Insect Herbivore Populations Research Articles

Predation, but not herbivory, declines with elevation in a tropical rainforest

AbstractNaturally, insect herbivore populations are controlled by their plant hosts and predators. These ‘bottom-up’ and ‘top-down’ controls influence leaf area lost to herbivory. Bottom-up control of herbivory may be driven by leaf nutrients and plant defences. Top-down control can be driven by abundance and species richness of natural enemies, host or prey specificity, and predation strategies (e.g., active searching or sit-and-wait ‘ambush’ predation). The relative importance of bottom-up and top-down controls is unresolved but likely to vary spatially and temporally and under different environmental conditions such as changing temperature. We surveyed leaf carbon and nitrogen, leaf area loss, and attacks on plasticine caterpillars across a tropical elevational gradient in Xishuangbanna, Yunnan Provence, China. We show that predatory foraging activity decreases with elevation and temperature, whereas leaf nutrients and leaf area loss from herbivory remains more or less constant. Predation patterns were driven by ants, which are thermophiles and therefore more active, abundant, and diverse at warmer, lower elevations. Leaf nutritional values are important in driving herbivory patterns as herbivory was stable across this gradient, but other factors such as mechanical defences and herbivore-induced plant volatiles demand further study. Elevational studies provide insight into how ecosystem function will shift under climate change. As increasing temperatures following climate change allows predatory groups like ants to exploit higher elevations, top-down control in high elevation habitats could increase, resulting in re-wiring of these ecologically sensitive communities. At the same time, top-down control at lower elevations may be at risk if critical thermal maxima for natural enemies are exceeded.

Host plant-mediation of viral transmission and its consequences for a native butterfly.

Pathogens play a key role in insect population dynamics, contributing to short-term fluctuations in abundance as well as long-term demographic trends. Two key factors that influence the effects of entomopathogens on herbivorous insect populations are modes of pathogen transmission and larval host plants. In this study, we examined tritrophic interactions between a sequestering specialist lepidopteran, Euphydryas phaeton, and a viral pathogen, Junonia coenia densovirus, on its native host plant, Chelone glabra, and a novel host plant, Plantago lanceolata, to explore whether host plant mediates viral transmission, survival, and viral loads. A two-factor factorial experiment was conducted in the laboratory with natal larval clusters randomly assigned to either the native or novel host plant and crossed with either uninoculated controls or viral inoculation (20% of individuals in the cluster inoculated). Diapausing clusters were overwintered in the laboratory and checked weekly for mortality. At the end of diapause, all surviving individuals were reared to adulthood to estimate survivorship. All individuals were screened to quantify viral loads, and estimate horizontal transmission postmortem. To test for vertical transmission, adults were mated, and the progeny were screened for viral presence. Within virus-treated groups, we found evidence for both horizontal and vertical transmission. Larval clusters reared on the native host plant had slightly higher horizontal transmission. Survival probability was lower in clusters feeding on the native host plant, with inoculated groups reared on the native host plant experiencing complete mortality. Viral loads did not differ by the host plant, although viral loads decreased with increased sequestration of secondary compounds on both host plants. Our results indicate that the use of a novel host plant may confer fitness benefits in terms of survival and reduced viral transmission when larvae feeding on it are infected with this pathogen, supporting hypotheses of potential evolutionary advantages of a host range expansion in the context of tritrophic interactions.

Higher temperatures reduce the efficacy of a key biocontrol parasitoid

A primary facet of climate change is an increase in average temperatures which can have immense consequences for ectotherms, organisms that cannot physiologically regulate their own body temperatures, such as insects. Moreover, thermal stress can influence species interactions across trophic levels, especially if members of different trophic levels show differential responses. In agricultural systems, parasitoids are frequently used as biocontrol agents to reduce crop damage by regulating populations of insect herbivores, but climate change can potentially threaten this ecosystem service. In this study, we investigated the effects of diurnally fluctuating temperature regimes (average treatment temperatures = 23 °C, 25 °C, 27 °C) on the relationship between the common agricultural pest Plutella xylostella and its larval parasitoid Diadegma insulare. We found that at higher temperatures, both the host and the parasitoid developed faster. Host survival was not influenced by temperature, but higher temperatures resulted in increased parasitoid mortality in the pupal stage. Realized parasitism rates were also significantly decreased at higher temperatures, suggesting that D. insulare may become a less efficient biocontrol agent of P. xylostella as temperatures continue to increase.

