Soil-dwelling Insects Research Articles

Perbanyakan Nematoda Patogenik Serangga (Rhabditida: Steinernema dan Heterorhabditis) pada Media In Vitro Cair Statik

<p>Entomopathogenic nematodes belonging<br />to genera Steinernema and Heterorhabditis are potentially<br />most effective and safe biological control agents for insect<br />pests, especially for soil dwelling insects and those living in<br />cryptic habitats. Field application of the nematodes is still<br />hampered by supply of large number of infective juvenile<br />(IJ) nematodes. Five published in vitro media along with its<br />two modifications were tested for mass propagations of two<br />indigenous nematodes (H. indicus PLR2 and Steinernema<br />T96) and one commercial strain (S. carpocapsae #25).<br />Varying levels of IJ yields were observed across the<br />replications and experiments. Medium F that contained 1.0%<br />yeast extract, 2.5% egg yolk, and 4.0% soy oil yielded the<br />highest IJ numbers of H. indicus PLR2 (1.5×105 IJ ml-1) and<br />of S. carpocapsae #25 (2.9×105 IJ ml-1), whereas the widely<br />used medium B, which is based on homogenized chicken<br />offal (40%), yielded the highest number of Steinernema T96<br />(5.8×104 IJ ml-1). The IJ’s quality, as measured by their<br />morphometrics and pathogenicities, were generally<br />impaired, indicating the lack of essential nutrient(s) in the<br />media. Optimization of the propagation media is therefore<br />still needed to increase IJ’s quantity and quality to achieve<br />the required standard for commercial scale of artificial<br />propagation.</p>

Complete mitochondrial genomes of four entomopathogenic nematode species of the genus Steinernema.

BackgroundNematodes belonging to the genus Steinernema are insect parasites and are used as effective biological agents against soil-dwelling insect pests. Although the full nuclear genomes of multiple Steinernema species have become available recently, mitochondrial genome information for the genus is limited. In this study, we sequenced the complete mitochondrial genomes of four species of Steinernema and analysed their structure, codon usage and phylogenetic relationships.ResultsMitochondrial genomes of Steinernema carpocapsae, S. glaseri, S. kushidai and S. litorale comprised 13,924, 13,851, 15,182 and 21,403 bp, respectively, with highly AT-rich nucleotide contents (AT ratio of 71.05–76.76 %). All the expected genes, including 12 protein-coding genes (encoding ATP6, CYTB, COX1-3, ND1-6 and ND4L), two rRNA genes and 22 tRNA genes were identified in the four genomes. Phylogenetic analyses based on the amino acid sequences of the 12 protein-coding genes identified the Steinernema species as monophyletic, representing a sister clade of Rhabditina and Ascaridida. In addition, they were more closely positioned to other Clade 10 nematodes, including Bursaphelenchus xylophilus, Aphelenchoides besseyi and Panagrellus redivivus, than to Strongyloides species. Gene arrangements and codon usage analyses supported this relationship. Mitochondrial genome comparison of two distinct strains of S. carpocapsae detected high intra-specific diversity.ConclusionsThe mitochondrial genomes of four species of Steinernema determined in this study revealed inter- and intra-species divergences/diversities of mitochondrial genomes in this genus. This information provides useful insights into the phylogenetic position of the genus Steinernema within the Nematoda and represents a useful resource for selecting molecular markers for diagnosis and population studies. These data will increase our understanding of the interesting biology of insect parasites.Electronic supplementary materialThe online version of this article (doi:10.1186/s13071-016-1730-z) contains supplementary material, which is available to authorized users.

