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Interactions between galling insects and leaf-feeding insects: the role of plant phenolic compounds and their possible interference with herbivores

The objective of this study was to determine the effects of gall induction on leaf phenolic compounds and their indirect effects on the subsequent attack of folivorous insects in Achatocarpus gracilis, Cordia alliodora, Guapira macrocarpa, Guettarda elliptica and Ruprechtia fusca that occur in both hillside and riparian sites at Chamela-Cuixmala tropical dry forest in western Mexico. There are differences in soil water content between riparian and hillside sites where trees in the first are mainly evergreen and deciduous in the second. A few tree species occur in both sites and their intraspecific phenological response is also different between sites. In this case, trees of a given species that occur in riparian sites will be evergreen whereas trees on hillsides of the same species will be deciduous. Four plant species had significantly greater total phenol concentrations in galled than ungalled leaves in both deciduous hillside and riparian sites. In three plant species associated with galls, host total phenol concentrations were significantly greater in short than in tall plants. The frequency and amount of folivore damage were greater in leaves without galls than leaves with galls in these four plant species. These results indicate that galling insect species may directly affect leaf phenolic concentrations and indirectly may affect the incidence and consumption of folivorous insects in tropical plant species. This may have important consequences on the preference of leaves by folivorous insects that might be excluded by galling insect species in this dry tropical system.

Contrasting herbivory patterns and leaf fluctuating asymmetry in Heliocarpus pallidus between different habitat types within a Mexican tropical dry forest

Leaf fluctuating asymmetry (FA) is considered an important indicator of environmentally induced stress in plants, but the relationship between herbivory and FA levels is not clear. In this study we compared leaf size and shape, leaf area consumed by herbivorous insects, and FA levels between individuals of Heliocarpus pallidus (Tiliaceae) from two adjacent and contrasting habitat types (deciduous and riparian) in the Chamela–Cuixmala tropical dry-forest biosphere reserve. Ten individuals of H. pallidus were collected in each habitat type. Leaf shape was assessed using geometric morphometric techniques. Results indicated statistically significant differences in leaf shape between individuals from the two habitat types. In individuals from the riparian habitat leaf area (mean = 42.3 ± 1.2 cm 2 ), herbivory levels (mean = 25.5% ± 1.8%) and FA levels (mean = 0.38 ± 0.04 cm) were significantly higher than in individuals from the deciduous habitat (17.2 ± 3.5 cm 2 ; 9.6% ± 1.0% and 0.18 ± 0.04 cm, respectively). Within habitats, significant correlations were found between total leaf area and percentage leaf area removed by insects ( R 2 = 0.92 in riparian habitats, R 2 = 0.9 in deciduous habitats), and between percentage leaf area removed and FA ( R 2 = 0.70 in riparian habitats, R 2 = 0.79 in deciduous habitats). As has been suggested for other plant species, it is possible that the more favourable conditions in the riparian habitat enhance leaf growth, resulting in higher FA. Also, individuals in this habitat might be characterized by lower chemical defence and/or higher nutritional quality, which would explain the higher herbivory levels.

Abundance of gall-inducing insect species in sclerophyllous savanna: understanding the importance of soil fertility using an experimental approach

Although many studies have now demonstrated that both richness and abundance of gall-inducing insect species are directly and indirectly (via the host plant) influenced by soil quality, the empirical evaluation of it in the field remains anecdotal at best. The effects of soil fertility on richness and abundance of gall-inducing insects associated with a widespread savanna species, Eremanthus glomerulatus , were evaluated under experimental field conditions in Brasilia, central Brazil. The effect of soil fertility on gall-inducing insects species richness was evaluated using three treatments: (1) plots fertilized with nitrogen; (2) plots fertilized with phosphorus; and (3) control plots: soils without fertilization. Species richness of gall-inducing insects (six species of Cecidomyiidae) did not differ among the treatments. Leaves with galls had higher nitrogen concentrations (mean = 15.0 ± 0.5 mg g −1 ), compared with leaves without galls (mean = 9.0 ± 0.7 mg g −1 ) on plants that occurred in soils with addition of nitrogen. Similarly, leaves with galls had higher foliar phosphorus concentration (mean = 1.0 ± 0.04 mg g −1 ) than leaves without galls (mean = 0.6 ± 0.05 mg g −1 ) in plots with addition of phosphorus. In galled leaves, a negative relationship between gall density and nitrogen concentration was found for one gall-inducing insect species, while three species showed a positive relationship between gall density and leaf nitrogen concentration. A negative relationship between gall density and concentration of leaf phosphorus was observed for four of the six gall-inducing insect species studied. No relationship was found between gall density and leaf nitrogen and phosphorus concentration in ungalled leaves. We argue that foliar nitrogen and phosphorus concentration respond to gall density in galled leaves and therefore, gall-inducing insect species are capable of manipulating their host plant, modifying the foliar nutrients of E. glomerulatus in sclerophyllous savanna.

