What can lead to chilled brood in a beehive?

Chilled brood occurs when bee larvae or pupae are exposed to temperatures that are too low for healthy development. The main reason for this condition is the lack of enough adult bees to generate and maintain the necessary warmth within the hive. Adult worker bees are responsible for thermoregulation, and when their numbers are insufficient, they cannot adequately cluster to generate heat, especially in cooler weather.

In a scenario where the population of adult bees is low, they are unable to provide the necessary warmth for the brood, leading to a failure in normal development. This can be particularly concerning in early spring or late fall when temperatures drop significantly. The other factors mentioned in the choices, while they may impact hive health, do not directly relate to the primary cause of chilled brood. Therefore, the connection between insufficient adult bees and chilled brood effectively showcases the importance of colony strength in the maintenance of appropriate temperature conditions for optimal brood rearing.