Impact of Bt-cotton on soil microbiological and biochemical attributes

Transgenic Bt-cotton produces Bt-toxins (Cry proteins) which may accumulate and persist in soil due to their binding ability on soil components. In the present study, the potential impacts of Bt- and non-Bt genotypes of cotton on soil microbial activity, substrate use efficiency, viable microbial population counts, and nutrient dynamics were studied. Two transgenic Bt-cotton genotypes (CIM-602 CIM-599) expressing cry1 Ac gene and two non-Bt cotton genotypes (CIM-573 and CIM-591) were used to evaluate their impact on biological and chemical properties of soil across the four locations in Punjab. Field trials were conducted at four locations (Central Cotton Research Institute-Multan, Naseer Pur, Kot Lal Shah, and Cotton Research Station-Bahawalpur) of different agro-ecological zones of Punjab. Rhizosphere soil samples were collected by following standard procedure from these selected locations. Results reveled that Bt-cotton had no adverse effect on microbial population (viable counts) and enzymatic activity of rhizosphere soil. Bacterial population was more in Bt-cotton rhizosphere than that of non-Bt cotton rhizosphere at all locations. Phosphatase, dehydrogenase, and oxidative metabolism of rhizosphere soil were more in Bt-cotton genotypes compared with non-Bt cotton genotypes. Cation exchange capacity, total nitrogen, extractable phosphorous, extractable potassium, active carbon, Fe and Zn contents were higher in rhizosphere of Bt-cotton genotypes compared with non-Bt cotton genotypes. It can be concluded from present study that the cultivation of Bt-cotton expressing cry1 Ac had apparently no negative effect on metabolic, microbiological activities, and nutrient dynamics of soils. Further work is needed to investigate the potential impacts of Bt-cotton on ecology of soil-dwelling insects and invertebrates before its recommendation for extensive cultivation.

Use of habitat odour by host‐seeking insects

Locating suitable feeding or oviposition sites is essential for insect survival. Understanding how insects achieve this is crucial, not only for understanding the ecology and evolution of insect-host interactions, but also for the development of sustainable pest-control strategies that exploit insects' host-seeking behaviours. Volatile chemical cues are used by foraging insects to locate and recognise potential hosts but in nature these resources usually are patchily distributed, making chance encounters with host odour plumes rare over distances greater than tens of metres. The majority of studies on insect host-seeking have focussed on short-range orientation to easily detectable cues and it is only recently that we have begun to understand how insects overcome this challenge. Recent advances show that insects from a wide range of feeding guilds make use of 'habitat cues', volatile chemical cues released over a relatively large area that indicate a locale where more specific host cues are most likely to be found. Habitat cues differ from host cues in that they tend to be released in larger quantities, are more easily detectable over longer distances, and may lack specificity, yet provide an effective way for insects to maximise their chances of subsequently encountering specific host cues. This review brings together recent advances in this area, discussing key examples and similarities in strategies used by haematophagous insects, soil-dwelling insects and insects that forage around plants. We also propose and provide evidence for a new theory that general and non-host plant volatiles can be used by foraging herbivores to locate patches of vegetation at a distance in the absence of more specific host cues, explaining some of the many discrepancies between laboratory and field trials that attempt to make use of plant-derived repellents for controlling insect pests.

A below-ground herbivore shapes root defensive chemistry in natural plant populations.

Plants display extensive intraspecific variation in secondary metabolites. However, the selective forces shaping this diversity remain often unknown, especially below ground. Using Taraxacum officinale and its major native insect root herbivore Melolontha melolontha, we tested whether below-ground herbivores drive intraspecific variation in root secondary metabolites. We found that high M. melolontha infestation levels over recent decades are associated with high concentrations of major root latex secondary metabolites across 21 central European T. officinale field populations. By cultivating offspring of these populations, we show that both heritable variation and phenotypic plasticity contribute to the observed differences. Furthermore, we demonstrate that the production of the sesquiterpene lactone taraxinic acid β-D-glucopyranosyl ester (TA-G) is costly in the absence, but beneficial in the presence of M. melolontha, resulting in divergent selection of TA-G. Our results highlight the role of soil-dwelling insects for the evolution of plant defences in nature.

