Advanced methods to increase the egg production of flour beetles (Tenebrio molitor)

A constant quantity of healthy and robust young larvae is essential for industrial insect rearing. In this article, the egg production and young larval hatching of flour beetles (Tenebrio) will be explained in more detail.

Laying performance – how many eggs a flour beetle lays

The literature is divided on how many eggs a flour beetle carries. Depending on the source, this varies from a few hundred (Riberio N. Abelho, M. / Costa, R. A, 2018) to thousands of eggs per female flour beetle. There are also very large differences in the egg-laying rate (how many eggs are laid per day by a female). While some experts assume 1-2 eggs per day per female, studies from other sources have shown up to 13 eggs (Cotton, 1927 and Frooninckx et al., 2022) per day. It is self-explanatory that these differences have a significant influence on the size of reproduction, parent rearing and the number of eggs laid.

The differences are partly due to the differences in climatic conditions. At lower temperatures, the beetles are less active and therefore lay fewer eggs. At the same time, the eggs are relatively small and difficult to count. At the same time, the eggs are eaten by the beetles and young larvae, which makes counting even more difficult. Another important factor is the food that the beetles receive. A distinction should be made between wet and dry food. Carrots are often used as moist food. They are relatively inexpensive and are available and storable in large quantities. When feeding, they can be easily crushed and retain their moisture for a relatively long time, allowing the beetles to feed on them for up to two days. They also do not stick to surfaces or to the beetles and do not go mouldy if not eaten. This makes them very attractive as a source of moist food. In addition to water, they also provide the beetles with vitamins and fibre.

The additional feeding of dry feed also has a positive effect on laying performance. For example, pellets with a relatively high moisture content and soft structure (for easy eating by the beetles) can be used as dry food. Care must be taken to ensure that the amino acid profile, fatty acids and important minerals are tailored to the beetles. There should always be enough food for the beetles to fully utilise before new food is added.

While only the female can lay eggs, egg production in females is up to 5 times higher when they are constantly surrounded by males, rather than when they are fertilised only at the beginning of the cycle and then separated (Drnevich et al, 2001).

Beetle density can also significantly influence the number of eggs if eggs are eaten through cannibalistic behaviour (Kuriwada, 2009 / Zim et al., 2022). The more beetles there are in an area, the greater the loss of eggs.

The age of the beetles also plays a major role in the number of eggs. Morales-Ramos et al. (2012) were able to show that laying performance falls after just three weeks.

How long does a flour beetle lay eggs

In addition to the laying performance, the laying period is also significantly dependent on the climatic conditions and the beetle feed. Experts agree that the laying period depends on the fixed number of eggs in a female. This means that if the beetles are not fed for a long period of time, for example, they will lay little or no eggs during this time, but will start laying again as soon as they have enough food. As they have laid few eggs and therefore carry even more eggs, their laying period is increased.

The aim of industrial breeding facilities is to ensure that the flour beetles lay as many eggs as possible in as short a period of time as possible. As a flour beetle lives for up to a year (Macalester College. (2023), he must be brought to lay eggs.

To achieve this, it is important that the flour beetle is exposed to little stress, can safely lay eggs, finds the optimal environmental conditions and always has sufficient (and balanced) food (Rho, M. S. & Lee, K. P. (2016)). Especially if pellets are also fed, the laying period can be increased by up to a month. However, it is important to note that the age of the beetle also has an influence on the survival rate and robustness of the young larvae, which can also affect the growth of the larvae in the first few weeks.

Hatching from the eggs

Young larvae hatch from freshly laid eggs after an average of seven days (Macalester College. (2023)). It should be noted that some hatch two to three days before and after. Here, too, the environmental conditions are decisive for egg development. When it is cooler, the development and hatching of the young larvae takes considerably longer and the hatching range (when the eggs laid on the same day hatch) also increases significantly.

During hatching, the young larva forces its way out of the egg, leaving the egg shell behind. This is relatively soft and can be compared to a larval skin in terms of consistency.

Cannibalistic behaviour – voracious triangular relationship between eggs, young larvae and beetles

Cannibalistic behaviour is often observed in the absence of moisture. Beetles eat eggs that have already been laid (even their own) as well as newly hatched young larvae. The young larvae themselves can also eat eggs or significantly younger young larvae. Stress, for example by sieving the nutrient substrate, can encourage this behaviour.

One way to reduce this behaviour somewhat is to always provide the beetles with sufficient moisture. This reduces their search for food and the likelihood that they will come across a laid egg.

Solution and insights from SmartBreed

SmartBreed has developed an egg depot for the beetles, where the beetles live throughout their entire egg-laying cycle. They are fed there and can lay their eggs in the egg tray at any time. This means that the beetles are not exposed to stress and have optimum access to food and egg-laying sites. The eggs cannot be reached by the beetles, which means that they cannot be lost through feeding. The eggs remain in the egg tray until they hatch, which means that they cannot be damaged by sieving or other vibrations.

As soon as the young larvae hatch from the eggs, they automatically fall from the egg tray into the collecting box below. This also prevents the young larvae from eating the eggs. The SmartBreed solution maximises the survival rate of the young larvae. Laying performances can be achieved that are at the top end of the literature. At the same time, regular feeding with pellets can keep the laying period high for up to 3 months.