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

Soumen Mallick, Freerk Molleman, Benjamin Yguel, Jörg Müller, Romain Georges, Stéphanie Llopis, Marjorie Levant, Frédéric Jean, Yasin Gundesli, Jaad Besse, et al.

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Soumen Mallick, Freerk Molleman, Benjamin Yguel, Jörg Müller, Romain Georges, et al. Unappar- ent trees: escaping enemies in time by being discreet, unpredictable and inaccessible. Oikos, 2025, pp.e11218. 10.1002/oik.11218 . hal-05100769

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OIKOS

Research article

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

Soumen Mallick<fnref n="1" />,<fnref n="2" />, Freerk Molleman<fnref n="3" />, Benjamin Yguel<fnref n="1" />,<fnref n="4" />, Jörg Müller<fnref n="2" />,<fnref n="5" />, Romain Georges<fnref n="1" />,
Stéphanie Llopis<fnref n="1" />, Marjorie Levant<fnref n="1" />, Frédéric Jean<fnref n="1" />, Yasin Gundesli<fnref n="1" />, Jaad Besse<fnref n="1" />,
Agnès Schermann-Legionnet<fnref n="1" /> and Andreas Prinzing<fnref n="1" />

Correspondence: Soumen Mallick (soumenmallick1994@gmail.com)

Oikos

2025: e11218

doi: 10.1002/oik.11218

Subject Editor:

Antonino Cusumano

Editor-in-Chief:

Paulo R. Guimaraes

Accepted 28 April 2025

<figure><img src="image_4.png" /></figure>

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 are 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.

Keywords: budburst, caterpillar, deme, insect-plant interaction, oak (Quercus), phenology matching, species sorting, unapparent trees

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www.oikosjournal.org

© 2025 The Author(s). Oikos published by John Wiley & Sons Ltd on behalf of Nordic Society Oikos.

This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

<footnote n="1">¹Centre National de la Recherche Scientifique, Université de Rennes, Research Unit UMR 6553, Ecosystèmes Biodiversité Evolution (ECOBIO), Campus de Beaulieu, Rennes, France</footnote>

<footnote n="2">²Field Station Fabrikschleichach, Chair of Conservation Biology and Forest Ecology, Biocenter, University of Würzburg, Rauhenebrach, Germany</footnote>

<footnote n="3">³Department of Systematic Zoology, Institute of Environmental Biology, Faculty of Biology, A. Mickiewicz University Poznań, Poznań, Poland</footnote>

<footnote n="4">⁴IGEPP, Institut Agro Rennes-Angers, Université de Rennes, INRAE, Angers, France</footnote>

<footnote n="5">⁵Bavarian Forest National Park, Grafenau, Germany</footnote>
**Introduction**

For five decades, biologists have distinguished between appar-
ent and unapparent plants, i.e. those easily found by their
herbivore enemies or not. Biologists argued that this appar-
ency impacts the relative importance of top-down versus bot-
tom-up control and the evolution of plant-defence (Feeny
1976, Forkner et al. 2004, Strauss et al. 2015). Adult trees are
considered as particularly apparent hosts, given their size and
stature in space (Feeny 1976, Smilanich et al. 2016, Dai et al.
2017, Kharouba and Wolkovich 2023), while tree saplings
might be unapparent (Castagneyrol et al. 2013, Zverev et al.
2017). Nevertheless, some deciduous trees appear to escape
enemies by becoming unapparent in time: these trees develop
leaves too early or too late for their herbivores which hence
suffer either from the absence of leaves or from leaves hav-
ing already turned tough or stuffed with defensive chemicals
(Feeny 1970, Hunter and Leehowiez 1992, Chen et al. 2003,
Forkner et al. 2004, Wesołowski and Rowiński 2006, Van
Asch and Visser 2007, Posledovich et al. 2015). The herbi-
vores on these trees have hatched eggs at the wrong moment
and herbivores that developed at the right moment on other
trees had not been able to immigrate e.g. by floating or walk-
ing (Edland 1971, Doane and Leonard 1975, Liebhold et al.
1986, Murakami and Wada 1997, Nealis and Régnière 2004,
Schafellner et al. 2005, Sarvašová et al. 2021).

Such mismatching of herbivores to tree phenology is
usually studied in relation to climate change (Visser and
Holleman 2001, Singer and Parmesan 2010, Ekholm 2017,
Büntgen et al. 2020, Ren et al. 2020, Zhou et al. 2023)
or to how different tree species respond to climate change
(Nakazawa and Doi 2012, Renner and Zohner 2018).
However, even within the same tree species, climate, and site,
host trees exhibit significant phenological variation (Crawley
and Akhteruzzaman 1988, Van Dongen et al. 1998, Tikkanen
and Julkunen-Tiitto 2003, Cole and Sheldon 2017). This
variation arises from genetic differences among trees, micro-
climatic and pathogen, and local adaptations, contributing
to asynchronous resource availability within plant popula-
tions (Dantec et al. 2015, Malyshev et al. 2022, Lin et al.
2024). Such local heterogeneity in tree phenology has major
consequences for the capacity of tree populations to adapt to
future climate change, and for the interactions of trees with
their enemies (Dantec et al. 2015, Cole and Sheldon 2017,
Malyshev et al. 2022), as herbivores closely match tree phe-
nology on some trees but not on others (Van Dongen et al.
1997, Van Asch and Visser 2007).

The phenology matching of herbivores to a tree could
result from the selection of matching individuals within
populations or the sorting of matching species within guilds
(Leibold and Chase 2017 for discussion of population vs
community processes). For instance, for the particularly
abundant guild of early-spring ectophagous folivorous cat-
erpillars (Southwood et al. 2004), their development needs
to match the bursting of buds. This ensures that caterpillars
have access to leaves when they are available but still tender
and low in tannins (Hunter and Leehowiez 1992, Chen et al.

2003, Wesołowski and Rowiński 2006, Van Asch and Visser
2007, Posledovich et al. 2015). Therefore, we hypothesize that
caterpillars of this guild develop early on early-bursting trees
(hypothesis 1). Such overall matching could be achieved by
both, the sorting of species or the selection of genotypes that
hatch at the right moment (Fig. 1). We predict that if match-
ing of caterpillar to tree phenology results from sorting of spe-
cies into guilds, such matching should be evident at the level
of interspecific variation of caterpillar phenology (hypoth-
esis 1 interspecific). If it results from selection of individuals
within populations, matching should be evident at the level
of intraspecific variation of caterpillar phenology (hypoth-
esis 1 intraspecific). These two processes are not mutually
exclusive and may operate simultaneously. Moreover, such
processes of selection or sorting might be ongoing or have
accumulated across years (Joshi et al. 2003, Bazykin and
Kondrashov 2011). Ongoing selection and sorting would
involve influx of matching herbivores on apparent trees and
exclusion of mismatching herbivores (Fig. 2a) (Saccheri and
Hanski 2006). These processes can accumulate across years
if a given tree in situ harbours a local population or guild of
herbivores (Fig. 2b). Localized populations adapted to specific
trees (termed 'demes' in Mopper 1996, Mopper and Strauss
1998) have indeed been documented (Schneider 1984, Van
Dongen et al. 1998). We predict that after transferring herbi-
vores from different trees into shared laboratory conditions –
a 'common garden' approach using field-collected individuals
– they will still match the phenology of their respective host
trees (hypothesis 2). If phenology matching reflects accumu-
lated past sorting of herbivore species within guilds, this pat-
tern will be evident at the level of interspecific variation of
caterpillar phenology (hypothesis 2 interspecific). Conversely,
if phenology matching reflects accumulated past selection
of herbivore individuals within populations – such as local
adaptation to the phenology of specific host trees – then this
pattern will be evident at the level of intraspecific variation of
caterpillar phenology (hypothesis 2 intraspecific).

