Soft rodent beetle

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Soft rodent beetle
Soft rodent beetle

Soft rodent beetle

Systematics
Order : Beetle (Coleoptera)
Subordination : Polyphaga
Family : Rodent beetle (Ptinidae)
Subfamily : Ernobiinae
Genre : Ernobius
Type : Soft rodent beetle
Scientific name
Ernobius mollis
( Linnaeus , 1758)
Ernobius mollis above.jpg Ernobius mollis antennae.jpg
Fig. 3: Feeler
A: male, B: female
Ernobius mollis puncture.jpg
Fig. 1: Top view, female Fig. 4: Puncture of the pronotum
in two different places
Ernobius mollis front.jpg Ernobius mollis under.jpg
Fig. 2: Front view Fig. 5: Bottom

The soft rodent beetle ( Ernobius mollis ) is a common but inconspicuous beetle from the family of rodent beetles . It is brown, mainly active at night, and can be harmful to barked timber. Like most rodent beetles, Ernobius mollis is very variable in shape and therefore there are many synonyms .

The name "rodent beetle" can be explained by the fact that the larva gnaws passages into the wood. The name part "soft" is the translation of the species name mollis ( lat. ) Into German and refers to the hairiness of the species. The generic name Ernobius is from Altgr. ἔρνος érnos, branch and βίος bíos, place of residence and means that the drill holes prefer to run in thin branches. The genus Ernobius is represented with 32 species in Europe and around seventy species worldwide.

The first description by Linnaeus in 1758 is: brick-colored dermestes with dark eyes and thread-like antennae, lives in Europe. Similar to a leaf beetle, but the stature of Dermestes pertinacis (lat. D. testaceus, oculis fuscis, antennis filiformibus. Habitat in Europe. Affinis Chrysomelis, sed statura D. pertinacis ).

Characteristics of the beetle

The beetle's body length varies widely - from 2.8 to 6.2 mm - because it correlates with the food available for the larva. Females are on average slightly larger than males. The top of the body is fine and closely covered with golden hair, which gives the beetle a vivid golden brown color. The pronotum is often a little more hairy and therefore appears lighter. However, with age, the hair wears off and the beetle appears matt dark brown.

The species similar to the soft rodent beetle can be grouped according to how much the body structure allows the animal to "make itself small". However, this does not reflect the taxonomic relationship. In the genus Ernobius , the fore chest has no recess to accommodate the head. In the resting position, the head is bent down and hidden under the pronotum, but in the active beetle the head is visible from above. The legs cannot be placed in the recesses provided for this purpose. The long antennae can be pulled under the body, but there are no special grooves in which the antennae are inserted. The head of the beetle shown in Fig. 1 is not visible. Only the left front leg of the legs is drawn in and therefore not visible. The left antenna is almost stretched out, the right antenna runs backwards under the body, then it is bent. The last limbs of the antennae are visible to the right of the body and, lying under the front and middle legs, point forward again.

The construction of the antennae (Fig. 3) is a good aid to delimiting the numerous Central European species of the genus Ernobius , but a genital examination is often unavoidable. In both sexes the three terminal links of the eleven-link antennae are noticeably elongated. The ninth link is about as long as the three preceding combined, in any case shorter than the four preceding combined. The 8th antennae-segment is shorter than the seventh, the last three antennae-segments in the male longer than all the rest together, in the female, however, not. The antennae of the male are slightly longer and narrower than those of the female. The three terminal links are more parallel in the male, but widen in the female. Lying under the body, the antennae of the male reach just under the middle of the abdomen, while those of the female do not protrude beyond the chest. As a further gender difference, the back plate (tergite) of the last visible abdominal segment in the female is slightly bent downwards at the end, whereas in the male it is slightly bent upwards. This tergite protrudes about 0.36 millimeters above the posterior end of the elytra in males and 0.45 millimeters in females.

The feelers are inserted in front of the eyes. The compound eyes are round and, as with many crepuscular insects, large and hemispherical. The mouthparts point downwards or forwards, depending on the position of the head. The upper jaws have a two-tooth tip. The jaw probes are four-part, the lip probes are three-part.

