Are insect pollinators more generalist than insect herbivores?

Recent community-level studies have acknowledged that generalist species are more widespread than previously thought and highlighted their preponderant impact on community functioning and evolution. It is suggested that the type of interaction, trophic versus mutualistic, should affect species generalization level; however, no direct comparison has been made yet. Here, we performed such a comparison using 44 plant–insect networks describing either pollination or herbivory communities. Our analysis shows that the type of interaction does indeed have an impact on various aspects of species generalism, from the distribution of generalism in the community to the phylogenetic diversity of the plants with which a given insect species interacts. However, the amplitude of the observed differences depends on the aspect of species generalism studied. While the non-quantitative and quantitative measures of generalism suggest that pollinators interact with more plant species and more evenly than herbivores, phylogenetic measures clearly show that herbivores interact with plant species far more closely related to each other than pollinators. This comparative approach offers a promising perspective to better understand the functioning and evolution of multispecies assemblages by pointing out some fundamental singularities of communities depending on the type of interaction considered.     

Crab spiders deter insect visitations to slickspot peppergrass flowers

Insects visiting the flowers of slickspot peppergrass, Lepidium papilliferum (Brassicaceae), risk predation by crab spiders, Misumena vatia (Thomisidae). In a field study conducted at two sites in southwestern Idaho, 7.5±2.7% of L. papilliferum plants (range 0–30%, N=16 surveys of up to 40 randomly selected plants) harbored a crab spider. However, through 205 minutes of observations at plants with a spider, only 15 predation attempts were observed, with only 3 of those being successful. Despite the relatively low incidence of predation by crab spiders, an experiment revealed that the number of insects visiting L. papilliferum flowers was significantly lower at plants that harbored a crab spider than at plants free of spiders. In another experiment, floral visits increased significantly following the removal of crab spiders from individual plants. The deterrent effect of spiders was not due to a disproportionate avoidance response by certain types of insects; all insect families included in our analysis showed decreases in visitations to flowers when spiders were present, although none of these differences were statistically significant at the individual level. We found no significant change in the duration of visits to plants harboring a spider, implying either that the visitors were oblivious to the predator's presence, or that they were aware of the predator but kept their distance. Our study is one of a growing number to find a decrease in floral visits in response to predators, suggesting that the phenomenon is more widespread than was previously recognized.     

Beyond pollinator attraction: extra-floral displays deter herbivores in a Mediterranean annual plant

The evolution of several floral traits is thought to be driven by multiple selective agents, including pollinators and herbivores. Similar combinations of selection pressures may have shaped extra-floral traits. The conspicuous purple tufts of leaves (“flags”), which often terminate vertical inflorescences in the Mediterranean annual Salvia viridis, were shown to attract insect pollinators to the flowering patch. Here we test whether they also function as anti-herbivore signals. We determined the aposematic potential of S. viridis flags on three levels: concentrations of anthocyanins, suggested to function as aposematic visual signals, in leaves and flags; spectrometry to estimate whether the color-vision system of two common Mediterranean generalist herbivores (locusts and goats) can discriminate flags from leaves; and choice experiments to determine food preferences of the same herbivores. Anthocyanin concentrations in flags were >10-fold higher than in leaves. Flags exhibited peak reflectance at 450 and 700 nm wavelengths, while leaves reflected maximally at 550 nm. According to the Vorobyev-Osorio color vision model, these differences in color reflection are likely to allow visual discrimination by herbivores. Goats preferred feeding on clipped inflorescences over control inflorescences. Locusts preferred leaves over flags. To test whether this was due to deterrence from the flags’ coloration, we also offered them choice between leaves and a rare, white morph, of the flags. The locusts chose both equally immediately after presentation, but leaves attracted more individuals after 5 min of feeding. The locusts also preferred green cabbage over anthocyanin-rich red cabbage. These results support the function of colorful extra-floral displays as warning signals.     

Mechanisms of plant defense against insect herbivores

Plants respond to herbivory through various morphological, biochemicals, and molecular mechanisms to counter/offset the effects of herbivore attack. The biochemical mechanisms of defense against the herbivores are wide-ranging, highly dynamic, and are mediated both by direct and indirect defenses. The defensive compounds are either produced constitutively or in response to plant damage, and affect feeding, growth, and survival of herbivores. In addition, plants also release volatile organic compounds that attract the natural enemies of the herbivores. These strategies either act independently or in conjunction with each other. However, our understanding of these defensive mechanisms is still limited. Induced resistance could be exploited as an important tool for the pest management to minimize the amounts of insecticides used for pest control. Host plant resistance to insects, particularly, induced resistance, can also be manipulated with the use of chemical elicitors of secondary metabolites, which confer resistance to insects. By understanding the mechanisms of induced resistance, we can predict the herbivores that are likely to be affected by induced responses. The elicitors of induced responses can be sprayed on crop plants to build up the natural defense system against damage caused by herbivores. The induced responses can also be engineered genetically, so that the defensive compounds are constitutively produced in plants against are challenged by the herbivory. Induced resistance can be exploited for developing crop cultivars, which readily produce the inducible response upon mild infestation, and can act as one of components of integrated pest management for sustainable crop production.     

