Северный хрущ: феромоны, поведение и микробиом

Species of scrarab

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. 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.

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

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.

Once adult female M. hippocastani lay their eggs in the soil, the larvae spend 36 months underground feeding on plant roots.

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.

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.