Scale insect allozyme differentiation within and between host trees

Summary Allelic frequencies and genotypic distributions in three polymorphic enzyme systems demonstrated genetic differentiation over extraordinarily short distances in a population of black pineleaf scale insects infesting ponderosa pine trees. A hierarchical analysis of the population genetic structure showed significant differences between demes on different twigs within individual host trees, between demes on neighboring trees, and between demes in pine plots on adjacent city blocks. Allelic frequencies at a malic enzyme locus were associated with deme-to-deme variation in ecological correlates of insect fitness, suggesting adaptive hypotheses about the causes of population subdivision.     

EVOLUTIONARY ECOLOGY OF DATURA STRAMONIUM L. IN CENTRAL MEXICO: NATURAL SELECTION FOR RESISTANCE TO HERBIVOROUS INSECTS

It has been assumed that herbivores constitute a selective agent for the evolution of plant resistance. However, few studies have tested this hypothesis. In this study, we look at the annual weed Datura stramonium for evidence of current natural selection for resistance to herbivorous insects. Paternal half-sib families obtained through controlled crosses were exposed to herbivores under natural conditions. The plants were damaged by two folivorous insects: the tobacco flea beetle Epitrix parvula and the grasshopper Sphenarium purpurascens. Selection was estimated using a multiple-regression analysis of plant size and of damage by the two herbivores on plant fitness measured as fruit production for both individual phenotypes and family breeding values (genetic analysis). Directional phenotypic selection was detected for both larger plant size and lower resistance to the flea beetles, whereas stabilizing phenotypic selection was revealed for resistance to S. purpurascens. However, performing the same analyses on the breeding values of the characters revealed directional and stabilizing selection only for plant size. Thus, no agreement existed between the results of the two types of analyses, nor was there any detectable potential for genetic change in the studied population because of selection on herbivore resistance. The narrow-sense heritability of every trait studied was small (all <0.1) and not different from zero. The potential for evolutionary response to natural selection for higher resistance to herbivores in the studied population of D. stramonium is probably limited by lack of genetic variation. Natural selection acts on phenotypes, and the detection of phenotypic selection on resistance to herbivores confirms their ecological importance in determining plant fitness. However, evolutionary inferences based solely on phenotypic selection analyses must be interpreted with caution.     

Soil legacy effects of plants and drought on aboveground insects in native and range-expanding plant communities

Soils contain biotic and abiotic legacies of previous conditions that may influence plant community biomass and associated aboveground biodiversity. However, little is known about the relative strengths and interactions of the various belowground legacies on aboveground plant–insect interactions. We used an outdoor mesocosm experiment to investigate the belowground legacy effects of range-expanding versus native plants, extreme drought and their interactions on plants, aphids and pollinators. We show that plant biomass was influenced more strongly by the previous plant community than by the previous summer drought. Plant communities consisted of four congeneric pairs of natives and range expanders, and their responses were not unanimous. Legacy effects affected the abundance of aphids more strongly than pollinators. We conclude that legacies can be contained as soil ‘memories’ that influence aboveground plant community interactions in the next growing season. These soil-borne ‘memories’ can be altered by climate warming-induced plant range shifts and extreme drought.     

Evidence of biotic resistance to exotic plant invasion in degraded Bornean forests

Intact tropical forests are generally considered to be resistant to invasions by exotic species, although the shrub Clidemia hirta (Melastomataceae) is highly invasive in tropical forests outside its native range. Release from natural enemies (e.g., herbivores and pathogens) contributes to C. hirta invasion success where native melastomes are absent, and here we examine the role of enemies when C. hirta co-occurs with native Melastomataceae species and associated herbivores and pathogens. We study 21 forest sites within agricultural landscapes in Sabah, Malaysian Borneo, recording herbivory rates in C. hirta and related native Melastoma spp. plants along two 100-m transects per site that varied in canopy cover. Overall, we found evidence of enemy release; C. hirta had significantly lower herbivory (median occurrence of herbivory per plant = 79% of leaves per plant; median intensity of herbivory per leaf = 6% of leaf area) than native melastomes (93% and 20%, respectively). Herbivory on C. hirta increased when closer to native Melastoma plants with high herbivory damage, and in more shaded locations, and was associated with fewer reproductive organs on C. hirta. This suggests host-sharing by specialist Melastomataceae herbivores is occurring and may explain why invasion success of C. hirta is lower on Borneo than at locations without related native species present. Thus, natural enemy populations may provide a “biological control service” to suppress invasions of exotic species (i.e., biotic resistance). However, lower herbivory pressures in more open canopy locations may make highly degraded forests within these landscapes more susceptible to invasion.     