Mechanisms of seedling mortality by subterranean insect herbivores

Summary The nature of the mortality of germinating seeds of Vicia sativa, the common vetch, was investigated in a pot trial under controlled conditions. Chafer and tipulid larvae were restricted by nylon mesh partitions to enable radicle and/or hypocotyl herbivory to occur. The effects of the two insects were very similar. In control situations, an average of 88% of the viable seed sown recruited successfully. Hypocotyl and radicle herbivory had similar effects on seedling mortality, with recruitment of viable seed sown being reduced to 52%. When both modes of attack occurred together, successful recruitment was only 34%. Feeding on both plant parts resulted in an average post-emergence mortality of 14% of the seed sown, but the effect on pre-emergence mortality was of greater importance, amounting to 41%. The mechanisms by which soil-dwelling herbivores may cause seedling mortality in the field are discussed.     

Host specificity of insect herbivores in tropical forests

Studies of host specificity in tropical insect herbivores are evolving from a focus on insect distribution data obtained by canopy fogging and other mass collecting methods, to a focus on obtaining data on insect rearing and experimentally verified feeding patterns. We review this transition and identify persisting methodological problems. Replicated quantitative surveys of plant-herbivore food webs, based on sampling efforts of an order of magnitude greater than is customary at present, may be cost-effectively achieved by small research teams supported by local assistants. Survey designs that separate historical and ecological determinants of host specificity by studying herbivores feeding on the same plant species exposed to different environmental or experimental conditions are rare. Further, we advocate the use of host-specificity measures based on plant phylogeny. Existing data suggest that a minority of species in herbivore communities feed on a single plant species when alternative congeneric hosts are available. Thus, host plant range limits tend to coincide with those of plant genera, rather than species or suprageneric taxa. Host specificity among tropical herbivore guilds decreases in the sequence: granivores > leaf-miners > fructivore > leaf-chewers = sap-suckers > xylophages > root-feeders, thus paralleling patterns observed in temperate forests. Differences in host specificity between temperate and tropical forests are difficult to assess since data on tropical herbivores originate from recent field studies, whereas their temperate counterparts derive from regional host species lists, assembled over many years. No major increase in host specificity from temperate to tropical communities is evident. This conclusion, together with the recent downward revisions of extremely high estimates of tropical species richness, suggest that tropical ecosystems may not be as biodiverse as previously thought.     

Plant tannins and insect herbivores: an appraisal

Abstract. 1. The bioassays with tannins and insects, and the ecological studies on insects implicating tannins, are summarized and discussed.
2. Because of the great variation now shown in all aspects of the insect-tannin relationship, the difficulty of making generalizations is stressed.
3. The significance of plant tannins for insect herbivores is reconsidered in the light of recent work and little-known older work, which illustrate the very varied nature of its effects.     

植物诱导性直接防御

众所周知,植物对植食性昆虫危害的反应表现在3个方面：直接防御,间接防御,和耐害性。直接防御是指植物自身所具有的能影响寄主植物感虫性的所有特性。植物对昆虫危害的直接防御包括：限制食物供给,降低营养价值,减少偏嗜程度,破坏组织结构和抑制害虫代谢途径。目前已知的防御化合物主要包括植物次生代谢物质、昆虫消化酶（蛋白）抑制剂、蛋白酶、凝集素、氨基酸脱氨酶和氧化酶。植物在防御某种昆虫为害时多个因素往往具有累加效应或协同作用,并且对一种昆虫起主导作用的因素在防御另一种昆虫时可能仅仅起次要作用甚至根本不起作用。因此,对寄主植物基因表达、蛋白水平和活性以及代谢物含量在不同时空条件下进行广泛的定量和定性的高通量分析,不仅可以促进对植物直接防御机制的全面理解,而且有助于在农业生产中加快对作物抗性的特定靶标的鉴定。     

Plant polyploidy and non-uniform effects on insect herbivores

Genomic duplication through polyploidy has played a central role in generating the biodiversity of flowering plants. Nonetheless, how polyploidy shapes species interactions or the ecological dynamics of communities remains largely unknown. Here we provide evidence from a 4 year study demonstrating that the evolution of polyploidy has reshaped the interactions between a widespread plant and three species of phytophagous moths. Our results show that polyploidy has produced non-uniform effects, with polyploids less attacked by one insect species, but significantly more attacked by two other species. These results suggest that the evolution of plant polyploidy may not generally confer uniform resistance to multiple species of insect herbivores. In the absence of such a uniform release, the extreme evolutionary success of polyploid plants is probably due to factors other than escape from herbivory. Together, these results suggest that a primary consequence of plant polyploidy may be to shape the ecological structure of plant-insect interactions, thereby providing opportunities for diversification in both plant and insect taxa.     

