Beta-Rays are a means to multiplying Metroids within 24 hours. Normally, the species reproduce via a Queen, but if they are exposed to extremely high Beta-Rays, Metroids are capable of asexual reproduction via division, in a process much like a cell undergoing mitosis. This method of propagating Metroids seems to be restricted to the Infant and Larva stages of a Metroid; whether Metroids in the later stages of their natural life cycle can be multiplied in the same manner is never explored.
The Space Pirates have used beta-radiation since the time they stole the first discovered Metroid specimens from the Galactic Federation's Space Research Vessel Marina. When the Pirates reached Tourian, their headquarters on planet Zebes, they continued to expose them to beta-rays in order to greatly increase their initially low number of Metroids. This allowed them to perform any number of research and experiments on the species in the hopes of exploiting their unique powers and ultimately achieve ways in using the species as bioweapons against galactic civilizations.
This beta-ray method is likely what helped the Pirates maintain a steady supply of Metroids throughout the years that consist the entirety of the Metroid Prime series following the downfall of Zebes. Interesting to note is that the highly radioactive mutagen known as Phazon has demonstrated on several occasions the power to multiply Metroid larvae. In fact, some Phazon-mutated strains of Metroid acquire the ability to spawn larvae.
Following the Space Pirate organization's multiple defeats at the hands of bounty hunter Samus Aran and the subsequent destruction of important research bases along with their Metroids, their supply of specimens eventually ran out. To prevent future misuse of the now-dreaded species, the Galactic Federation ordered the extinction of the Metroids on their natural homeworld, SR388; this effectively left the Pirates with no more specimens to acquire anywhere until the discovery of the last Infant Metroid on the Ceres Space Colony. After successfully stealing it, the Pirates were quick to use the tried-and-true beta-rays on it, only this time instead of exposing several Metroids to the radiation simultaneously, the sole Infant served as the entire backbone of the Metroid propagation.
Time would be against the Space Pirates however, and in their desperation to resupply themselves with a satisfactory amount of Metroids before a major retaliation from Samus and/or the Galactic Federation occurred, the marauding race attempted to include genetic cloning alongside the beta-radiation. The clones would turn out to be failures while the Metroids born from beta-rays had no issues, but the problem remained of the last Metroid being the initial sole target of all radiation exposure.
Continued exposure to beta-Rays may have given side effects on the last Metroid: growth through the natural life cycle is known to have become stunted, and it unnaturally increased in size. It is never confirmed in official media if the beta-rays was the main factor behind this anomaly. The non-canon Super Metroid comic presented excessive beta-ray exposure as being the sole cause.
- The Super Metroid PAL guide's backstory implies that the Federation were the first to use Beta-Rays on Metroids when the species was first discovered. This would thus be followed by the Pirates emulating the work achieved by the Federation.