Cockchafer

Cockchafers are prevalent across Europe, including in Germany, France, and the United Kingdom. They are particularly prevalent in temperate regions with suitable soil conditions for larval development. However, they have also been reported in parts of Asia, including Turkey and the Caucasus region. Geographical barriers, climatic conditions, and ecological factors may limit their dispersal to other continents.^[10]

Adults of M. melolontha reach sizes of 25 to 30 millimetres (1 to 1+1⁄4 inches) in length.^[5] Behind their heads they have a black pronotum covered with short hairs. This black coloration distinguishes them from their close relative M. hippocastani, whose pronotum is brown. The top of their bodies have hard, brown elytra and a black thorax, while their underside is black and partly white on the sides. They have a dark head with two antennae with ten segments each. Male cockchafers have seven "leaves" on their antennae, whereas the females have only six.^[5]

Larvae have 3 stages of development over the course of 3–4 years. In the first stage, they are 10–20 mm long, then grow to 30–35 mm in the second year of development, and finally reach their full size of 40–46 mm in their final year of development before emerging.^[5] In some areas of Eastern Europe the larvae develop for a fourth year. They have white bodies that curve into an arc with a black coloration at the abdomen and long, hairy, and well developed legs.^[5] They have large orange heads with strong, grabbing mandibles. On their heads they have 2 small antennae which they use to smell and taste their surroundings while underground.^[8]

Cockchafer feeds on deciduous plant and fruit tree leaves, including oaks, maple, sweet chestnut, beech, plum, and walnut trees. The feeding behaviour of larvae can cause severe damage to the plants. They feed on both the small roots of field plants such as grain, grass, tree, beet roots and the large part of crop rootlets. Larvae can gnaw the root for 30 cm each day, which quickly kills the plant.^[9]

Adults appear at the end of April or in May and live for about five to seven weeks. After about two weeks, the female begins laying eggs, which she buries about 10 to 20 cm deep in the earth. She may do this several times until she has laid between 60 and 80 eggs. Most typically, the female beetle lays its eggs in fields. The preferred food for adults is oak leaves, but they will also feed on conifer needles.

The larvae, known as "chafer grubs" or "white grubs", hatch four to six weeks after being laid as eggs. They feed on plant roots, for instance potato roots. The grubs develop in the earth for three to four years, in colder climates even five years, and grow continually to a size of about 4–5 cm, before they pupate in early autumn and develop into an adult cockchafer in six weeks.^[5]

The cockchafer overwinters in the earth at depths between 20 and 100 cm. They work their way to the surface only in spring.

Because of their long development time as larvae, cockchafers appear in a cycle of every three or four years; the years vary from region to region. There is a larger cycle of around 30 years superimposed, in which they occur (or rather, used to occur) in unusually high numbers (10,000s).

The European mole is a natural predator of cockchafers. Moles are known to feed on cockchafer larvae. They can detect them using their keen sense of smell and specialised digging behaviour. This predation can help regulate cockchafer populations in mole-inhabited areas. ^[11]

M. melolontha adults are predated by ground beetles and ants. Larvae are predated by click beetles while underground. Starlings, crows, and gulls also predate M. melolontha larvae, often after a field has been plowed.^[5]

Dexia rustica is a parasitic fly that uses M. melolontha larvae as their hosts. D. rustica eggs hatch underground and look for cockchafer larvae to hibernate within over the winter. Their presence will ultimately kill the beetle larvae in the spring. One to six fly larva can parasitise a single host.^[5]

Males leave the soil when the temperature is favourable in April or May. Sexual dimorphism is observed as male beetles, at dusk, will begin to swarm and locate around groups of trees at forest edges.^[5] On the other hand, females will stay in place and feed on leaves until they reach sexual maturity. Males primarily fly around the branches looking for females to mate with.^[6][7] This behaviour occurs for several hours until darkness for about 10-20 days.^[5] These swarms typically have minimal damage to the trees, but they are occasionally harmful in cherry or plum orchards because of their consumption of blossoms. Once the females have matured and mated, they return to the fields to lay their eggs in the soil. Only a third of females will survive this trip, but any survivors will make a second, and occasionally third, swarming trip and return to the field to lay eggs again.^[5]

Green leaf volatiles (GLVs) are a series of saturated and monounsaturated six-carbon aldehydes, alcohols, and esters released by vascular plants in response to stresses.^[12] GLVs have been found to act as a kairomone, which is a compound released by an organism that only benefits the receiver.^[6][7] This enhances the attractiveness of toluquinone, a sex pheromone in scarab beetles. Only male M. melolontha are attracted to GLVs, using its release to identify leaves that female beetles are feeding on. Females have the ability to detect GLV, but any change in behaviour that it may cause is unclear.^[6][7] M. melolontha males are more sensitive to lower GLV concentrations, possibly due to the anatomical differences between male and female antennae.^[7] Due to this phenomenon, sexual dimorphism can be observed in flight behaviour. During swarming behaviour, males will hover around the foliage while females remain on twigs and branches to feed. Males then use GLVs to identify which leaves have females that they can mate with.^[7] GLVs are being investigated as a possible pest control technique to attract males and prevent mating.^[5]

