Melolontha hippocastani

Melolontha hippocastani, the northern cockchafer, is a species of scarab beetle native to Eurasia, with its range spanning from Western Europe to the Pacific coast of China. It is one of several species in the genus Melolontha known as cockchafers, alongside the common cockchafer (Melolontha melolontha) and Melolontha pectoralis, but generally at more northern latitudes, or at higher altitudes in upland woods further south. The adults are around 20–29 mm in length. It is distinguished from Melolontha melolontha by the shape of its pygidium, which is primarily black in colour. It primarily dwells in forests, and as such is also known as the forest cockchafer. The mate-finding behaviour in M. hippocastani is facilitated by plant volatiles and sex pheromones. Mating activities primarily takes place during the evening flight periods. Females lay their eggs in soil, and the larvae feed on decaying organic matter and later small plant roots, including the roots of young trees like pines and firs. The larvae usually develop between 3 and 5 years. They emerge between late April and the end of June. Like other cockchafers, they have been considered a serious pest of crops and trees.

Geographic range
Melolontha hippocastani primarily resides in forest ecosystems in northern Eurasia. Its distribution range spans across most of Europe, excluding the northernmost and southernmost regions. This species extends its habitat into Mongolia, parts of Central Asia, and Siberia. Unlike its counterpart, the common cockchafer, which occupies diverse environments including forested and open areas, M. hippocastani is primarily a forest species, often in taiga forest.

Genetics
Research has probed the genetic diversity in M. hippocastani. Researchers analysed microsatellite loci in over 200 individuals from three primary outbreak areas in Poland, and found significant genetic diversity within subpopulations. This research compared the genetic makeup of M. hippocastani to a different cockchafer beetle, the Melolontha melolontha. They demonstrated that both M. hippocastani and M. melolontha show considerable genetic diversity within subpopulations within sampling sites with minor effects of past bottlenecks possibly masked by current population sizes, which genetic similarities between the species. The estimates of effective population sizes indicated a large number of individuals contributing to further generations of the population, indicating a limited impact of forest management practices on M. hippocastani and M. melolontha alike. However, while genetic differentiation was observed in M. melolontha populations, M. hippocastani populations were genetically similar across populations suggesting a possible historical divergence between the two species. Further research is required to understand M. hippocastani genetic population structure.

Mate searching
Mating behaviour primarily occurs during flight periods at dusk, during which beetles hover around tree tops. During this time, females and males mate several times, and mating lasts for several hours. Then the females oviposit in the soil, which causes severe tree damage from their feeding on tree roots. The swarming flights are mainly performed by males, while females stay in their host trees and continue feeding. Males are subsequently drawn towards damage-induced green leaf volatiles allowing location of mechanically damaged foliage, which allows males to locate females based on the leaves they have eaten. In order to distinguish between nonspecific leaf damage and damage caused by feeding females, male cockchafers orientate by a sex attractant. Research has investigated the role of sex pheromones of the scarab beetle on mating behaviour.

Volatiles and sexual dimorphism
One study demonstrates that male M. hippocastani are attracted to volatiles emitted by damaged leaves and conspecific females. Although both male and female beetles showed a physiological response to these volatiles, only males exhibited behavioural responses, suggesting that the volatile response reflects a mate-finding strategy rather than a search for feeding resources.

A variety of compounds have been analysed in research to identify which compound serves as a sex pheromone in M. hippocastani, and the mechanism by which it takes action. One research study determined that mate-finding behaviour in M. hippocastani is driven by males locating females using olfactory cues such as green leaf volatiles and 1,4-benzoquinone, along with the species' sexual dimorphism. Both males and females contain 1,4-benzoquinone; however studies showed that female extracts provoked a higher number of landings than male extracts. This finding led to further analysis on the quantities of 1,4-benzoquinone in each sex, which were found to be higher in females, suggesting its role in attracting males. This compound, known for its defense function in arthropods, is hypothesised to have evolved into a sex pheromone in M. hippocastani. Thus, research suggests a dual function of benzoquinones as both mate attractants and defense compounds. Further research is exploring other compounds' roles to develop semiochemical-based methods for controlling M. hippocastani populations.

The role of (Z)-3-hexen-1-ol
Additional research investigated whether volatiles from freshly damaged leaves are more attractive to males than those from older damaged leaves. Analysis of volatiles from freshly damaged leaves revealed typical leaf aldehydes, while older damaged leaves predominantly emitted (Z)-3-hexen-1-ol and (Z)-3-hexenyl acetate. Surprisingly, males were equally attracted to volatiles from both fresh and old damaged leaves, with a preference for the latter in synthetic mixture experiments. Further experiments identified (Z)-3-hexen-1-ol as highly attractive to male beetles, while other tested compounds were behaviourally inactive. However, all tested compounds elicited comparable electrophysiological responses on male antennae. Thus, (Z)-3-hexen-1-ol plays a vital role in the sexual communication of M. hippocastani, attracting both sexes of an insect.