Foraging wireworms are attracted to root-produced volatile aldehydes

Soil-dwelling insects are known to react to chemical cues they encounter in the rhizosphere. Whether wireworms (Coleoptera, Elateridae) use root-emitted volatile organic chemicals to localize their host plant remains, however, poorly understood. Here, we aimed at identifying chemical cues released by barley roots that attract Agriotes sordidus. In a first behavioral experiment, we assessed the ability of wireworms to orient towards live barley roots, using dual-choice olfactometers suitable for belowground insects. Then, we collected the volatile organic compounds (VOC) produced by barley roots using a dynamic head-space sampling approach. VOC were quantified and identified using gas chromatography—mass spectrometry (GC–MS). The odorant blend is composed of four aldehydes, namely hexanal, (E)-hex-2-enal, (E)-non-2-enal, and (E,Z)-nona-2,6-dienal. In a second set of dual-choice bioassays, wireworms were attracted towards a synthetic blend of these four major compounds. However, the synthetic blend was not as attractive as live roots, which is partially explained by the absence of CO2, commonly known as a strong attractant for soil-dwelling insects. While CO2 indicates the presence of living material in the vicinity, we hypothesize that additional VOC inform about the plant suitability. A better understanding of these belowground signals would contribute to the development of new integrated control strategies against wireworms.

Unraveling the intraguild competition between Oscheius spp. nematodes and entomopathogenic nematodes: Implications for their natural distribution in Swiss agricultural soils

Entomopathogenic nematodes (EPN) are excellent biological control agents to fight soil-dwelling insect pests. In a previous survey of agricultural soils of Switzerland, we found mixtures of free-living nematodes (FLN) in the genus Oscheius, which appeared to be in intense competition with EPN. As this may have important implications for the long-term persistence of EPN, we studied this intraguild competition in detail. We hypothesized that (i) Oscheius spp. isolates act as scavengers rather than entomopathogens, and (ii) cadavers with relatively small numbers of EPN are highly suitable resources for Oscheius spp. reproduction. To study this, we identified Oscheius spp. isolated from Swiss soils, quantified the outcome of EPN/Oscheius competition in laboratory experiments, developed species-specific primers and probe for quantitative real-time PCR, and evaluated their relative occurrence in the field in the context of the soil food web. Molecular analysis (ITS/D2D3) identified MG-67/MG-69 as Oscheius onirici and MG-68 as O. tipulae (Dolichura-group). Oscheius spp. indeed behaved as scavengers, reproducing in ∼64% of frozen-killed cadavers from controlled experiments. Mixed infection in the laboratory by Oscheius spp. with low (3 IJs) or high (20 IJs) initial EPN numbers revealed simultaneous reproduction in double-exposed cadavers which resulted in a substantial reduction in the number of EPN progeny from the cadaver. This effect depended on the number of EPN in the initial inoculum and differed by EPN species; Heterorhabditis megidis was better at overcoming competition. This study reveals Oscheius spp. as facultative kleptoparasites that compete with EPN for insect cadavers. Using real-time qPCR, we were able to accurately quantify this strong competition between FLN and EPN in cadavers that were recovered after soil baiting (∼86% cadavers with >50% FLN production). The severe competition within the host cadavers and the intense management of the soils in annual crops readily explain the low EPN numbers in Swiss field samples. The developed molecular tools can be used to elucidate the extent to which the competitive interactions affect EPN populations. This can help to develop strategies to achieve good persistence and natural EPN recycling, in particular in systems where native EPN levels are low, such as annual crops.

Soil substrates affect responses of root feeding larvae to their hosts at multiple levels: Orientation, locomotion and feeding

The role of soil characteristics for cue-directed behaviour of rhizophagous insects remains largely unexplored to date. Here, we studied behavioural responses of polyphagous cockchafer larvae (Melolontha melolontha; Scarabaeidae) to roots of one of its preferred host plants, dandelion (Taraxacum sectio ruderalia; Asteraceae). Plants were grown in substrates with different adsorptive capacities, i.e. in vermiculite or sand. A behavioural assay was developed which allowed both monitoring the release of attractive CO2 from roots and observation of larval behaviour at the same time. In sand, larvae got closer to the roots than in vermiculite where they eventually stopped orienting towards the roots: However, their total locomotory activity and the number of turning events/stationary points were higher in vermiculite. Larval behaviour was not correlated with CO2 concentrations in both vermiculite and sand. On average, larvae kept a greater distance to vermiculite-grown roots. However, a feeding bioassay revealed that the larvae consumed more of vermiculite-grown roots than of sand-grown ones. This result showed that the weaker orientation of larvae towards vermiculite-grown roots was not due to the lower palatability of these roots. Vermiculite might adsorb foraging-relevant cues of dandelion roots, alter composition of root exudates, and/or physically impair oriented movements of soil-dwelling insects. Our study shows that the type of substrate affects responses of rhizophagous larvae to host roots at multiple levels independently of root-derived CO2 gradients.