Sorting and selection of herbivore phenologies by tree
phenologies may be more efficient when trees are more appar-
ent. We hypothesize that apparenty depends on the speed
with which the buds of a tree burst (time between first and
last buds to burst on the tree), a character that varies drasti-
cally among trees (Denéchère et al. 2021). We hypothesize
that higher speed may increase tree apparenty by increasing
homogeneity of budburst within a crown (all buds burst-
ing simultaneously Harper 1989, Quiring 1993, Quiring
and McKinnon 1999). Rapidly bursting trees thus offer an
apparent signal of high concentration of freshly emerging
leaves to floating, flying or walking herbivores that match
phenology, whereas herbivores that do not match experi-
ence a scarcity of edible leaves (Fig. 2a). Consequently, we
hypothesize that matching herbivores are attracted by rapidly
bursting trees, while mismatching herbivores are excluded.
Accumulating over years, such selective attraction and exclu-
sion would result in efficient selection of matching individu-
als within populations or sorting of matching species within
guilds (Fig. 2b). Attraction can work for herbivores with
<figure><img src="image_6.png" /><figcaption>Figure 1. Two processes that contribute to the phenology matching of herbivores to trees: the sorting of matching species within guilds (top panel) and the selection of matching individuals within populations (bottom panel). The x-axis represents the progression of tree and herbivore phenology from early to late stages. Different colours of herbivores distinguish species.</figcaption></figure>

different life cycles: caterpillars hatching from overwintered eggs in early spring may float the air and use tree odours to land on suitable trees (McClain et al. 2006), and species that overwinter as pupae or adults can use sensory cues to select suitable trees for oviposition in spring. However, these mechanisms may not be effective everywhere. While exclusion may operate universally as mismatching herbivores might simply die or move away, attraction requires easy movement between adjacent trees (Fig. 2c). Therefore, we hypothesize that attraction is ineffective when the host trees are isolated from potential sources of herbivores: when conspecific trees are spatially distant or neighbouring trees that are phylogenetically distant and hence do not share herbivores with the focal tree (Vialatte et al. 2010). We also hypothesise that accumulation of local selection and sorting is ineffective when trees exhibit unpredictable timing of budburst. Trees are unpredictable when they vary their relative timing of budburst across years, e.g. a tree being among the first to burst buds in one year while being average or late in another year. Taken together,

we predict that herbivores will match phenology more on rapidly bursting trees (hypothesis 3), particularly if trees are easily accessible (hypothesis 3a) or show little variation in relative timing of budburst among years (hypothesis 3b).

Phenology matching is commonly assessed by synchrony between events (e.g. budburst and egg hatching), but this approach has limitations (Visser et al. 2012, Visser and Gienapp 2019). Successful matching can also be measured based on individual body size and its community-level consequence: herbivory impact on the plant. Herbivore body size and impact provide different strengths and weaknesses compared to egg hatching as indicators of phenology matching. While egg hatching offers an unbiased measure of relative timing, it does not directly inform about fitness outcomes. In contrast, body size and herbivory impact might introduce biases in assessing relative timing due to mortality from factors like food availability, competition, and species-specific growth rates (but see cases where body size did relate to hatching date of the dominant species, Supporting information).
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Figure 2. Three ways in which tree characteristics can favour apparenty of a tree in time and thereby matching of timing of budburst by herbivores. (a) Rapid budburst. A rapidly bud-bursting tree (left) has the highest peak concentration of freshly emerged leaves. This high concentration increases tree apparenty and thus attracts herbivores that need freshly emerged leaves (happy herbivores). Herbivores that are arriving earlier or later (unhappy herbivores) are strongly mismatching with the budburst stage and are hence excluded (skull). On a tree that is unapparent in time due to ‘discreet’, slow bud burst (right), such ongoing attraction and exclusion of herbivores are weak (emotion-less herbivores). (b) Consistent budburst across years (e.g. consistently being the first tree to burst). In case of consistent budburst (left), matching budburst in one year ensures matching budburst during the next years. Thus, the tree accumulates matching herbivores over years. On a tree that is unapparent in time due to unpredictable budburst (right), such accumulation of past selection and sorting is absent. (c) Phylogenetically non-isolated from neighbourhood. A phylogenetically non-isolated focal tree (left) is apparent to the more specialized herbivores present on neighbouring trees so that if the focal tree better matches their phenology than the neighbours, herbivores can relatively easily immigrate through e.g. ballooning. On a tree that is phylogenetically isolated (right), such immigration of matching specialist herbivores is rare, and thus the tree's phenology is unapparent in time. A similar argument can be made for strict specialists and spatial isolation from conspecific trees.
Nevertheless, body size or impact allow inferences about
individual fitness and population performance of those that
did survive (Osorio-Canadas et al. 2016, Macphie et al.
2023 who use body size). Additionally, measuring body size
or impact allows us to account for herbivores that hatched
elsewhere and immigrated by floating in the air or walking
(Edland 1971, Doane and Leonard 1975, Liebhold et al.
1986, Murakami and Wada 1997, Nealis and Régnière 2004,
Schaffellner et al. 2005, Sarvašová et al. 2021). Importantly,
it allows us to study the entire guild of early-spring caterpil-
lars in the field, while monitoring the egg hatching is feasible
only for a single (or few) species with wingless females (Van
Asch and Visser 2007).

Monitoring size and herbivory impact permitted us to dif-
ferentiate between two scenarios (Fig. 3). Scenario 1: on a
tree that is apparent due to high budburst speed, the herbi-
vores match the timing of the trees: an early bud bursting
corresponds to an early hatching and hence large body size

and herbivory impact across season, and the inverse for late
bursting. On trees that are unapparent due to low budburst
speed, herbivores did not succeed in establishing on the early-
bursting trees, to the advantage of the early bursting tree.
Scenario 2: on trees that are apparent due to high budburst
speed, herbivores that had hatched elsewhere succeed in find-
ing and immigrating on a late-bursting tree with fresh, young
leaves, to the detriment of that tree (Edland 1971, Doane and
Leonard 1975, Liebhold et al. 1986, Murakami and Wada
1997, Nealis and Régnière 2004, Schaffellner et al. 2005,
Sarvašová et al. 2021 for movement of caterpillars between
trees, also beyond youngest caterpillar stages). In contrast, on
trees that are unapparent due to slow budburst speed, the
herbivores match the timing of the trees, where a late bud
bursting corresponds to late hatching, small body size and
little impact, to the advantage of the trees.

We studied the phenology matching between oak trees *Quercus petraea*, *Q. robur* and their hybrids) and the entire

<figure><img src="image_8.png" /></figure>

Further statistical effects:

The combination of scenarios 1 and 2 should result in a positive main effect of speed and no interaction term.

The respective effects of high *budburst speed* should disappear on isolated trees, i.e. high spatial distance of conspecifics or high phylogenetic distance of neighbours. This results in negative interaction *distance: speed* and positive (scenario 1) or negative (scenario 2) interaction *distance: speed: budburst-timing*.

The respective effects of high *budburst speed* should be reinforced on trees bursting predictably, i.e. consistently across years. This results in positive interaction *consistency: speed* and negative (scenario 1) or positive (scenario 2) interaction *consistency: speed: budburst-timing*.