Though adults can damage some fruit trees, M. melolontha larvae are the primary agricultural pests.^[5] Larva hatch from their eggs 4–6 weeks after being laid and develop into adults over the course of 3–4 years. Immediately after hatching, larvae will gnaw on small roots. It will continue feeding on roots, particularly grasses, cereals, and other crops, during its three larval stages, only pausing to burrow deep into the soil for winter hibernation.^[5]

In their first stage, M. melolontha larvae identify roots by CO₂ release. They will only do damage at extreme densities.^[5] In their second stage, larva will cause the most damage to crops.^[8] In their third stage, larva will do less but still severe damage to crops. They most prominently use structures on their antennae called pore plates to smell. This structure is a thin layer of cells that covers a number of sensory units consisting of dendrite bundles. These and other olfactory organs on the head of the larva can identify CO₂ and plant volatiles. They've also been found to push their heads into the walls of their burrows and probe with their antennae, likely to taste the soil with bristle-like sensilla.^[8]

In the Middle Ages, pest control was rare, and people had no effective means to protect their harvest. This gave rise to events that seem bizarre from a modern perspective. In 1320, for instance, cockchafers were brought to court in Avignon and sentenced to withdraw within three days onto a specially designated area, otherwise they would be outlawed. Subsequently, since they failed to comply, they were collected and killed. Similar animal trials also occurred for many other animals in the Middle Ages.^[13]

Both the grubs and adults have a voracious appetite and thus have been and sometimes continue to be a major problem in agriculture and forestry. In the pre-industrialised era, the main mechanism to control their numbers was to collect and kill the adult beetles, thereby interrupting the cycle. They were once very abundant: in 1911, more than 20 million individuals were collected in 18 km² of forest.^[1] Collecting adults was an only moderately successful method.

In some areas and times, cockchafers were served as food. A 19th-century recipe from France for cockchafer soup reads: "roast one pound of cockchafers without wings and legs in sizzling butter, then cook them in a chicken soup, add some veal liver and serve with chives on a toast". A German newspaper from Fulda from the 1920s tells of students eating sugar-coated cockchafers. Cockchafer larvae can also be fried or cooked over open flames, although they require some preparation by soaking in vinegar in order to purge them of soil in their digestive tracts.^[14] A cockchafer stew is referred to in W. G. Sebald's novel The Emigrants.

In Sweden the peasants looked upon the grub of the cockchafer as furnishing an unfailing prognostic whether the ensuing winter will be mild or severe; if the animal has a bluish hue (a circumstance which arises from its being replete with food), they affirm it will be mild, but if it is white, the weather will be severe: and they carry this so far as to foretell, that if the anterior be white and the posterior blue, the cold will be most severe at the beginning of the winter. Hence they call this grub Bemärkelse-mask—prognostic worm.^[15]

Only with the modernisation of agriculture in the 20th century and the invention of chemical pesticides did it become possible to effectively combat the cockchafer. Combined with the transformation of many pastures into agricultural land, this has resulted in a decrease of the cockchafer to near-extinction in some areas in Europe in the 1970s.

Since the 1970s, agriculture has generally reduced its use of pesticides. Because of environmental and public health concerns (pesticides may enter the food chain and thus also the human body) many chemical pesticides have been phased out in the European Union and worldwide. In recent years, the cockchafer's numbers have been increasing again, causing damage to agricultural use of over 1,000 square kilometres (400 sq mi) of land all over Europe (0.001% of land).

Due to legal provisions from the European Union for the sustainable use of pesticides, aerial treatment, which had been used to successfully control M. melolontha populations, is now banned.^[16] Light traps have been successful in attracting M. melolontha adults, particularly males, when put at height (4 m). If a peak swarming time can be identified, shaking isolated trees and collecting feeding adults can reduce population, though it is time consuming.^[16] Azadirachtin is a chemical that inhibits maturation feeding and egg development, but low persistence and difficulty spraying it high enough in trees prevents widespread use.^[16] Soil tilling has been a historically successful method, particularly in early June when larvae are first hatching.^[17] Pre-cropping is also a promising possibility, with buckwheat being of particular interest because it can reduce grub weight and population density before the crop of interest is planted.^[16] Sex pheromones have been used for mass trapping, mating disruption, and "Attract and Kill" methods. The unlikelihood of developing resistance due to the sex pheromones being produced by the beetles makes this a promising method of pest control.^[5]

