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More about our AI transparencyRound hole in a hazelnut? The hazelnut weevil (Curculio nucum) leaves the nut from the inside. Identification guide, feeding traces and garden practice in the natural garden.
In brief: At the end of August, you sometimes find small, almost perfectly round holes of about two millimetres on hazelnuts (genus Corylus). They are usually not an entrance, but the exit of the hazelnut weevil (Curculio nucum). Its larva develops inside the nut, eats the kernel and later gnaws its way out through the hard shell. Afterwards it digs into the soil; some of the larvae can remain in diapause for several years, in exceptional cases up to four years. Not every hole comes from the beetle: hazel dormice (Muscardinus avellanarius), mice and red squirrels (Sciurus vulgaris) leave different traces. For you as a garden owner, this means: look closely, compare traces and understand the cycle in a natural garden.
If you stand under your hazel tree in late summer, you may discover empty shells or nuts with a small hole. At first glance this looks like damage from outside. But appearances can be deceptive. Here the perspective shifts: the culprit did not crawl in from outside, but came out. The hole is therefore not an entrance but an exit. Only when you understand this direction does a round hole become a biological trace.
For nature-interested garden owners this is exciting, because you can observe the process right on your doorstep. You do not need a microscope. A magnifying glass, a ruler and a little patience are often enough to learn more about the hazelnut weevil and other animals on your hazelnuts. What matters is that you do not interpret the traces too hastily. A hole can come from a beetle, a broken-open shell from a red squirrel, a round, gnawed opening from a hazel dormouse. The differences are subtle, but often recognisable.
The hazelnut weevil (Curculio nucum) is a small weevil (family Curculionidae). Weevils are a group of beetles with a more or less elongated snout. It is not in your garden to annoy you — hazelnuts are simply its nursery. Much earlier in the year, the female lays her eggs on or in young, still soft hazelnuts. At this point the nutshell is not yet hard. The larva hatches and develops protected inside the fruit. There, something happens that you cannot see from outside: it eats the growing nut kernel.
That is the crucial point. An infested hazelnut can look remarkably inconspicuous from outside. You do not automatically recognise infestation from a withered surface or an early hole. The development takes place hidden away. Only when the larva has grown sufficiently does it become active from inside. For you this means: if you find a nut with a small round hole, the actual story is almost over. The larva has lived inside for weeks and eaten the kernel. The hole is only the final act.
Once the larva is sufficiently developed, it must leave the hazelnut. To do this it gnaws its own exit through the now hard nutshell. What remains is a characteristic small, almost perfectly round hole about two millimetres in diameter. This hole normally does not show where the hazelnut weevil entered. It shows where its larva came out.
This is more than splitting hairs. Many people assume that a hole in a nut is always an entry hole from outside. In the hazelnut weevil, however, the visible round opening is the exit. The larva has worked its way from inside to outside. At this point the nutshell was already hard enough that it could not simply be broken open. The larva had to gnaw. That is why the edge is often round and comparatively even. If you open such a nut, the kernel is often missing, because that is exactly what the larva had fed on.
An important note: there is also an earlier egg-laying hole on young, soft nuts. But this is not the conspicuous round hole that you find at the end of August. Egg laying takes place much earlier, larval development occurs inside, and only later does the larva leave the nut through the exit it has gnawed itself. So if you see a round hole in late summer, many signs point to the larva's exit.
After leaving the hazelnut, the story is far from over. The larva falls or otherwise reaches the ground and burrows into it. There it spends the next stage of development. Particularly astonishing: not every larva continues its development immediately in the following year. Some of the larvae can remain in the soil for several years. In exceptional cases, periods of up to four years are described. Diapause here means a resting state in which development is delayed.
For the garden this means: an empty hazelnut is not just a leftover from last autumn. It can be part of a life cycle that extends over several years. The larva disappears into the soil, and something takes place there that you usually cannot see with the naked eye. So if you find holes again next year, they may come from an earlier generation. The hazelnut weevil is thus an example of how strongly garden ecology is connected to the soil. What appears above ground as a small hole has an underground continuation.
Empty hazelnut shells are biological traces. Not every hole comes from the hazelnut weevil. Hazel dormice (Muscardinus avellanarius), for example, open hazelnuts in a different way. Typical are larger, round gnawed openings with a comparatively smooth inner edge and characteristic tooth marks. Various mice also leave their own gnawing marks. Red squirrels (Sciurus vulgaris) process hazelnuts completely differently and can break open or split the shell. A nutshell lying around is therefore not simply rubbish. It can be a biological trace.
