Multiple interaction types determine the impact of ant predation of caterpillars in a forest community.

Direct and indirect effects of predators are highly variable in complex communities, and understanding the sources of this variation is a research priority in community ecology. Recent evidence indicates that herbivore community structure is a primary determinant of predation strength and its cascading impacts on plants. In this study, we use variation in herbivore community structure among plant species to experimentally test two hypotheses in a temperate forest food web. First, variation in the strength of predator effects, such as ant predation of caterpillars, is predicted to be density dependent, exhibiting stronger effects when prey abundance is high (density-dependent predation hypothesis). Second, mutualistic interactions between ants and sap-feeding herbivores are expected to increase the abundance of predatory ants, strengthening predation effects on herbivores with cascading effects on host plants (keystone mutualism hypothesis). Using a large-scale predator exclusion experiment across eight dominant tree species, we tracked changes in insect density on 862 plants across two years, recording 2,322 ants, 1,062 sap-feeders, 5,322 caterpillars, and quantifying herbivory on 199, 338 leaves. In this experiment, density-dependent predation did not explain variation in the direct or indirect effects of ants on caterpillars and herbivory. In partial support of the keystone mutualism hypothesis, sap-feeders strengthened top-down effects of ants on caterpillars under some conditions. However, stronger ant predation of caterpillars did not lead to measurable trophic cascades on trees occupied by sap-feeders. Instead, the presence of sap-feeders was associated with increased per capita feeding damage by caterpillars, and this bottom-up effect attenuated the indirect effects of ants on host plants. These findings demonstrate that examining the multi-trophic impacts of mutualisms and predation in the context of the broader community can reveal patterns otherwise masked by compensatory interactions.

Boom and bust: rapid feedback responses between insect outbreak dynamics and canopy leaf area impacted by rainfall and CO2.

Frequency and severity of insect outbreaks in forest ecosystems are predicted to increase with climate change. How this will impact canopy leaf area in future climates is rarely tested. Here, we document function of insect outbreaks that fortuitously and rapidly occurred in an ecosystem under free-air CO2 enrichment. Over the first 2 years of CO2 fumigation of a naturally established mature Eucalyptus woodland, we continuously assessed population responses of three sap-feeding insect species of the psyllid genera Cardiaspina, Glycaspis and Spondyliaspis for up to ten consecutive generations. Concurrently, we quantified changes in the canopy leaf area index (LAI). Large and rapid shifts in psyllid community composition were recorded between species with either flush (Glycaspis) or senescence-inducing (Cardiaspina, Spondyliaspis) feeding strategies. Within the second year, two psyllid species experienced significant and rapid population build-up resulting in two consecutive outbreaks: first, rainfall stimulated Eucalyptus leaf production increasing LAI, which supported population growth of flush-feeding Glycaspis without impacting LAI. Glycaspis numbers then crashed and were followed by the outbreak of senescence-feeding Cardiaspina fiscella that led to significant defoliation and reduction in LAI. For all three psyllid species, the abundance of lerps, protective coverings excreted by the sessile nymphs, decreased at e[CO2 ]. Higher lerp weight at e[CO2 ] for Glycaspis but not the other psyllid species provided evidence for compensatory feeding by the flush feeder but not the two senescence feeders. Our study demonstrates that rainfall drives leaf phenology, facilitating the rapid boom-and-bust succession of psyllid species, eventually leading to significant defoliation due to the second but not the first outbreaking psyllid species. In contrast, e[CO2 ] may impact psyllid abundance and feeding behaviour, with psyllid species-specific outcomes for defoliation severity, nutrient transfer and trophic cascades. Psyllid populations feeding on Eucalyptus experience rapid boom-and-bust cycles depending on availability of suitable foliage driven by rainfall patterns and leaf phenology.

Phenology, Seasonal Abundance, and Host-Plant Association of Spittlebugs (Hemiptera: Aphrophoridae) in Vineyards of Northwestern Italy.

Spittlebugs (Hemiptera: Aphrophoridae) are the vectors of the bacterium Xylella fastidiosa (Xf) in Europe. Xf may cause severe epidemics in cultivated plants, including grapevines. To assess the threat represented by the bacterium to grapevines, detailed information on the vectors' phenology, density, and ecology in vineyards is needed. The aim of the present work was to describe spittlebug diversity, phenology, and host-plant association in the vineyard agroecosystem. Two separate field surveys of nymphal and adult spittlebug populations, i.e., a two-year survey of a single site and a one-year survey of three sites, were performed in vineyards of northwestern Italy in three consecutive years. Philaenus spumarius was the most common species, reaching average nymph densities on herbaceous cover up to 60-130 nymphs/m2. Adults were sampled on grapevines from May to September, with a peak in June (up to 0.43 insects/sweep). Herbaceous cover was colonized after egg hatching and in late summer for oviposition, while wild woody hosts represented a refuge during summer. The results show that spittlebugs can reach high population levels in vineyards, at least in the areas where the ground is covered by herbaceous plants for the whole season and the use of insecticides is moderate. The extended presence of P. spumarius adults on grapevines represents a serious risk factor for the spread of Xf. The scenarios of Xf establishment in vineyards in northwestern Italy and Europe are discussed in relation to the abundance, phenology, and plant association of spittlebugs.

