Insect Life History Traits Research Articles

With a little help from my friends: the roles of microbial symbionts in insect populations and communities.

To understand insect abundance, distribution and dynamics, we need to understand the relevant drivers of their populations and communities. While microbial symbionts are known to strongly affect many aspects of insect biology, we lack data on their effects on populations or community processes, or on insects' evolutionary responses at different timescales. How these effects change as the anthropogenic effects on ecosystems intensify is an area of intense research. Recent developments in sequencing and bioinformatics permit cost-effective microbial diversity surveys, tracking symbiont transmission, and identification of functions across insect populations and multi-species communities. In this review, we explore how different functional categories of symbionts can influence insect life-history traits, how these effects could affect insect populations and their interactions with other species, and how they may affect processes and patterns at the level of entire communities. We argue that insect-associated microbes should be considered important drivers of insect response and adaptation to environmental challenges and opportunities. We also outline the emerging approaches for surveying and characterizing insect-associated microbiota at population and community scales. This article is part of the theme issue 'Towards a toolkit for global insect biodiversity monitoring'.

Ecological strategies of Hyphantria cunea (Lepidoptera: Arctiidae) response to different larval densities

Population density is an essential factor affecting the life history traits of insects and their trade-off relationships, as increasing density intensifies intraspecific competition. It decreases the average resources available to individuals within a population, affecting their morphology, physiology, behavior, and fitness. The fall webworm, Hyphantria cunea (Drury) (Lepidoptera: Arctiidae), has been an invasive pest of forest trees, ornamental plants, and fruit trees in China for many years. The larvae have a typical aggregation habit before the fourth instar and keep spitting silk to gather the damaged leaves into silk webs. However, the fitness of H. cunea in response to population density remains unclear. In this study, the critical biological parameters, food utilization, and population parameters of H. cunea in response to different rearing densities were investigated. The results showed that under high population density, H. cunea larvae showed better performance, with faster development, higher survival rates, and shorter generation time, but pupal weight and female fecundity decreased as population density increased. In contrast, for larvae raised in low density, the developmental period was prolonged, and mortality was increased, while higher food utilization, greater body size, and female fecundity were observed. Both males and females had similar development strategies in response to the density, but females may be more resistant to crowding than males. In conclusion, H. cunea could adopt different ecological strategies against the stress of density. High population densities result in shorter generation cycles and higher survival rates. Conversely, the low-density generation period becomes longer but with greater fecundity. The results may help determine the possible outbreak mechanism and develop effective population monitoring and forecasting measures for H. cunea.

Population-specific effects of temperature and photoperiod on development and body mass in Cassida vibex (Coleoptera: Chrysomelidae).

An interplay of genetic divergence and phenotypic plasticity in shaping geographic variation is increasingly receiving attention in the entomological literature. Two major environmental variables that govern life histories are temperature and photoperiod. Studies of thermal and photoperiodic reaction norms help us understand how insect diversity evolved and how insects respond to environmental change. We studied survival, development, and body mass in three geographic populations of the beetle Cassida vibex reared in the laboratory under several combinations of constant temperature (16, 19, 22, 25, and 28°C) and photoperiod (short-day and long-day). The three collection sites are situated along a climatic gradient and separated by hundreds of kilometers. Each population subtly but significantly differs in the absolute values of survival rate, developmental rate, and body mass as well as in the thermal and photoperiodic plasticity of these traits, but the geographic differences do not form a latitudinal cline. The southernmost population from a relatively warm climate survives worse at low temperatures than the other two, but the overall survival is lowest in the latitudinally intermediate population. Short-day conditions tend to accelerate postembryonic development and increase the slope of the developmental rate-temperature relationship, especially so in the intermediate population, followed by the southernmost population and then by the northernmost population. The latter, which inhabits a harsh climate, has the fastest and most temperature-sensitive development, regardless of photoperiod, and attains the largest body mass among the three populations. The intermediately located and photoperiodically plastic population, which lives in a cool but mild climate, in contrast, has the smallest body size. Hence, although the importance of short-day conditions as a seasonal cue increases poleward, the photoperiodic responses do not always become more pronounced in colder, high-latitude environments. Our results emphasize that insect life-history traits can exhibit quite sophisticated patterns of variation along climatic gradients.

