White worm beetle

The white worm beetle (Hylamorpha elegans) is a species of beetle in the family Scarabaeidae. It is the only species in the genus Hylamorpha. This beetle is native to South America, particularly in regions of Argentina, Brazil, and Uruguay.

This beetle is recognized for its unique appearance and intriguing ecological role within its habitat as a decomposer. Its glossy green exoskeleton, with variations in color across its head, pronotum, and elytra, makes it easily identifiable. It feeds on decaying organic matter, aiding in the breakdown of dead plant material. This process contributes to nutrient recycling and soil enrichment. This species is primarily nocturnal, so is often observed foraging for food at night.

Their mating behavior is influenced by host-plant volatiles, with males being attracted to females emitting specific pheromones while feeding on leaves. Once mating occurs, females lay eggs on preferred host plants, such as red clover.

These beetles are agricultural pests, causing damage to crops by feeding on grass and cereal roots in their larval stage. Additionally, they defoliate trees like Nothofagus species during adulthood, impacting forest ecosystems. Predators such as the Black-faced Ibis and rainbow trout contribute to controlling their population. Understanding their genetics, including chemoreception mechanisms mediated by odorant-binding proteins and chemoreceptors, provides insights into their behavior and potential management strategies. Their interactions with humans and livestock pose challenges in agriculture and forestry, driving the exploration of alternative pest management approaches like disrupting mating behavior or enhancing soil organic matter to reduce larval damage.

Description
The white worm beetle has a distinct appearance due to its long body, usually measuring between 11.8 to 18.2 mm long, and 5.8 to 10.8 mm wide across the shoulders. It showcases an oval physique and a smooth, glossy green exoskeleton. Its head, pronotum, and elytra (wing covers) can range from light to dark green, sometimes with shiny silver or bronze reflections, and rarely orange. Under certain conditions or after death, it can turn blue, red, orange, or purple.

Males' front legs are usually green or light brown to match their body color. While females' front legs are mostly light brown, their back legs are green like their bodies. Male beetles have all green legs with a shiny appearance, especially on the middle and back legs, while females have green legs with varying degrees of shine. The ends of their front leg teeth are always black.

The beetle's head has small bumps or dots and is usually green or brown. Its antennae have ten sections, with one section longer in males than in females. The beetle's mandibles are triangular and end in a sharp or rounded point. Its back has tiny bumps or dots, and its sides have a small edge. The back legs and the lower body have long white hairs. The front legs have three small teeth at the end, and the back legs have a bump on the outer edge. The bottom part of the legs have small bumps, and the very last part of the body is slightly curved, both in males and females.

Habitat
Similar to other scarab beetles, they likely rely on hygro and thermoreceptive sensilla to assess their surroundings. These sensory structures, such as sensilla coeloconica or sensilla styloconica, are known to respond to changes in humidity and temperature. This suggests that the beetle is adapted to habitats where these sensory cues play a significant role, favoring areas with specific moisture and temperature levels conducive to their emergence and activity.

White worm adults typically emerge from the ground during the spring and summer, particularly when evenings are warmer.

Distribution
The white worm beetle inhabits both sides of the Andes in central and southern Chile and southwestern Argentina. Its range notably corresponds to the distribution of Nothofagus species, which serve as its primary food source. It's an agricultural pest in Chile where the larvae feed on grain and the adults defoliate young trees.

Food resources
The white worm beetle exhibits a broad diet encompassing various plant parts, making it a significant agricultural pest. In its larval stage, this beetle primarily feeds on the roots of grasses and small grain cereals, leading to severe agricultural damage in regions like Chile.

Additionally, during its adult stage, H. elegans occasionally causes significant defoliation on its secondary host plants, which predominantly include species of the genus Nothofagus, such as Nothofagus antarctica, betuloides, dombeyi, and obliqua. This feeding behavior can result in the death of young trees, particularly when there are large numbers of the white worm beetles on them.

This shift in dietary preference signifies a change in ecological niche and resource utilization as the beetle progresses through its life cycle.