Insect Herbivore Populations and Plant Damage Increase at Higher Elevations.

Simple SummaryIt is vitally important to understand the effects of climate change on insect pest populations and crop losses. Using elevation as a proxy for climate change, a field study was conducted in farmer’s fields in Nepal at various elevations in the Himalayan Mountains. At higher elevations, natural herbivore populations and plant damage from herbivory were significantly higher compared to their low-elevation counterparts. Temperature varied with elevation in the field and significantly affected both insect populations and plant damage. A geographical shift of insect pests towards higher elevations is predicated, so it is important to better understand how biotic and abiotic ecological factors and evolutionary processes will act together to affect ecosystem dynamics to reliably predict future pest problems.Elevation gradients are used as a proxy to simulate climate change effects. A field study was conducted along an elevational gradient in Nepal to understand the effects of abiotic conditions on agriculturally important insect herbivore populations (tobacco caterpillar: Spodoptera litura, tomato fruit worm: Helicoverpa armigera, and South American leaf miner, Tuta absoluta) and herbivory damage on tomatoes. Elevation ranged from 100 m to 1400 m above sea level, representing different climatic zones where tomatoes are grown. Contrary to our hypothesis, natural herbivore populations and herbivory damage significantly increased at higher elevations. Individual insect species responses were variable. Populations of S. litura and T. absoluta increased at higher elevations, whereas the H. armigera population was highest at the mid-elevational range. Temperature variations with elevation also affected insect catch numbers and the level of plant damage from herbivory. In the context of climate warming, our results demonstrate that the interactive effects of elevation and climatic factors (e.g., temperature) will play an important role in determining the changes in insect pest populations and the extent of crop losses.

Bug-bitten leaves from the early Miocene of Ethiopia elucidate the impacts of plant nutrient concentrations and climate on insect herbivore communities

High-resolution paleoecological studies elucidate the effects of biotic and abiotic factors on the diversity and distribution of insect herbivory in deep time. Previous analyses of fossil leaf assemblages from intervals of significant environmental change have shown that insect damage frequency and diversity are strongly influenced by temperature and carbon dioxide levels. Similar studies that span intervals of environmental quiescence are a necessary complement and can help disentangle the relative influences of biological versus abiotic factors. The 21.73 ± 0.03 million year old (early Miocene) Mush Valley lagerstätten, Ethiopia, provides one such example. Insect damage censuses were conducted at six stratigraphic levels that span ~50,000 years and whose plant ecology has been studied in detail. A total of 2200 leaves (47 morphotypes) were examined and 35 damage types (DTs) observed. We documented variations in insect damage frequency, diversity, and composition among plant host taxa and across stratigraphic levels and tested for correlations with abiotic and biotic factors. Interspecific variations in damage frequency and diversity among host plants at Mush with >20 leaves in the damage census could not be explained solely by leaf mass per area, presence/absence of trichomes, symbiosis with nitrogen-fixing bacteria, or host plant abundance. The strongest predictor of insect herbivore damage on the bulk floras (all leaves) was the prevalence of legumes, whose symbiotic relationships with nitrogen-fixing bacteria likely made them more nutrient-rich. This pattern was also observed across the climatically-variable Paleocene-Eocene boundary in the Bighorn Basin, USA, suggesting that the soil microbiome, plant nutrient composition, and climate work synergistically in regulating insect herbivore distribution, population sizes, and feeding. • A new Miocene record of fossil insect herbivory from the African tropics. • Insect damage frequency and diversity strongly correlate to abundance of plants with N 2 -fixing symbionts. • Predicting variability in herbivore damage among fossil plant hosts is challenging.

