Insect consumption in late middle-ages in the Northern Europe

The evidence for this is overall scarce, given the low status of peasant food and most of the animals involved in such a frame, even though their sheer number of species to consider is extremely vast.

But despite this comparative lack of evidence, there is some evidence and so my guess from what evidence is available suggests that this was indeed quite common, not only in cases of famine, but also in a few cases as sought after specialties.

What we want to exclude is the very common accidental ingestion of bugs or larvae when eating berries or other food near spoilage. Several insect products, like bee honey, bumblebee honey, honey dew, wax are surely too common now to be considered part of this question for 'traditional European specialties'?

But we have evidence — sometimes very localised, some quite widespread — for a range of habits and customs relating to 'on-purpose eating of insects'.

Very famous and still practised is finishing cheeses with the help of insects, that are then of course eaten with the remains of the cheese. In Italy we still find the banned-for-hygiene-reasons Casu Marzu, In Germany we find the Milbenkäse, in France the Mimolette (though those would technically be arachnides: today not strictly grouped into 'insects'). While the specific 'trademark names' may be a bit later than the medieval period, the practice of eating what certainly many readers here will consider 'just spoiled cheese, inedible' is surely much older.

A list for many other food habits and culinary peculiarities may be:

1. eating head lice (with even some reports from Spain, Hungary, Romania that this was regarded as having medicinal properties)
2. Melolontha bug species were reported as variously famine food or food for children, especially from Ireland, Italy, Romania. In Germany this seems to have been an even somewhat common dish served in a 'crayfish soup style', Cockchafer soup with a rule of thumb being '30 of them satisfy one customer'…)
3. Salted and smoked grasshoppers were eaten in Russia and the Crimea up to the 19th century
4. Locusts — as also already mentioned in the bible, Leviticus & Matthew — were eaten in Southern France up until the 19th century as well
5. Adult Scarab beetles were eaten in Wallachia and Moldavia (Amphimallon pini (Ol.)).
6. The peasants in Lombardy have used the beetle, Rhizotrogus assimilis (Herbst) as food

7. Galls of the ground ivy, Glechoma hederacea L., which are produced by the Cynips glechoma L., have been eaten in France (Cowan 1865). On Crete, inhabitants gathered the very juicy galls of sage (Salvia spp.) for food. Collecting them at the beginning of May, the people of Chania also sold them to neighboring villagers (Fagan 1918). The galls, which were caused by Aulax sp., were esteemed for their aromatic and acid flavors. They were used locally, but also formed a considerable trade product in the eastern Mediterranean region

8. A local traditional habit for children in the historical-geographical region of Carnia in northeastern Italy is eating the sweet ingluvies (the crop) from day-flying moths of the genus Zygaena and its mimic, the moth Amata phegea (L.)

9. Hungarians in Central Europe have consumed the honey stomachs of black-colored carpenter bees, Xylocopa spp.

10. Hungarian children also used to harvest sweet paste from the reed nests made by solitary bees, Hoplitis adunca.

11. In Sweden, ants were used to flavour schnaps, specifically red wood ants, Formica rufa L.: myrbrännvin, or historically known as "Spiritus formicarum". But apparently these exact ant species was also valued for their taste when eaten directly and still alive:

    Consett (1789, p. I 18), in his travels in. Sweden, mentions a young Swede who ate live ants with the greatest relish.

While we have to be consider a relative lack of direct evidence for northern European entomophagy, the indirect inferences that can be drawn from material before and material from other parts of the world suggest this extrapolation as reasonable:

In ancient Greece, cicadas constituted a very popular dish and, according to Aristotle, females full of eggs were juicier than males. […]
A character in a play by Aristophanes remarks: “Are locusts superior in flavor to thrushes? Why! Do you want to fool me? Everybody knows that locusts taste much better!” His compatriot, Alexis, mentions the locust (a grasshopper, whereas cicadas are often erroneously called “locusts” or “locuses”) among the provisions of a poor Athenian family:

For our best and daintiest cheer,
Through the bright half of the year,
Is but acorns, onions, peas,
Ochros, lupines, radishes,
Vetches, wild pears nine and ten,With a locust now and then.