Larvae
The life history of H. elegans begins with its larval stage, which develops in the soil, specifically in the root layer of various plants, including crops. The larvae feed on plant roots, making them significant pests in agricultural settings. The larvae of H.elegans have been found alongside the larvae of other Chilean beetle scarab species including the larvae of Brachysternus prasinus.

Hylamorpha elegans undergoes a complex life cycle, beginning with the oviposition of eggs by females. These eggs give rise to larvae, which constitute the third instar stage.

The third instars of H. elegans larvae exhibit specific morphological characteristics that distinguish them from larvae of related species, such as Aulacopalpus punctatus. These distinctions include coloration, roughness of the cephalic capsule, and the distribution and shape of setae on the raster''. H. elegans'' larvae typically display a slightly rugose, shiny, red surface with a head width averaging 4.2 ± 0.12 mm.

Enemies
The enemies of the Hylamorpha elegans beetle consist of avian predators such as the Black-faced Ibis (Theristicus melanopis). The Black-faced Ibis is known for its tactile, non-visual hunting strategy, utilizing its long and slender bill to probe the ground for prey such as H. elegans larvae and other terrestrial invertebrates.

The study found that larvae of H. elegans, commonly known as the Southern Green Chafer, are among the important prey items consumed by the Black-faced Ibis. The green chafer larvae were found to provide a higher mean biomass and energetic content compared to other prey, making them a preferred food source for the ibis. H. elegans larvae are not immune to predation by the ibis.

In January, at Lago Traful in Argentina's Neuquén Province, rainbow trout (Oncorhynchus mykiss) prey on Hylamorpha elegans that inadvertently plummet into the lake from the overhanging Nothophagus trees. H. elegans is incredibly abundant along the shores of this lake and likely others.

Genetics
The genetics of Hylamorpha elegans in the context of chemoreception is crucial for insect behavior such as host-seeking, mate finding, and aggregation. Chemoreception in insects involves specific proteins located in the antennae, particularly in hair-like sensory organs called sensilla. These proteins are responsible for recognizing volatile organic compounds (VOCs) from the environment, and among them, odorant receptors (ORs) play a central role in transducing olfactory information toward the central nervous system of insects.

Recent studies have shed light on the genetic basis of chemoreception in coleopterans, including Hylamorpha elegans. While ORs have been the primary focus, attention has also been given to gustatory receptors (GRs) and ionotropic receptors (IRs). A transcriptomic analysis of its antennal transcriptome revealed chemoreceptors ORs, GRs, and IRs. This analysis identified a total of 102 OR transcripts, 22 GR transcripts, and 14 IR transcripts for the species. The identification of these transcripts provides valuable insights into the genetic mechanisms underlying chemoreception in H. elegans.

Phylogenetic analysis delineated five main clades corresponding to different sensory functions, including bitter, sugar, and carbon dioxide (CO2) reception. Transcript abundance analysis revealed HeleGR21a as the most abundant GR transcript, followed by HeleGR7, indicating their potential importance in chemosensory signaling.

Ionotropic receptors (IRs) play crucial roles in mediating olfactory and gustatory responses in insects. Transcriptomic analysis of H. elegans antennae identified 14 HeleIRs and 3 BpraIRs. These transcripts exhibited 3-4 transmembrane domains (TMDs) and were distributed across different clades associated with various sensory functions, including olfaction, hygrosensation, and thermosensation. Transcript abundance analysis revealed HeleIR8a, HeleIR25a, and HeleIR76b as the most abundant IR transcripts in H. elegans, highlighting their potential significance in chemosensory signaling pathways.

Odorant receptors (ORs) are critical for detecting volatile compounds in the environment and are essential for various behaviors, including host-seeking and mating. Transcriptomic analysis revealed 65 OR transcripts plus Orco in H. elegans.

The presence of a larger number of GRs in H. elegans suggests a potential role in mediating feeding behaviors and host selection, consistent with the generalist feeding behavior observed in H. elegans. Furthermore, the identification of specific GRs associated with carbon dioxide detection implies their involvement in environmental sensing and possibly mate location.