Response of specialist and generalist predators to nonprogressive annual fluctuations in herbivorous insect populations

To control sawfly populations that cause a forest management problem due to feeding damage, it is necessary to understand the mechanism of eruptive outbreak in erratic intervals by clarifying the responses of natural enemies to the host population fluctuations. In this study, we investigated annual beech sawfly population fluctuations and its predator, ichneumonid parasitoid wasps (as specialists) and predatory soldier beetles (as generalists). In addition, yearly responses of predators to the host emerging patterns were examined using structural equation modeling. The results revealed that ichneumonid wasps showed a non-delayed synchronous response to erratic fluctuation of the host population, resulting in the maintenance of a constant parasitism ratio. In contrast, soldier beetle population, which tends to moderately fluctuate while maintaining a specific density, showed a low-intensity delayed response without significance in the model. These predator populations without strong delayed density-dependent response are quite unlikely to highly densify causing mass mortality to the host population in the both the current and following years of outbreak. In places where eruptive outbreaks of sawfly species repeatedly occur in erratic intervals, sawfly populations are possibly maintained in high density due to the low mortality ratio caused by predators.

Bt cotton area contraction drives regional pest resurgence, crop loss, and pesticide use.

SummaryGenetically‐modified crops expressing Bacillus thuringiensis (Bt) proteins have been widely cultivated, permitting an effective non‐chemical control of major agricultural pests. While their establishment can enable an area‐wide suppression of polyphagous herbivores, no information is available on the impact of Bt crop abandonment in entire landscape matrices. Here, we detail a resurgence of the cosmopolitan bollworm Helicoverpa armigera following a contraction of Bt cotton area in dynamic agro‐landscapes over 2007–2019 in North China Plain. An 80% reduction in Bt cotton was mirrored in a 1.9‐fold increase of ambient H. armigera population levels, culminating in 1.5–2.1‐fold higher yield loss and a 2.0–4.4‐fold increase in pesticide use frequency in non‐Bt crops (i.e. maize, peanut, soybean). Our work unveils the fate of herbivorous insect populations following a progressive dis‐use of insecticidal crop cultivars, and hints at how tactically deployed Bt crops could be paired with agro‐ecological measures to mitigate the environmental footprint of crop production.

Arboreal arthropod assemblages in chili pepper with different mulches and pest managements in freshwater swamps of South Sumatra, Indonesia

Abstract. Herlinda S, Tricahyati T, Irsan C, Karenina T, Hasbi, Suparman, Lakitan B, Anggraini E, Arsi. 2021. Arboreal arthropod assemblages in chili pepper with different mulches and pest managements in freshwater swamps of South Sumatra, Indonesia. Biodiversitas 22: 3057-3066. In the center of freshwater swamps in South Sumatra, three different chili cultivation practices are generally found, namely differences in mulch and pest management that can affect arthropod assemblages. The effect of mulches and pest management on arboreal arthropod assemblages specific to chili production centers in the freshwater swamps of South Sumatra has never been investigated. This study aimed to observe arboreal arthropod assemblages in chili with different mulches and pest management. Arboreal arthropods were sampled using sweep nets in three locations with plots treated with leaf litter mulch and bioinsecticide, plastic mulch and synthetic insecticide, and weedy plot without mulch with synthetic insecticide. The species number of arboreal arthropods found was 28 species of Arachnids and 23 species of Insects, and consisting of 6 families of the Arachnids and 25 families of Insects. The abundance of arboreal arthropods was 65.60 individuals/5 nets per observation. In the chili field without mulch but with the insecticide, the species biodiversity and abundance of arboreal predatory arthropods were the highest. In contrast, in the chili field, that applied with synthetic insecticides and plastic mulch, the abundance of arboreal predatory arthropods was the lowest. The herbivorous insect populations in chili with plastic mulch and synthetic insecticides and the chili with the leaf litter mulch were higher than those in the chili without mulch. In the chili with the leaf litter mulch and bioinsecticide, the species number and abundance of the spiders were the highest compared to the other chili fields. The weedy chili field without mulch and chili with the leaf litter mulch has proved ideal habitats for the arboreal predatory arthropods.

