, our results show that global warming
could strongly increase pest outbreaks and destabilize biological pest
control, which would likely result in accrued yield losses.