The differential expression of ORs between male and female H. elegans suggests a potential role in sex-specific behaviors and reproductive strategies. Sex-biased expression analysis of selected ORs revealed differential expression patterns between male and female antennae, suggesting potential roles in mediating sex-specific behaviors or responses to environmental cues. HeleOR14, HeleOR64, and HeleOrco were significantly upregulated in female antennae, while HeleOR46 showed significant upregulation in male antennae. Similarly, BpraOR1 exhibited significant upregulation in female antennae, indicating potential differences in chemosensory responses between sexes. These findings provide valuable insights into the genetic basis of sex-specific behaviors and chemical communication in H. elegans and contribute to our understanding of the molecular mechanisms underlying insect chemoreception.

Chemical compounds
The mating behavior of the white worm beetle is intricately linked to chemical communication facilitated by pheromones and sensory structures on their antennae. Virgin females release 1,4-hydroquinone and 1,4-benzoquinone, which are compounds that elicit attraction response from males. The ability of females to release these pheromones likely evolved as a mechanism to increase their reproductive success by attracting males for mating. On the other hand, males do not attract other males, indicating specific response to female cues. Olfaction plays a crucial role in scarab beetle mating behavior, aiding in the location of potential mates, food, and oviposition sites.

Mating behavior in H. elegans is notably influenced by host-plant volatiles, suggesting a mechanism by which adults locate potential mates. Male beetles exhibit a remarkable ability to recognize females while they are feeding on leaves, indicating the presence of sexual kairomones released by the attacked plants.

After copulation, females of H. elegans engage in a distinct behavior of flying to specific crops, such as red clover (Trifolium pratense), to deposit their fertilized eggs. The seasonal flight period typically occurs from 11th November to 27th January. This oviposition behavior indicates a deliberate selection of oviposition sites, likely driven by volatile compounds emitted by preferred host plants.

Chemosensory and pheromones in relation to mating
Males, equipped with a larger abundance of chemosensory sensilla on their antennae compared to females, demonstrate a heightened ability to detect pheromones, reflecting their active role in mate searching. Sensilla placodea, sensilla coeloconica, and sensilla basiconica are identified as the main sensory structures involved in chemoreception, particularly for detecting pheromones and plant volatiles.

This selective response to female odors suggests a form of sexual selection among males. Those individuals capable of detecting and responding to female pheromones may have a reproductive advantage by increasing their chances of encountering and mating with receptive females. Over time, this preference for specific chemical cues released by females may drive the evolution of male sensory mechanisms, enhancing their ability to locate mates in the environment.

Observations suggest that mating encounters occur on foliage, where females remain feeding on host leaves, potentially enhancing male response towards the sex pheromone released by females. Compounds released from the host tree, such as those found in the essential oil of Nothofagus obliqua leaves, interact with female pheromones to increase male responsiveness. This behavior underscores the significance of plant volatiles in facilitating mating interactions.

Reproduction
One of the notable aspects of the white worm beetle is its reproductive behavior. During the mating season, males engage in elaborate courtship displays to attract females. Once mating occurs, females lay their eggs in moist soil or decaying vegetation, where the larvae hatch and develop.

Sexual dimorphism
Male beetles exhibit pronounced dimorphism in flight behavior compared to females. Light trap data indicates that males are primarily involved in dispersal flights, as they are predominantly captured in traps. This suggests that males engage in active flight to search for mates. Females, on the other hand, are less frequently caught in light traps, indicating a more sedentary behavior that is likely associated with feeding on host leaves during the flight period.

Larvae
Hylamorpha elegans larvae exhibit a selective feeding behavior, preferring particulate organic matter (POM) over fine organic matter particles for their nutritional requirements. This preference for POM suggests a specialized physiological adaptation to efficiently utilize certain types or qualities of organic matter present in the soil. The larvae's ability to selectively ingest POM indicates a sophisticated physiological mechanism for nutrient acquisition and energy metabolism.

Research indicates a strong correlation between larval growth and the quality of organic matter present in the soil rather than its quantity. Larvae feeding on soils rich in POM exhibit enhanced growth rates and increased live weight compared to those on soils with lower POM content, particularly evident in second instar larvae feeding on high POM soil. This underscores the importance of organic matter composition in facilitating optimal growth and development.