Entomopathogenic organisms—organisms that produce disease in insects—are an active area of research for the control of M. melolontha grub populations.^[18] Entomopathogenic fungi is currently being studied as a way to control M. melolontha grub populations. Beauveria brongniartii has been found to work on the Melolontha species, and B. bassiana has been successful with other agricultural pests. There have been difficulties with determining the best strategy to apply the fungi to the fields.^[18] Entomopathogenic nematodes have been found to be particularly successful ways of reducing populations, particularly when larvae are in the first and second stage.^[17] entomopathogenic bacteria from the genera Steinernema and Heterorhabditis are also being investigated, but they have been difficult to apply to fields as opposed to laboratory settings.^[19] The focus on entomopathogenic bacteria has been on their symbiosis with entomopathogenic nematodes and their ability to act together as a larval control strategy.^[17] Poor results with the application of these methods have stemmed intensive research into the gut enzymes and microbiome of M. melolontha to determine if they are acting as defense against entomopathogenic organisms.^[19]

The gut enzymes and microbiota of M. melolontha larvae allow them to exploit a variety of ecological niches unique to their phylogenetic family. These are low energy foods such as grass roots and rotting organic matter in the soil.^[20] There are two major compartments in the scarabaeid larvae intestinal tract. The first is a tubular midgut that secretes hydrolytic enzymes for macromolecule breakdown, and the second is a bulbous hindgut used for fermentation. High bacterial diversity between individuals of M. melolontha in the intestinal tract reflects the diversity of food sources.^[21]

In the midgut, glucose is broken down and absorbed by the epithelium. It has been shown that proteolytic breakdown of toxins is a common resistance mechanism for agricultural pests.^[19] Proteolytic activity of enzymes in the midgut is hypothesised to increase resistance to entomopathogenic bacteria in the beetle larvae. Trypsin-like enzymes from the midgut of M. melolontha have been found to break down certain bacterial toxins and inactivate them.^[19]

The hindgut has a high density of bacteria that ferment recalcitrant residues such as cellulose, with the byproducts being absorbed by the beetle.^[21] Acetate is a major product of this fermentation, suggesting that much of the bacteria in the hindgut is homoacetogenic. High abundance of species in the bacterial genus Desulfovibrio in the hindgut suggests that sulphate reduction is an important process, but the source of this sulphate in the diet is unknown.^[21]

Some research on the M. melolontha microbiome has been focused on increasing the entomopathogenic properties of nematodes used as pest control due to their symbiosis.^[20] Bacteria such as Xenorhabdus nematophila are transported by nematodes and released into the insect's midgut. The bacteria will release lytic enzymes and other antimicrobial substances to decrease competition from the beetle's native microbiome. This creates an optimal environment for nematode development. Bacterial species in the midgut of M. melolontha such as Pseudomonas chlororaphis have been found to fight back, acting as antagonists to entomopathogenic bacteria. These bacteria have been identified differentially in different larval stages, with P. chlororaphis usually being found in the third and final larval stage.^[20]

Environmental factors such as temperature, humidity, and plant type have a considerable impact on the existence and behaviour of cockchafers in wooded environments. It indicates that cockchafer populations are strongly influenced by climatic conditions, with warmer temperatures and higher humidity level favouring their occurrence. Additionally, specific vegetation types, including deciduous trees and shrubs, provide suitable habitats for cockchafers, facilitating their survival and reproduction within forest stands.^[11]

The name "cockchafer"^[22] derives from the late-17th-century usage of "cock"^[23] (in the sense of expressing size or vigour) + "chafer"^[24] which simply means an insect of this type, referring to its propensity for gnawing and damaging plants. The term "chafer" has its root in Old English ceafor or cefer, of Germanic origin and is related to the Dutch kever, all of which mean "gnawer" as it relates to the jaw. As such, the name "cockchafer" can be understood to mean "large plant-gnawing beetle" and is applicable to its history as a pest animal.

Children since antiquity have played with cockchafers. In ancient Greece, boys caught the insect, tied a linen thread to its feet and set it free, amusing themselves to watch it fly in spirals. English boys in Victorian times played a very similar game by sticking a pin through one of its wings.^[25] Nikola Tesla recalls that as a child he made one of his first "inventions", an "engine" made by harnessing four cockchafers in this fashion.^[26]

Cockchafers appear in the fairy tales "Thumbelina" by Hans Christian Andersen and "Princess Rosette" by Madame d'Aulnoy.

The cockchafer is featured in a German children's song similar to the English Ladybird, Ladybird:

The verse dates back to the Thirty Years' War in the first half of the 17th century, in which Pomerania was pillaged and suffered heavily. Since World War II, it is associated in Germany with the closing months of that war as well, when Soviet troops advanced into eastern Germany.

According to one source, the dumbledore in Thomas Hardy's 1899 poem An August Midnight^[27] is a cockchafer.^[28] However, in his novel The Mayor of Casterbridge, Hardy uses the dialect word dumbledore to mean a bumble bee.^[29]

There have been four Royal Navy ships named HMS Cockchafer.

• Red-headed cockchafer, native to Australia