Carbon dioxide receptor genes and their expression profile in Diabrotica virgifera virgifera

Plant-associated CO2 mediates long-distance host location and foraging behaviour of a root herbivore

Insect herbivores use different cues to locate host plants. The importance of CO₂ in this context is not well understood. We manipulated CO₂ perception in western corn rootworm (WCR) larvae through RNAi and studied how CO₂ perception impacts their interaction with their host plant. The expression of a carbon dioxide receptor, DvvGr2, is specifically required for dose-dependent larval responses to CO₂. Silencing CO₂ perception or scrubbing plant-associated CO₂ has no effect on the ability of WCR larvae to locate host plants at short distances (<9 cm), but impairs host location at greater distances. WCR larvae preferentially orient and prefer plants that grow in well-fertilized soils compared to plants that grow in nutrient-poor soils, a behaviour that has direct consequences for larval growth and depends on the ability of the larvae to perceive root-emitted CO₂. This study unravels how CO₂ can mediate plant–herbivore interactions by serving as a distance-dependent host location cue.

Living deep in the ground and surrounded by darkness, soil insects must rely on the chemicals released by plants to find the roots they feed on. Carbon dioxide, for example, is a by-product of plant respiration, which, above ground, is thought to attract moths to flowers and flies to apples; underground, however, its role is still unclear. This gaseous compound can travel through soil and potentially act as a compass for root-eating insects. Yet, it is also produced by decaying plants or animals, which are not edible. It is therefore possible that insects use this signal as a long-range cue to orient themselves, but then switch to another chemical when closer to their target to narrow in on an actual food source.

To test this idea, Arce et al. investigated whether carbon dioxide guides the larvae of Western corn rootworm to maize roots. First, the rootworm genes responsible for sensing carbon dioxide were identified and switched off, making the larvae unable to detect this gas. When the genetically engineered rootworms were further than 9cm from maize roots, they were less able to locate that food source; closer to the roots, however, the insects could orient themselves towards the plant. This suggests that the insects use carbon dioxide at long distances but rely on another chemicals to narrow down their search at close range.

To confirm this finding, Arce et al. tried absorbing the carbon dioxide using soda lime, leading to similar effects: carbon dioxide sensitive insects stopped detecting the roots at long but not short distances. Additional experiments then revealed that the compound could help insects find the best roots to feed on. Indeed, eating plants that grow on rich terrain – for instance, fertilized soils – helps insects to grow bigger and faster. These roots also release more carbon dioxide, in turn attracting rootworms more frequently.

In the United States and Eastern Europe, Western corn rootworms inflict major damage to crops, highlighting the need to understand and manage the link between fertilization regimes, carbon dioxide release and how these pests find their food.

Comparative Susceptibility of Western Corn Rootworm (Coleoptera: Chrysomelidae) Neonates to Selected Insecticides and Bt Proteins in the Presence and Absence of Feeding Stimulants

The susceptibility of western corn rootworm, Diabrotica virgifera virgifera LeConte, larvae to nine insecticides from five different classes and to Bt proteins eCry3.1Ab and mCry3A in the presence or absence of feeding stimulants, was estimated in filter paper and diet toxicity assays, respectively. The use of a synthetic feeding stimulant blend of the sugars glucose, sucrose, and fructose plus linoleic acid at a ratio of 30:4:4:0.3 mg/ml of distilled water was evaluated to determine whether they increase the efficacy of insecticides and Bt proteins. The efficacy of thiamethoxam diluted in solutions with feeding stimulants was significantly increased when compared to thiamethoxam dilutions in water (>60-fold). Differences in the efficacy of the other insecticide classes when diluted in feeding stimulant solutions were no greater than fivefold when compared to the insecticides diluted in water. The presence of corn root juice as a natural feeding stimulant diminished toxicity of the insecticides, except for thiamethoxam, even though larval fresh weight was higher when fed on root juice compared to feeding stimulant or water. The use of feeding stimulants in diet toxicity assays did not enhance efficacy of eCry3.1Ab nor mCry3A proteins. Feeding stimulants can be recommended in combination with thiamethoxam to increase larval mortality. These results are discussed in terms of applicability of feeding stimulants to improve susceptibility of western corn rootworm larvae to pesticides in general.

Access to the odor world: olfactory receptors and their role for signal transduction in insects

The sense of smell enables insects to recognize and discriminate a broad range of volatile chemicals in their environment originating from prey, host plants and conspecifics. These olfactory cues are received by olfactory sensory neurons (OSNs) that relay information about food sources, oviposition sites and mates to the brain and thus elicit distinct odor-evoked behaviors. Research over the last decades has greatly advanced our knowledge concerning the molecular basis underlying the reception of odorous compounds and the mechanisms of signal transduction in OSNs. The emerging picture clearly indicates that OSNs of insects recognize odorants and pheromones by means of ligand-binding membrane proteins encoded by large and diverse families of receptor genes. In contrast, the mechanisms of the chemo-electrical transduction process are not fully understood; the present status suggests a contribution of ionotropic as well as metabotropic mechanisms. In this review, we will summarize current knowledge on the peripheral mechanisms of odor sensing in insects focusing on olfactory receptors and their specific role in the recognition and transduction of odorant and pheromone signals by OSNs.

Identification of odorant binding proteins in Carpomya vesuviana and their binding affinity to the male-borne semiochemicals and host plant volatiles

Carpomya vesuviana (Costa; Diptera: Tephritidae) is an agricultural pest that causes serious damage to jujube fruits. However, the mechanism of olfaction, which is critical for host identification, is not well understood in this pest. In this study, we have identified for the first time five protein types involved in the olfactory signal transduction of C. vesuviana by using transcriptome sequencing. These include 6 odorant-binding proteins (OBPs), 15 odorant receptors (ORs), 22 gustatory receptors (GRs), 2 chemosensory proteins (CSPs), and 2 sensory neuron membrane proteins (SNMPs). Amino acids alignment and phylogenetic analysis showed that all 6 OBPs have a signal peptide at their respective N-termini with four OBPs belonging with the classic OBPs, and OBP2 and OBP5 belonging to the Minus-C family. OBP3 clustered with the OBP83a/83b clade, which comprised pheromone binding protein related proteins (PBPRPs). Moreover, volatiles from C. vesuviana adults and its host plants were collected and identified by using solid phase microextraction (SPME) and gas-chromatography/mass spectrometry (GC/MS). The results indicated that male adults emitted nonanal, and five other compounds, caryophyllene, chamigrene, camphene, (Z)-3-hexen-1-ol acetate, and ocimene were identified in the fruits of jujubes. Electroantennogram (EAG) assays revealed that adult C. vesuviana responded to all six compounds along with two additional pheromones (geranyl acetate and α-farnesene) from other tephritids and the values ranged from 0.50mV to 1.26mV. To further explore the interaction between OBPs and volatiles, competitive binding assays were carried out. The results showed that only CvesOBP2 had binding affinity to (Z)-3-hexen-1-ol acetate. OBP5 and OBP6 exhibited broad spectrum binding to compounds with relatively low molecular weights, and OBP1 and OBP4 had some affinity to caryophyllene and chamigrene. However, OBP3 exhibited relatively high binding affinity to α-farnesene. The findings of this study provide insights into the olfactory mechanisms and the potential functions of OBPs in the olfactory reception pathway in C. vesuviana. The OBPs identified in this study could be used as potential targets to develop attractants to monitor this insect pest for effective pest control.