Figure 3. Scenarios how transparency of trees may modify the relationship between timing of budburst and caterpillar size or their herbivory impact, the consequences for tree performance (happy/sad tree), and the corresponding statistical pattern. See Introduction for further explanations of processes.
guild of early-spring ectophagous caterpillars (free-living and
shelter-building Lepidoptera) in a forest in western France.
We selected individual trees with varying levels of isolation
and measured their budburst timing, speed, and among-year
consistency, as well as, later in June, their degree of leaf matu-
ration. Forty-five species of early-spring ectophagous cater-
pillars were sampled, and we measured the body size of the
individuals. Subsequently, the caterpillars were reared under
shared conditions in a common garden to determine the
date of pupation and to identify adults. Herbivory impact
of all herbivores was determined at the end of the season.
Our research aimed to test the above predictions and thereby
address the following questions: 1) do caterpillar guilds
develop earlier on early bursting trees, and is this driven by
selection within populations or sorting within guilds? 2)
When reared under 'common garden' conditions, do cater-
pillars still pupate earlier if they originated from trees that
reduce leaf quality early? 3) Do caterpillars exhibit stronger
phenology matching on rapidly bursting trees, particularly
when these trees are a) spatially or phylogenetically proxi-
mate to neighbours, or b) exhibit little among-year variation
in relative budburst timing?

# Material and methods

## Field site and study trees

We conducted the study in a temperate forest in western France (48°11′N, 01°34′W; 90 m a.s.l., oceanic climate with a mean annual temperature and precipitation of 11.3°C and 632 mm, respectively), which has mostly oak or pine-dominated parcels (average parcel size approximately 14.5 ha). We selected pairs of oaks that were 30–150 m apart where one grows in an oak-dominated parcel and the other in a pine-dominated parcel. A total of 10 tree species were in direct contact with our focal species. We studied eleven pairs in 2010, 2011 and 2019 (at that time ca 60-year-old with the mean circumference at breast height 62.1 cm). Seven trees from 2010 and 2011 (earlier used in Yguel et al. 2011) had become unavailable in 2019 due to forestry activity, and we replaced them with seven new trees. Thus, the study covers 22 trees every year and 29 across years. For each tree, we quantified accessibility to herbivores as both spatial and phylogenetic isolation of individual trees. Spatial isolation was measured as trunk-to-trunk distance to the nearest oak (not to be confused with within-pair distance). Phylogenetic isolation was calculated by averaging the phylogenetic distances of the focal oak to its neighbouring trees (details in the Supporting information and in Vialatte et al. 2010).

## Determining tree phenology

To determine the timing of budburst, we observed bud devel-
opment every 2–3 days from early March (budburst starting
mid-March) until all trees had completed budburst (Fig. 4).
Specifically, we selected ten buds each from the upper and
lower stratum of each studied tree (i.e. the upper- and lower-
half of the crown) and determined their developmental stage

using binoculars, with a three-rank scale: 0 for closed bud, 1
for folded leaves and 2 for unfolded leaves (Wesołowski and
Rowiński 2006). So, the total score of a tree ranged from
0 to 40 at a given date. We defined ‘timing of budburst’ as
the days of years when the tree reached score 20, calculated
by interpolation of a linear regression excluding all but the
last days with budburst scores zero and all but the first day
with budburst scores 40. We defined ‘speed of budburst’ as
the slope of the regression line explaining budburst score by
the day of the year. To calculate ‘consistency of budburst’ we
first calculated the relative timing for each of the three years
(2010, 2011 and 2019) and then used the inverse of the stan-
dard deviation of relative timing across years. We obtained
relative timing, in a given year by subtracting the average tim-
ing of budburst of all the trees from an individual tree’s bud-
burst timing. As expected, the speed of budburst correlated
to homogeneity of budburst across tree at a given moment
(standard deviation of bud burst stages across a crown, aver-
aged across visits; r²=0.37; Supporting information). At
the same time, speed of budburst was much less strongly
related to budburst date (r²=0.07; Supporting information),
strongly reducing the risk of multicollinearity.

Some trees might mature their leaves faster than others
and irrespective of their timing of budburst. Therefore, in
2019, we also quantified late-spring phenology of trees as
‘leaf maturation’ (Fig. 4). Between 17 and 18 June, we sam-
pled ten leaves from each of the upper and lower strata, and
characterised the physical and chemical properties. The leaves
were randomly selected, potentially including those from the
(very rare) starting second flush, resulting in a realistic rep-
resentation of the within-tree variation in nutritional qual-
ity of leaves for herbivores(Gaytán et al. 2022). The physical
properties (leaf toughness, thickness, dry matter content)
were characterised following Pérez-Harguindeguy et al. 2013.
Specifically, leaf toughness was assessed using a penetrometer
(Halda Gram Gauge, Halda), leaf thickness was measured
with a digital calliper , and dry matter content was deter-
mined by calculating the oven-dry mass of a leaf divided by its
water-saturated mass. The chemical properties (chlorophyll,
nitrogen balance index, flavonols and anthocyanins) were
characterised using a Dualex sensor following Goulas et al.
(2004). These measurements were taken directly in the field,
allowing for rapid and accurate assessment of leaf chemical
traits. We conducted a principal component analysis (PCA)
with these physical and chemical parameters. The first axis of
the PCA explained over 80% of the variation in leaf param-
eters, so we used PC1 as leaf maturation scores, reflecting
higher toughness and thickness, and higher concentrations
of flavonols and anthocyanins and lower chlorophyll content
and nitrogen balance index.

## Caterpillar collection and rearing

We sampled the guild of early-season ectophageous folivores
caterpillars that are active early in the season during bud-
burst which is a critical period for the youngest stages of these
ectophages (Southwood et al. 2004). In each year, we col-
lected caterpillars, within a span of two to seven days, starting
<figure><img src="image_9.png" /></figure>

Figure 4. Methodological approach to monitor the phenology of trees and their early-spring caterpillars in the field and in the laboratory. Starting in early March, we monitored budburst phenology every 2–3 days and buds began to burst in mid-March i.e. week 1. The timing of budburst for each tree was recorded when it reached its midpoint of budburst. In week 3, when all the trees had busted buds, we collected caterpillars from all the trees within 2 days and measured the size of the caterpillars. Then we transferred the caterpillars in a shared laboratory condition (common garden) and monitored their timing of pupation. In the second half of June in week 12, we sampled leaves from all the trees and monitored leaf maturation. Finally, in mid-September in week 24, we sampled leaves and quantified herbivory by ectopha-gous herbivores. Budburst and caterpillar size were studied in three years, with exact timing that could vary between years.

from the day when the last tree reached budburst score 40 (day of years 131 in 2010, 110 in 2011 and 116 in 2019). From each tree, we cut two one-to-two meters long branches (one from upper and one lower stratum). In 2019 we char-acterised ten leaves per branch by chemical composition (chlorophyll, nitrogen balance index, flavonols and antho-cyanins) and (later in lab) leaf toughness, thickness and dry matter content. We transported branches to the lab in plastic bags and searched for caterpillars. We measured caterpillar length (in mm with the precision of three decimal points) in ImageJ from their images taken two days after collection (Fig. 4). The advantage of such length measurements is that unlike mass measurements, they do not depend on the gut-filling of caterpillars at the time of measurement. A potential concern is that sexual dimorphism, particularly in species of *Lymantriidae*, where females often undergo an extra instar compared to males (Wallner and McManus 1986), could introduce bias, as the same body length could indicate differ-ent developmental stages between sexes. However, this bias is minimized in our study because we primarily measured the size of early-instar caterpillars, where sexual dimorphism is less pronounced (Hunter and Willmer 1989, Teder and Tammaru 2005), and the sex ratio of caterpillars is unlikely to vary between trees (Hamilton 1967). As parasitism can reduce caterpillar growth (Morales et al. 2007), we excluded parasitized caterpillars from analyses. To monitor parasitoid infection, we reared the caterpillars until they reached adult-hood, which also permitted to identify the caterpillars to spe-cies (details in the Supporting information).