Novel miR-108 and miR-234 target juvenile hormone esterase to regulate the response of Plutella xylostella to Cry1Ac protoxin

Insect hormones, such as juvenile hormone (JH), precisely regulate insect life-history traits. The regulation of JH is tightly associated with the tolerance or resistance to Bacillus thuringiensis (Bt). JH esterase (JHE) is a primary JH-specific metabolic enzyme which plays a key role in regulating JH titer. Here, we characterized a JHE gene from Plutella xylostella (PxJHE), and found it was differentially expressed in the Bt Cry1Ac resistant and susceptible strains. Suppression of PxJHE expression with RNAi increased the tolerance of P. xylostella to Cry1Ac protoxin. To investigate the regulatory mechanism of PxJHE, two target site prediction algorithms were applied to predict the putative miRNAs targeting PxJHE, and the resulting putative miRNAs were subsequently verified for their function targeting PxJHE using luciferase reporter assay and RNA immunoprecipitation. MiR-108 or miR-234 agomir delivery dramatically reduced PxJHE expression in vivo, whilst only miR-108 overexpression consequently increased the tolerance of P. xylostella larvae to Cry1Ac protoxin. By contrast, reduction of miR-108 or miR-234 dramatically increased PxJHE expression, accompanied by the decreased tolerance to Cry1Ac protoxin. Furthermore, injection of miR-108 or miR-234 led to developmental defects in P. xylostella, whilst injection of antagomir did not cause any obvious abnormal phenotypes. Our results indicated that miR-108 or miR-234 can be applied as potential molecular targets to combat P. xylostella and perhaps other lepidopteran pests, providing novel insights into miRNA-based integrated pest management.

Life history traits of spotted lanternfly (Hemiptera: Fulgoridae) when feeding on grapevines and tree of heaven.

The invasive planthopper, spotted lanternfly (SLF), Lycorma delicatula (White) (Hemiptera: Fulgoridae), feeds on a broad range of plants including species of economic importance such as grape. Although SLF feeds on wild and cultivated grape, the effect of grapevines on the insect's life history traits is unknown. This study examined the effect of cultivated Concord grapevines (Vitis labrusca) and the insect's preferred host tree of heaven (TOH), Ailanthus altissima, on SLF development, survival, reproduction, and body mass. Newly emerged nymphs were allowed to feed on either TOH, Concord grapevines or a mixed diet of Concord grapevines plus TOH through adulthood until death. Development, mortality, and oviposition of paired adults were tracked daily to calculate the SLF rate of development, survival, and reproduction among treatments. When feeding exclusively on Concord grapevines, SLF was able to develop and reproduce but had higher mortality, slower development, and produced fewer eggs. SLF fed on the mixed diet of grapevines plus TOH exhibited faster nymphal development, laid more eggs, and had higher body mass compared with those fed only on grape or TOH. SLF had greater survival when fed on either the mixed diet or on TOH alone. We conclude that Concord grapevines are a poor-quality host for SLF, but when combined with TOH, SLF fitness increases above that of feeding on TOH alone. This study supports the elimination of TOH as a part of SLF vineyard management practices.

Predicting age and mass at maturity from feeding behavior and diet in Manduca sexta: An empirical test of a life history model.

Feeding for most animals involves bouts of active ingestion alternating with bouts of no ingestion. In insects, the temporal patterning of bouts varies widely with resource quality and is known to affect growth, development time, and fitness. However, the precise impacts of resource quality and feeding behavior on insect life history traits are poorly understood. To explore and better understand the connections between feeding behavior, resource quality, and insect life history traits, we combined laboratory experiments with a recently proposed mechanistic model of insect growth and development for a larval herbivore, Manduca sexta. We ran feeding trials for 4th and 5th instar larvae across different diet types (two hostplants and artificial diet) and used these data to parameterize a joint model of age and mass at maturity that incorporates both insect feeding behavior and hormonal activity. We found that the estimated durations of both feeding and nonfeeding bouts were significantly shorter on low-quality than on high-quality diets. We then explored how well the fitted model predicted historical out-of-sample data on age and mass of M. sexta. We found that the model accurately described qualitative outcomes for the out-of-sample data, notably that a low-quality diet results in reduced mass and later age at maturity compared with high-quality diets. Our results clearly demonstrate the importance of diet quality on multiple components of insect feeding behavior (feeding and nonfeeding) and partially validate a joint model of insect life history. We discuss the implications of these findings with respect to insect herbivory and discuss ways in which our model could be improved or extended to other systems.

