Deutscher Wetterdienst DWD, German Federal Ministry of Transport, Building and Urban Affairs. † MeteoSwiss, Swiss Federal Offi ce of Meteorology and Climatology. * Statistical analyses were conducted for each plant series with 'drought stress' and 'plant population' as fi xed factors in two-way ANOVAs (signifi cant effects in bold) followed by LSD post hoc tests (shown in brackets).

1. Water stress may increase or reduce the suitability of plants for herbivores. The recently proposed 'pulsed stress hypothesis' suggests consideration of stress phenology (pulsed vs. continuous stress) to explain these conflicting effects of plant water stress on herbivore performance.2. This hypothesis was tested for the effect of differing stress intensity on performance and preference of insect herbivores belonging to different feeding guilds, namely leaf-chewing insects ( Spodoptera littoralis caterpillars) and phloem-feeding insects ( Aphis pomi aphids), on apple plants ( Malus domestica ). The plants were non-stressed or exposed to a low or high intensity of pulsed water stress.3. Plant responses to the different stress levels were generally monotonic. Growth, stomatal conductance ( g s ), leaf water, and old-leaf nitrogen concentration decreased, whereas young-leaf nitrogen concentration and leaf mass per area (LMA) increased with increasing stress intensity. The stable isotope composition of foliar carbon ( ␦ 13 C) responded non-monotonically to the drought treatments. The ␦ 13 C values were highest in low-stress plants, intermediate in high-stress plants, and lowest in non-stressed plants.4. The preference and performance responses of the caterpillars were also non-monotonic. Non-stressed plants were intermediately, low-stress plants least, and high-stress plants most attractive or suitable. Aphid population growth was highest on non-stressed plants and lowest on low-stress plants.5. The results highlight the importance of water stress intensity for the outcome of interactions between herbivores and drought-affected plants. They show that pulsed water stress may enhance or reduce insect herbivore performance and plant resistance, depending on stress intensity.

Heterospecific neighbors may reduce damage to a focal plant by lowering specialist herbivore loads (associational resistance hypothesis), or enhance damage by increasing generalist herbivore loads (associational susceptibility hypothesis). We tested the associational effects of tree diversity on herbivory patterns of the tropical focal tree Tabebuia rosea in an experimental plantation setup, which contained tree monocultures and mixed stands. We found higher herbivore damage to T. rosea at higher tree diversity, indicating that T. rosea did not benefit from associational resistance but rather experienced associational susceptibility. The specific consideration of the two dominant insect herbivore species of T. rosea, the specialist chrysomelid Walterianella inscripta and the specialist pyralid Eulepte gastralis, facilitated understanding of the detected damage patterns. Tree diversity exerted opposite effects on tree infestation by the two herbivores. These findings point to resource concentration effects for the chrysomelid beetle (favored by tree monoculture) and to resource dilution effects for the pyralid caterpillar (favored by tree mixture) as underlying mechanisms of herbivore distribution. A strong contribution of the pyralid to overall damage patterns in diversified stands suggests that associational susceptibility may not necessarily be related to higher abundances of generalist herbivores but may also result from specialized herbivores affected by resource dilution effects. Thus, the identity and biology of herbivore species has to be taken into account when attempting to predict damage patterns in forest ecosystems.

Abstract. Land use and corresponding habitat loss are major drivers of local species extinctions. Orthoptera as important grassland herbivores showed different responses to land-use intensity in different studies, and the susceptibility of this group remains unclear. We sampled annually for seven years 150 temperate grassland sites across three regions in Germany, for which land-use gradients were quantified as mowing, grazing, and fertilization intensity. We analyzed the effects of land-use intensity on orthopteran diversity and community abundance. To describe species-specific responses to environmental gradients, we employed a new approach termed "niche model," coupled with a randomization procedure, which is sensitive even for rare species for which trends may otherwise be difficult to detect. Based on abundanceweighted means for each species, we quantified the species' occurrence along land-use gradients and identified potential losers and winners of intensive land use. Overall, high land-use intensity negatively affected orthopteran diversity across years and regions, corresponding to decreases with high fertilization, mowing, and grazing intensity. Intensive mowing and grazing negatively affected abundance. Diversity and abundance increased with the time after the last cut. The niche model detected 15 of 29 Orthoptera species as losers of land use, showing significantly higher abundance in grasslands with low-intensity land use. Two species were winners of high land-use intensity, whereas the remaining 12 were assigned as opportunists. Most species were losers of high fertilization intensity, followed by frequent mowing. Grazing intensity was least detrimental at the species level. Omnivorous, herbivorous, and graminivorous species did not differ in their response to land-use intensity, whereas bryovorous/lichenivorous Tetrix species showed consistently negative responses to intensive land use. Our highly replicated, long-term and large-scale survey suggests that further land-use intensification threatens many Orthoptera and causes a consistent diversity loss. Low intensity of fertilization, infrequent mowing, and variable grazing will help to maintain a high diversity of orthopterans. The generality of our niche model approach advances studies on species' susceptibility in various study systems.

Abstract.  1. The effectiveness of ants as plant defenders is equivocal for plants that attract ants via extrafloral nectaries (EFNs). 2. This study focused on the myrmecophilic savannah tree Pseudocedrela kotschyi that attracts ants to EFNs and on the arthropod fauna associated with P. kotschyi. Herbivory and arthropod community composition were compared between trees that were dominated by one of three congeneric ant species, Camponotus acvapimensis, C. rufoglaucus, and C. sericeus, and between trees where ants were experimentally excluded and untreated control trees. 3. Short‐term ant‐exclusion experiments failed to demonstrate a consistent effect of ants on herbivory. 4. Plants dominated by different ant species differed significantly in leaf damage caused by herbivorous insects. The relative ranking of herbivory levels of the trees dominated by different ant species was persistent in three consecutive years. 5. Ants significantly reduced the abundance of different arthropod groups (Araneae, Blattodea, Coleoptera, Homoptera, non‐ant Hymenoptera). Other groups, including important herbivores, seemed not to be affected (Lepidoptera, Orthoptera, Thysanoptera, Heteroptera). 6. The study suggests that the presence of ants only benefits plants when specific ant species are attracted, and protection by these ants is not counterbalanced by their negative effect on other beneficial arthropods.