Aphids, including the bird cherry-oat aphid (Rhopalosiphum padi), are significant agricultural pests. The wild relative of barley, Hordeum spontaneum 5 (Hsp5), has been described to be partially resistant to R. padi, with this resistance proposed to involve higher thionin and lipoxygenase gene expression. However, the specificity of this resistance to aphids and its underlying mechanistic processes are unknown. In this study, we assessed the specificity of Hsp5 resistance to aphids and analysed differences in aphid probing and feeding behaviour on Hsp5 and a susceptible barley cultivar (Concerto). We found that partial resistance in Hsp5 to R. padi extends to two other aphid pests of grasses. Using the electrical penetration graph technique, we show that partial resistance is mediated by phloem- and mesophyll-based resistance factors that limit aphid phloem ingestion. To gain insight into plant traits responsible for partial resistance, we compared non-glandular trichome density, defence gene expression, and phloem composition of Hsp5 with those of the susceptible barley cultivar Concerto. We show that Hsp5 partial resistance involves elevated basal expression of thionin and phytohormone signalling genes, and a reduction in phloem quality. This study highlights plant traits that may contribute to broad-spectrum partial resistance to aphids in barley.

Summary Insect herbivore damage and abundance are often reduced in diverse plant stands. However, few studies have explored whether this phenomenon is a result of plant diversity effects on host plant traits. We explored indirect effects of tree species diversity on herbivory via changes in leaf traits in a long‐term forest diversity experiment in Finland. We measured 16 leaf traits and leaf damage by four insect guilds (chewers, gall formers, leaf miners and rollers) on silver birch ( Betula pendula ) trees growing in one‐, two‐, three‐ and five‐species mixtures. A decline in the frequency of birch in mixed stands resulted in reduced leaf area. This, in turn, mediated the reduction in chewing damage in mixed stands. In contrast, associational resistance of birch to leaf miners was not trait‐mediated but driven directly by concurrent declines in birch frequency as tree species richness increased. Our results show that leaf trait variation across the diversity gradient might promote associational resistance, but these patterns are driven by an increase in the relative abundance of heterospecifics rather than by tree species richness per se . Therefore, accounting for concurrent changes in stand structure and key foliar traits is important for the interpretation of plant diversity effects and predictions of associational patterns.

The Jarman-Bell principle states that large-bodied mammalian herbivores can subsist on lower quality diets because of their lower metabolism requirement/gut capacity ratio. Two major hypotheses for sexual segregation (the behaviour in which animals of the same species aggregate by sex) base their foundations on extending this principle to the intraspecific level, despite the lack of experimental evidence to support this. The first proposes that the larger males can process fibre (low-quality diet) more efficiently than the smaller females, leading to sexual segregation by habitat partitioning due to selection of different food quality and/or quantity (sexual dimorphism-body size hypothesis). The second suggests that the longer time and extra rumination required to digest low-quality food will cause asynchrony of behaviour between males and females, which then leads to sexual segregation (activity budget hypothesis). To provide experimental evidence for the Jarman-Bell principle at the intraspecific level we carried out a set of digestibility trials in Soay sheep (Ovis aries) using grass hay as the diet to test whether sexual dimorphism in body mass can produce significant sexual differences in the efficiency of food digestion. Males were slightly more efficient in digesting forage than females that were at least 30% smaller than the males. Overall, there was a decrease in faecal output of 1 g/kg body mass in favour of males. These differences were not due to differences in food selection, passage rates or faecal particle size and it was not clear why males were more efficient in digesting forage. Although these results do not directly support arguments for either the sexual dimorphism-body size or activity budget hypotheses, they do indicate that the physiological argument upon which the Jarman-Bell principle is founded also operates at the intraspecific level and may be an important factor influencing sexual segregation.

No abstract

Sexual segregation is the behavior in which animals of different sex in a species live in separate groups outside the mating season. Recently a new concept, namely, the “activity‐budget hypothesis,” has claimed to be the ultimate explanation of this behavior. The new hypothesis explains not only sexual segregation, but also segregation between animals of different size within sex (i.e., social segregation). The hypothesis states that the activity patterns of animals will differ when big differences in body mass exist between them, because of the associated difficulties of the synchronization in behavior making it costly to form groups, leading to segregation by size. Here we tested the assumptions and predictions of the activity‐budget hypothesis using 40 Soay sheep (Ovis aries) as the model species in a 2.3‐ha experimental arena. Sheep were divided into treatment groups to test the effect of sex, body mass, and food supplementation in their activity budgets, behavioral synchronization, diet composition, intake, food digestibility, and spatial segregation. Our animals segregated by sex but not by size, and food supplementation did not affect the spatial distribution of any sex, which is all against the predictions of the hypothesis. We also found sexual differences in dry‐matter digestibility independent of body mass, which questions the Jarman‐Bell principle at the intra‐specific level, which is the foundation for some other hypotheses of sexual segregation. Increasing behavioral synchronization led to segregation, but at the same time forming groups facilitates synchronization, so it is unclear which (i.e., synchronization or segregation) is the cause and which the consequence. Our results do not support the activity‐budget hypothesis and clearly indicate that there is no strong association between behavioral synchronization and segregation.