Highly Potent Extracts from Pea (Pisum sativum) and Maize (Zea mays) Roots Can Be Used to Induce Quiescence in Entomopathogenic Nematodes.

Root exudates can play an important role in plant-nematode interactions. Recent studies have shown that the root cap exudates obtained from several plant species trigger a state of dormancy or quiescence in various genera of nematodes. This phenomenon is not only of fundamental ecological interest, but also has application potential if the plant-produced compound(s) could be used to control harmful nematodes or help to prolong the shelf-life of beneficial entomopathogenic nematodes (EPNs). The identification of the compound(s) involved in quiescence induction has proven to be a major challenge and requires large amounts of active material. Here, we present a high-throughput method to obtain bioactive root extracts from flash-frozen root caps of green pea and maize. The root cap extract obtained via this method was considerably more potent in inducing quiescence than exudate obtained by a previously used method, and consistently induced quiescence in the EPN Heterorhabditis megidis, even after a 30-fold dilution. Extracts obtained from the rest of the root were equally effective in inducing quiescence. Infective juveniles (IJs) of H. megidis exposed to these extracts readily recovered from their quiescent state as soon as they were placed in moist soil, and they were at least as infectious as the IJs that had been stored in water. Excessive exposure of IJs to air interfered with the triggering of quiescence. The implications of these results and the next steps towards identification of the quiescence-inducing compound(s) are discussed from the perspective of applying EPN against soil-dwelling insect pests.

Effects of Neonicotinoids and Crop Rotation for Managing Wireworms in Wheat Crops.

Soil-dwelling insects are severe pests in many agroecosystems. These pests have cryptic life cycles, making sampling difficult and damage hard to anticipate. The management of soil insects is therefore often based on preventative insecticides applied at planting or cultural practices. Wireworms, the subterranean larvae of click beetles (Coleoptera: Elateridae), have re-emerged as problematic pests in cereal crops in the Pacific Northwestern United States. Here, we evaluated two management strategies for wireworms in long-term field experiments: 1) treating spring wheat seed with the neonicotinoid thiamethoxam and 2) replacing continuous spring wheat with a summer fallow and winter wheat rotation. Separate experiments were conducted for two wireworm species--Limonius californicus (Mannerheim) and Limonius infuscatus (Motschulsky). In the experiment with L. californicus, spring wheat yields and economic returns increased by 24-30% with neonicotinoid treatments. In contrast, in the experiment with L. infuscatus, spring wheat yields and economic returns did not increase with neonicotinoids despite an 80% reduction in wireworms. Thus, the usefulness of seed-applied neonicotinoids differed based on the wireworm species present. In experiments with both species, we detected significantly fewer wireworms with a no-till summer fallow and winter wheat rotation compared with continuous spring wheat. This suggests that switching from continuous spring wheat to a winter wheat and summer fallow rotation may aid in wireworm management. More generally, our results show that integrated management of soil-dwelling pests such as wireworms may require both preventative insecticide treatments and cultural practices.

Persistence of Beauveria bassiana sensu lato and Metarhizium anisopliae sensu lato in Vermont (USA) forest soil

Persistence of granular formulations of Beauveria bassiana sensu lato, strains GHA726 and ERL1170, and Metarhizium anisopliae sensu lato, strains ERL824 and ERL701, applied to forest soil was documented. One formulation was prepared by producing fungi on millet and at application adding cooked, dried millet to provide additional nutrients for fungal growth. The other was made by producing fungi on corn starch ‘packing peanuts’, a biodegradable packaging material, adding additional packing peanuts as a nutrient base. The trial was conducted in three sites in a forest predominating in sugar maple (Acer saccharum) in Underhill, VT, USA. In each site, 33 dominant sugar maples were randomly selected. A 1-m2 plot located 1 m from the bole of the tree was established. Soil samples were collected before application, and 1 h, 2, 4, 8, 12 and 16 wk post-application. Fungal levels were estimated based on number of colony-forming units (CFUs). A significant increase in CFUs was observed 2 wk post-application in plots treated with isolates GHA726, ERL1170 and ERL824 as a result of fungal reproduction on nutrients in the formulations. Evidence of greater persistence for the B. bassiana than the M. anisopliae isolates was observed, based on CFU levels over time. Isolates formulated with millet proliferated more than those on packing peanuts. Use of entomopathogenic fungi in forest soil has potential for managing soil-dwelling insect pests such as pear thrips, Taeniothrips inconsequens, and the black vine weevil, Otiorhynchus sulcatus, though the type of formulation and the specific isolate used must be selected carefully.