In addition, in 2019, we monitored late-spring caterpil-lar phenology as the timing of pupation of the caterpillars sampled before (Fig. 4). Pupation is a distinct phenological event. Data on pupation were not limited to the caterpillars collected shortly after budburst, but also included caterpil-lars collected earlier and later: we collected these caterpillars from the first tree reaching budburst score 40 until mid-May. Collection was conducted two-three times a week, and a subset of trees was sampled on any given day. We reared all caterpillars (separately in petri dishes) in shared laboratory conditions in a common garden mimicking average diurnal change in temperature, daylight and relative humidity of the study forest. We always fed the caterpillars with freshly emerged leaves from oak trees from the Beaulieu Campus of Université de Rennes. We recorded pupation every one-two days.

Determining herbivore impact

We quantified herbivore impact on the trees as ‘rate of her-bivory’ (in 2010 and 2011, Fig. 4). For this, we sampled forty leaves (covering the upper and lower stratum, details in Yguel et al. 2011) from each tree in mid-September, which allowed us to cover all the herbivory caused by caterpillars as most of the leaf herbivory is caused by early-season leaf chew-ing caterpillars (Southwood et al. 2004) and almost all leaf herbivory on oaks is caused before the end of August (Rinker and Lowman 2004). We measured (using a 1 × 1-cm dot grid) the area of the intact leaves including the reconstructed missing parts presumed to be eaten. We calculated the ‘rate of
herbivory' by dividing the summed area of missing parts by that of the intact leaves.

## Replication statement

We investigated how the phenological advancement of a guild of ectophagous caterpillars – assessed via body size, herbivory impact and pupation – is influenced by host tree characteristics. Body size measurements were taken from caterpillars collected from 22 individual trees over three years, totaling 366 non-parasitized individuals (an average of 5.54 caterpillars per tree per year). Herbivory was assessed at the tree level, as described above. Pupation timing of 213 individuals was recorded under shared laboratory conditions. Our primary focus was on how body size, herbivory impact and pupation timing varied among trees, with 366 caterpillars serving as replicates for all analyses involving caterpillar size and herbivory, and 213 replicates for all analyses involving pupation timing (note that tree, herbivores species and year were accounted for in the analyses). The 366 caterpillars spanned 45 species (Supporting information). We also note, while herbivory was measured at the tree level, our analyses were conducted at the caterpillar level for consistency with body size assessments. However, reanalyzing herbivory at the tree-year level did not alter the core relationship between herbivory rate and budburst timing (t value = -2.069, p value = 0.045, df = 41).

## Data analyses

To assess phenology matching between caterpillars and their host trees, we employed linear mixed-effects models (LMMs) using the 'lme4' package in R ver. 4.4.1 (www.r-project.org). Our analyses focused on two key response variables: individual caterpillar size and herbivory rate. For overall phenology matching (hypothesis 1), we modelled each response variable as a function of the fixed effects relative budburst timing and year of study, while including tree identity and caterpillar species identity as random effects. To examine phenology matching arising from species sorting within guilds (hypothesis 1 interspecific) and selection within populations (hypothesis 1 intraspecific) we applied the method of van de Pol and Wright (2009) – we replaced relative budburst timing with species mean budburst timing and within-species budburst timing, respectively. Species mean budburst timing was calculated as the average relative budburst timing of host trees for all individuals of a caterpillar species, whereas within-species budburst timing was computed as the difference between an individual caterpillar's relative budburst timing and its species' mean budburst timing. This way, by explicitly separating species means and within-species variations, the mixed model provides unbiased estimates of both between-species sorting and within-species selection (van de Pol and Wright 2009).

A similar approach was used to assess matching between pupation timing in a shared lab environment and leaf maturation of the trees of origin in June in the forest. For overall phenology matching (hypothesis 2), we modeled timing of pupation as a function of the fixed effect leaf maturation score, with tree identity and caterpillar species identity

included as random effects. Since data were collected only in 2019 there was no variation to be explained by year. To evaluate whether this matching arose from species sorting within guilds (hypothesis 2 interspecific) and selection within populations (hypothesis 2 intraspecific), we replaced leaf maturation score with species mean leaf maturation score and within-species leaf maturation score (calculated using the same approach as for budburst timing).

To investigate the effect of apparenty on phenology matching between caterpillars and host trees (hypothesis 3, 3a, 3b), we used the model for hypothesis 1 as the baseline, as it had a lower AIC compared to the models for hypothesis 1 interspecific and hypothesis 1 intraspecific. We extended the baseline models by incorporating fixed effects representing apparenty parameters. Specifically, we included budburst speed, its interaction with relative budburst timing, with spatial isolation, and with relative budburst timing and spatial isolation. Importantly, this final model excluded species mean budburst timing, which had received little support in the baseline model compared to within-species budburst timing. Thus, the extended models primarily support intraspecific processes (e.g. selection within populations or local adaptation) rather than interspecific sorting as the main driver of phenology matching. The effect of increased budburst speed on matching between body size of caterpillars or their herbivory impact (hypothesis 3) was identified as explained in Fig. 3. The effect of budburst speed on matching may in turn depend on the spatial isolation or the phylogenetic isolation (hypothesis 3a) or on the between-year consistency of budburst (hypothesis 3b). These effects were identified as explained in Fig. 3. Random factors included into the model were tree identity and caterpillar species, as before.

## Results

### Data range

Caterpillar size ranged from 1.62 to 39.38 mm, with an average of 8.99 mm. Herbivory rate ranged between 0.009 and 0.143, averaging 0.074. Pupation date ranged from 113 to 246 days of the year (DOY), with a mean of 153 DOY. Budburst timing ranged from 91 to 130 DOY, averaging 103 DOY. The speed of budburst, measured as score gain per day, ranged from 1.24 to 6.55, with an average of 2.79. Leaf advancement in mid-June varied considerably, for instance nitrogen balanced index varied between 16.02 to 44.57. Spatial isolation varied between 0.7 and 16.8 m, with a mean of 5.1 m. Phylogenetic isolation ranged from 10 to 125.67 million years (MY), averaging 42.15 MY. Between-year consistency of budburst ranged from 0.075 to 0.632, with an average of 0.215. All averages were calculated across caterpillar samples (see full dataset in the Supporting information).