[…] Pliny the Elder (23 AD–79 AD) often referred to the larva of an insect known as “cossus” (probably the larvae of the stag beetle species Lucanus cervus L. and similar species as Cerambyx cerdo L.), considered a delicacy by the Romans. He tells that the epicures of his time considered insects on a par with the daintiest meats and even fed them on flour and wine in order to fatten them and heighten their flavor.

— E.M. Costa-Neto & F.V. Dunkel: "Insects as Food: History, Culture, and Modern Use around the World", in: "Insects as Sustainable Food Ingredients. Production, Processing and Food Applications", Elsevier, 2016. (p29-60) doi

More generally:

There is no reason to believe that European peasants and herders differed from native societies in other parts of the world regarding their ecological knowledge (Lévi-Strauss 1962). Although herders and peasants in traditional Europe had a vast knowledge of the biota including invertebrates, the folk knowledge and use of insects is very little researched (Svanberg et al. 2011). As a result, information about European folk knowl- edge of the wild invertebrate fauna, including their use in healing and nutrition, is scarce. As in other cultures around the world, some insects were appreciated, others disliked or even feared, and some were utilized for various purposes (Brøndegaard 1985; Ulicsni, Svanberg, and Molnár 2016). The Spanish fly, Lytta vesicatoria L., for instance, has been widely used in folk medicine and for increasing sexual pleasure in many parts of Europe (Sandroni 2000; Łuczaj 2005; Stokker 2007; Ulicsni, Svanberg, and Molnár 2016). Only a few ethnographical studies mention edibleness in relation to insects.

[…] Several insects have also been part of the pharmacopeia in pre-industrial Sweden and other Scandinavian countries (and probably in most other European countries), e.g., remedies and tinctures made of Lucanus cervus (L.) (stag beetle), Lytta vesicatoria (L.) (Spanish fly), Dactylopius coccus Costa (cochineal), Kermes ilicis (L.) (kermes), Bombyx mori L. (silk worm), Apis mellifera L. (honey bee), Formica rufa L. (red wood ant), Cynips quercusfolii L. (oak gall wasp) and Diplolepis rosae (L.) (bedeguar) (Linnaeus 1750). Spanish fly and bee products were available in Swedish pharmacies still in the end of the nineteenth century (Rosendahl 1897).

This is based on (including all quotes):
— Ingvar Svanberg & Åsa Berggren: "Insects as past and future food in entomophobic Europe", in: Food, Culture & Society, 16 Mar 2021, (2021) DOI: 10.1080/15528014.2021.1882170

This is summarised as:

From ancient times to the present day, insects have been consumed in many societies throughout the world. In fact, the almost total absence of insects from European menus can be regarded as relatively recent. […]

It was not merely lack of larger food, however, that made man eat insects. The fact that devouring insects was once so widespread and to be found in almost any culture on earth, irrespective of food or protein shortages, indicates that there must have been other reasons that made it worthwhile to look for and collect insects.

— Victor Benno Meyer-Rochow: "Edible insects in three different ethnic groups of Papua and New Guinea", The American Journal of Clinical Nutrition, Volume 26, Issue 6, June 1973, p673–677, doi

Since the focus of this question is now on Northern Europe: we have to acknowledge that there are and were 'a lot of insects', probably also available as food. However, using them as a food source was and is restricted locally somewhat by the climatic conditions. While in Summer small flying things may mount up being a huge pest in the numbers, a large part of the year will be without flying insects in the cold open spaces.

So seasonal variation may be considerable. To then gauge any volume and method for getting at them, as an example:

[…] Melolontha in Europe suggest themselves. The former have been eaten by so many peoples that their food value may be accepted as established, though the civilized world needs convincing about the white grubs. And a boy following a plough could pick up a day's ration of the latter sufficient for a family in a short time.

— Friedrich Simon Bodenheimer: "Insects as Human Food. A Chapter of the Ecology of Man", Springer: Dordrecht, 1951, p61.