In search of ecological determinants of fungal infections: Asemi-field experiment with folivorous moths.

Natural enemies shape the fate of species at both ecological and evolutionary time scales. While the effects of predators, parasitoids, and viruses on insects are well documented, much less is known about the ecological and evolutionary role of entomopathogenic fungi. In particular, it is unclear to which extent may the spatiotemporal distribution patterns of these pathogens create selective pressures on ecological traits of herbivorous insects. In the present study, we reared three lepidopteran species in semi‐natural conditions in a European hemiboreal forest habitat. We studied the probability of the insects to die from fungal infection as a function of insect species, food plant, study site, (manipulated) condition of the larvae, and the phenological phase. The prevalence of entomopathogenic fungi remained low to moderate with the value consistently below 10% across the subsets of the data while as many as 23 fungal species, primarily belonging to the families Cordycipitaceae, Aspergillaceae, and Nectriaceae, were recorded. There were no major differences among the insect species in prevalence of the infections or in the structure of associated fungal assemblages. The family Cordycipitaceae, comprising mainly obligatory entomopathogens, dominated among the pathogens of pupae but not among the pathogens of larvae. Overall, there was evidence for a relatively weak impact of the studied ecological factors on the probability to be infected by a fungal pathogen; there were no effects of food plant, study site, or phenology which would be consistent over the study species and developmental stages of the insects. Nevertheless, when the prevalence of particular fungal taxa was studied, Akanthomyces muscarius was found infecting insects fed with leaves of only one of the food plant, Betula spp. Feeding on a particular plant taxon can thus have specific fitness costs. This demonstrates that fungus‐mediated effects on insect life history traits are possible and deserve attention.

Strong Fitness Costs of Fall Armyworm Resistance to Dual-Gene Bt Maize Are Magnified on Less-Suitable Host-Crop Cultivars

We examined the fitness costs of resistance to transgenic Cry1A.105+Cry2Ab2 Bacillus thuringiensis (Bt) maize in the fall armyworm, Spodoptera frugiperda, a globally invasive pest species. Using Bt-resistant and -susceptible populations of similar genetic backgrounds, we compared insect life-history traits and population growth rates on the foliage of Bt or non-Bt cultivars of maize, soybean, and cotton. We found that (i) the resistance alleles led to a major reduction in insect fitness on seven of the eight cultivars of three host crops studied; (ii) developmental time was the life-history trait that accounted for most of the fitness variation of the armyworm, and (iii) the magnitude of fitness reduction of the resistant individuals was stronger on cotton foliage, on which the insects did not pass the resistance alleles to subsequent generations. These results show that fall armyworm resistance to Cry1A.105+Cry2Ab2-expressing Bt maize comes with strong fitness costs, which were magnified on less-suitable host plants for the insects. Thus, natural selection may help maintain or even restore the insect susceptibility to the pesticidal proteins and perhaps is a significant factor helping manage fall armyworm resistance to Cry1A.105+Cry2Ab2 Bt maize. These findings indicate that fitness costs of insect resistance to multi-toxin Bt crops can be strong, and host plants or conditions that magnify the fitness differential of susceptible insects could be specifically deployed with refuge to improve resistance management to Bt crops.

Diversity and Functions of Yeast Communities Associated with Insects.

Following the concept of the holobiont, insect-microbiota interactions play an important role in insect biology. Many examples of host-associated microorganisms have been reported to drastically influence insect biological processes such as development, physiology, nutrition, survival, immunity, or even vector competence. While a huge number of studies on insect-associated microbiota have focused on bacteria, other microbial partners including fungi have been comparatively neglected. Yeasts, which establish mostly commensal or symbiotic relationships with their host, can dominate the mycobiota of certain insects. This review presents key advances and progress in the research field highlighting the diversity of yeast communities associated with insects, as well as their impact on insect life-history traits, immunity, and behavior.