Landscape-dependent effects of varietal mixtures on insect pest control and implications for farmer profits.

Intraspecific plant diversity can significantly impact insect herbivore populations in natural systems. Yet, its role as an insect pest control strategy in agriculture has received less attention, and little is known about which crop traits are important to herbivores in different landscape contexts. Moreover, empirical economic analyses on the cost‐effectiveness of varietal mixtures are lacking. We used varietal mixtures of Brassica oleracea crops on working farms to examine how two metrics of intraspecific crop diversity, varietal richness and number of plant colors (color richness), affect crop damage and the incidence and abundance of two insect pest species: Pieris rapae and Phyllotreta spp. We evaluated the context‐dependency of varietal mixtures by sampling early‐ and late‐season plantings of B. oleracea crops in farms across a gradient of landscape composition. We developed crop budgets and used a net present value analysis to assess the impact of varietal mixtures on input and labor costs, crop revenues, and profit. We found context‐dependent effects of varietal mixtures on both pests. In early‐season plantings, color richness did not affect Phyllotreta spp. populations. However, increasing varietal richness reduced Phyllotreta spp. incidence in simple landscapes dominated by cropland, but this trend was reversed in complex landscapes dominated by natural habitats. In late‐season plantings, color richness reduced the incidence and abundance of P. rapae larvae, but only in complex landscapes where their populations were highest. Varietal richness had the same effect on P. rapae larvae as color richness. Unexpectedly, we consistently found lower pest pressure and reduced crop damage in simple landscapes. Although varietal mixtures did not affect crop damage, increasing color richness corresponded with increased profits, due to increased revenue and a marginal reduction in labor and input costs. We demonstrate varietal mixtures can significantly impact pest populations, and this effect can be mediated by intraspecific variation in crop color. However, the strength and direction of these effects vary by season, landscape composition, and pest species. The association between varietal color richness and profitability indicates farmers could design mixtures to enhance economic returns. We recommend additional research on the benefits of intraspecific trait variation for farmers.

Diet-mediated effects of cadmium on fitness-related traits and detoxification and antioxidative enzymes in the oriental armyworm, Mythimna separata

Cadmium (Cd) is a heavy metal that is of great concern in the agroecosystem, because of its toxicity to plants and herbivores. Previous research has suggested that populations of herbivorous insects can be affected by Cd, via disrupting tri-trophic interactions comprising plants, herbivores, and predators. However, there is less understanding of the direct effects of dietary Cd on the life history traits and the detoxification or antioxidant enzyme systems of insects. Here we show that the life history traits of the herbivorous oriental armyworm, Mythimna separata (Walker), can be impacted by Cd. The body mass of larvae was decreased under an artificial diet containing a low (0.15 mg kg -1 ) Cd concentration, but was increased by intermediate (0.30 mg kg -1 ) and high (0.60 mg kg -1 ) concentrations of Cd. In contrast, pupal weight declined under intermediate and high concentrations of Cd compared to either low Cd concentration or the control. When Cd concentrations increased, larvae survival rate and fecundity decreased and the pupation rate and pupal deformity rate increased. Low and intermediate concentrations of Cd extended the length of the larval stage while high concentrations of Cd shortened the larval period. Dietary Cd inhibited the activity of antioxidant enzyme polyphenol oxidase (PPO), but increased acetylcholinesterase (AChE) activity and the activities of detoxifying enzymes carboxylesterase (CarE) and glutathione S -transferase (GST) of third-instar larvae. Overall, our results indicate that for Cd concentration there is a physiological trade-off between its adverse effects on life history traits and elevated detoxification, acetylcholinesterase sensitivity and antioxidation of M. separata , thus representing a potential adaptation strategy.