Fluctuating asymmetry and herbivory in two ontogenetical stages of Chamaecrista semaphora in restored and natural environments

Reintroduced environments represent stressful conditions to plants that can be observed in different ways. We evaluated the relationships between fluctuating asymmetry (FA), herbivory, and plant ontogeny of Chamaecrista semaphora (Fabaceae) under natural and restored habitat conditions. The patterns of leaf FA and herbivory by folivorous insects (chewing) on saplings and mature plants in each habitat were determined. No relationship was found between FA and herbivory on the two ontogenetic stages in both environments, suggesting that FA did not represent an indicator of stress. The frequency and amount of leaf area removed by folivores were higher in saplings compared to adult plants under the natural habitat, while the opposite trend was observed on restored habitat for adult plants. The restored habitat did not represent an environmental stress condition to C. semaphora, indicating that this endemic plant may represent a good candidate to restoration programs in harsh environments. However, we observed great differences in leaf FA and herbivory among individuals within habitats. Knowledge on plant quality, competition, physiology, and interactions with natural enemies are highly needed to support long lasting programmes on restoration of harsh environments.

Relationships among plant genetics, phytochemistry and herbivory patterns in Quercus castanea across a fragmented landscape

Herbivorous insects respond to the chemical variation of their host plants which, in turn, usually has a genetic component. Therefore, it is expected that individual host plants with similar genotypes will have similar secondary chemistries and herbivore communities. However, natural or anthropogenic environmental variation can also influence secondary chemistry and herbivore abundance and composition. Here, we determined the relationships among plant genetics, phytochemistry, and herbivory levels by leaf chewers and miners in the red oak Quercus castanea, across a fragmented landscape. Ten oak individuals were sampled at each of four sites in the Cuitzeo basin, central Mexico. Two sites were small and fragmented forest patches and two were large and continuous patches. Individuals were genotyped with six nuclear microsatellites, and analyzed chemically to determine foliar concentrations of water, total nonstructural carbohydrates, and secondary compounds. Damage by leaf chewers was higher in the small fragments than in the large fragments. Mantel tests indicated significant correlations of the genetic distance among individuals with their chemical similitude, and also of chemical similitude with damage levels by leaf miners, but not with damage by folivores. There was no direct relationship between genetic distance and herbivory levels by any of the two insect guilds. Our results suggest that variation in concentration of secondary metabolites in Q. castanea has a genetic component and that plant chemistry acts as an intermediate link between plant genes and the community of associated herbivores. However, this effect was only apparent for herbivory by leaf miners, probably because these insects interact more intimately with the host, while free-living leaf chewers may be more responsive to environmental variation.

Morphological Variations of Gall-Forming Insects on Different Species of Oaks ( Quercus ) in Mexico

The knowledge of the interaction between gall-forming insects and oaks in Mexico is still very poor. However, the results presented in this chapter indicate that Mexican oaks have great gall morph diversity, and that external and internal gall morphology is structurally variable. More studies on gallwasp taxonomy, gall morphology and physiology are necessary to understand the adaptive significance of gall induction in oaks of temperate and tropical forests. Hybrids have the potential to act as new adaptive zones for the diversification of wasp species. Considering that hybridization is one of the main factors explaining the striking species richness of Mexican oaks, many gall-forming insects are also expected to have coevolved. Co-phylogenetic studies on Mexican oaks are now underway, and particularly aim at comparing endemic species with a restricted distribution to species with wider latitudinal and altitudinal distributions. This analysis is expected to contribute to our understanding of the co-evolutionary dynamics of this interaction at different spatial scales. Gall-forming insects associated to oaks may represent even greater species richness than found among oak species themselves. Therefore, this should be considered an important biodiversity component, which has its own specific conservation requirements within the diverse Mexican montane oak forests.