The pronotum adjoins the elytra in its entire width. At the base it is barely noticeably narrower than the wing-coverts (0.1 mm). It is evenly curved with rounded sides. The rounded posterior angles of the pronotum are close to the base of the pronotum. The sides of the pronotum are completely edged, but not raised with a sharp edge when viewed from below. The hair on the pronotum is not conspicuously coiled, but only slightly curved. The pronotum is deeply punctured and , according to another source, covered with granules (Fig. 4).

The elytra are disordered finely dotted. Like the whole beetle, they have a matt sheen. Their sides are parallel and at the back they are rounded together. They do not completely cover the end of the abdomen, but are protruded by about 0.4 millimeters from the last abdominal segment.

The legs are slim, never noticeably broadened. All tarsi are clearly five-limbed, long and narrow, the first tarsal the longest. The section of the fourth tarsal phalanx of the hind legs extends over half its length.

larva

The larva shows the shape typical of the family. Like most larvae living in wood, it is white and thin-skinned. Only the front part of the head with the chewing apparatus is severely sclerotized and dark. It is roughly semicircular and about twice as wide as it is long. The larva has six well-developed five-limbed legs. The first segments are thickened, the abdomen consists of ten segments and is curved downwards in the shape of a horseshoe.

The caterpillar is doubly hairy. The long hair is gray or very dark red, longest on the end of the body. There are also numerous tiny dark bristles on the side of at least the last segment. Upright bristles that point slightly backwards at the tip are also found on the third breast section in the fold towards the abdomen and on the first seven to eight segments of the abdomen.

The bristles allow the larva to move in the feeding tunnel.

Eggs

The eggs are whitish and oval. They are rounded at one end and the micropyle is located at the other, somewhat longer end . The eggs are 0.45 to 0.65 mm long and 0.25 to 0.35 mm wide. The shell is sculptured in the shape of pentagons or hexagons. This sculpting is particularly pronounced at the end with the micropyle, but is missing at the points of contact with the bark. When laying eggs, the eggs are often pressed into cracks and change their shape considerably.

biology

General

The beetle is counted among the old wood colonists. It is polyphagous and attacks dry, still barked softwood, less often the cones. These are often thin branches that are about to die. Infested wood can still be used for development in several years to come. Obviously the development conditions deteriorate and the average height of the hatched adults decreases. The animals are most active at dusk and at night. During the day they avoid light. In the event of danger, they pull their legs and feelers to the body and remain stiff.

Mating and laying eggs

The information on reproductive biology varies slightly. There can be many reasons for this, such as the host plant or the geographical location of the location of the investigation or the climatic conditions in the investigation period. The data given relate mainly to a long-term study in the Czech Republic with the host plant Pinus nigra .

When you leave the doll's cradle, the genital organs are completely or almost completely developed. In the latter case, they are functional for a few hours to a maximum of two days afterwards. Both sexually mature and immature specimens begin to mate soon after leaving the pupa cradle. Mating takes just under a day, and the beetles retreat into cracks in the wood or hideout holes in larger insects. Occasionally the mating position is maintained even if the animals fall into the fixative fluid of the collecting device. The same beetles can mate repeatedly, with the female laying eggs in between.

Immediately after fertilization, the females run over the bark in search of suitable places to lay their eggs and check it with their antennae. Sugars favor the choice of storage place, esters , fatty acids and volatile oils, on the other hand, have a negative effect. Twigs where the bark is rough and flaky are often chosen as suitable places. With a laying tube up to two millimeters long, one or two, maximum six eggs are laid under protruding bark scales or in cracks in the bark. The number of eggs laid depends on the size of the crack and the thickness of the bark. The egg-laying begins on the first or second day after leaving the doll's cradle during the night and continues for almost two weeks.