The effect of vineyard groundcover on the abundance of naturally occurring entomopathogenic fungi isolated using a quantitative Galleria bait method

Although sold commercially as myco-insecticides, naturally occurring soil borne entomopathogenic fungi (EPF) pose a unique opportunity for regulating soil-dwelling insect herbivore populations via conservation biological control. Plant identity has a significant effect on soil borne fungi, which may includee EPF. We asked whether the choice of covercrops could affect the diversity and infectivity of EPF in a vineyard. We studied the effect of different mixtures of groundcover vegetation on the infectivity and community composition of the naturally occurring EPF community in a semi-arid vineyard in British Columbia, Canada. Soil was collected from a groundcover field trial and used in a quantitative laboratory bioassay with the model insect, Galleria mellonella. Death by mycosis was greater when Galleria was exposed to soil collected from groundcovers containing native versus exotic grasses. EPF communities isolated from Galleria cadavers also differed among groundcovers. This study presents the first evidence that manipulating vegetative identity can alter the composition and infectivity of soil-dwelling EPF. Future work should investigate the potential for groundcovers and cover crops to increase conservation biological control of soil-dwelling insect pests via changes to EPF communities in specific crop-pest scenarios.

Woody vegetation remnants within pastures influence locust distribution: Testing bottom-up and top-down control

Agriculture is a major factor in landscape fragmentation, altering nutrient cycling and animal and plant populations through increasing habitat edge density. Most research on insect herbivores in agroecosystems has focused on the top-down effects of predators moving throughout the habitat mosaic. Few studies have focused on the top-down and bottom-up factors modulating the distribution of insect herbivore populations between natural and agricultural patches. For example, despite an understanding that Australian plague locusts (Chortoicetes terminifera) avoid tree patches, the underlying mechanisms remain unknown. Here, we explored how wooded remnants within pastures affect locust density and the potential top-down and bottom-up mechanistic explanations. We tested three hypotheses: 1) grasses near wooded areas are nutritionally suboptimal, 2) predator density is higher near wooded areas, and 3) temperatures are cooler underneath trees. We measured locust density, grass nutrient content, predator abundance, temperature, and ground cover along 50 m transects from wooded areas to open grassy areas. We ran those transects in three fields and had four transects per field. We confirmed locust avoidance for trees at a 20 m periphery, however none of the variables tested independently explained this trend. Grass nutrient content was similar underneath wooded areas and in open patches. Predator abundance did not differ between the two habitats. The ground was warmer under wooded areas than in grassy areas potentially due to woody vegetation negating windchill. Further, we found that locust density was negatively correlated with plant protein content and was highest in areas with approximately 20 % bare ground cover. Both plant protein and ground cover are important for grasshopper performance and reproduction. It is likely a complex interaction between these variables and others that drive the distribution of this species and other insect herbivores in agroecosystems. The small-scale mechanisms driving the response of insect herbivores to landscape changes is critical to understanding and predicting population dynamics at large-scales.

Varying degree of physiological integration among host instars and their endoparasitoid affects stress‐induced mortality