Mexican Gall-Inducing Insects: Importance of Biotic and Abiotic Factors on Species Richness in Tropical Dry Forest

In this chapter we provide a brief overview of the major abiotic and biotic factors that influence the diversity of gall-inducing insects in a Mexican tropical dry forest. We tested some current hypotheses (plant species richness, plant structural complexity, plant age, resource concentration and soil fertility) that seek to explain the variation in gall-inducing insect species richness in plant communities of deciduous and riparian habitats. Gall-inducing insect species maintain a great specificity on their host plant species; each gall-inducing insect species is associated with a different plant species. A significant positive correlation was found between gall-inducing insect species richness and plant species richness in both habitats. This suggests that radiation of gall-inducing insect species may be associated with plant species richness. Most of the galling species occurred on trees and shrubs but not on herbs and climbers. The structural complexity of these life forms may provide more potential sites to be colonized by gall insects. The frequency of gall-inducing insects was greater on saplings of most host plants, suggesting that early ontogenetic stages hold more sites with undifferentiated tissues or meristems susceptible to gall induction.

Landscape genetics reveals inbreeding and genetic bottlenecks in the extremely rare short-globose cacti Mammillaria pectinifera (Cactaceae) as a result of habitat fragmentation

Mammillaria pectinifera is an endemic, short-globose cactus species, included in the IUCN list as a threatened species with only 18 remaining populations in the Tehuacán-Cuicatlán Valley in central Mexico. We evaluated the population genetic diversity and structure, connectivity, recent bottlenecks and population size, using nuclear microsatellites. M. pectinifera showed high genetic diversity but some evidence of heterozygote deficiency (FIS), recent bottlenecks in some populations and reductions in population size. Also, we found low population genetic differentiation and high values of connectivity for M. pectinifera, as the result of historical events of gene flow through pollen and seed dispersal. M. pectinifera occurs in sites with some degree of disturbance leading to the isolation of its populations and decreasing the levels of gene flow among them. Excessive deforestation also changes the original vegetation damaging the natural habitats. This species will become extinct if it is not properly preserved. Furthermore, this species has some ecological features that make them more vulnerable to disturbance such as a very low growth rates and long life cycles. We suggest in situ conservation to prevent the decrease of population sizes and loss of genetic diversity in the natural protected areas such as the Tehuacán-Cuicatlán Biosphere Reserve. In addition, a long-term ex situ conservation program is need to construct seed banks, and optimize seed germination and plant establishment protocols that restore disturbed habitats. Furthermore, creating a supply of living plants for trade is critical to avoid further extraction of plants from nature.

Diversity of gall wasps (Hymenoptera: Cynipidae) associated with oak trees (Fagaceae: Quercus) in a fragmented landscape in Mexico

Abstract Habitat fragmentation reduces the available habitat area and increases both the distance between fragments and the amount of fragment edges. Therefore, there are more probabilities of plant population size reduction and species extinction. In the same way, biotic and abiotic changes associated with forest fragmentation can dramatically alter plant growth and phenological patterns. We conducted a 3-year study to analyze effects of habitat fragmentation and seasonal variation on host plant quality (quantity of leaves, diameter at breast height, tree height), gall abundance and species richness in a temperate oak forest. Our results show that host plant quality was significantly higher in isolated oaks and small fragments, increasing the abundance and species richness of oak gall wasp species in most fragmented habitats. Oak canopy cover is altered by forest fragmentation, there being higher production of leaves on trees that are more exposed to fragmentation, and can provide important resources for maintaining gall wasp species diversity in a fragmented landscape. We found higher gall wasp richness and abundance in autumn than in the spring, which matches with the higher quantity of leaves in this season.

Tri-trophic interactions among congeneric sympatric host plants of Chamaecrista, seed predators and parasitoids

The defensive role against seed predation of a set of plant traits in 13 congeneric sympatric taxa (Chamaecrista: Leguminosae), and the influence of the third trophic level on seed predators’ performance and host range were investigated. Taxa co-occur in rupestrian grasslands in Serra do Cipó, Brazil, and belong to three taxonomic sections. Fruit production, fruit pubescence, and seed size were analyzed. Measures of these traits in the 13 taxa were regressed separately against seed predation rates by endophagous and ectophagous insects. Time of seed production and fruit pubescence showed no influence on seed predation rates by either predator type. Seed size was positively correlated to bruchid seed predation, but negatively related to ectophagous seed predation. There was a negative correlation between glandular fruit trichome length and parasitism rates of bruchids, suggesting that seed predation pressure may have produced evolutionary responses from plants (fruit trichome reduction), which should facilitate parasitoid action.