The influence of Salix leaf abscission on leaf-miner survival and life history

Abstract. 1. Early abscission of mined leaves was an important mortality factor of a Phyllonorycter species (Lepidoptera: Gracillariidae) on Salix lasiolepis Benth. (Salicaceae). A larger percentage of mined leaves abscised early (34.4% in 1990; 24.5% in 1991), and Phyllonorycter survival was greatly reduced in these abscised leaves.
2. Leaf-mining by Phyllonorycter was associated with increased early leaf abscission. An egg removal experiment demonstrated that leaf mining induced this increase in leaf abscission.
3. The induction of early leaf abscission was dependent upon the timing of herbivory and simulated herbivory (mechanical damage). Early mechanical damage induced leaf abscission, late mechanical damage did not. Mines which expanded early were more likely to induce leaf abscission than mines which expanded more slowly.     

Effects of simulated shoot and leaf herbivory on vegetative growth and plant defense in Acacia drepanolobium

Plants have considerable ability to respond to herbivory, both with (above-ground) regrowth and with increased defense. We simulated both leaf and shoot herbivory in controlled, replicated experiments on individuals of Acacia drepanolobium in Laikipia, Kenya. These experiments were carried out on individuals that had experienced different, experimentally controlled histories of large mammalian herbivory. Both forms of simulated herbivory were associated with compensatory regrowth. Branches whose shoots had been removed grew significantly more over the next year than paired control branches, fully compensating for the lost shoot length. Branches whose leaves were removed both grew faster and had more leaves one year later than did control branches. Shoot removal, but not leaf removal, increased the production of side shoots. However, because past herbivore pressure was negatively associated with net shoot growth, there may be a long-term cost of herbivory even when plants appear to fully compensate for herbivory in the short term. In contrast to the effects on growth, simulated herbivory did not significantly increase physical (spines) or chemical (tannins) defenses, and there were no significant negative correlations between compensatory growth and plant defense.     

Delayed greening,leaf expansion,and damage to sympatric<Emphasis Type="Italic"> Shorea</Emphasis> species in a lowland rain forest

The function of delayed greening in the seedlings of canopy tree species in a lowland tropical rain forest was examined in terms of its potential defensive value against herbivory. To explore the ecological and evolutionary backgrounds for delayed greening, we chose eight sympatric congeneric (Shorea) dipterocarp species that were either normal-greening or delayed-greening species. Expansion and toughening of leaves took approximately 30 days for all species, and did not differ between the normal- and delayed-greening species. The main factors that affected leaf damage during expansion were insect herbivory and fungal infection. Levels of leaf damage were significantly lower for delayed-greening species than for normal-greening species, but proportions of heavily damaged leaves and leaf abscission during expansion did not differ. In addition, no significant difference was found in damage levels on leaves (aged 1–2 months) of naturally occurring seedlings between normal- and delayed-greening species. Therefore, delayed greening may effectively reduce the level of leaf damage in young expanding leaves, but may not necessarily reduce leaf abscission and damage to mature leaves. The existence of delayed greening could not be simply explained by the phylogenetic and ecological backgrounds of the trees. Consequently, delayed greening may have a function in reducing damage during expansion, but more information (such as knowledge of the secondary metabolites involved in this phenomenon) is needed to explain fully why these species exhibit delayed greening.     

Simulated herbivory advances autumn phenology in Acer rubrum

To determine the degree to which herbivory contributes to phenotypic variation in autumn phenology for deciduous trees, red maple (Acer rubrum) branches were subjected to low and high levels of simulated herbivory and surveyed at the end of the season to assess abscission and degree of autumn coloration. Overall, branches with simulated herbivory abscised ～7 % more leaves at each autumn survey date than did control branches within trees. While branches subjected to high levels of damage showed advanced phenology, abscission rates did not differ from those of undamaged branches within trees because heavy damage induced earlier leaf loss on adjacent branch nodes in this treatment. Damaged branches had greater proportions of leaf area colored than undamaged branches within trees, having twice the amount of leaf area colored at the onset of autumn and having ～16 % greater leaf area colored in late October when nearly all leaves were colored. When senescence was scored as the percent of all leaves abscised and/or colored, branches in both treatments reached peak senescence earlier than did control branches within trees: dates of 50 % senescence occurred 2.5 days earlier for low herbivory branches and 9.7 days earlier for branches with high levels of simulated damage. These advanced rates are of the same time length as reported delays in autumn senescence and advances in spring onset due to climate warming. Thus, results suggest that should insect damage increase as a consequence of climate change, it may offset a lengthening of leaf life spans in some tree species.     