AbstractIn natural populations of insect herbivores, genetic differentiation is likely to occur due to variation in host plant utilization and selection by the local community of organisms with which they interact. In parasitoids, engaging in intimate associations with their host during immature development, local variation may exist in host quality for parasitoid development. We compared the development of a gregarious endoparasitoid, Cotesia glomerata L. (Hymenoptera: Braconidae), collected in The Netherlands, in three strains and three caterpillar instars (L1–L3) of its main host, Pieris brassicae L. (Lepidoptera: Pieridae). Hosts had been collected in The Netherlands and France, and were reared in the laboratory for one generation. We also used an established Dutch laboratory strain that had not been exposed to parasitoids for at least 24 generations. Parasitoid survival to adulthood was inversely correlated with host instar at parasitism. Adult parasitoid body mass was largest when hosts were parasitized as L1 and smallest when hosts were parasitized as L3, whereas egg‐to‐adult development time was quickest on L3 hosts and slowest on L1 hosts. Higher survival and faster development of C. glomerata on French L2 hosts also showed that there is variation in host‐instar‐related suitability. Many L2 and most L3 caterpillars that were parasitized exhibited signs of pathogen infection and perished within a few days of parasitism, whereas this never happened when hosts were parasitized as L1 or in non‐parasitized control caterpillars. Our results reveal that, irrespective of the host strain, L1 hosts are optimally synchronized with C. glomerata development. By contrast, the high precocious mortality of L3 larvae may be due to stress‐induced regulation by the parasitoid in order to ‘force’ its developmental program into synchrony with the developing parasitoid larvae. Our results underscore a potentially important role played by pathogens in mediating herbivore–parasitoid interactions that are host‐instar‐dependent in their expression.

Temporal variation in bottom-up and top-down effects differ among herbivores with different seasonality

How variations in the strength of bottom-up and top-down forces are mediated by life-history trait differences of insect herbivores is an important but underexplored topic in ecology. Here, we implemented a factorial experiment manipulating plant quality by nitrogen fertilization and attack rates of parasitoids and spiders, respectively. We used sticky traps and mesh barriers in a salt marsh system to examine bottom-up and top-down effects on populations of three hemipteran sap-feeding monophagous species of Phragmites australis with different seasonality. The species included Chloriona alaica and Stenocranus matsumurai most abundant in spring and autumn, respectively, and Dimorphopterus pallipes abundant in both summer and autumn. Fertilization increased the density of S. matsumurai but had no effect on C. alaica and D. pallipes. Its negative effects on densities of the latter two species were even observed in earlier months. On the other hand, parasitoid removal not only increased abundance of all three herbivores, but also reduced the culm diameter, shoot height and biomass of P. australis, suggesting a trophic cascade. The main effects of parasitoid removal were greatest in months when those herbivores had their peak densities. In contrast, spider removal only affected the abundance of S. matsumurai with the greatest effect in September. Our results indicate that parasitoids have strong top-down effects on populations of insect herbivores in P. australis-dominated salt marshes and the strength of their effects is positively correlated with the host density. Furthermore, effects of plant quality and spider predation are more varied among herbivores with different seasonality.

Strong positive effects of termites on savanna bird abundance and diversity are amplified by large herbivore exclusion.

Vast areas of the African savanna landscapes are characterized by tree‐covered Macrotermes termite mounds embedded within a relatively open savanna matrix. In concert with termites, large herbivores are important determinants of savanna woody vegetation cover. The relative cover of woody species has considerable effects on savanna function. Despite the potentially important ecological relationships between termite mounds, woody plants, large herbivores, and birds, these associations have previously received surprisingly little attention. We experimentally studied the effects of termites and large herbivores on the avian community in Lake Mburo National Park, Uganda, where woody vegetation is essentially limited to termite mounds. Our experiment comprised of four treatments in nine replicates; unfenced termite mounds, fenced mounds (excluding large mammals), unfenced adjacent savanna, and fenced savanna. We recorded species identity, abundance, and behavior of all birds observed on these plots over a two‐month period, from late dry until wet season. Birds used termite mounds almost exclusively, with only 3.5% of observations occurring in the treeless intermound savanna matrix. Mean abundance and species richness of birds doubled on fenced (large herbivores excluded) compared to unfenced mounds. Feeding behavior increased when large mammals were excluded from mounds, both in absolute number of observed individuals, and relative to other behaviors. This study documents the fundamental positive impact of Macrotermes termites on bird abundance and diversity in an African savanna. Birds play crucial functional roles in savanna ecosystems, for example, by dispersing fruits or regulating herbivorous insect populations. Thus, the role of birds in savanna dynamics depends on the distribution and abundance of termite mounds.