Spittlebugs of Mediterranean Olive Groves: Host-Plant Exploitation throughout the Year.

Spittlebugs are the vectors of the bacterium Xylella fastidiosa Wells in Europe, the causal agent of olive dieback epidemic in Apulia, Italy. Selection and distribution of different spittlebug species on host-plants were investigated during field surveys in 2016-2018 in four olive orchards of Apulia and Liguria Regions of Italy. The nymphal population in the herbaceous cover was estimated using quadrat samplings. Adults were collected by sweeping net on three different vegetational components: herbaceous cover, olive canopy, and wild woody plants. Three species of spittlebugs were collected: Philaenus spumarius L., Neophilaenus campestris (Fallén), and Aphrophora alni (L.) (Hemiptera: Aphrophoridae). Philaenus spumarius was the predominant species both in Apulia and Liguria olive groves. Nymphal stages are highly polyphagous, selecting preferentially Asteraceae Fabaceae plant families, in particular some genera, e.g., Picris, Crepis, Sonchus, Bellis, Cichorium, and Medicago. Host-plant preference of nymphs varies according to the Region and through time and nymphal instar. In the monitored sites, adults peak on olive trees earlier in Apulia (i.e., during inflorescence emergence) than in Liguria (i.e., during flowering and beginning of fruit development). Principal alternative woody hosts are Quercus spp. and Pistacia spp. Knowledge concerning plant selection and ecological traits of spittlebugs in different Mediterranean olive production areas is needed to design effective and precise control strategies against X. fastidiosa vectors in olive groves, such as ground cover modifications to reduce populations of spittlebug vectors.

Characterization of Feeding Injuries Caused by Ceresa nigripectus Remes Lenicov (Hemiptera: Membracidae) on Alfalfa Stems.

Piercing-sucking insects cause mechanical and physiological injury to plants. Ceresa nigripectus Remes Lenicov is a pest of alfalfa in subtropical regions of South America and a carrier of the ArAWB phytoplasma. The aim of this study was to determine the feeding habits of this treehopper and to describe the effects of the feeding injuries on stem vascular tissues in alfalfa. Adults and nymphs of C. nigripectus inserted their stylets repeatedly girdling the stem. One week after feeding, alfalfa stems exhibited numerous feeding canals with salivary deposits, most of which reached the phloem. Two weeks after feeding, cortex and phloem cells next to the salivary sheath collapsed, mature tracheal elements became sparse and appeared with an increased cross-section area, and phenolic compounds increased in cells and cell walls compared to undamaged plants. Three weeks after feeding, an annular callus, formed by abnormal cell division and hypertrophy of preexisting cortex and vascular cambium cells, appeared immediately above the stem girdle. Parenchyma cells from the outer layers of the callus differentiated to form secondary anomalous amphicribal bundles in the wound. The aerial parts above the stem girdle eventually withered and died.

Inhibition of α-glucosidase activity by N-deoxynojirimycin analogs in several insect phloem sap feeders.

Secondary metabolites and synthetic iminosugars that structurally resemble monosaccharides are potent inhibitors of α-glucosidase activity. The enzyme is core in cleaving sucrose in phloem feeding insects and it also plays a crucial role of reducing osmotic stress via the formation of oligosaccharides. Inhibition of hydrolysis by iminosugars should result in nutritional deficiencies and/or disruption of normal osmoregulation. Deoxynojirimycin (DNJ) and 2 N-alkylated analogs [N-butyl DNJ (NB-DNJ) and N-nonyl DNJ (NN-DNJ)] were the major iminosugars used throughout the study. The extensive experiments conducted with α-glucosidase of the whitefly Bemisia tabaci indicated the competitive nature of inhibition and that the hydrophilic DNJ is a potent inhibitor in comparison to the more hydrophobic NB-DNJ and NN-DNJ compounds. The same inhibitory pattern was observed with the psyllid Cacopsylla bidens α-glucosidase. In contrast to the above pattern, enzymes of the aphids, Myzus persicae and Aphis gossypii were more sensitive to the hydrophobic iminosugars as compared to DNJ. In vivo experiments in which adult B. tabaci were fed dietary iminosugars, show that the hydrophilic DNJ was far less toxic than the lipophilic NB-DNJ and NN-DNJ. It is proposed that this pattern is attributed to the better accessibility of the hydrophobic NN-DNJ to the α-glucosidase membrane-bound compartment in the midgut. Based on the inhibitory effects of certain polyhydroxy N-alkylated iminosugars, α-glucosidase of phloem feeding hemipterans could serve as an attractive target site for developing novel pest control agents.