Summary1. Anthropogenic noise is known to affect acoustic signal production in birds, frogs and mammals. These animals use different mechanisms to adjust their signals to elevated background noise levels (increase in signal amplitude, shift to higher frequencies, etc.). Previous studies have concentrated on behaviourally plastic changes in signal production as a result of elevated background noise levels. To our knowledge, long-term effects of anthropogenic noise on signal production have not yet been investigated. Moreover, strategies of invertebrate species to ensure acoustic signal transmission under anthropogenic noise have not been examined, so far. 2. We tested whether and how male Chorthippus biguttulus grasshoppers from noisy roadside habitats may adjust acoustic courtship signals to elevated background noise levels, compared with conspecifics from quiet control habitats. In this species, sexually selected male courtship signals serve to attract potential mating partners, which make the undisturbed transmission of signals in habitats with increased background noise levels crucial for male reproductive success. 3. Compared to males from control populations, males from roadside habitats produced songs with a significantly higher local frequency maximum under standardized, quiet recording conditions. This local frequency maximum (in the range of c. 6-9 kHz) overlaps with low-frequent road noise that has the potential to degrade or mask this part of the signals' frequency spectrum. 4. To our knowledge, this is the first evidence that insects from noisy habitats produce different acoustic signals than conspecifics from quiet habitats, possibly using a more permanent mechanism for signal adjustment than behavioural plasticity, which was found in different bird species adjusting to high background noise levels. Such an effect of anthropogenic noise has not been shown for any invertebrate species before, and our results suggest that similar strategies to avoid degradation or masking by noise (i.e. increase in carrier frequency) are used over a wide range of taxa, including both, vertebrates and invertebrates.

Theory predicts negative effects of increasing plant diversity on the abundance of specialist insect herbivores, but little is known about how plant diversity affects the performance and abundance of generalist insect herbivores. We studied oviposition rates and offspring numbers in females of the generalist grasshopper Chorthippus parallelus that were collected in 15 montane grasslands in 2005 and 2007 along a gradient of plant species richness in central Germany. In addition to plant species richness, we determined evenness and plant community composition in the grasslands and measured aboveground plant biomass and other habitat variables such as leaf area index, vegetation height, and solar radiation. There was substantial variation among sites in grasshopper fecundity and the number of nymphs that hatched from the egg pods. Both fitness measures were positively influenced by plant species richness at the sites, while female fitness did not correlate with any of the other habitat parameters. Abundance of C. parallelus in the grasslands was positively correlated with plant species richness, plant community composition, and incident solar radiation of the sites. There were no phenological differences between grasshoppers from the different study sites. Our results suggest that decreasing biodiversity threatens the persistence not only of specialist, but also of generalist insect herbivores via a variety of mechanisms including a decrease in diversity of the generalists' food plants.

1. Generalist insect herbivores occupy a variety of habitats that differ in food plant composition. Dietary mixing has been proposed as a possibility for generalists to overcome nutritional deficiencies of single plant species, but only a few studies have investigated herbivore feeding and fitness for diets that resemble natural scenarios. We studied feeding behaviour, survival, and reproduction of the generalist grasshopper Chorthippus parallelus raised on food plants of four typical habitats. 2. Grasshopper diet consisted of grasses (92.5%), legumes (6.7%) and, in small quantities, other forbs (0.8%). Diet selection differed between the four food plant mixtures, and depended on grasshopper sex and developmental stage. There was no correlation between the relative abundance of plant species in the field and the fraction of these species in the grasshopper diet. 3. Grasshoppers survived on average for 40.4 ± 1.0 days before maturity, grew 106.8 mg until maturity moult, and females laid 4.1 ± 0.4 egg pods, each of which contained 8.5 ± 0.4 eggs. However, despite the differences in feeding behaviour, grasshopper fitness was the same in all of the four food plant mixtures. While the digestibility of ingested food was similar in the four different treatments, indices indicated differences in the conversion efficiency to body mass. 4. Our results show that C. parallelus is a plastic feeder with no fixed preferences in diet composition. The results emphasise that generalist herbivores can counteract putative quality deficiencies of single food plants by selective dietary mixing.

Citation: Franzke, A., and K. Reinhold. 2011. Stressing food plants by altering water availability affects grasshopper performance. Ecosphere 2(7):art85. doi:10.1890/ES11-00095.1Abstract. Extreme weather events like drought and heavy rain are likely to increase with climate change in Central Europe and may affect nutrient content in plants. They may therefore influence the performance (growth rate, developmental time, mortality, body size/mass, fecundity) and population dynamics of herbivorous insects. We conducted a common-garden experiment on food plants to investigate effects of severe drought and moisture events on reproduction and fitness components in the insect herbivore Chorthippus biguttulus (Orthoptera, Acrididae). Periodic irrigations of food plants were used to simulate a 60% decrease in average summer precipitation (drought treatment), a 40% increase in average summer precipitation (moisture treatment) and a normal summer precipitation (control treatment). Individuals of C. biguttulus that fed on drought-stressed plants showed beneficial effects on life-history traits including an increased reproductive success than grasshoppers that fed on control plants. The opposite was true for individuals feeding on plants grown under severe moisture conditions. We propose that herbivore performance is influenced by increased concentrations of soluble proteins and amino acids in plants under drought stress conditions. Our results suggest that drought events may increase population performance and consequently population density in the grasshopper species C. biguttulus, while extreme moisture events may cause negative population trends.

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Genome size is largely uncorrelated to organismal complexity and adaptive scenarios. Genetic drift as well as intragenomic conflict have been put forward to explain this observation. We here study the impact of genome size on sexual attractiveness in the bow-winged grasshopper Chorthippus biguttulus. Grasshoppers show particularly large variation in genome size due to the high prevalence of supernumerary chromosomes that are considered (mildly) selfish, as evidenced by non-Mendelian inheritance and fitness costs if present in high numbers. We ranked male grasshoppers by song characteristics that are known to affect female preferences in this species and scored genome sizes of attractive and unattractive individuals from the extremes of this distribution.We find that attractive singers have significantly smaller genomes, demonstrating that genome size is reflected in male courtship songs and that females prefer songs of males with small genomes. Such a genome size dependent mate preference effectively selects against selfish genetic elements that tend to increase genome size. The data therefore provide a novel example of how sexual selection can reinforce natural selection and can act as an agent in an intragenomic arms race. Furthermore, our findings indicate an underappreciated route of how choosy females could gain indirect benefits.

In many animal species, male acoustic signals serve to attract a mate and therefore often play a major role for male mating success. Male body condition is likely to be correlated with male acoustic signal traits, which signal male quality and provide choosy females indirect benefits. Environmental factors such as food quantity or quality can influence male body condition and therefore possibly lead to condition‐dependent changes in the attractiveness of acoustic signals. Here, we test whether stressing food plants influences acoustic signal traits of males via condition‐dependent expression of these traits. We examined four male song characteristics, which are vital for mate choice in females of the grasshopper Chorthippus biguttulus. Only one of the examined acoustic traits, loudness, was significantly altered by changing body condition because of drought‐ and moisture‐related stress of food plants. No condition dependence could be observed for syllable to pause ratio, gap duration within syllables, and onset accentuation. We suggest that food plant stress and therefore food plant quality led to shifts in loudness of male grasshopper songs via body condition changes. The other three examined acoustic traits of males do not reflect male body condition induced by food plant quality.

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