Reduced resistance of invasive varieties of the alien tree<Emphasis Type="Italic"> Sapium sebiferum</Emphasis> to a generalist herbivore

Invasive plants are often larger in their introduced range compared to their native range. This may reflect an evolved reduction in defense and increase in growth in response to low herbivory in their introduced range. Key elements of this scenario include genetic differences in defense and growth yet uniformly low rates of herbivory in the field that dissociate defense and herbivore damage for alien species. We conducted a laboratory experiment with Melanoplus angustipennis grasshoppers and Chinese Tallow Tree seedlings ( Sapium sebiferum) from its native range (China) and its introduced range (Texas, USA) where it is invasive. We caged grasshoppers with pairs of Sapium seedlings from the same continent or different continents. The amounts of leaf area removed from Texas and China seedlings, and their height growth rates, were indistinguishable when both seedlings in the pair were from the same continent. However, when grasshoppers had a choice between seedlings from different continents, they removed more Texas Sapium foliage than China Sapium foliage and height growth rates were higher for China Sapium seedlings compared to Texas seedlings. Grasshopper growth rates increased with greater Sapium foliage consumption. In a common garden in Texas, Sapium seedlings from Texas grew 40% faster than those from China. Chewing insect herbivores removed little Sapium foliage in the field experiment. Although grasshoppers preferred to feed on Texas Sapium when offered a choice in the laboratory, extremely low herbivory levels in the field may have allowed the Texas seedlings to outperform the China seedlings in the common garden. These results demonstrate post-invasion genetic differences in herbivore resistance and growth of an invasive plant species together with a decoupling of defense and herbivore choice in the introduced range.     

Interspecific competition affects growth and herbivore damage of Brassica napus in the field

Resource competition can influence plant fitness either directly, or indirectly by influencing the amount of herbivore damage received by plants in the field. We previously found that competition could constrain the constitutive and woundinduced expression of defensive trypsin inhibitors in pot-grown Brassica napus seedlings in the greenhouse, suggesting that the ability of a plant to chemically defend itself could be constrained by competition in the field. Guided by these results, we investigated whether competition would affect growth and the presence of herbivores and herbivore damage on B. napus plants in the field. We established sixteen 1 m 2 plots in the field in a 7 x7 mgrid. Nine two-week-old B. napus seedlings were transplanted from the greenhouse into each 1 m 2 plot. Half of the plots were kept weed-free and half were left to develop interspecific weed competi-tors.After six weeks, three randomly chosen plants in each plot were measured for height, number of leaves, leaf area removed by herbivores, and the presence of aphids, leaf miners, and eggs of ladybird beetles. Consistent with the induction of the shade-avoidance response, plants in plots with weed competitors were significantly taller and had half as many leaves as plants in weed-free plots. Competing plants also had 60% more leaf arearemoved by herbivores, an 80% higher proportion of leaves with aphids, and an equal proportion of leaves with leaf miners. In this study, weed competition had dramatic effects on growth, leaf area removal by herbivores, and the presence of aphids on B. napus plants in the field. Together with our demonstration that competition can constrain the expression of trypsin inhibitor activity, these results suggest that resource competition may limit theability of a plant to defend itself from natural enemies, leading to greater herbivory. In turn, increased herbivory on competing plants could exacerbate the direct effects of competition on plant fitness.     