The physiological response of larvae to varying soil conditions is evident in their live weight dynamics over time. The study highlights that the magnitude of the effect of POM content on live weight varies at different sampling times, suggesting temporal variations in larval growth patterns in response to changes in soil quality. Furthermore, the growth rates of larvae feeding on soils with different POM contents exhibit significant differences, indicating distinct physiological responses to nutrient availability and organic matter composition.

Third instar larvae feeding on soils with lower POM content exhibit diminished live weight compared to those on high POM soil, underscoring the influence of soil quality on larval development. The observed weight loss nearing pupation suggests a transition phase characterized by reduced feeding rates and metabolic adjustments, impacting larval growth dynamics.

Exploration of faecal production reveals physiological adjustments in nutrient utilization or metabolic processes during prolonged feeding periods, impacting faecal output in third instar larvae. While the POM content of soils does not significantly affect relative faecal weight per larva, the negative effect of feeding duration highlights the dynamic nature of larval physiology in response to changing environmental conditions.

Odorant-binding proteins
The physiological aspects of Hylamorpha elegans, a native Chilean scarab beetle, are tied to its ecological interactions, particularly its olfactory perception and behavior. As a relevant agricultural pest, H. elegans exhibits cryptic habits, making conventional control methods challenging. Consequently, there's a pressing need for alternative, environmentally friendly strategies. Understanding the beetle's sensory mechanisms offers promising avenues for modifying its behavior. One key aspect of this is the study of odorant-binding proteins (OBPs), which play a crucial role in the beetle's olfactory perception. These proteins, characterized by their structure and function, facilitate the recognition of odorants and their transport to olfactory receptors. Through the identification of OBPs and their interactions with volatile compounds released by its native host plant Nothofagus obliqua, researchers aim to uncover new semiochemicals that could potentially disrupt pest behavior.

Among the proteins involved in olfactory perception, OBPs stand out as crucial players. These small, soluble proteins are present in sensory organs and are characterized by their structure, which allows them to bind to odorant molecules. OBPs serve as carriers for lipophilic odorants, transporting them to olfactory receptors and initiating the olfactory process. Researchers aim to identify candidate compounds for pest control by leveraging the affinity of OBPs for specific odorants. By combining computational tools such as molecular docking and molecular dynamics, researchers can predict the interactions between OBPs and volatile compounds, providing insights into potential pest management strategies.

The focus on odorant-binding proteins (OBPs) sheds light on the beetle's olfactory perception mechanisms. Despite the prediction of six OBPs from the genome draft, only four were successfully amplified via polymerase chain reaction (PCR) from cDNA extracted from adult individuals. Notably, these transcripts exhibited higher abundance in chemosensitive tissues compared to hindleg tibia, indicating their crucial role in olfactory processes. The characterized OBPs, namely HeleOBP1, HeleOBP3, HeleOBP4, and HeleOBP6, vary in length, isoelectric point (pI), and molecular mass, reflecting their diverse functions in odorant detection and discrimination.

The identification of four novel odorant-binding proteins (OBPs) in H. elegans represents a crucial advancement, particularly considering the limited genomic information available for scarab beetle species. RT-PCR analyses revealed that these OBPs are predominantly expressed in organs associated with olfactory and gustatory senses, indicating their pivotal role in chemosensation. Notably, HeleOBP1 and HeleOBP3 exhibit heightened expression levels in the antenna, suggesting their involvement in detecting semiochemicals relevant to host-seeking and oviposition behaviors. Conversely, HeleOBP4 and HeleOBP6 show increased expression in mouthparts, potentially implicating them in the perception of feeding-related cues. These tissue-specific expression patterns provide valuable insights into the functional roles of OBPs in mediating insect behaviors related to mating, feeding, and oviposition.

The discovery of novel volatile compounds from Nothofagus obliqua leaves further enriches our understanding of H. elegans physiology. Six compounds, including beta-myrcene, beta-ocimene, dodecane, tetradecane, alpha-gurjunene, and aromadendrene, were identified, expanding the repertoire of chemicals relevant to the beetle's olfactory perception and behavior. These compounds hold promise for future analysis aimed at elucidating their roles as attractants, repellents, or pheromones in H. elegans ecology and communication.

