Overall, our results provide strong evidence that the relationship between leaf size and insect herbivory does not follow a constant rule across seasons; a pattern of monotonic increase occurred in September, while a hump-shaped pattern occurred in December. More complex patterns was detected among species, showing a variety of patterns including hump-shaped and U-shaped patterns, and patterns with monotonic decrease and increase, although most species showed a leaf-size-independent pattern.
As discussed previously, various mechanisms determine both the positive and negative effects of leaf size on herbivory (Feeny 1976; Bogacheva 1994; Moles and Westoby 2000; Low et al. 2009; Moreira et al. 2016). In this study, the leaves differed greatly not only in leaf size, but also in other traits such as specific leaf area, and carbon and nitrogen content (unpublished data), all of which may affect the foraging preferences of insect herbivores (Ackerly et al. 2002; Cardenas et al. 2014; Zava and Cianciaruso 2014), thus influencing the effects of leaf size on herbivory. Despite potential interference from other factors, our results showed a clear positive relationship between herbivory and leaf size in September; larger leaves were more likely to be damaged by insects and suffered heavier damage than smaller ones, indicating that the positive effects of leaf size may overcome the negative effects. However, a hump-shaped pattern was found in December, that is, the consumption of medium-sized leaves was higher than that of both large and small ones. In our study area, the temperature was much lower in December than in September (7.2 ℃ vs. 15.3 ℃). In general, ectothermic insect herbivores are sensitive to temperature reduction, and often show a decrease in abundance, activity, and metabolic intensity, as well as reduced energy requirements at lower temperatures (Sinclair et al. 2003; Deutsch et al. 2008; Rho and Lee 2017). In such situations, based on the predator satiation hypothesis (Janzen 1971), large-sized leaves may show a smaller proportion of insect damage than medium-sized leaves, as the absolute nutrient content of large leaves far exceeds the energy requirement of the insect herbivores.
Our results showed that the leaf size preference varied considerably among species in both seasons, indicating that the effect of leaf size on insect herbivory was species-specific. Similar interspecific variation has been reported in other species interactions such as seed dispersal and predation by small mammals (Feng et al. 2021; Thein et al. 2021). The species measured in our study differed greatly in the mean value (0.83–42.30 cm2) and range (0.46–104.69 cm2) of leaf size, and the ratio of maximum to minimum leaf size (2–617-fold). However, most of them did not show clear effects of leaf size on herbivory within the species. Here, we propose several possible explanations: 1) the limited range of leaf size within a single species may not be sufficient to exhibit a clear effect of size on herbivory. 2) Plant leaves usually possess physical and chemical defense traits in varying degrees (Moles et al. 2013; Cardenas et al. 2014; Zava and Cianciaruso 2014; Zhao et al. 2021), which may influence the effects of leaf size on herbivory; for example, insect herbivores may reject a leaf regardless of whether it is large or small, if the leaf contains a large amount of highly toxic chemicals. 3) Leaves often differ in nutrient content among species (Kattge et al. 2011; Cardenas et al. 2014), and the nutrient content of a leaf may also introduce biases on the effects of leaf size on herbivory. 4) Many insects are specialist herbivores (Ali and Agrawal 2012), and different species of herbivores may show different preferences for leaf size (Bogacheva 1994). Furthermore, the neighbor effect also affects insect-leaf interactions; for example, whether a given species of plant would be consumed by an insect herbivore can be influenced by the presence of neighboring plants (Baraza et al. 2006; Hahn and Orrock 2016; Moreira et al. 2017). The neighbor effect often manifests as different or even opposite patterns, depending on the species present in the neighborhood (Wang et al. 2010, Castagneyrol et al. 2018, Wang 2020). In our study, the sample plots were randomly selected; therefore, for any given species of leaves, the neighbor effect may differ among plots because of the differences in species composition among the plots. Such variations in the neighbor effect may dilute the effect of leaf size on the preference of insect herbivores for a specific species of leaves.
The effect of leaf size on insect herbivory varied across seasons and among plant species, In conclusion, the effects of leaf size on the foraging preferences of insect herbivores may be contingent on both external (e.g., temperature) and intrinsic (e.g., other leaf traits) factors, indicating that a one-off survey with a few species may bias our understanding on the overall pattern of the effect of leaf size on herbivory. Similar variations may also exist in other systems (e.g., pollination, frugivory, seed predation, etc.), and should be considered in future studies on biotic interactions.