### Hypothesis 1: in the field, herbivores match budburst phenology of host trees, both within and across species

In support of hypothesis 1, herbivore guilds exhibited overall phenology matching. Individual caterpillar size in the field
decreased as the relative budburst timing of the host tree was
delayed (estimate = -0.192 ± 0.058 SE; Table 1a, Fig. 5a top-
left). This decrease of size with budburst timing was distinctly
stronger than any of the relationships of caterpillar size with
nutritional qualities of host trees as characterised from the
leaves in 2019 (p value 0.001 versus 0.012–0.488, on average
0.140). Furthermore, the herbivory rate in the field decreased
with a delay in relative budburst timing (estimate = -0.003
± 0.001 SE; Table 1b, Fig. 5a bottom-left). Hypothesis 1
interspecific received partial support. Individual caterpillar
size decreased with a delay in species mean budburst timing
(estimate = -0.346 ± 0.148 SE; Table 1a, Fig. 5a, top-mid-
dle). However, herbivory rate was not affected by the spe-
cies mean budburst timing (estimate = -0.002 ± 0.001 SE;
Table 1b, Fig. 5a bottom-middle). Hypothesis 1 intraspecific
was supported. Individual caterpillar size decreased with a
delay in within-species budburst timing (estimate = -0.174
± 0.060 SE; Table 1a, Fig. 5a top-right). Likewise, herbivory
rate also decreased with a delay in within-species budburst
timing (estimate = -0.003 ± 0.001 SE; Table 1b, Fig. 5a
bottom-right).

Hypothesis 2: in the lab, herbivores match leaf-maturation of host trees, mainly within species

In support of hypothesis 2, herbivore guilds exhibited overall
phenology matching. In the shared laboratory environment:
individuals pupated early when the leaf maturation score of
the host tree in the field was high i.e. on trees whose leaves in
mid-June were particularly tough and rich in chemicals that
can play a role in defence against caterpillars such as flavonols
and anthocyanins (Table 1c, Fig. 5b left). Hypothesis 1 inter-
specific was not supported. Pupation timing was not affected
by the species mean leaf maturation (Table 1b, Fig. 5b mid-
dle). Hypothesis 1 intraspecific was supported. Individuals
pupated early when within-species leaf maturation score was
high (Table 1c, Fig. 5b right). Note that the maturation of
leaves in mid-June was not related to the date of budburst
of the same trees (t=-1.373, p=0.187, df=18, adj. R² =
0.04). The observed early pupation under shared laboratory
conditions in caterpillars originating from trees of early leaf
maturation is hence not a side effect of large body size in the
field in caterpillars sampled from early bursting trees. And
late bursting constraints the time window where caterpillar
suitable resources.

Hypothesis 3: in the field, low budburst speed
reduces phenology matching, notably on isolated or
unpredictable trees

Hypothesis 3 invoked the effect of budburst speed as such,
hypotheses 3a and 3b invoke in addition tree isolation and
consistency, respectively. We will here verify the predictions
from Fig. 3. As to hypothesis 3: caterpillar size significantly
increased with budburst speed (main effect) in one out of
three analyses (Table 2a, Fig. 6a left column) but was not
affected by budburst speed: budburst timing. Together this is
consistent with a combination of scenario 1 and 2 in Fig. 3.
Herbivory rate was (marginally) significantly increased by

budburst speed (main effect) in two out of three cases
(Table 2b, Fig. 6b left column, one opposite effect that is hard
to represent graphically and hence unconvincing). Herbivory
rate was in both analyses affected by a positive interaction
budburst speed: budburst timing: on fast bursting trees late
bursting did not decrease herbivory. Together this is consis-
tent with scenario 2 in Fig. 3.

As to hypothesis 3a: caterpillar size was significantly
affected by a negative interaction budburst speed: isolation,
for spatial but not for phylogenetic isolation. Caterpillar size
was not affected by the interaction speed: timing: isolation.
(Table 2a, Fig. 6a left column). Taken together this is con-
sistent with a combination of scenario 1 and 2 in Fig. 3 for
spatial but not for phylogenetic isolation. Herbivory rate
was not affected by budburst speed: isolation. Herbivory rate
was negatively affected by speed: timing: isolation for spatial
isolation (Table 2b, Fig. 6b left column). Together, this is
consistent with scenario 2 in Fig. 3 for spatial but not for
phylogenetic isolation.

As to hypothesis 3b, caterpillar size was not affected by
any of the terms entered into the model (Table 2a, Fig. 6a left
column). Herbivory rate was positively affected by budburst
speed:consistency and by speed:timing:consistency (Table
2b, Fig. 6b left column). Together, this is consistent with sce-
nario 2 in Fig. 3.

Discussion

We studied the phenology of an entire guild of herbivores,
sampling in total 45 species. Herbivores matched tree phe-
nology in the field: larger caterpillars and higher herbivory
on trees with earlier budburst. Albeit this relationship might
appear self-evident at first sight, it has to our knowledge not
been demonstrated before, it permitted to identify within
and between-species effects, ongoing and past processes, and
it systematically disappeared on apparent, late bursting trees.
Specifically, phenology matching was maintained in the com-
mon garden (delayed pupation in caterpillars initially from
trees with slower leaf maturation). Phenology matching was
consistently observed at the intraspecific level (3 out of 3
cases) but was less evident at the interspecific level (1 out of
3 cases). Notably, herbivores appeared most constrained by
late bursting of host trees that burst buds slowly and unpre-
dictably and are difficult to access. Moreover, lower matching
related to decreased herbivory rates. To our knowledge none
of these patterns has been shown before.

Limitations

Our study has several limitations. First, we focused on the
patterns of matching rather than the underlying processes,
such as hatching, attraction of immigrants, exclusion, and
accumulation). While we do have evidence that small body
is a good proxy for recent hatching in a dominant species
(Supporting information) our other process-related inferences
<table><caption>Table 1. Phenology matching of caterpillar guilds with individual host trees. Explaining (a) caterpillar size and (b) herbivory rate in the field by relative budburst timing, and (c) explaining pupation timing under shared conditions in the laboratory by leaf maturation in the field. Phenology matching is indicated by significant negative relationships. The ‘overall models’ tested the existence of phenology matching, whereas the ‘species models’ differentiate between two mechanisms of such matchings. The species-mean budburst timing (or leaf maturation) detects phenology matching due to sorting of matching species within guilds, whereas within-species budburst timing (or leaf maturation) detects phenology matching due to selection of matching individuals within populations. Models (a) and (b) include factor Year, all models include random factors Tree and Herbivore species (not shown). See Fig. 5 for illustration.</caption><thead><tr><th></th><th></th><th>Estimate</th><th>SE</th><th>t-value</th><th>p-value</th><th>df</th><th>R<sup>2</sup>m</th><th>R<sup>2</sup>c</th></tr></thead><tbody><tr><td rowspan="6">(a) Effects on caterpillar size of</td><td>Overall model</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Species model</td><td>Relative budburst timing</td><td>-0.192</td><td>0.058</td><td>-3.331</td><td><strong>0.001</strong></td><td>359</td><td>0.08</td><td>0.50</td></tr><tr><td></td><td>Species mean</td><td>-0.346</td><td>0.148</td><td>-2.334</td><td><strong>0.021</strong></td><td>358</td><td>0.10</td><td>0.47</td></tr><tr><td></td><td>budburst timing</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td></td><td>Within species</td><td>-0.174</td><td>0.060</td><td>-2.888</td><td><strong>0.005</strong></td><td>66</td><td></td><td></td></tr><tr><td>Species model</td><td>budburst timing</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td rowspan="6">(b) Effects on herbivory rate of</td><td>Overall model</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Species model</td><td>Relative budburst timing</td><td>-0.003</td><td>0.001</td><td>-4.522</td><td><strong>&lt;0.001</strong></td><td>186</td><td>0.21</td><td>0.82</td></tr><tr><td></td><td>Species mean</td><td>-0.002</td><td>0.001</td><td>-1.224</td><td>0.223</td><td>185</td><td>0.21</td><td>0.83</td></tr><tr><td></td><td>budburst timing</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td></td><td>Within species</td><td>-0.003</td><td>0.001</td><td>-4.576</td><td><strong>&lt;0.001</strong></td><td>188</td><td></td><td></td></tr><tr><td>Species model</td><td>budburst timing</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td rowspan="6">(c) Effects on pupation timing of</td><td>Overall model</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Species model</td><td>Leaf maturation</td><td>-2.606</td><td>1.271</td><td>-2.049</td><td><strong>0.046</strong></td><td>208</td><td>0.02</td><td>0.07</td></tr><tr><td></td><td>Species mean</td><td>0.786</td><td>2.454</td><td>0.320</td><td>0.749</td><td>207</td><td>0.04</td><td>0.07</td></tr><tr><td></td><td>leaf maturation</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td></td><td>Within species</td><td>-3.703</td><td>1.403</td><td>-2.640</td><td><strong>0.010</strong></td><td>79</td><td></td><td></td></tr><tr><td></td><td>leaf maturation</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr></tbody></table>
<figure><img src="image_10.png" /><figcaption>Figure 5. Phenology of individual trees matched by phenology (a) of herbivores in the field, and (b) of herbivores sampled from these trees and kept in the laboratory. In (a) phenology matching means profiting from early budburst and being constrained by late budburst (Fig. 3), i.e. a negative relationship between caterpillar size (top row) and herbivory rate (middle row) with relative budburst timing. In (b) phenology matching means profiting from slow deterioration of leaf quality and pupating late or being constrained by quick deterioration and pupating early, i.e. a negative relationship between pupation timing (bottom row) in the laboratory with leaf maturation. Note that in the laboratory all herbivores are fed with the same food so that a present pattern of phenology matching likely reflects past processes in the field. The overall phenology matching effect (left column) can result from two processes: Sorting of species (middle column, where tree phenology is represented by means within herbivore species), and selection within species (right column, where tree phenology is represented by the deviations from species means). Further details are provided in the Introduction and Methods. Statistical results are summarised in Table 1.</figcaption></figure>

