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Abstract:
1. Induced responses in plants, initiated by herbivory, create potential for trait‐mediated
indirect interactions among herbivores. Responses to an initial herbivore
may change a number of plant traits that subsequently alter ecological processes
with additional herbivores. Although common, indirect interactions between
taxonomically distant herbivores, such as mammals and insects, are less studied
than between taxonomically related species (i.e., insect–insect). In terms of mammal–
insect interactions, effects on insect numbers (e.g., density) are relatively
well studied, whereas effects on performance (e.g., fecundity) are rarely explored.
Moreover, few studies have explored mammal–insect interactions on coniferous
plants.
2. The aim of this study was to investigate the effect of mammalian induced responses
on insect performance. We specifically investigated the effect of moose
(Alces alces) browsing on Scots pine (Pinus sylvestris) and subsequent effects on
sawfly (Neodiprion sertifer) performance.
3. Sawfly larvae were reared on browsed, clipped, and unbrowsed control pine trees
in a controlled field experiment. Afterward, cocoon weight was measured. Needle
C:N ratio and di‐terpene content were measured in response to browsing.
4. Sawfly performance was enhanced on trees browsed by moose. Cocoon weight
(proxy for fecundity) was 9 and 13% higher on browsed and clipped trees compared
to unbrowsed trees. Cocoon weight was weakly related to needle C:N ratio,
and browsed trees had lower a C:N ratio compared to unbrowsed trees. Needle
di‐terpene content, known to affect sawfly performance, was neither affected by
the browsing treatments nor did it correlate with sawfly weight.
5. We conclude that mammalian herbivory can affect insect herbivore performance,
with potential consequences for ecological communities and with particular importance
for insect population dynamics. The measured plant variables could not
fully explain the effect on sawfly performance providing a starting point for the
consideration of additional plant responses induced by mammalian browsing affecting
insect performance.