Herbivory on Temperate Rainforest Seedlings in Sun and Shade: Resistance,Tolerance and Habitat Distribution

Differential herbivory and/or differential plant resistance or tolerance in sun and shade environments may influence plant distribution along the light gradient. Embothrium coccineum is one of the few light-demanding tree species in the temperate rainforest of southern South America, and seedlings are frequently attacked by insects and snails. Herbivory may contribute to the exclusion of E. coccineum from the shade if 1) herbivory pressure is greater in the shade, which in turn can result from shade plants being less resistant or from habitat preferences of herbivores, and/or 2) consequences of damage are more detrimental in the shade, i.e., shade plants are less tolerant. We tested this in a field study with naturally established seedlings in treefall gaps (sun) and forest understory (shade) in a temperate rainforest of southern Chile. Seedlings growing in the sun sustained nearly 40% more herbivore damage and displayed half of the specific leaf area than those growing in the shade. A palatability test showed that a generalist snail consumed ten times more leaf area when fed on shade leaves compared to sun leaves, i.e., plant resistance was greater in sun-grown seedlings. Herbivore abundance (total biomass) was two-fold greater in treefall gaps compared to the forest understory. Undamaged seedlings survived better and showed a slightly higher growth rate in the sun. Whereas simulated herbivory in the shade decreased seedling survival and growth by 34% and 19%, respectively, damaged and undamaged seedlings showed similar survival and growth in the sun. Leaf tissue lost to herbivores in the shade appears to be too expensive to replace under the limiting light conditions of forest understory. Following evaluations of herbivore abundance and plant resistance and tolerance in contrasting light environments, we have shown how herbivory on a light-demanding tree species may contribute to its exclusion from shade sites. Thus, in the shaded forest understory, where the seedlings of some tree species are close to their physiological tolerance limit, herbivory could play an important role in plant establishment.     

Distance-dependence in herbivory and foliar condition for juvenile Shorea trees in Bornean dipterocarp rain forest

We assessed density- and distance-dependence in herbivore effects and juvenile condition for four species of Shorea, the most speciose genus in the dominant canopy family of southeast Asian rain forest trees (Dipterocarpaceae). Herbivore damage was quantified as partial leaf loss on young leaves, and whole plant foliar condition as the product of the fraction of leaf nodes containing leaves and the fraction of tissue remaining on extant leaves. Adults of the four species were centers of high total, as well as conspecific, density of juveniles (<1 m tall). For two species, S. hopeifolia and S. pinanga, herbivore damage declined significantly with distance, decreasing by 40% and 51% respectively, between 5 m and 35 m from the parent. For the same two species, foliar condition improved significantly between 5 m and 35 m, increasing by 45% for S. hopeifolia and 24% for S. pinanga. If foliar condition influences juvenile survival and growth, more widely dispersed seeds of these species are more likely to recruit to the canopy. In contrast, there was no significant distance-dependence for S. parvifolia or S. longisperma. Among species, herbivore damage was greatest in those species with greatest local juvenile abundances, i.e., those with highest densities, leaf size, juvenile foliar mass and/ or foliar mass/m² ground area, but was unrelated to the toughness of mature leaves. However, distance was a better predictor of herbivore damage than was conspecific juvenile density, as evaluated by backward elimination regressions, for both S. hopeifolia and S. pinanga. For foliar condition, the best predictor was distance for S. pinanga, but conspecific density for S. hopeifolia, whose juveniles were smallest and occurred at the highest densities. Total juvenile density (all woody plants) was eliminated as a factor in all cases. The species-specificity of effects (i.e., their dependence on conspecific distance or density), together with the marked differences among congeneric species, caution against generalizations regarding distance-dependent effects in diverse forests. Received: 21 April 1998 / Accepted: 8 July 1998     

The Herbivore Assemblage, Herbivory and Leaf Chemistry of the Mangrove Kandelia obovata in Two Contrasting Forests in Hong Kong