At least 120 eggs are laid in the ovaries of the females, but only twenty to thirty eggs are laid. The maximum observed was 66 eggs. The size of the females is determined by the food available during development and also affects the number of eggs laid. In a study with six size classes, the number of eggs developed in the ovaries was positively correlated with the size of the females. In females in the group of the largest specimens, the ovaries, with an average of 68 eggs, contained 2.7 times as many eggs as the ovaries of the females in the smallest size class, with an average of 25 eggs. Depending on the temperature, the larvae hatch from the eggs under natural circumstances after two to three weeks.

development

Shortly before the embryo hatches, the egg becomes cloudy. The young larva bites a hole in the egg shell and penetrates through it to the outside. Then she consumes the egg shell in whole or in part. The newly hatched larva is barely curved and the breast section is only slightly thickened. In addition, the larva can still move around on a flat surface without any problems, a property that it later loses. The larva can travel several centimeters in search of a place where it can penetrate the bark and extend this search over a day. Unsuccessful attempts are also possible. Usually, however, the larvae immediately find a cracked spot where they can penetrate the bark. The larva bores its way through the bark to the growth layer of the host plant between the bark and wood ( cambium ), which is almost the shortest route . The entry holes into the bark are covered by small accumulations of brown to black disc-shaped to loaf-shaped pieces of excrement together with drilling dust. Later the pieces of feces become lenticular (fecal lenses). When the larva reaches the cambium, the walk between wood and bark is continued. In this layer the tunnels run up to pupation.

Characteristics of the larval stages
(length in millimeters)
stage larva Fecal lenses Feeding passage
Head width length colour diameter height width filling Gnawed layers
1st stage 0.37 0.7-0.8 brown to dark not pronounced lenticular not pronounced lenticular 0.3-0.5
oval cross-section 		Dark drill dust compacted into chunks,
chunk size 0.07 × 0.05 		initially bark, then cambium, phloem
2nd stage 0.53 1.5-2.2 brown to dark 0.25 0.14 0.4-0.9 Fecal lenses Cambium, phloem
3rd stage 0.75 1.9-3.3 rarely light and mostly dark 0.4 0.2 0.7-1.4 Fecal lenses, dark and light drill dust Cambium, phloem, boundary layer of xylem and bark
4th stage 1.1 2.6-6.0 light or dark 0.57 0.3 depends on the space available Fecal lenses, dark and light drill dust Cambium, phloem, xylem up to 2 mm deep, bark

There are four larval stages, the characteristics of which are summarized in the table above. The ducts of the 1st larval stage with a width of 0.3 to 0.5 millimeters and an oval cross-section run between the wood and bark. The larva now eats phloem and cambium and produces dark excrement. After about a week, the molt occurs to the second larval stage.

Due to the increasing size of the larvae in the later stages, the feeding tunnels also enlarge. The enlargement mainly affects the width, but the aisles also become higher. As a result, the layers of wood adjacent to the phloem and cambium are increasingly affected. Both the drill dust and the fecal lenses are light yellowish to white when they originate from the xylem . Correspondingly, in the late larval stages, the proportion of white drill dust (duct content “donuts with sugar”) then also increases in light-colored fecal lenses (duct content “pepper and salt”). However, even in the last larval stage, the wood is usually only scraped to a depth of two millimeters, the bark, on the other hand, can be so consumed that only a paper-thin layer remains. In the last (fourth) larval stage, the tunnels are very irregular and can form a complicated labyrinth, especially if the infestation is intense. In addition, their course is determined by the space available. In the case of thin branches, the feeding ducts can penetrate into the core.

Pupation takes place in the coming year. Pupations take place over a long period of time, but mainly in spring and early summer. The doll's cradle is placed just below the surface and runs parallel to it. If there is not enough space, the situation can be different. The doll's cradle is six to eight millimeters long, 2.5 to 3 millimeters wide and lined with a firmly glued mixture of excrement pellets and drill dust. The beetle pupates about two days after the doll's cradle has been completed. It hatches after about two weeks.

After hatching, the imago remains in the pupa’s cradle for about two weeks. The beetle gnaws an escape hole only after the skeleton has hardened. This is round to oval. With a diameter of just under two millimeters, it allows the shoulders to be passed straight. Males and females hatch at the same time, about the same number of males as females. However, the male to female ratio can vary widely.