The Population Dynamics of Insects in Sugarcane Field at Aundipatti, Theni District, Tamil Nadu

Sugarcane (Saccharum offecinarum L) is one of the most important commercial crops of the tropical countries and is the main source of sugar in the world. Sugarcane crop is affected by more than 60 insects of which about 10 insects are rather more important as far as the yield loss is concerned. Among the major insects damaging sugarcane, borers, sucking pests, soil dwelling insects are more prevalent in Tamil Nadu and adjoining states which account for 20% loss in cane yield and 15% loss in sugar yield. About 200 species of insects have been reported as damaging sugarcane crop in India. Light trap serves as an important survey tool to sample population of nocturnal insects effectively. Trapping methods are principal tools in insect pest management programs. The goal of this study was to assess population dynamics of insects in a sugarcane field at Aundipatti, Tamil Nadu. A 6 month study was conducted to sample insects using light trap at the farmer’s field from July 2011 to December 2011. Collected specimens were identified to species level and predominant species were identified. Predominant herbivore species were Diatraea saccharalis, Melanotus communis and Agrostis capillaries L. The result of this study provides important information related to the status of insects in sugarcane field.

Traditional and molecular detection methods reveal intense interguild competition and other multitrophic interactions associated with native entomopathogenic nematodes in Swiss tillage soils

Background and aims As part of a research consortium that explores ways to improve soil health, we study how entomopathogenic nematodes (EPNs) can be better exploited for the biological control of soil-dwelling insect pests in annual crops. Methods We evaluated how tillage might affect belowground interactions in two 30-year running Swiss field trials by combining traditional (insect bait) and molecular (novel real-time qPCR protocols) methods. Soil samples (April and October 2013) were evaluated for the presence and activity of EPN soil food web assemblage comprising 13 EPN species, six nematophagous fungi, one ectoparasitic bacterium, and the free-living nematodes (FLN) of the Acrobeloides group. Results Mortality of sentinel larvae, as well as qPCR analyses (for which we provide seven new primers/ probes sets) found only trace levels of six EPN species, dominated by heterorhabditids species. Analysis of nematode progeny revealed that EPN compete intensely with FLN for insect cadavers. Overall, it appears that temperate annual cropping systems provide poor environments for EPN and that tillage does not negatively affect the natural occurrence of EPN. Conclusions Natural occurrence of EPN in Swiss tillage soils was very low, and augmentation may be a promising strategy to improve the control of root pests of annual crops.

An insect pupal cell with antimicrobial properties that suppress an entomopathogenic fungus

Soil-dwelling insects have developed various mechanisms to defend against pathogen infection. The pecan weevil, Curculio caryae, spends two to three years in the soil inside an earthen cell. We hypothesized that the cell may possess antimicrobial properties. In a laboratory study, we tested the hypothesis using the fungus Beauveria bassiana as a model. B. bassiana is a common endemic pathogen of C. caryae. We compared the number of colony-forming-units on selective media when B. bassiana was exposed to autoclaved soil, non-autoclaved soil, or soil from a C. caryae pupal cell. Soil from C. caryae cells was suppressive to B. bassiana. To our knowledge this is the first report of antimicrobial properties associated with an insect soil cell. The findings expand our knowledge of host–pathogen relationships. Additional research is needed to determine the basis for the suppressive effects observed.

