From a strictly evolutionary perspective, there's no reason why female beetles should be promiscuous. Unlike males, taking on multiple partners won't allow females to spread their genes more widely, and too much sex can actual shorten the female beetle lifespan.
But according to research from the UK's University of East Anglia, there may be a very good evolutionary reason for female beetles to seek out multiple partners: it's a way of warding off the ill effects of inbreeding. In small populations, beetles are often forced to breed with those closely related to them. This means that any dangerous genetic defects carried by the mother are also likely to be carried by the father, resulting in a greatly increased risk of illness for their children.
Matthew Gage and his research team discovered how exactly mating with multiple partners can work to counteract this. Writing for ScienceNOW, Elizabeth Pennisi describes the process:
When allowed to mate with just one male, inbred females have 50% fewer surviving offspring than outbred counterparts that mated with non-kin. But the number of offspring is equal if the inbred females breed with five males, the team reports today in Science. The inbred females are apparently able to weed out sperm from kin that would lead to less fit offspring. If they mate with just one male, however, they don't have that option.
The researchers also ran an experiment in which half of a beetle population was forced to inbreed for several generations, while the other half was allowed to breed with unrelated partners. When the researchers then left these two populations alone for several more generations, the inbred half quickly developed promiscuous behavior, while the females of the outbred half generally just stuck to single partners.
This research is some of the first to provide a strong evolutionary reason behind multiple mating in females, something that otherwise defied clear biological explanation. It's very possible that other species in small populations could develop this behavior, and depending on how deeply this behavior is woven into the evolutionary past, it may have something to do with the existence of promiscuous behavior, even in species that have lost this ability to mix and match genetic material.