The beetles live for about a month in nature, and for a maximum of two weeks in the laboratory. The males usually live shorter than the females. At most, they absorb water. After successful reproduction, they die in a few days.

Number of generations

The beetles usually appear in April and May and are most common in June and July. But they can be found practically all year round. Due to the long presence of the adult and the associated large time interval between laying eggs, several developmental stages can occur in parallel. In a statistical evaluation in November, only one pupa was found among 729 individuals, 4% were larvae of the first instar, 12% were in the second instar, 29% were in the third instar, and 55% were in the fourth instar. From this simultaneity of different larval stages it was concluded earlier that the species is two years old. Today it is assumed that the animal hibernates in the larval stage in which it is currently. If this is the fourth, it will pupate early next year, which is why you can find newly hatched adults early. If it winters at an earlier stage, it pupates accordingly later. Exceptionally late-hatched beetles can only start reproducing after the inactive phase during winter.

Harmfulness

In contrast to the common rodent beetle, the larvae of the soft rodent beetle dig the outer sapwood no more than 1–3 mm deep. Therefore, despite the beetle's frequency, the damage is generally low. In addition, they only attack dead wood or, more rarely, dying wood and can only develop if this is at least partially barked. The beetle, for example, only slightly reduces the quality of firewood. If timber that has not been debarked is used, the damage is also insignificant.

Under optimal conditions, a larva destroys an average of 7.2 cm² of phloem and 5.5 cm² of cambium. In view of the fact that usually existing passages are widened and some of the larvae die before food consumption increases dramatically in the fourth larval stage, the area of ​​the destroyed phloems is on average 2.4 cm², that of the destroyed cambium 1.8 cm² estimated. These low values ​​are due to the fact that the burrows of the larvae are generally quite short.

Only in the event that wood that has not been completely debarked is used for plywood and this is then used in furniture construction, a depreciation can occur through the escape holes of the developed beetles. In its urge to go outside, the hatched beetle not only penetrates wood, but almost all materials, including metallic ones. In the case of museum pieces, the damage can be considerable.

The best way to combat it is to completely debark the built-up wood. Hot air treatment or, in the case of furniture, heating to 60 ° C for at least three hours already cause the larvae to die.

Occurrence and frequency

The woolly rodent beetle is the most common type of the genus and can be found regularly at suitable breeding sites. The species is distributed in the Palearctic with the distribution center in Europe and Siberia, but is now to be found almost worldwide through introduction. His wide range of foods makes it easier for him to feel at home.

literature

Web links

Commons : Ernobius mollis  - collection of images, videos and audio files

Individual evidence

  1. ^ A b Fauna Europaea, synonyms and distribution
  2. ↑ Sigmund Schenkling: Explanation of the scientific beetle names (species)
  3. ↑ Sigmund Schenkling: Explanation of the scientific beetle names (genus)
  4. ↑ Ernobius at Fauna Europaea. Retrieved February 13, 2013
  5. ↑ Ernobius at BioLib
  6. ↑ C.Linnaeus: Systema Naturae per Regna tria naturae, secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis. Tomus I. Editio decima, reformata first description by GDZ
  7. ↑ Heinz Freude , Karl Wilhelm Harde , Gustav Adolf Lohse (ed.): Die Käfer Mitteleuropas . tape  8 . Teredilia Heteromera Lamellicornia . Elsevier, Spektrum, Akademischer Verlag, Munich 1969, ISBN 3-8274-0682-X .
  8. ↑ Coleo-net, erroneous "deep dotting"
  9. ^ Schmidl J & Bussler H 2004: Ecological guilds xylobionter beetles Germanys.-Nature protection and landscape planning 36 (7); Stuttgart as XLS (No. 854) ( Memento from December 27, 2015 in the Internet Archive )
  10. ↑ Information on biological pest control ( Memento from February 11, 2015 in the Internet Archive )