Integration of transcriptome and proteome reveals molecular mechanisms underlying stress responses of the cutworm, Spodoptera litura, exposed to different levels of lead (Pb)

Heavy metals are major environmental pollutants that affect organisms across different trophic levels. Herbivorous insects play an important role in the bioaccumulation, and eventually, biomagnification of these metals. Although effects of heavy metal stress on insects have been well-studied, the molecular mechanisms underlying their effects remain poorly understood. Here, we used the RNA-Seq profiling and isobaric tags for relative and absolute quantitation (iTRAQ) approaches to unravel these mechanisms in the polyphagous pest Spodoptera litura exposed to lead (Pb) at two different concentrations (12.5 and 100 mg Pb/kg; PbL and PbH, respectively). Altogether, 1392 and 1630 differentially expressed genes (DEGs) and 58, 114 differentially expressed proteins (DEPs) were identified in larvae exposed to PbL and PbH, respectively. After exposed to PbL, the main up-regulated genes clusters and proteins in S. litura larvae were associated with their metabolic processes, including carbohydrate, protein, and lipid metabolism, but the levels of cytochrome P450 associated with the pathway of xenobiotic biodegradation and metabolism were found to be decreased. In contrast, the main up-regulated genes clusters and proteins in larvae exposed to PbH were enriched in the metabolism of xenobiotic by cytochrome P450, drug metabolism-cytochrome P450, and other drug metabolism enzymes, while the down-regulated genes and proteins were found to be closely related to the lipid (lipase) and protein (serine protease, trypsin) metabolism and growth processes (cuticular protein). These findings indicate that S. litura larvae exposed to PbL could enhance food digestion and absorption to prioritize for growth rather than detoxification, whereas S. litura larvae exposed to PbH reduced food digestion and absorption and channelized the limited energy for detoxification rather than growth. These contrasting results explain the dose-dependent effects of heavy metal stress on insect life-history traits, wherein low levels of heavy metal stress induce stimulation, while high levels of heavy metal stress cause inhibition at the transcriptome and proteome levels.

Patterns of Genetic Diversity and Mating Systems in a Mass-Reared Black Soldier Fly Colony.

The black soldier fly (BSF), Hermetia illucens, is a promising candidate for the emerging insect farming industry with favourable characteristics for both bioremediation and production of animal delivered nutritive and industrial compounds. The genetic management of commercial colonies will become increasingly important for the sustainability of the industry. However, r-selected life history traits of insects pose challenges to conventional animal husbandry and breeding approaches. In this study, the long-term genetic effects of mass-rearing were evaluated as well as mating systems in the species to establish factors that might influence genetic diversity, and by implication fitness and productivity in commercial colonies. Population genetic parameters, based on microsatellite markers, were estimated and compared amongst two temporal wild sampling populations and four generations (F28, F48, F52, and F62) of a mass-reared colony. Furthermore, genetic relationships amongst mate pairs were evaluated and parentage analysis was performed to determine the oc-currence of preferential mate choice and multiple paternity. The mass-reared colony showed a reduction in genetic diversity and evidence for inbreeding with significant successive generational genetic differentiation from the wild progenitor population. Population-level analysis also gave the first tentative evidence of positive assortative mating and genetic polyandry in BSF. The homoge-neity of the mass-reared colony seems to result from a dual action caused by small effective popu-lation size and increased homozygosity due to positive assortative mating. However, the high ge-netic diversity in the wild and a polyandrous mating system might suggest the possible restoration of diversity in mass-reared colonies through augmentation with the wild population.

Development time mediates the effect of larval diet on ageing and mating success of male antler flies in the wild.