are bound to remain speculative. Second, we compared pupa-
tion in common garden to leaf maturation in the field only
for one year. However, none of our conclusions depends
solely on this finding. Notably, the importance of past pro-
cesses (adaptation, species sorting) is supported also by the

significant effects of among-year predictability of budburst
on phenology matching of caterpillars (Fig. 6b). Lastly, while
we found significant effects, some explained only a limited
amount of variance. Although our study does not encompass
all potential drivers of these traits, the variance explained is
<table><caption>Table 2. Effects of apparent on host-tree phenology matching by caterpillar guilds in the field. The matching is measured based on individual caterpillar size (a), and their cumulative herbivory impact (b). The interpretations of apparent variables are detailed in Fig. 3. All models include factor Year and random factors Tree and Herbivore species (not shown). See Fig. 6 for visual representations.</caption><thead><tr><th></th><th></th><th>t-value</th><th>p-value</th><th>df</th><th>R<sup>2</sup>m</th><th>R<sup>2</sup>c</th></tr></thead><tbody><tr><td rowspan="10">(a) Effects on caterpillar size of</td><td rowspan="5">Spatial isolation oak model</td><td>Relative budburst timing</td><td>-2.450</td><td><strong>0.016</strong></td><td>354</td><td>0.09</td><td>0.54</td></tr><tr><td>Budburst speed</td><td>2.730</td><td><strong>0.007</strong></td><td>117</td><td></td><td></td></tr><tr><td>Spatial isolation</td><td>2.650</td><td><strong>0.009</strong></td><td>161</td><td></td><td></td></tr><tr><td>Relative budburst timing: Budburst speed</td><td>0.630</td><td>0.530</td><td>114</td><td></td><td></td></tr><tr><td>Budburst speed: Spatial isolation</td><td>-2.623</td><td><strong>0.009</strong></td><td>182</td><td></td><td></td></tr><tr><td rowspan="5">Phylogenetic isolation model</td><td>Relative budburst timing</td><td>0.442</td><td>0.659</td><td>86</td><td></td><td></td></tr><tr><td>Budburst speed</td><td>-1.560</td><td>0.121</td><td>114</td><td></td><td></td></tr><tr><td>Phylogenetic isolation</td><td>0.760</td><td>0.451</td><td>126</td><td></td><td></td></tr><tr><td>Relative budburst timing: Budburst speed</td><td>-0.400</td><td>0.694</td><td>78</td><td></td><td></td></tr><tr><td>Budburst speed: Phylogenetic isolation</td><td>0.100</td><td>0.924</td><td>102</td><td></td><td></td></tr><tr><td rowspan="5">Budburst consistency model</td><td>Relative budburst timing: Budburst speed: Phylogenetic isolation</td><td>0.255</td><td>0.799</td><td>85</td><td></td><td></td></tr><tr><td>Relative budburst timing</td><td>-1.630</td><td>0.106</td><td>311</td><td>0.10</td><td>0.56</td></tr><tr><td>Budburst speed</td><td>0.210</td><td>0.830</td><td>85</td><td></td><td></td></tr><tr><td>Budburst consistency</td><td>-0.210</td><td>0.833</td><td>124</td><td></td><td></td></tr><tr><td>Relative budburst timing: Budburst speed</td><td>1.430</td><td>0.156</td><td>92</td><td></td><td></td></tr><tr><td rowspan="5">Phylogenetic isolation model</td><td>Budburst speed: Budburst consistency</td><td>0.701</td><td>0.484</td><td>145</td><td></td><td></td></tr><tr><td>Relative budburst timing: Budburst speed: Budburst consistency</td><td>-0.563</td><td>0.575</td><td>159</td><td></td><td></td></tr><tr><td>Relative budburst timing</td><td>-5.660</td><td><strong>&lt;0.001</strong></td><td>181</td><td>0.25</td><td>0.85</td></tr><tr><td>Budburst speed</td><td>1.850</td><td>0.067</td><td>180</td><td></td><td></td></tr><tr><td>Spatial isolation</td><td>0.770</td><td>0.445</td><td>169</td><td></td><td></td></tr><tr><td rowspan="5">Phylogenetic consistency model</td><td>Relative budburst timing: Budburst speed</td><td>4.580</td><td><strong>&lt;0.001</strong></td><td>142</td><td></td><td></td></tr><tr><td>Budburst speed: Spatial isolation</td><td>-0.908</td><td>0.365</td><td>182</td><td></td><td></td></tr><tr><td>Relative budburst timing: Budburst speed: Spatial isolation</td><td>-2.518</td><td><strong>0.013</strong></td><td>173</td><td></td><td></td></tr><tr><td>Relative budburst timing</td><td>-4.830</td><td><strong>&lt;0.001</strong></td><td>173</td><td></td><td></td></tr><tr><td>Budburst speed</td><td>2.320</td><td><strong>0.021</strong></td><td>175</td><td></td><td></td></tr><tr><td rowspan="5">Budburst consistency model</td><td>Phylogenetic isolation</td><td>-0.460</td><td>0.650</td><td>102</td><td></td><td></td></tr><tr><td>Relative budburst timing: Budburst speed</td><td>2.670</td><td><strong>0.008</strong></td><td>180</td><td></td><td></td></tr><tr><td>Budburst speed: Phylogenetic isolation</td><td>-1.135</td><td>0.258</td><td>177</td><td></td><td></td></tr><tr><td>Relative budburst timing: Budburst speed: Phylogenetic isolation</td><td>-0.768</td><td>0.443</td><td>183</td><td></td><td></td></tr><tr><td>Relative budburst timing</td><td>-8.320</td><td><strong>&lt;0.001</strong></td><td>181</td><td>0.37</td><td>0.86</td></tr><tr><td rowspan="5">Phylogenetic consistency model</td><td>Budburst speed</td><td>-2.430</td><td><strong>0.016</strong></td><td>173</td><td></td><td></td></tr><tr><td>Budburst consistency</td><td>-4.220</td><td><strong>&lt;0.001</strong></td><td>176</td><td></td><td></td></tr><tr><td>Relative budburst timing: Budburst speed</td><td>4.630</td><td><strong>&lt;0.001</strong></td><td>179</td><td></td><td></td></tr><tr><td>Budburst speed: Budburst consistency</td><td>4.129</td><td><strong>&lt;0.001</strong></td><td>176</td><td></td><td></td></tr><tr><td>Relative budburst timing: Budburst speed: Budburst consistency</td><td>5.906</td><td><strong>&lt;0.001</strong></td><td>178</td><td></td><td></td></tr></tbody></table>
<figure><img src="image_11.png" /><figcaption>Figure 6. Effects of apparent size of caterpillar guilds in the field, i.e. caterpillar can grow large (a) and consume a lot (b) on trees that burst early but not trees that burst late. Each panel is divided into three columns. The left column illustrates the effect of budburst speed on phenology matching, with the red line representing trees with below-median budburst speed and the blue line representing trees with above-median budburst speed. High budburst speed may increase apparent size of trees to herbivores and permit</figcaption></figure>
Figure 6. Continued

