Unapparent trees: escaping enemies in time by being discreet, unpredictable and inaccessible

For half a century, biologists considered trees as particularly apparent
to their enemies. But why then do some trees escape herbivorous enemies by
bursting buds either too early or too late, leading to phenological
mismatch? We hypothesize that such mismatches occur on trees that are
unapparent in time—those that burst buds “discreetly” (slowly) and
unpredictably (inconsistently across years), and are difficult to access
(host trees among non-hosts). We studied herbivores on oaks in Western
France. We for the first-time characterized phenology matching of an
entire guild of herbivores, early spring ectophagous caterpillars (45
species). We examined whether, on a given date, caterpillars have a large
body size and impact (herbivory), and whether this size or impact is
largest on trees that burst buds earliest. Furthermore, we distinguished
the contributions of within-guild sorting of species and within-population
selection of individuals to phenology matching, we investigated whether
present phenology matching was determined during past generations by
testing if caterpillars from trees with early-maturing foliage pupate
early, even when transferred into a common garden. We found that
caterpillar size and herbivory impact in the field and pupation date in
the lab generally matched leaf phenology in the field. Such phenology
matching was consistently observed at the intraspecific level but was less
evident at the interspecific level. Together this indicates that phenology
matching is mainly driven by past within-population selection rather than
present, within-guild sorting. Furthermore, herbivores size and impact
were most constrained by late bursting of host trees if these trees burst
buds slowly and unpredictably and are difficult to access. We suggest
that, during the assembly of early-spring herbivore guilds, trees can best
constrain development of herbivore populations by late budburst and
remaining unapparent in time—bursting buds discreetly, unpredictably
across years, or in inaccessible neighbourhoods.