High-quality developmental environments often improve individual performance into adulthood, but allocating toward early life traits, such as growth, development rate and reproduction, may lead to trade-offs with late-life performance. It is, therefore, uncertain how a rich developmental environment will affect the ageing process (senescence), particularly in wild insects. To investigate the effects of early life environmental quality on insect life-history traits, including senescence, we reared larval antler flies (Protopiophila litigata) on four diets of varying nutrient concentration, then recorded survival and mating success of adult males released in the wild. Declining diet quality was associated with slower development, but had no effect on other life-history traits once development time was accounted for. Fast-developing males were larger and lived longer, but experienced more rapid senescence in survival and lower average mating rate compared to slow developers. Ultimately, larval diet, development time and body size did not predict lifetime mating success. Thus, a rich environment led to a mixture of apparent benefits and costs, mediated by development time. Our results indicate that 'silver spoon' effects can be complex and that development time mediates the response of adult life-history traits to early life environmental quality.

Functional Analysis of Nuclear Factor Y in the Wing-Dimorphic Brown Planthopper, Nilaparvata lugens (Hemiptera: Delphacidae).

Nuclear factor Y (NF-Y) is a heterotrimeric transcription factor with the ability to bind to a CCAAT box in nearly all eukaryotes. However, the function of NF-Y in the life-history traits of insects is unclear. Here, we identified three NF-Y subunits, NlNF-YA, NlNF-YB, and NlNF-YC, in the wing-dimorphic brown planthopper (BPH), Nilaparvata lugens. Spatio-temporal analysis indicated that NlNF-YA, NlNF-YB, and NlNF-YC distributed extensively in various body parts of fourth-instar nymphs, and were highly expressed at the egg stage. RNA interference (RNAi)-mediated silencing showed that knockdown of NlNF-YA, NlNF-YB, or NlNF-YC in third-instar nymphs significantly extended the fifth-instar duration, and decreased nymph-adult molting rate. The addition of 20-hydroxyecdysone could specifically rescue the defect in adult molting caused by NlNF-YARNAi, indicating that NlNF-Y might modulate the ecdysone signaling pathway in the BPH. In addition, NlNF-YARNAi, NlNF-YBRNAi, or NlNF-YCRNAi led to small and moderately malformed forewings and hindwings, and impaired the normal assembly of indirect flight muscles. Adult BPHs treated with NlNF-YARNAi, NlNF-YBRNAi, or NlNF-YCRNAi produced fewer eggs, and eggs laid by these BPHs had arrested embryogenesis. These findings deepen our understanding of NF-Y function in hemipteran insects.

The combined effect of host and food availability on optimized parasitoid life-history traits based on a three-dimensional trade-off surface.

The reproductive success of many insects is considered to be limited by two main factors: the availability of mature eggs to lay (termed egg limitation) and the time to locate suitable hosts (termed time limitation). High host density in the environment is likely to enhance oviposition opportunities, thereby selecting for higher investment in egg supply. In contrast, a shortage of food (e.g. sugar sources) is likely to increase the risk of time limitation, thereby selecting for higher allocation to initial energy reserves. To our knowledge, the combined effect of host and food availability on these optimal life-history allocations has never been investigated. We thus modelled their simultaneous effects on a three-dimensional trade-off between initial investment in energy reserves, egg number and egg size, while focusing on insect parasitoids. The model was based on Monte Carlo simulations coupled with genetic algorithms, in order to identify the optimal life-history traits of a single simulated parasitoid female in an environment in which both hosts and food are present in varying densities. Our results reproduced the simple predictions described above. However, some novel predictions were also obtained, especially when specific interactions between the different factors were examined and their effects on the three-dimensional life-history surface were considered. The work sheds light on long-lasting debates regarding the relative importance of time versus egg limitation in determining insect life-history traits and highlights the complexity of life-history evolution, where several environmental factors act simultaneously on multiple traits.