them to maximise growth and consumption (Fig. 3). The middle and right columns show the same relationships but for different subsets of trees. The middle column includes only trees with below-median values for (i) spatial isolation (top row), (ii) phylogenetic isolation (middle row), and (iii) budburst consistency (bottom row). The right column includes only trees with above-median values for these same factors. Statistical results are summarised in Table 2. Low spatial and phylogenetic isolation and high consistency should facilitate the access to apparent, rapidly bursting trees as outlined in Fig. 3.

comparable to or greater than that of earlier studies focused on variation across years rather than across trees (Crawley and Akhteruzzaman 1988, Watt and McFarlane 1991). Additionally, caterpillar body size was more strongly related to the host's budburst phenology than to its nutritional quality. This body size-budburst relationship thus serves as a valuable proxy for phenology matching across an entire caterpillar guild, unlike previous measures limited to single-species populations with wingless females that couldn't account for herbivore movement between trees (Van Asch and Visser 2007) nor consequences for the trees.

## Possible mechanisms

In support of hypothesis 1 and hypothesis 2, we found clear evidence of phenology matching by the herbivore guild, both in the field and under shared conditions in a common garden. Is this overall phenology matching driven by the sorting of matching species into guilds, or by the selection of matching individuals within populations? Supporting hypothesis 1 interspecific, at the interspecific level in the field, larger caterpillars were found on early-bursting trees. Similarly, Crawley and Akhteruzzaman (1988) reported that budburst phenology plays a key role in shaping the composition of early-spring caterpillar guilds on trees. However, at the interspecific level, herbivory impact in the field was not significantly higher on early-bursting trees, nor did caterpillars pupate earlier in the common garden when their host trees in the field exhibited faster leaf maturation. In support of hypothesis 1 intraspecific, in the field, larger caterpillars were again found on early-bursting trees, a pattern consistent with the formation of local demes on trees (Mopper 1996). Additionally, at the intraspecific level, herbivory impact in the field was significantly higher on early-bursting trees, and caterpillars pupated earlier in the common garden when their host trees in the field exhibited faster leaf maturation. The fact that phenology matching was consistently observed at the intraspecific level (3 out of 3 cases) but was weaker at the interspecific level (1 out of 3 cases) suggests that overall phenology matching in the caterpillar guild is primarily driven by selection for individuals within populations species (i.e. deme formation), rather than sorting into guilds.

Is the observed matching of herbivore phenology to tree phenology due to present or past processes? In support of hypothesis 2 we found that after transfers of the herbivores into shared laboratory conditions in a common garden, the pupation of herbivores still matched the leaf maturation of their respective host tree. Notably, in June, leaves of late-bursting trees were no less advanced than those of

early-bursting trees, which means that late bursting constraints the time-window available to herbivores on a given tree, consistent with our results on herbivore size and herbivory for non-apparent trees. Late bursting of an oak, in turn, might be the outcome of interactions with neighbours, in particular with distantly related trees (Yguel et al. 2014, Perot et al. 2021), possibly reflecting reduced recycling of soil nutrients (Santonja et al. 2025) or availability of light (Heinecke et al. 2024). While we cannot entirely rule out early-life or maternal effects from field sampling, the maintenance of phenology matching in a common garden suggest a strong outcome of selection or sorting that accumulated over the past years (de Villemereuil et al. 2016). Without selection and sorting by past leaf maturation, the herbivores would match the laboratory conditions rather than initial host trees. Past selection of individuals within populations may be more important than past sorting of species within guilds (hypothesis 2 interspecific versus 2 intraspecific), given that caterpillars pupated earlier in the common garden when their original host trees had faster leaf maturation in the field, but only at the intraspecific level – not at the interspecific level. This also suggest that on many trees, herbivores form a relatively persistent population with a persistent phenology. This model of locally persistent populations (Mopper 1996) is not incompatible with some extant of changes in species composition on individual trees between years (Supporting information). First, these changes likely reflect replacement of individuals of early (or late) phenology by other individuals of early (or late) phenology irrespective of their species identity. Second, present-day presence of a given individual may partly reflect matching to tree during previous-year(s).

In support of hypothesis 3 the guild of early season caterpillars better tracked available resources of young leaves on rapidly bursting trees, in particular when the trees were easily accessible i.e. closer to conspecifics (hypothesis 3a) and showed little variation in relative timing of budburst among years (hypothesis 3b). Counter to expectation, we found not so much a tracking of opportunity due to early bursting trees and more of a relief from phenological constraint by late bursting trees: high budburst speed appears to facilitate the establishment of large, impactful caterpillars, likely through immigration from earlier bursting trees (Murakami and Wada 1997, Sarvašová et al. 2021). This form of phenology matching of herbivores on rapidly bursting trees could be due to ongoing exclusion of mismatching herbivores, ongoing attraction of matching herbivores, or the accumulation of attraction and exclusion over past years. Ongoing exclusion may operate universally as mismatching herbivores might simply die or move away. Rapid budburst, however, favoured phenology matching by herbivores only
under certain conditions (better accessibility and predictability). It is thus less likely that ongoing exclusion played a key role here. Ongoing attraction can potentially include both flying adults and floating or walking caterpillars, which both happens across short distances (Schroeder and Degen 2008) and from one potential host to another. Accessibility of host trees for herbivores from neighbours, and suitability of these neighbours as hosts, would permit effective attraction (suggested by Vialatte et al. 2010). Indeed, we found that rapid budburst favoured impactful caterpillars especially on the trees that are not spatially proximate to a conspecific tree. Finally, accumulation of local selection across years (Mopper and Strauss 1998) would require host phenologies to be predictable between years. Indeed, we found that rapid budburst favoured herbivores especially on the rapidly bursting trees that time their budburst consistently between years. Altogether, these observations suggest that ongoing attraction of matching herbivores and the accumulation of local selection over past years both contribute to herbivores being less constrained on late bursting trees.

