Lesson #3: Harvest season

>> Just like with any other livestock, raising the larvae ends with their harvesting and euthanizing. Rearing mealworms – harvest.

rearing mealworms harvest

This chapter does not concern you, of course, if you plan to keep the current population of larvae for further breeding. In that case, please proceed to the next chapter.

The harvest time can be recognized when the first mealworms begin to pupate. Pupation is hard to miss, fresh pupae are snow-white. We can even recognize pupation of mealworms in advance. Large and plump larvae will “fall asleep” – they assume the shape of the letter “C” and stop moving until they pupate (approximately 1 week).

Rearing mealworms – harvest

Larval size

Unfortunately, even though all individuals in one box are roughly the same age, they never grow at the same rate. This is due to several factors.

The most important of these is, of course, the age of the larvae. Larvae born earlier have a better chance in the competitive battle for food. Although there seems to be enough food in the box at first glance, it’s not the case. Freshly hatched larvae are miniature and, as a result, consume the smallest pieces of substrate first. If there aren’t enough such pieces in the substrate, older larvae will gain a slight growth advantage. This can be multiplied in the subsequent competition for food and, especially, water. Water intake is crucial for the speed of larval development, and the uneven distribution of large pieces of vegetables in the box provides additional opportunities for stronger individuals to develop faster. So, while some voracious eaters are already morphing into pupae, we will still have a significant portion of delayed stunted individuals in the same box, even by weeks.

rearing mealworms harvest

The diversity of larval sizes can be reduced by several factors.

  1. Raising individuals of the same age in one rearing box. This means, in practice, harvesting egg clusters from the parental system more frequently (preferably every 2-3 days).
  2. Adequately fine substrate
  3. Uniform distribution of liquids – for smaller larvae, spray the entire surface of the box. For larger larvae, chop vegetables into small pieces and spread them evenly.


Although we adhere to the three principles mentioned above, achieving complete uniformity in mealworm size in the box is not possible. We have several options for the harvest:

  1. Harvest the entire box at once, regardless of losses.
  2. Get a sorting sieve with a 2.5 mm mesh size and harvest only sufficiently grown individuals.
  3. Manually sort large worms from small ones.
  4. Harvest pupae for several weeks (for further breeding) until at least the majority of the population reaches the appropriate size. Then harvest the entire box at once.

In order to allow their digestive tract to be completely cleansed (in an empty container or, preferably, on a sorting sieve for frass), we always let the mealworms fast for 24 hours before their consumption.


Biologists argue that insects do not feel pain, certainly not in the same way as mammals. Insects have sensitive thermal senses, but some scientific studies suggest that insects will avoid heat that might harm them, but not unconditionally. For example, when it comes to food, insects are willing to voluntarily expose themselves to high temperatures that could injure them.

The ethical question of euthanizing a cultivated animal is always a matter of lengthy ethical discussions. The EU permits various methods for insects, including “freezing, dry freezing, crushing, suffocation with carbon dioxide, immersion in boiling water, drying, and euthanizing with hot steam.

Principles of Insect Production

You can find not only the methods of euthanizing but also other general principles of insect farming for human consumption on the food safety information website.

Rearing mealworms harvest

Rearing mealworms – harvest

Most farmers/cooks euthanize mealworms by plunging them into boiling water. Death occurs within a fraction of a second, and it simultaneously sterilizes the insects. We strain the water through a kitchen sieve, and the mealworms are then ready for further processing.

A somewhat more humane option to some might be the “cooling method” (1 hour at 6-7°C), followed by freezing (24 hours at -20°C). This method requires some extra energy, but if you plan to store the harvested mealworms in the freezer anyway, you can choose this method.

Pupae >>

Did you like the article about Rearing mealworms – harvest? Do you have any questions or comments? Join the discussion!

Leave a Review

Your email address will not be published. Required fields are marked *

Scroll to Top