Different seasonal feeding activity of diverse herbivores in two temperate forests

The aim of the present study was to understand the effects of abiotic conditions on seasonal feeding activity of diverse herbivores on the same oak tree species in two different forests. We tracked changes in herbivore feeding activities on an oak tree species (Quercus serrata) in two localities: a low elevation small hillock forest patch (Muan, MN) and a middle elevation mountain forest patch (Mt. Jirisan, JR). A total of five sites were selected in each of two forest localities. Data for leaf expansion, leaf chemical qualities, leaf damage ratio, and numbers of lepidopteran caterpillars were collected during spring (May) and summer (July to August), 2012. Leaf expansion rate was higher at the low hillock forest than the mid‐mountain forest from spring to summer. Nitrogen and carbon content decreased seasonally at both localities. Lepidopteran larval diversity was high in the mid‐mountain forest, and two‐way ANOVA showed that species richness of lepidopteran larvae was significantly affected by the interaction between season and locality. Leaf damage by all herbivores was higher in the low hillock forest than the mid‐mountain forest in spring, but was higher in the mid‐mountain forest in summer. Relative proportion of general herbivores increased from spring to summer in the mid‐mountain forest, but not in the low hillock forest. Canonical Correspondence Analysis (CCA) ordination showed that altitude‐ and season‐related variables were significant species and environment interaction factors. Our data indicate that locality and temperature disproportionally affected the feeding activities of diverse herbivores in two different temperate forests.     

Plants and insects in early old-field succession: comparison of an English site and an American site

The plant and insect communities of early, secondary succession beginning with bare ground in an Old World site (southern Britain) and a New World site (Iowa, U.S.A.) shared a number of characteristics. Both sites showed similar temporal patterns of plant species cover and species richness, although overall richness was greater at the Old World site. Annuals dominated at both sites during the first year of succession and were largely replaced by perennials in the second year. Monocotyledons were more abundant at the Old World site, especially in the second year. The two sites differed markedly in the contribution of native and introduced plant species, with the Old World site dominated by natives and the New World site by alien plant species. Insect herbivore load was greater at the Old World site, when expressed in terms of structural complexity of the vegetation, suggesting that there may be major differences in the influence of herbivores on the direction and rate of succession at the two sites.     

Interactions between spatially separated herbivores indirectly alter plant diversity

Above‐ and belowground herbivores promote plant diversity when selectively feeding on dominant plant species, but little is known about their combined effects. Using a model system, we show that neutral effects of an aboveground herbivore and positive effects of a belowground herbivore on plant diversity became profoundly negative when adding these herbivores in combination. The non‐additive effects were explained by differences in plant preference between the aboveground‐ and the belowground herbivores and their consequences for indirect interactions among plant species. Simultaneous exposure to aboveground‐ and belowground herbivores led to plant communities being dominated by a few highly abundant species. As above‐ and belowground invertebrate herbivores generally differ in their mobility and local distribution patterns, our results strongly suggest that aboveground–belowground interactions contribute to local spatial heterogeneity of diversity patterns within plant communities.     

Herbivorous insect species in the tree canopy of a relict South African forest

Abstract.

• 1 The herbivorous insects on twelve species of evergreen broadleafed trees were repeatedly sampled over a period of 11 months in a small relict forest on the east coast of South Africa. This extraordinarily speciose forest patch has an unusually high proportion of endemic tree species, some of which are extremely rare.
• 2 The insect herbivore fauna (number of species) seems to be markedly depauperate compared to that reported on native, broadleafed trees from other parts of the world. Some possible reasons for this are discussed.
• 3 The total number of herbivorous insect species on each tree species was strongly correlated with the local relative abundance of the host plant species.
• 4 There was no relationship between the total number of insect herbivore species on each tree species and the relative taxonomic isolation of the trees. The proportion of seemingly unique (= specialist) herbivorous insect species (i.e. those that occurred on one tree species only) was greatest on taxonomically isolated trees.
• 5 A fundamental deficiency in the interpretation of the data in this study, and of many other similar studies that report on the number of insect species on plants, is discussed, namely the lack of clarity on the closeness of the association between individual insect herbivore species and their respective host plants.

     

Herbivorous insects: model systems for the comparative study of speciation ecology

Does ecological divergence drive species-level evolutionary diversification? How so and to what degree? These questions were central to the thinking of the evolutionary synthesis. Only recently, however, has the ecology of speciation become an important focus of empirical study. Here, we argue that ecologically specialized, phylogenetically diverse, and experimentally tractable herbivorous insect taxa offer great opportunities to study the myriad mechanisms by which ecology may cause reproductive isolation and promote speciation. We call for the development and integrated experimental study of a taxonomic diversity of herbivore model systems and discuss the availability and recent evaluation of suitable taxa. Most importantly, we describe a general comparative framework that can be used to rigorously test a variety of hypotheses about the relative contributions and the macroevolutionary generality of particular mechanisms. Finally, we illustrate important issues for the experimental analysis of speciation ecology by demonstrating the consequences of specialized host associations for ecological divergence and premating isolation in Neochlamisus bebbianae leaf beetles.