The following table summarises the most important distinguishing features. It is intended as a guide, not as an absolutely reliable identification characteristic. Feeding traces can vary, and not every nut can be clearly assigned.
| Animal | Typical trace on the hazelnut | What to look out for |
|---|---|---|
| Hazelnut weevil (Curculio nucum) | small, almost perfectly round hole of about 2 mm; often as an exit hole | hole in hard shell; nut may have looked inconspicuous externally for a long time; kernel often missing |
| Hazel dormouse (Muscardinus avellanarius) | larger, round gnawed opening; comparatively smooth inner edge; characteristic tooth marks | do not confuse with the 2-mm hole |
| Mice and voles (various rodents; voles = Arvicolinae) | their own gnawing marks | identification is often difficult; traces can vary depending on species |
| Red squirrel (Sciurus vulgaris) | shell is broken open or split | usually not a small round hole |
If you find a nut, first look at the size of the opening. A hole of about two millimetres that is almost perfectly round fits well with the hazelnut weevil. Then check the edge: is it smooth? Are there tooth marks? Is the shell broken open over a large area? Such details help you to classify the trace better. You will not identify every nut with certainty, but with a little practice you can recognise typical patterns.
Would you like to discover the hazelnut weevil and other traces yourself? Then you can start with a simple method. It is suitable for any garden and requires no special equipment.
Collect empty shells and nuts with holes. Note whether they are still hanging on the tree or already lying on the ground. Record the date, because the timing can help with interpretation. The end of August is the typical period for round holes. This can vary depending on region and weather.
Look at the opening with a magnifying glass. Measure it with a ruler or compare it with a known measurement. A hole of about two millimetres is a strong indication of the hazelnut weevil.
Check the shape. Is the hole almost perfectly round? Does it look gnawed?
Examine the inner edge. For the hazel dormouse, larger, round gnawed openings with a comparatively smooth inner edge and characteristic tooth marks are typical. For the hazelnut weevil, the small round opening is the focus.
Look out for broken-open shells. Red squirrels can break open or split the shell. That looks different from a clean small hole.
Open the nut carefully. If the kernel is missing and the larva has left the nut, that fits with the hazelnut weevil. The larva eats the growing nut kernel before it emerges.
Photograph your find. Place a ruler next to it. That way you can later compare it with other pictures or document your observation.
Observe over the season. If you look again and again over several weeks, you may recognise when the holes appear. That is more exciting than just a single find.
Do not disturb any animals. If you suspect that a hazel dormouse has been at the nuts, just observe. A reliable identification often needs more than a single nut.
Let nature take its course. The hazelnut weevil is part of garden life. You do not have to fight it. If you want to harvest nuts, you can sort out infested nuts and leave them in an undisturbed spot. Pesticides are not a solution in a natural garden.
A natural garden is not a place where everything goes according to plan. It is a mosaic of habitats. The hazelnut weevil shows you how closely above-ground and underground processes are connected. The larva lives first in the nut, then in the soil. It is therefore part of a cycle that can extend over years. Such microhabitats make a garden interesting for many species.
Further information on this is available on NaturKompass. Around 70–75% of wild bee species nest in the ground, and over 1,500 beetle species depend on deadwood habitats. These figures make clear how important undisturbed soil and wood areas are. So if you leave one corner of the garden untidy, many species often benefit. The hazelnut weevil is only one example. It is a reminder that a garden is not only there for us, but also for animals that live hidden away.
As helpful as feeding traces are, they have limits. Not every nut can be clearly assigned. Feeding traces allow only partial interpretation. This means: you can find clues, but sometimes you need several nuts or additional observations.
No. It attacks hazelnuts, not people. You can observe it safely.
From a natural garden perspective, no. The beetle is part of ecological processes. If you want to harvest, you can sort out infested nuts. We do not recommend pesticides.
The larva has eaten the growing nut kernel and later left the nut through the round hole.
Not in the way that is often assumed. The larva gnaws its own exit. The hole normally shows where it came out, not where the beetle crawled in.
The larva reaches the ground and burrows into it. There it spends the next stage of development. The larvae may remain in diapause for several years — exceptionally, for as long as four years.
Egg laying takes place much earlier on young, still soft hazelnuts. The conspicuous round hole in late summer is the larva's exit.
No. Check size, edge, tooth marks and whether the shell has been broken open. The table above gives you initial pointers.
Just observe them without disturbing them. Reliable identification based on feeding traces is possible, but not always easy. For official surveys there are special methods, such as the Dormouse nut hunt of the People's Trust for Endangered Species.
If you like, pick up an empty hazelnut on your next walk. Look closely. Perhaps the small round hole is not a sign that something crawled in. Perhaps it is the door through which something went out. It is precisely this perspective that turns the garden into an expedition.
original video: Der Täter kam nicht von außen – er kam aus der Haselnuss - Gartenexpedition #naturgarten #shorts
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Last updated: 10/09/2026. Corrections can be reported via the feedback form.