The expected effect of apparency on phenology match-
ing was stronger for cumulative herbivory impact than for
individual caterpillar size. This discrepancy likely stems from
the interplay between resource availability, competition, and
population dynamics. When egg hatching is synchronized
with budburst, larger herbivore populations can form, some-
times leading to outbreaks (Van Asch and Visser 2007). In
such cases, intense competition for food may slow individual
growth, limiting caterpillar size. However, despite this con-
straint on individual growth, the cumulative impact of the
larger population can still result in significantly higher over-
all herbivory. This suggests that, while apparency strengthens
overall herbivory matching, its effect of apparency on indi-
vidual-size matching is moderated by resource competition.

Implications

To mitigate excessive herbivory, trees can become less appar-
ent by adopting at least three strategies. Firstly, slowing down
budburst so that they discreetly burst only few buds at a time
can reduce apparency. Consistently, Quiring (1993) found
that herbivory of first-instar caterpillars is reduced by slower
budburst. Secondly, growing away from conspecifics or
closely related species can reduce apparency of tree phenology
so that trees attract fewer herbivores such as floating caterpil-
lars or gravid adults. Consistently, Yguel et al. (2011) found
lower herbivore pressure on phylogenetically isolated trees.
Thirdly, fluctuating relative timing of budburst across years
can reduce apparency by reducing predictability of bud burst.
Altogether, to avoid excessive herbivory, trees should be dis-
creet by bursting buds slowly across a crown, inaccessible by
growing among distantly related neighbours, and unpredict-
able by bursting buds inconsistently among year. The speed
and consistency of plant phenology would be a valid comple-
ment to the phenology literature, which is mainly focused

on timing (Crawley and Akhteruzzaman 1988, Rousi and Heinonen 2007, Van Asch and Visser 2007). In addition, interactions between tree neighborhood and phenology matching of herbivores have not been studied before. Also, foresters might support these three strategies of trees by selecting tree genotypes that burst buds slowly and unpredictably, and associating tree species that are phylogenetically distant or burst at different moments to prevent late budbursting trees from attracting somewhat more advanced herbivores. In the evolutionary past, selection for being unapparent might have contributed to the formation of phylogenetically diverse forests and niche convergence among phylogenetically distant lineages of trees (Yguel et al. 2011).

Being discreet, unpredictable and inaccessible may also
have costs. First, being discreet by slowing down budburst
can reduce tree performance as it exposes buds to different
risks. Early bursting buds are more vulnerable to late frost
damage (Svystun et al. 2021), while late bursting buds expe-
rience shading from earlier bursting buds, limiting their
growth opportunities (Carabelli et al. 2007). Second, being
unpredictable by fluctuating relative timing of budburst
might cost energy. To advance budburst in the following
year, a tree might first need to advance leaf senescence in the
current year (Marchand et al. 2020), which may reduce the
carbohydrate production in the current year. Third, being
inaccessible by growing among distantly related species may
increase the inter-specific competition and reduce the sup-
port from mutualists. These costs could outweigh the benefits
of reduced matching, explaining why many trees do not fully
adopt these strategies of inapparency and rather burst rapidly
and consistently and grow with close relatives or conspecifics.

Conclusions

Our unique approach allowed us to study the phenology of
the entire guild of early-spring ectophagous caterpillars and
revealed that present phenology matching of caterpillars to
spring phenology of trees is probably primarily driven by
past selection of matching individuals within herbivore spe-
cies, rather than species-sorting within guilds. Most impor-
tantly, in contrast to the long-held believe that trees are
apparent given their size and stature in space (Feeny 1976,
Forkner et al. 2004, Strauss et al. 2015), we showed that trees
can avoid extensive herbivory by being unapparent in time,
i.e. inaccessible to early-developing herbivores by growing
among distant neighbours, discreet by bursting buds slowly
across a crown, and unpredictable by bursting buds inconsis-
tently among years. Thus, if trees can be inapparent in time,
then ‘tree’ may be a poor proxy of high apparency in studies
aiming to identify the consequences of apparency – possibly
explaining why these studies came to inconsistent conclu-
sions (Schuldt et al. 2012, Zverev et al. 2017). Beyond trees,
all kinds of hosts might apply slow and unpredictable phe-
nology, or isolation from conspecifics as a strategy to evade
phenology-matching parasite guilds.
***Acknowledgements*** – We thank Adrian Hackenbeck, Romuald Cloteau, Marie-Claire Martin, Benoît Dujol, and Valérie Gouesbet for help in the field and with the rearing, Nicole Lepertel and Mael Garrin from GRoupe d'ETude des Invertébrés Armoricais (GRETIA) for the visual identification of herbivore species in 2019. We thank Advanced Identification Methods GmbH from Leipzig, Germany for conducting the barcoding of our samples from 2019. We thank Mickael Pihain and Lorène Julia Marchand for fruitful discussions and insightful comments. We sincerely thank Ally Phillimore for valuable time and insightful comments, which have greatly improved the quality of this manuscript. The constructive feedback and suggestions have helped refine our analysis and strengthen our interpretations.

***Funding*** – This study was funded by an ACOMB grant from the Région Bretagne, an ATIP grant from CNRS. BY was supported by a doctoral grant from Région Bretagne and CNRS, and SM was supported by a doctoral grant from the Ministry of Research and Education (France). FM received support from the National Science Centre (NCN, Poland), grant no. 2018/29/B/NZ8/00112. ***Permits*** – Fieldwork was conducted with permission from the Office National des Forêts (ONF) at the Forest of Rennes. All activities complied with national and institutional guidelines for ethical research and biodiversity conservation. The study followed established protocols for the non-destructive sampling of herbivorous insects and host trees.

## Author contributions

**Soumen Mallick:** Conceptualization (equal); Data curation (equal); Formal analysis (lead); Funding acquisition (equal); Investigation (equal); Methodology (equal); Software (equal); Validation (equal); Visualization (equal); Writing – original draft (lead); Writing – review and editing (equal). **Freerk Molleman:** Conceptualization (equal); Funding acquisition (supporting); Supervision (supporting); Writing – review and editing (equal). **Benjamin Yguel:** Data curation (equal); Writing – review and editing (equal); **Jörg Muller:** Conceptualization (equal); Supervision (supporting); Writing – review and editing (equal). **Romain Georges:** Data curation (supporting); Writing – review and editing (supporting). **Stéphanie Llopis:** Data curation (equal).

**Marjorie Levant:** Data curation (supporting); Writing – review and editing (supporting). **Frédéric Jean:** Data curation (supporting); Writing – review and editing (supporting).

**Yasin Gundesli:** Data curation (supporting); Writing – review and editing (supporting). **Jaad Besse:** Data curation (equal). **Agnès Schermann-Legionnet:** Data curation (supporting). **Andreas Prinzing:** Conceptualization (lead); Data curation (equal); Funding acquisition (lead); Investigation (equal); Methodology (equal); Project administration (equal); Resources (equal); Supervision (lead); Validation (equal); Visualization (equal); Writing – original draft (supporting); Writing – review and editing (lead).

## Data availability statement

The datasets used and/or analysed during the current study are given in the appendix, and will be deposited in Dryad along with the R-codes used for the data analyses.

Data are available from the Dryad Digital Repository: https://doi.org/10.5061/dryad.nzs7h452t (Mallick et al. 2025).

## Supporting information

The Supporting information associated with this article is available with the online version.

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