The herbivore assemblage, intensity of herbivory and factors determining herbivory levels on the mangrove Kandelia obovata (previously K. candel, Rhizophoraceae) were studied over a 13-month period at two forests with contrasting growing conditions in Hong Kong. Mai Po was part of an eutrophic embayment in the Pearl River estuary and generally offered more favourable conditions for mangrove growth, whereas Ting Kok had a rocky substratum and oceanic salinity. Twenty-four insect herbivore species were recorded on K. obovata, with lepidopteran larvae that consume leaf lamina being the dominant species. While leaf litter production was similar at the two forests, herbivory level at Mai Po (mean = 3.9% in terms of leaf area loss) was more severe than that at Ting Kok (mean = 2.3%). Peak herbivory levels were found in summer at both locations (6.5% for Mai Po and 3.8% for Ting Kok). Young leaves of K. obovata at both locations were generally preferred by the herbivores from the period of late spring to summer. Concentrations of most feeding deterrents (ash, crude fibre, and total soluble tannins) were significantly higher in both young and mature leaves at Ting Kok, whereas leaf nutrients (total nitrogen and water) were the same at the two sites. Young leaves at Ting Kok contained about 30% more tannins than their counterparts at Mai Po. Significant differences in leaf chemistry also existed between young and mature leaves at either site. The differences were concomitant with the observed patterns of leaf herbivory on K. obovata, and suggest a potential relationship between environmental quality and plant defence against herbivory.     

Herbivory-induced stress: Leaf developmental instability is caused by herbivore damage in early stages of leaf development

Herbivory is a major source of plant stress and its effects can be severe, decreasing plant fitness, or subtle, affecting the development of leaves by influencing the normal pattern of growth and expansion of leaf blades. Fluctuating asymmetry (FA) analysis is recognized as a measure of plant stress, and can be used to evaluate subtle effects of herbivory on the imperfect growth of bilaterally symmetrical traits, such as leaves. One general issue is that authors usually consider FA as an indicator of stress, which can attract herbivores (plant stress hypothesis), and studies showing that herbivores themselves affect leaf symmetry (herbivory-induced stress hypothesis) are scarce, with mixed results. Here, we investigated the relationship between herbivory by thrips and leaf FA in Banisteriopsis malifolia and Heteropterys escallonifolia (Malpighiaceae). Pseudophilothrips obscuricornis is a free-living, non-pest, sucking species that feeds mainly on leaf buds. We hypothesized that herbivory by thrips in the early stages of leaf development would provoke increased FA levels in mature leaves. The results showed that thrips herbivory rate was low, affecting barely more than 1% of the leaf blade. Nonetheless, thrips-attacked leaves of B. malifolia and H. escallonifolia presented increases of 15 and 27% in leaf asymmetry, respectively, compared to uninjured leaves, corroborating the herbivory-induced stress hypothesis. Since herbivory by thrips in leaf buds was related to significant increases in the stress of mature leaves, we assume that under these circumstances, FA can be used as a biomarker for plant stress following herbivory damage. To be useful as a biomarker of stress, FA in plants must be investigated with caution, taking into account the natural history of the herbivore species and timing of leaf damage.     

常绿阔叶林中栲树展叶期叶片的虫食格局

叶片的虫食主要发生在展叶期。虽然展叶期短暂, 它却可能是了解植食性昆虫和植物之间相互关系的关键。为了解栲树(Castanopsis fargesii)在展叶期叶片的虫食格局和展叶方式对叶片虫食的影响, 研究了栲树展叶期内的虫食动态变化, 结果表明: 栲树展叶的两个阶段(折叠期和打开期), 虫食叶片的格局存在较大的差异, 打开阶段的日虫食频度和日虫食率显著高于折叠阶段(F_{1, 32}=8.97, p=0.005 4; F_{1, 32}=12.38, p=0.001 4), 展叶期最终叶片虫食频度为50.72%, 叶片虫食率为8.25%。折叠期叶片主要受到低强度的虫食, 打开期叶片虫食则以较大强度的虫食为主。展叶期叶片的虫食主要发生在夜间, 夜间虫食率显著高于日间虫食率(t=2.51, p=0.017), 变化趋势与日虫食率一致。栲树叶片在展叶的两个阶段可能采用了不同的防御对策。     

Ontogenetic changes in anti-herbivore defensive traits in leaves of four Mediterranean co-occurring <Emphasis Type="Italic">Quercus</Emphasis> species