Immune defence strategies of generalist and specialist insect herbivores

Ecological immunology examines the adaptive responses of animals to pathogens in relation to other environmental factors and explores the consequences of trade-offs between investment in immune function and other life-history traits. Among species of herbivorous insects, diet breadth may vary greatly, with generalists consuming a wide variety of plant families and specialists restricted to a few species. Generalists may thus be exposed to a wider range of pathogens exerting stronger selection on the innate immune system. To examine whether this produces an increase in the robustness of the immune response, we compared larvae of the generalist herbivore Heliothis virescens and the specialist Heliothis subflexa challenged by entomopathogenic and non-pathogenic bacteria. Heliothis virescens larvae showed lower mortality, a lower number of recoverable bacteria, lower proliferation of haemocytes and higher phagocytic activity. These results indicate a higher tolerance to entomopathogenic bacteria by the generalist, which is associated with a more efficient cell-mediated immune response by mechanisms that differ between these closely related species. Our findings provide novel insights into the consequences of diet breadth and related environmental factors, which may be significant in further studies to understand the ecological forces and investment trade-offs that shape the evolution of innate immunity.     

Similar responses of insect herbivores to leaf fluctuating asymmetry

Fluctuating asymmetry (FA) represents small, random variation from symmetry and it has been used as an indicator of plant quality and susceptibility to herbivory. In this study, the effects of FA on the responses of distinct herbivore species belonging to several guilds were examined along an environmental gradient in south Florida. This approach was chosen because it relies on a multi-species approach to the study of fluctuating asymmetry and patterns of herbivory between and within plants along an environmental gradient of salinity and plant stress. To examine differences in FA between and within plant communities, seven plant species were investigated. Four of these plants were coastal species and three species occurred in upland communities. Levels of FA were assessed before herbivory and plants were followed for the whole herbivory season in 2006. Coastal plants exhibited significantly higher salt concentration, higher percentage of asymmetric leaves and higher asymmetry levels than upland plants. Herbivore abundance varied widely amongst the seven species studied, but quantitative syntheses of our results indicated significant and positive responses of insect herbivores to leaf asymmetry: insects were 25.11% more abundant on more asymmetric plants and stronger effects of asymmetry were observed for leaf miners compared to gall-formers. As demonstrated by other recent studies, FA might be used as a reliable stress indicator, leading to similar responses of insect herbivores to variation in leaf symmetry.     

Indirect plant defense against insect herbivores: a review

Plants respond to herbivore attack by launching 2 types of defenses: direct defense and indirect defense. Direct defense includes all plant traits that increase the resistance of host plants to insect herbivores by affecting the physiology and/or behavior of the attackers. Indirect defense includes all traits that by themselves do not have significant direct impact on the attacking herbivores, but can attract natural enemies of the herbivores and thus reduce plant loss. When plants recognize herbivore‐associated elicitors, they produce and release a blend of volatiles that can attract predators, parasites, and other natural enemies. Known herbivore‐associated elicitors include fatty acid–amino acid conjugates, sulfur‐containing fatty acids, fragments of cell walls, peptides, esters, and enzymes. Identified plant volatiles include terpenes, nitrogenous compounds, and indoles. In addition, constitive traits including extrafloral nectars, food bodies, and domatia can be further induced to higher levels and attract natural enemies as well as provide food and shelter to carnivores. A better understanding of indirect plant defense at global and componential levels via advanced high throughput technologies may lead to utilization of indirect defense in suppression of herbivore damage to plants.     

Rapid adaptation of insect herbivores to an invasive plant

Introduced plant success often is attributed to release from natural enemies in their new ranges. However, herbivores may accumulate over time and reduce invasiveness but evidence for this process to date is weak. We report here that enemy release is indeed limited to the early stages of introduction of the Chinese tallow tree (Sapium sebiferum). In bioassays and gardens along a geographical gradient of time since tallow tree introduction, herbivory was highest and tree performance was poorest where tallow tree has been present longer (i.e. introduced earlier). Additionally, Asian ecotypes (grown from seeds collected in Asia) had lower survival than North American ecotypes (seeds collected in North America), which is consistent with genetic responses to low herbivory in the introduced range (EICA Hypothesis). Release from insect herbivores appears to contribute to early success of the tallow tree, but accumulation of insect herbivores has apparently reduced this benefit over time.     

硅对植物抗虫性的影响及其机制

硅不是植物必需营养元素,但硅在提高植物对一系列非生物和生物胁迫的抗性方面都具有重要作用。综述了硅对植物抗虫性的影响及其机制。在多数植物中,增施硅肥可增强其抗虫性;所增强的抗性与硅肥种类和施用方式之间存在关系。植物组织中沉积的硅可增加其硬度和耐磨度,降低植物可消化性,从而增强植物组成性防御,包括延缓昆虫生长发育、降低繁殖力、减轻植物受害程度;植物体内的硅含量以及硅沉积的位点和排列方式影响组成性防御作用的强度。此外,硅可以调节植物诱导性防御,包括直接防御和间接防御,直接防御涉及增加有毒物质含量、产生局部过敏反应或系统获得抗性、产生有毒化合物和防御蛋白,从而延缓昆虫发育;间接防御主要通过释放挥发性化合物吸引植食性昆虫的捕食性和寄生性天敌而导致植食性昆虫种群下降。