Plant-animal interactions comprise the fundamental relationships of ecological research (Blue et al. 2011, Li et al. 2011, Liu et al. 2011, Martin and Maron 2012, Fang and Huang 2014, Mu et al. 2014). Changes in the relationships between animals and plants can affect plant growth and reproduction (Liu et al. 2011, Fang and Huang 2014, Mu et al. 2014, Shao et al. 2017), aboveground biomass (Blue et al. 2011, Li et al. 2011), community composition (Strauss and Irwin 2004, Nogales et al. 2015), nutrient cycling and energy flux (Heimann and Reichstein 2008, Defossez et al. 2011). Most previous studies have concentrated on effects of environmental change on plant-animal interactions (Morris et al. 2008, Malhi et al. 2009, Li et al. 2011, Liu et al. 2011, de Sassi et al. 2012, Hoover et al. 2012, Martin and Maron 2012) and have found that these relationships could be affected by rising temperatures (Li et al. 2011, Liu et al. 2011), changes in precipitation regimes (Morris et al. 2008, Malhi et al. 2009) and differential nitrogen deposition (de Sassi et al. 2012, Hoover et al. 2012). However, the effects of human activity also affect plant-animal interactions (Romero et al. 2006, Rosin and Poulsen 2016), especially in the case of plant and pollinator mutualisms (Aizen and Feinsinger 1994, Kearns et al. 1998).
Pruning is one widespread human activity that directly affects a plant’s ability to perform vital life processes, such as photosynthesis (Agrawal and Spiller 2004, Grechi et al. 2008). Plant traits, including nutrient content and physical or chemical defenses, may also be altered by pruning (Martinez and Wool 2002, Agrawal and Spiller 2004, Kumar et al. 2010, Saifuddin et al. 2010, Maltoni et al. 2012, Albarracín et al. 2017) and could subsequently affect a plant’s accessibility or attractiveness to herbivores (Nykänen and Koricheva 2004, Wen et al. 2006). However, the effects of pruning on the preference of herbivores are inconsistent in previous studies, and thus unknown. On one hand, light pruning may remove unhealthy tissue, and reduce pest damage. This is known as the “escape strategy” (Rattan 1992, Sivapalan 1999, Maltoni et al. 2012). On the other hand, pruning produces vigorous shoots and leaves, which could attract more herbivores. This is called the plant vigor hypothesis (Price 1991, Grechi et al. 2008). Furthermore, a plant undergoing a heavy pruning treatment produces more tender twigs and leaves, which leads to a higher water content in the plant than under undisturbed conditions. The tender twigs could attract fewer pests, according to the plant-stress hypothesis (White 1969), or it could attract more sap-feeders, according to the pulsed stress hypothesis (Huberty and Denno 2004). Actually, effects of defoliation, which is similar as pruning, have been widely studied. The results show that defoliation can affect the plant architecture, nutrient content, and defensive chemicals, and thus affect the community dynamics of herbivores (Mattson and Palmer, 1987; Leather et al. 1988; Leather et al., 1993; Leather et al., 1995; Riipi et al., 2005; Springer-Verlag et al., 2006). However, only few studies have tested these theories and investigated the difference in predator abundance between lightly- and heavily-pruned plants of the same species, or the influence of pruning on plant structure (e.g., leaf mass, twig length etc.) within a species. We will use these metrics to assess the impact of pruning on herbivorous insect abundance and to increase our understanding of the response mechanisms of trophic relationships to human activity. Considering both that heavy pruning induces more compensatory growth than does light pruning and that plant structure and insect attraction are tightly correlated, we hypothesized that heavy pruning would produce more vigorous shoot and leaf growth and thereby attract more sap-feeders.
Our study system consists of the aphid species (Aphis gossypii Glover) and one of its host species, the Box tree (Buxus megistophylla Levl.). The Box tree is a popular evergreen garden plant and is cultivated in most provinces of China. The Box tree is often pruned to one of two canopy shapes: a cubic canopy, like a hedge with heavy pruning and a spheroidal canopy, like a ball with light pruning. We investigated the abundance of aphids, and we measured plant structure with the leaf dry mass content, the length of the annual shoots and the inflorescence number of the differently-shaped plants. Our objectives were 1) to test whether the different shapes of the plants would affect the plant-animal interactions, and 2) to explore the response of animals to different branch pruning shapes in plants they often eat.