Investment in anti-herbivore defence in tree species has been one of the priority research topics in plant terrestrial ecology during the last decades. However, despite considerable experimental effort, interspecific differences in the ontogenetic trends in the investment in defence are still a matter of debate, as to date experimental evidence is contradictory. In the present work, insect herbivory levels were measured in seedlings and mature trees of four co-occurring Mediterranean Quercus species with differing leaf life spans, as well as several leaf characteristics that can determine herbivore preference. The measured leaf traits included nitrogen (N), fibre (cellulose, hemicellulose and lignin), total phenolic contents, leaf mass per unit area (LMA) and leaf thickness. The leaves of seedlings had a lower LMA and leaf thickness and lower concentrations of N and cellulose, but higher concentrations of lignin and phenols than those of mature trees. However, the loss of leaf area tended to be more severe for seedlings than for mature trees, although the differences were only significant for deciduous species. This constitutes a confirmation of the strong effects of physical traits on herbivore preferences. The greater resource limitations for defensive mechanisms in seedlings with respect to mature trees would explain that at intraspecific level we do observe a compromise between chemical and physical defences. As a result, seedlings rely on chemical rather than on physical defences.     

Do tropical forest leaves suffer more insect herbivory? A comparison of tropical versus temperate herbivory,estimated from leaf litter

It is generally believed that tropical forests suffer more herbivory, as a proportion of leaf area, than do temperate forests. Reviews so far have compared studies performed by different authors using very different methodologies. Here we carried out studies on 125 samples at 86 localities in eastern North America and on 75 samples taken at five localities in Malaysia and Singapore, including both mature secondary and primary forest. Samples in North America were spread over 3 years. In tropical Asia, the samples were taken at four time slices at least 8 months apart, scattered over a 4-year period. Total herbivore damage during the lifetime of tree leaves was estimated from the percentage area damaged in recently fallen, undecayed leaves from the forest floor, using scanner-linked software. In terms of percentage damage per leaf, the results suggest that lowland tropical forest has significantly higher leaf herbivory (5.82%) than temperate forest (5.48%). This is in accord with the general expectation that aseasonal tropical forests should have more herbivory damage. However, when percentage damage ‘per unit time of growing season’ is calculated based on an estimate of leaf lifetime in the tropics, tropical lowland herbivory damage turns out to be a fraction (about one half) of that in the temperate zone. Thus, these results tend to put in question the widely held view that herbivore damage is markedly more intense in the tropics. Over total leaf lifetime, the intensity of damage in the tropical area is only slightly higher than temperate regions. In terms of intensity of herbivory on leaves per unit of time, the opposite seems to be the case. It is uncertain which index should be taken as more significant in interpreting the selection pressure for anti-herbivore defenses in the tropics.     

Effects of insect attack to stems on plant survival,growth, reproduction and photosynthesis

Studies of insect herbivory have mostly focused on leaf‐feeding even though most woody plant biomass is stem tissue. Attack to stems has the potential to be more detrimental to plant performance than attack to leaves. Here we asked how severe is the impact of insect stem herbivory on plant performance. We quantify the effect of insect stem herbivory via a meta‐analysis of 119 papers in 100 studies (papers by the same authors were treated as the same study). These studies involved 92 plant species and 70 species of insect herbivore (including simulated herbivory). Attack to plant stems reduced plant performance by an average of approximately 22%. Stem herbivory had greatest impacts on plant and branch survival, which was reduced by 63%. Measures of plant reproduction and vegetative biomass were reduced by 33% and 16% respectively, while measurements of photosynthetic rate were not significantly different between plants with and without stem herbivore attack. Stem herbivory led to a decline in leader performance but an increase in performance of laterals, highlighting the importance of plant compensation. Juvenile plants were more severely affected by stem herbivory than adult plants, and studies conducted in greenhouses found more severe effects than studies conducted in the field. Stem herbivory did not have a significant effects on any of the non‐performance responses measured (defence compounds, SLA, root:shoot, phenology and plant carbon and nitrogen). We compare our results with results from various meta‐analyses considering herbivory on other plant parts. The impact of insect herbivory to stems on plant performance appears at least as severe as insect herbivory to roots and leaves, if not more.     