BIOEFFICACY OF TWO ENTOMOPATHGENIC NEMATODES AGAINST SPODOPTERALITTORALIS BOISDUVAL (LEPIDOPTERA) AND TEMNORHYNCHUS BAAL REICHE(COLEOPTERA) LARVAE

ABSTRACT Efficacy of the two entomopathogenic nematode species, Heterorhabditis bacteriophora Poinar (HP88 strain) and Steirnema glaseri Steiner (NJ strain), was tested on the fifth and third instar larvaeof Spodoptera littoralis (Boisd.) and Temnorhynchus baal (Reiche) under laboratory conditions. Experiments were conducted on filter paper and sandy soil substrates using nematode at 50, 100, 200and 400 infective juveniles/5ml of water. Heterorhabditis bacteriophora was most effective on thefifth instar larvae of S. littoralis whereas S. glaseri was effective against third instar larvae of T. ball third. Insect mortality was high (60-90%) and low (<45%) at higher and lower nematodeconcentrations respectively. Heterorhabditis bacteriophora treated larvae of S. littoralis succumbedto the infection at the higher rates (80-100%) as compared to those treated with S. glaseri. The rate of mortality of S. littoralis was lowest when treated with S. glaseri (2-20%). The differences in the rateof nematode infection and insect mortality under various experimental conditions are attributed tothe difference in the behavior, virulence, rate of penetration and host searching abilities of nematodes and the abilities of insect pest larvae to resist nematode penetration. Present study suggests that entomopathogenic nematodes are important and effective biological control agents of most soil dwelling insect pests.

Enhanced alginate capsule properties as a formulation of entomopathogenic nematodes

Despite many advantages of employing entomopathogenic nematodes (EPNs) as biocontrol agents against soil-dwelling insect pests, their use is still limited to home gardening or small-scale farming. Lack of cost-effective, durable formulations, as well as appropriate application methods have so far prevented the use of EPN in large-scale farming. A recent report on alginate capsules as a novel EPN formulation for application into the soil shows promise. However, EPNs often readily escaped from these capsules within a few days especially when they are not refrigerated. To improve EPN retainment, we adjusted capsule properties by changing the reaction temperature for capsule formation and by adopting post-treatment of alginate capsules with excessive Ca2+, and then evaluated EPN escape from the capsules manufactured under different conditions. While capsules with the hardest capsule shell were produced at 4 °C with the Ca2+ post-treatment, the 40 % enhanced EPN retainment was achieved only by lowering the reaction temperature.

Chemically mediated group formation in soil-dwelling larvae and pupae of the beetle Trypoxylus dichotomus.

Many insects form groups through interactions among individuals, and these are often mediated by chemical, acoustic, or visual cues and signals. In spite of the diversity of soil-dwelling insects, their aggregation behaviour has not been examined as extensively as that of aboveground species. We investigated the aggregation mechanisms of larvae of the Japanese rhinoceros beetle Trypoxylus dichotomus, which live in groups in humus soil. In two-choice laboratory tests, 2nd- and 3rd-instar larvae gathered at conspecific larvae irrespective of the kinship. The ablation of maxillae, which bear chemosensilla, abolished aggregation behaviour. Intact larvae also exhibited aggregation behaviour towards a larval homogenate. These results suggest that larval aggregation is mediated by chemical cues. We also demonstrated that the mature larvae of T. dichotomus built their pupal cells close to a mesh bag containing a conspecific pupal cell, which indicated that larvae utilize chemical cues emanating from these cells to select the pupation site. Thus, the larvae of T. dichotomus may use chemical cues from the conspecifics in two different contexts, i.e. larval aggregation and pupation site selection. Using conspecific cues, larvae may be able to choose suitable locations for foraging or building pupal cells. The results of the present study highlight the importance of chemical information in belowground ecology.

Genetic selection of the ambush foraging entomopathogenic nematode, Steinernema carpocapsae for enhanced dispersal and its associated trade-offs