In the context of H. elegans, understanding the physiological basis of olfactory perception is essential for developing effective pest management techniques. By mining the beetle's genome and characterizing novel OBPs, researchers can uncover key insights into its sensory mechanisms. Furthermore, by elucidating the interactions between OBPs and volatile compounds released by its preferred host plant, researchers can identify potential disruptors of pest behavior.

Chemoreceptors
Chemoreception, crucial for insect behavior such as host-seeking and mate finding, relies on specific proteins located in the Hylamorpha elegans’ antennae, particularly in sensilla, which are hair-like sensory organs. These proteins, including odorant receptors (ORs), gustatory receptors (GRs), and ionotropic receptors (IRs), play essential roles in detecting volatile organic compounds (VOCs) from the environment.

Olfaction is implicated in foraging, host selection, mate finding, aggregation, and reproductive behaviors in numerous insect species. The comparative analysis of chemoreceptors, including gustatory receptors (GRs), ionotropic receptors (IRs), and odorant receptors (ORs), sheds light on the molecular basis of olfactory perception in H. elegans. Notably, H. elegans has a larger number of identified GRs compared to Brachysternus prasinus, another South American scarab beetle that lives in similar areas as H. elegans. This larger number of identified GRs in H. elegans suggests a broader repertoire for detecting sugars from various tree species, potentially influencing feeding behavior, host selection, and defense mechanisms.

In the peripheral olfactory system, sensilla houses these chemoreceptors, which are responsible for recognizing and transducing olfactory information toward the central nervous system. Among these receptors, ORs are central to mediating various behavioral responses, such as attraction or repulsion, by transducing olfactory signals.

Gustatory receptors (GRs) and ionotropic receptors (IRs) also contribute to chemoreception in insects. GRs are involved in recognizing a wide range of tastants, including bitter and sweet molecules, sugars, nutrients, and carbon dioxide. Meanwhile, IRs play diverse roles depending on the insect species, including thermal and water sensations, as well as avoidance behaviors, such as selecting oviposition sites.

Phylogenetic analysis of ionotropic receptors (IRs) reveals a diverse array of receptors in H. elegans, distributed across clades associated with gustation, olfaction, hygrosensation, and thermosensation. The presence of coreceptors such as HeleIR8a and HeleIR25a shows their importance in mediating chemosensory responses, with implications for environmental sensing and behavioral adaptations.

Sex-biased expression analysis of selected ORs provided insights into potential differences in olfactory sensitivity between male and female H. elegans. Significant expression differences were observed for certain ORs, indicating a role in mediating sex-specific behaviors or responses to environmental cues. The differential expression of ORs between male and female H. elegans suggests a role in sex-specific behaviors, including mate location and reproductive strategies.

Interactions with humans and livestock
The beetle poses significant challenges for both humans and livestock in Central and Southern Chile and Southwestern Argentina. The larvae of these beetles inflict severe agricultural damage by feeding on the roots of grasses and small grain cereals in Chile. The beetle is also a significant pest in berry crops and common hazel orchards, reducing crop yields and forage availability. Livestock grazing on pastures infested with these larvae may also experience indirect effects. Damage to pasture plants' roots can lead to reduced forage availability and quality, affecting livestock nutrition and productivity. Additionally, irregular outbreaks of H. elegans larvae in grasses and small grain cereals can be challenging for farmers and ranchers in maintaining viable pasturelands and cereal crops.

Additionally, their adult counterparts occasionally cause extensive defoliation on secondary host plants, particularly species of Nothofagus trees such as Nothofagus antarctica, betuloides, dombeyi, and obliqua. These adult beetles often congregate, feed, and mate in the canopy of these trees. This defoliation can result in the death of young trees.

The use of semiochemicals, such as pheromones and kairomones, for pest management has been poorly developed, adding to the challenge of mitigating their impact. Alternative strategies based on ethology, such as disrupting mating or host finding, are being explored as options for pest management. Studies have also shown that adding manure to soil, which increases the availability of labile organic matter such as particulate organic matter (POM), can reduce larval damage to pasture plants. The presence of POM may serve as an alternative food source for scarab larvae, potentially decreasing their intensity of feeding on plant roots.