Thus, a combination of plant volatiles and sex pheromones allow for males to find feeding females in the trees, facilitating mating behaviour.

Physiology
The mate finding technique performed by these beetles using sex pheromones are performed using specific physiologies. Melolonthinae sex pheromone glands are everted from the abdominal tip. M. hippocastani show sexual dimorphism within the antenna. Male antennae consist of seven large lamellae, while female clubs have only six smaller lamellae, which suggests different olfactory abilities between sexes and the presence of a female derived sex pheromone. Pheromone-degrading enzymes are present in the antennae of M. hippocastani and show considerable substrate specificity.

Life cycle
Once adult female M. hippocastani lay their eggs in the soil, the larvae spend 36 months underground feeding on plant roots. During this time, their growth phase is divided into three distinct phases. They pupate in summer, the year before swarming, and spend their last winter as adults. The following year, in late April to early May, the adult insects emerge from the soil and feed on tree foliage. About 2 weeks after emergence, oviposition flights are observed. Females land on the ground at dusk and burrow into the soil to lay eggs in clusters. The females prefer sandy soils because they facilitate the females' digging, allow for larval movement, and allow volatile compounds from host to spread, facilitating the larval orientation and survival in the soil. Females lay an average of 24 eggs during their first egg-laying phase and 15 during their second egg-laying phase.

Microbiome
Diet and digestion in M. hippocastani are facilitated by microbial symbionts residing in their guts. M. hippocastani larvae guts consist of two large compartments-- a tubular midgut and an enlarged hindgut. The midgut releases hydrolytic enzymes into an alkaline and oxidative environment, which threaten the development of bacterial species. The hindgut is an expanded organ specialised for anaerobic fermentation. Both regions have diverse bacterial communities, which are responsible for gut pH modification, the detoxification of plant allelochemicals and the maintenance of the microbial community structure.

Specifically, these microbes play a crucial role in breaking down woody food components such as lignocelluloses and xylans. When comparing the larvae and adult microbiotes of M. hippocastani, the larvae microbiotes had harsher conditions, yet harboured a richer and more diverse bacterial community compared to adult guts. A core group of bacterial phylotypes was shared between larvae and adults, indicating some degree of stability despite different feeding habits. Little overlap was observed between bacterial species from food or soil contamination and those in the gut, suggesting minimal alteration of bacterial diversity upon ingestion. Some isolates from larvae exhibited enzymatic properties related to digestion. This symbiotic relationship underscores the importance of microbes in the digestive processes across organisms.

Further research on the reactions of M. hippocastani to high manganese (Mn) content in their diet revealed significant effects on their activity and fertility. As the Mn content in their food increased, the activity of adult cockchafers decreased, and females fed on high Mn diets showed zero fertility. Despite the cockchafers' ability for self-detoxification, the presence of Mn in their diet still influenced their activity. It was found that expelling Mn through the digestive system serves as the primary mechanism for the cockchafers' self-detoxification process, showing the relationship between diet, digestion, and physiological responses in M. hippocastani.

Interactions with humans
The parasitic nature of these beetles has caused damage on a wide variety of foliage, which has demanded an abundance of research for strategies to understand M. hippocastani in order to mitigate their harmful impacts and protect the forests in which these beetles are local. For example, in parts of southern Germany, specifically in the states of Hessen, Rheinland-Pfalz, and Baden-Württemberg, mass breeding of M. hippocastani has been observed every 30–40 years, which causes damage from adults feeding on the foliage between April and May. However, this damage can be compensated during a secondary sprouting period in June. Contrarily, grubs that develop within 3–4 years in the forest soil cause severe, long-term damage on young trees by feeding on roots. In northeastern France, specifically in the Vosges Mountains, M. hippocastani populations have been at epidemic levels since 2007, and high larval densities have been recorded, which induces a high mortality risk for forest plantations. Thus, these beetles can cause vast economic and ecological losses in oak-dominated forest. Extensive research continues to be performed on management of these pests. Specifically, research has shown that forests with a dense shrub layer have a negative effect on the density of egg clusters and the number of eggs in the soil, while forests with a canopy openness and the proportion of oak basal area have positive effects. Some theories for the destructive impacts of dense shrubbery on the ability of M. hippocastani to lay eggs in the soil include: the shrub acting as an impenetrable barrier, flight imprecision of the beetle, and the use of olfaction to sense the environment as methods with which the shrubbery could impact the egg laying behaviour. These techniques have potential to understand egg laying patterns to mitigate forest damage.