Dispersal of organisms is influenced by environmental and innate population variability. It results in redistribution of populations with potential consequences for gene flow, population resilience and stability, and evolutionary diversification of traits in response to specific selection pressures. However, dispersal behavior in soil-dwelling organisms is understudied. Species of entomopathogenic nematodes, a group of soil-inhabiting lethal insect parasites used in biological pest control show a dichotomy in foraging behavior. Some species have been classified as ambushers while others as cruisers. We previously discovered that the ambush foraging Steinernema carpocapsae possesses a small group of sprinters that disperse faster than the fastest moving cruisers. In this study, we genetically selected S. carpocapsae for enhanced dispersal in the absence of hosts by capturing the fastest and farthest reaching infective juveniles (IJs) emanating from a nematode-infected Galleria mellonella cadaver, in soil. S. carpocapsae showed positive response to selection for dispersal with 13–23 and 21–37 fold increase in the percent IJs dispersing to the farthest distance from the source cadaver, after five and ten rounds of selection, respectively. There was also a significant increase in the average displacement of the selected lines (6.85–7.54 cm/day) than the foundation population (5.54 cm/day) maintained by passing through G. mellonella larvae in Petri dishes. The overall mean realized heritability for dispersal was 0.60. The farthest reaching IJs of the selected lines comprised more males (72 %) than the foundation population (44 %) at most time points. Trade-offs associated with enhanced dispersal included reduced reproduction capacity and nictation ability, a trait associated with ambush foraging. In conclusion, this study revealed the costs and benefits associated with selection for enhanced dispersal in a soil-dwelling insect parasite, enhancing our understanding of the evolution of new behavioral patterns, which could have important implications in biological control.

Root damage by insects reverses the effects of elevated atmospheric CO2 on Eucalypt seedlings.

Predicted increases in atmospheric carbon dioxide (CO2) are widely anticipated to increase biomass accumulation by accelerating rates of photosynthesis in many plant taxa. Little, however, is known about how soil-borne plant antagonists might modify the effects of elevated CO2 (eCO2), with root-feeding insects being particularly understudied. Root damage by insects often reduces rates of photosynthesis by disrupting root function and imposing water deficits. These insects therefore have considerable potential for modifying plant responses to eCO2. We investigated how root damage by a soil-dwelling insect (Xylotrupes gideon australicus) modified the responses of Eucalyptus globulus to eCO2. eCO2 increased plant height when E. globulus were 14 weeks old and continued to do so at an accelerated rate compared to those grown at ambient CO2 (aCO2). Plants exposed to root-damaging insects showed a rapid decline in growth rates thereafter. In eCO2, shoot and root biomass increased by 46 and 35%, respectively, in insect-free plants but these effects were arrested when soil-dwelling insects were present so that plants were the same size as those grown at aCO2. Specific leaf mass increased by 29% under eCO2, but at eCO2 root damage caused it to decline by 16%, similar to values seen in plants at aCO2 without root damage. Leaf C:N ratio increased by >30% at eCO2 as a consequence of declining leaf N concentrations, but this change was also moderated by soil insects. Soil insects also reduced leaf water content by 9% at eCO2, which potentially arose through impaired water uptake by the roots. We hypothesise that this may have impaired photosynthetic activity to the extent that observed plant responses to eCO2 no longer occurred. In conclusion, soil-dwelling insects could modify plant responses to eCO2 predicted by climate change plant growth models.

Effects of releasing a generalist predator (Acari: Gaeolaelaps aculeifer) on a subterranean insect herbivore (Coleoptera: Diabrotica virgifera virgifera)

Enhancing biological control of soil-dwelling insect pests is desirable, but due to the nature of the subterranean environment, it is challenging to elucidate interactions between pests and their natural enemies. Western corn rootworms (Diabrotica virgifera virgifera) are a major pest of maize, and the larvae can cause substantial damage to maize roots. Research has shown that numerous arthropods feed on rootworm immatures, and in some cases enhancing predator densities has led to rootworm suppression. We conducted a 2-yr field experiment and examined effects of releasing a predatory mite (Gaeolaelaps aculeifer) on densities of pest rootworms, root damage ratings, and plant growth parameters in conjunction with an insecticide control. In contrast to our expectations, larval rootworm incidence and root damage was generally higher in plots where mites were released. Differences in pest density and root damage did not translate into negative effects on plant biomass or grain yield at the end of the season, likely indicating that environmental conditions were favorable for plant compensation after rootworm damage. We also explored how rootworm and mite presence impacted densities of other soil invertebrates. Rootworm infestation had positive impacts on densities of carabid beetles and acarid mites during the time when rootworm eggs were hatching. Addition of predatory mites had both negative and positive effects on densities of soil invertebrates, with the latter potentially due to disruption of naturally-occurring biocontrol. This research indicates that the soil invertebrate community can impact densities of rootworm larvae and subsequent root damage.