Different mechanisms explain feeding type-specific patterns of latitudinal variation in herbivore damage among diverse feeding types of herbivorous insects

To determine whether latitudinal variation in herbivore impact exists, we examined three major herbivorous insect feeding types (chewers, gallers, and miners) on/in leaves of Japanese beech. Herbivores were collected with litter traps deployed in forests across a latitudinal gradient of 10°. Leaf litter analyses demonstrated that chewing herbivory increased with increasing latitude of collection site. However, the densities of miners and gallers decreased with latitude. To test whether latitudinal variation in herbivore damage occurs in the absence of geographically differentiated environmental cueing (e.g., physical stresses or herbivore damage), we measured both genetically determined constitutive leaf traits and herbivore damage in a common-garden experiment. In this experiment, miner density decreased with latitude, but chewing herbivory did not vary latitudinally. Galler density was higher on trees from native provenances than on trees from unrelated provenances likely because of local adaptations. Leaf mass per unit area (LMA), tannin, and phenolics all decreased with latitude of provenance. The latitudinal variation in one constitutive leaf trait (LMA) best explained latitudinal variation in chewing herbivory. Thus, different mechanisms account for feeding type-specific patterns of latitudinal variation in herbivore damage among different herbivore feeding types.     

Herbivory pattern and induced responses to simulated herbivory inQuercus mongolica var.grosseserrata

Seasonal changes in leaf traits and the herbivory pattern ofQuercus mongolica var.grosseserrata were studied, and simulated herbivory experiments were carried out in order to evaluate leaf trait responses. Leaves ofQ. mongolica emerged simultaneously in spring and most were retained until autumn. Nitrogen concentration was highest when leaves first emerged and decreased rapidly with leaf age. Leaf mass per area (LMA) increased with leaf age. Herbivore attack was concentrated in the first 20 days after bud-break, which corresponded to the high nutritional value of the leaves for herbivores at this time. Simulated herbivory experiments indicated that LMA increased with artificial leaf damage, suggesting an increase in leaf toughness, and that nitrogen concentration decreased later in the season in comparison with intact leaves. As a result, herbivore attack following artificial leaf damage decreased with increasing initial leaf damage. However, leaf longevity was not affected by initial leaf damage. These responses were considered to be a strategy to disperse herbivory damage among leaves.     

A comparison of artificial defoliation techniques using canola (<Emphasis Type="Italic">Brassica napus</Emphasis>)

We conducted two experiments that investigated how the method and location of artificial defoliation influenced growth, reproduction, and allocation in canola, Brassica napus. In one experiment, 0%, 25%, or 50% of leaf area was removed by cutting circular holes at three possible locations: concentrated at either the base of leaves or at their tips, or dispersed throughout leaf blades. Plants fully compensated for such damage; reproduction and allocation were unaffected by either defoliation intensity or wound location. In a second experiment, we again initiated three intensities of defoliation: non-damaged plants served as controls, while others had 25% or 50% of their leaf areas removed. The method of removal in the second experiment consisted of cutting either multiple, similar-sized, circular holes or single, contiguous patches of a leaf blade. At the highest defoliation intensity reproductive output and allocation were significantly less in plants treated with the former method than the latter, even though an equivalent initial amount of leaf area was removed in both treatments. We conclude that simulated herbivory studies must account for not only how much of the plant is damaged, but also the pattern of leaf damage itself, since both factors contribute to a plant’s physiological and ecological responses to grazing.     

Do small leaves expand faster than large leaves,and do shorter expansion times reduce herbivore damage?

Leaves are most vulnerable to herbivory during expansion. We hypothesised that one factor favouring small leaves could be that smaller-leaved species have shorter expansion times and are therefore exposed to high levels of herbivory for a shorter period than large leaves. In order to test this hypothesis, leaf expansion time and leaf area loss were measured for 51 species from Sydney, Australia. Strong positive correlations were found between leaf length and area and leaf expansion time, confirming that small leaves do expand in a shorter time than large leaves. The amount of leaf area lost was highly variable (from 0.5 to 90% of total leaf area), but was significantly related to both leaf expansion time and log leaf area. The amount of leaf area lost was not significantly correlated with specific leaf area nor with the presence of distasteful substances in the leaves, but was lower on species with hairy expanding leaves.