Seasonal and diel variations in the activity of canopy insect herbivores differ between deciduous and evergreen plant species in a tropical dry forest

This study evaluated whether herbivorous insects can be expected to have particular adaptations to withstand the harsh dry season in tropical dry forests (TDFs). We specifically investigated a possible escape in space, with herbivorous insects moving to the few evergreen trees that occur in this ecosystem; and escape in time, with herbivores presenting an increased nocturnal rather than diurnal activity during the dry season. We determined the variation in the free-feeding herbivorous insects (sap-sucking and leaf chewing) between seasons (beginning and middle of both rainy and dry seasons), plant phenological groups (deciduous and evergreen trees) and diel period (diurnal and nocturnal) in a Brazilian TDF. We sampled a total of 5827 insect herbivores in 72 flight-interception traps. Contrary to our expectations, we found a greater herbivore diversity during the dry season, with low species overlap among seasons. In the dry season, evergreen trees supported greater richness and abundance of herbivores as compared to deciduous trees. Insects were also more active at night during the dry season, but no diel differences in insect abundance were detected during the rainy season. These results indicate that the strategies used by insect herbivores to withstand the severe climatic conditions of TDFs during the dry season include both small-scale escape in space and time, with evergreen trees playing a key role in maintaining resident insect herbivore populations in TDFs. Relatively more nocturnal activity during the dry season may be related to the avoidance of harsh climatic conditions during the day. We suggest that the few evergreen tree species occurring in the TDF landscape should be especially targeted for protection in this threatened ecosystem, given their importance for insect conservation.

Phylloplane bacteria increase the negative impact of food limitation on insect fitness

1. When populations of herbivorous insects increase in density, they can alter the quantity or quality of their food. The impacts of diet‐related stressors on insect fitness have been investigated singly, but not simultaneously.2. Foliage quantity and quality of red alder, Alnus rubra, were manipulated together with the presence of non‐entomopathogenic phylloplane bacteria to investigate their impacts, singly and in combination, on survival, pupal mass, growth rate, fecundity and egg quality of a cyclic forest insect, the western tent caterpillar, Malacosoma californicum pluviale.3. Food limitation (half food) had strong negative impacts on all life‐history traits. When the larvae were fed continuously, however, neither ingesting phylloplane bacteria nor eating leaves from damaged branches (induced foliage) affected survival. In the half‐food treatment, ingesting bacteria further increased mortality, while feeding on induced foliage improved survival.4. Growth rate and pupal mass of both sexes were reduced for larvae with food limitation compared with continuously fed insects and this was exacerbated when the larvae also ate bacteria‐treated leaves. A combination of bacteria and induced foliage also reduced larval growth rate by 5% in the full‐food treatment.5. Fecundity (eggs per egg mass) was 2.7 times greater in full‐food than in food‐limited treatments but neither phylloplane bacteria nor plant induction had an effect. Insects fed induced foliage produced smaller eggs. Overall, there was no evidence of a three‐way interaction between the three stressors, although there were negative synergistic effects, primarily between food limitation and the ingestion of phylloplane bacteria.

Spatial turnover in host-plant availability drives host-associated divergence in a South African leafhopper (Cephalelus uncinatus)