The Need for Alternative Insect Protein in Africa

AbstractAs the global population is expected to reach 9 billion people by 2050, food production must increase by 60% to meet demand. Increasing agricultural commodities to meet this demand for food products exacerbates several issues of human concern, such as over-fertilization and natural resource depletion. Further, changes in diets due to uncertainty in local crop availability change our food forecast. We are, however, poised to overcome agriculture and nutrition challenges, and become food secure by 2030. One challenge is to produce protein in a cost-effective, sustainable manner, especially in sub-Saharan Africa. Protein is an essential key ingredient of livestock feeds, and is necessary for animal growth, body maintenance, and producing offspring. The use and optimization of farming insects for protein-rich livestock feed is a transformative area of agriculture-based research that will contribute to improved food security and meeting global sustainable developmental goals. The resulting need is to minimize the anthropogenic impacts through research-driven approaches that will improve sustainable agricultural practices. This need will be addressed with insects. Larvae of certain insects feed on decomposing organic matter and can reduce associated bacterial (including pathogens) populations. The resulting larvae can be dried, milled, and used as feed for livestock, including poultry and aquaculture. Optimizing insect life history traits and their associated microbes as novel feed for livestock is currently understudied, but has tremendous impact to increase agricultural sustainability, improve feed security, and be easily introduced into local food production chains in Africa.

Effects of cold exposure on Tenebrio molitor (Coleoptera: Tenebrionidae) pupal period, proportion of adult emergence, weight and deformation percentage

Low temperatures affect many life history traits of insects. The purpose of this study was to assess the effects of duration of cold exposure on the length of Tenebrio molitor’s pupal period, proportion of adult emergence, adult weight and proportion of deformed adults. The trials were conducted in a laboratory in constant conditions with wheat flour and whole-wheat flour meal were used as food. Cold-exposed groups were achieved by exposing pupae taken on the first day after pupation for 1, 3, 5, 10 and 15 days at +4 °C. The results showed that as the period of cold increased, pupal period extended and proportion of deformed adults increased whereas there were no or less straightforward effects on adult weights and proportion of emerged adults, respectively

Geographical Variations in Life Histories of House Flies, Musca domestica (Diptera: Muscidae), in Punjab, Pakistan.

Musca domestica Linnaeus is an important public health pest with the ability to adapt to diverse climates. Assessment of variations in biology and life-history traits of insects along geographical gradients is important for a successful management plan in different regions. We investigated life-history traits and life table parameters of M. domestica from six different geographical regions of Punjab, Pakistan: Rahim Yar Khan (RYK), Bahawalpur (BWP), Multan (MTN), Lahore (LHR), Gujrat (GJT), and Murree (MRE). Overall, M. domestica from localities of lower latitude and elevations with higher mean temperatures completed their development faster than those from localities of higher latitude and elevations with lower mean temperatures. The immature developmental time was the longest for the MRE population that was collected from higher latitude and elevation with cooler climate, whereas the shortest for the RYK population from lower latitude with warmer climate. Pupal weights were heavier for the RYK, BWP, and MTN populations, all were from the lowest latitude and elevations with warmer climate, compared with rest of the field populations. Similarly, rate of adult eclosion, fecundity, egg hatching, longevity, and life table parameters such as intrinsic rate of population increase, mean relative growth rate, net reproductive rate, and biotic potential were significantly higher for the RYK, BWP, and MTN populations compared with the GJT, LHR, and MRE populations of M. domestica. The current results will probably be of importance when planning management of M. domestica in different geographical regions of Pakistan.

Impact of irradiation on the reproductive traits of field and laboratory An. arabiensis mosquitoes

BackgroundThe sterile insect technique (SIT) aims at suppressing or decreasing insect pest populations by introducing sterile insects into wild populations. SIT requires the mass-production of insects and their sterilization through, for example, radiation. However, both mass-rearing and radiation can affect the life history traits of insects making them less competitive than their wild counterparts. In the malaria mosquito Anopheles arabiensis, some progress has been made to improve the mating competitiveness of mass-reared irradiated males. However, to date, no study has explored the relative effects of colonization and irradiation on important reproductive traits in this species. Such data may help to focus research efforts more precisely to improve current techniques.MethodsTwo strains of An. arabiensis originating from the same locality were used: one reared in the laboratory for five generations and the second collected as late larval instars in the field prior to experimentation. Pupae were irradiated with 95 Gy and some adult reproductive traits, including insemination rate, fecundity, oviposition behavior, fertility and male survivorship, were assessed in different mating combinations.ResultsOur study revealed the different effects of mosquito strain and irradiation on reproductive processes. The insemination rate was higher in field (67.3%) than in laboratory (54.9%) females and was negatively affected by both female and male irradiation (un-irradiated vs irradiated: 70.2 vs 51.3% in females; 67.7 vs 53.7% in males). Irradiated females did not produce eggs and egg prevalence was lower in the field strain (75.4%) than in the laboratory strain (83.9%). The hatching rate was higher in the field strain (88.7%) than in the laboratory strain (70.6%) as well as in un-irradiated mosquitoes (96.5%) than in irradiated ones (49%). Larval viability was higher in the field strain (96.2%) than in the laboratory strain (78.5%) and in un-irradiated mosquitoes (97.6%) than irradiated ones (52%). Finally, field males lived longer than laboratory males (25.1 vs 20.5 days, respectively).ConclusionsOur results revealed that both irradiation and colonization alter reproductive traits. However, different developmental stages are not equally affected. It is necessary to consider as many fitness traits as possible to evaluate the efficacy of the sterile insect technique.

Differential life-history responses in Neolema abbreviata, a biological control agent for Tradescantia fluminensis under water and nitrogen gradients

Due to their sessile nature, plants inexorably endure both transient and chronic environmental stressors with subsequent impact on their interactions with other trophic groups. Water and nitrogen variability are key abiotic factors mediating the interaction between plants and insect herbivores. For insects used in classical biological control of plants, such factors may determine extent of herbivory, population dynamics and ultimately field establishment and persistence. We conducted a full factorial experiment to determine the impact of water and fertiliser variability on the performance of Neolema abbreviata (Lacordaire) (Coleoptera: Chrysomelidae) and its host plant Tradescantia fluminensis Vell (Commelinaceae). Water and fertiliser variability strongly influenced T. fluminensis biomass accumulation and foliar nitrogen content. These changes in host plant quality resulted in preferential oviposition selection by the beetles with optimally irrigated plants that received moderate fertilizer being most favoured. Larval performance, in terms of weight gain and time to pupation, was highest in this treatment providing support for the preference–performance hypothesis. There were similarities in feeding performance and subsequent performance among plant treatments between larvae and adults suggesting uniform nutritional quality requirements between the life-stages. Overall, our study reveals differential effects of paired abiotic stressors and performance among herbivorous insects with both juvenile and adult feeding life-stages. The study, therefore, calls for multi-trait and factor evaluation to understand the impact of plant quality on insect life history traits. Our findings also have direct implications on mass-rearing protocols of N. abbreviata and its potential field performance.

Loss of coevolved basal and plastic responses to temperature may underlie trophic level host-parasitoid interactions under global change

Climate change has complex impacts on insect life history traits, biogeography, survival, population dynamics and host-parasitoid seasonal phenologies that affect their synchrony. This is more pronounced in tropical southern Africa, where global warming may be relatively high. While some work on Plutella xylostella (L.) temperature tolerance has been reported, none focussed on African field insect populations and implications of trophic level interaction on biological control. To determine how climate change may affect coevolved trophic level interactions in a host-parasitoid trophic system, we compared the basal thermal tolerance of wild F1 populations of the parasitoid Cotesia vestalis (Haliday) to two life stages of herbivorous host P. xylostella (L.), a global economic pest of brassicas. Our results showed significantly lower C. vestalis critical thermal limits (P < .001) compared to the host. Similarly, the parasitoid heat knockdown time and chill comma recovery time tolerance were significantly lower than that of the host (P < .001). Lethal temperature assays revealed significantly lower survival rates (P < .001) for C. vestalis at both upper and lower temperature scales. Observed differential basal temperature responses may offset coevolved host-parasitoid synchrony through differences in host-parasitoid phenologies. Hence, future biological control programmes should aim at ‘creating’ resilience and climate-fitness in natural enemies through enhancing evolutionary potential to buffer them from the changing climate. This is the first report detailing trophic level thermal tolerance of P. xylostella and larval parasitoid, C. vestalis, focussing on ecological service implications of the field-wild parasitoid populations on the herbivore host.