BackgroundThe evolution of reproductive isolation between herbivorous insect populations is often initiated by shifts to novel host-plants, a process that underlies some of the best examples of ecological speciation. However, it is not well understood why host-shifts occur. Arguably the most common hypothesis is that host-shifts occur in response to competition, while a less frequently invoked hypothesis is that herbivores adapt locally to geographic differences in potential host-plant communities. Here we investigate whether geographic variation in host-plant availability is likely to have driven host-shifts in restio leafhoppers. We studied local adaptation of a camouflaged restio leafhopper species, Cephalelus uncinatus, to host-plants in the Restionaceae (restios); a family of plants with exceptional diversity in the anomalously species-rich Cape Floristic Region (CFR). To determine whether C. uncinatus experiences heterogeneous host communities across its range, we first quantified the degree of geographic overlap between C. uncinatus and each of its associated host-plant species. Then we quantified trait divergence (host preference, body size and colour) for three pairs of C. uncinatus populations found on different host-plant species differing in their degree of spatial overlap. Spectral reflectance was modelled in bird visual space to investigate whether body colour divergence in C. uncinatus corresponds to leaf sheath colour differences between restio species as perceived by potential predators.ResultsWe demonstrate that C. uncinatus is forced to use different restio species in different regions because of turnover in available host species across its range. Comparisons between geographically separated populations were consistent with local adaptation: restio leafhoppers had preferences for local host-plants over alternative host-plants and matched local plants better in terms of size and colour.ConclusionsSpatial turnover in host-plant availability has likely facilitated host-shifts in C. uncinatus. Spatial turnover in host-plant availability may be an important driver of insect diversification in the CFR and globally.

Insects and Bobwhite Quail Brood Habitat Management

Small insects were the most important foods eaten by quail chicks 2 to 20 days of age. The foods eaten, in order of importance, were beetles, leafhoppers, true bugs, spiders, grasshoppers, ants, larvae, snails, and flies. Important seeds ingested were Panicum spp., Carex spp., Scleria spp., Paspalum spp., and Setaria sp. The effect of fire, a major tool in southern quail management, on insect populations was studied by sampling burned and unburned plots with a sweep net and a D-vac machine. On an old-field type of habitat, population densities and biomass of herbivorous insect populations were significantly greater on February-burned plots than on 5-year-old unburned plots. Two peaks in numbers of insects were found. The first peak of ca. 64,000/acre (sweep net) occurred in mid-June. The second peak occurred in mid-August (D-vac) with a density 0f ca. 90,000/acre. Total insect biqmass, excluding individuals over 0.035 g dry weight, averaged 147 g/acre (sweep net) and 128 g/acre (D-vac). In the second phase of the study, in a longleaf pine forest habitat, grasshoppers were the only species of insect having significantly greater density and biomass on unburned, 3-year-old "roughs" than on annually burned plots. Lack of litter on annually burned plots probably caused this disparity. At peak density, in the period of mid-July to early August, sweep-net density was 19,500/acre and D-vac density was 58,500/acre. Total insect biomass averaged 79 g/acre (sweep net) and 52 g/acre (D-vac).The major considerations for brood habitat are abundance and availability of insects. In old-field habitat, fire increases insect abundance and removes accumulated litter, opening the area for ease of chick movement. If the soil is fertile, then annual burns are feasible. The interval of burning advocated is 1 or 2 years, but local problems may modify this. Burning annually in the poor-soil region of the longleaf forest type is not necessary.

Sex‐specific dispersal by a parasitoid wasp in the field

AbstractAlthough parasitoid wasps are important regulators of herbivorous insect populations, very little is known regarding their movement under field conditions. Here, we examined the vertical and cardinal directionality of dispersal by Binodoxys communis (Gahan) (Hymenoptera: Braconidae: Aphidiinae), an exotic Asian parasitoid released as a biological control agent of the soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae: Aphidinae), in North American soybeans, Glycine max (L.) Merr. (Fabaceae). The patterns are consistent with time‐dependent, sex‐specific dispersal strategies; whereas males show flight that is devoid of vertical or cardinal directionality, females consistently move at or above soybean canopy height toward the east and north. Male capture rates also appear to be more correlated with local conditions than those of females. These results suggest that females actively cross the flight boundary layer (the space above which insect flight is largely wind‐driven as opposed to self‐directed) and follow air currents away from soybean fields, whereas males engage in more localized movement. The data also suggest differing response potentials of the sexes to changing local resource conditions (host availability, female availability, host plant cultivar). We discuss the implications for sex‐specific movement on the basic biology of introduced parasitoid species, and their applied role as potential agents in importation biological control programs.