I have some experience working with P.brassicae, a close species. I have seen some unfertilised eggs, albeit rarely. They would be white-ish and easily distinguishable. My guess would be that since this phenomenon occurs in P. brassicae, which lays batches of eggs, it would be possible for P. rapae, although its oviposition mode is different.

I have experience working with Lobesia botrana, a monandrous species, and I’ve seen unfertilized eggs layed by unmated females. Similarly as Nicolas described them, those eggs are whitish and after 3 days you can easily recognize 2 black dot inside that represent the eyes of the embryo. It is quiet strange that polyandrous species do not mate even once. See Wedell and Cook 1999.

We also perform Pieris rapae oviposition experiments. I know that Pieris brassicae will lay unfertilized eggs after a few days if no mating occured. This will probably also be the case for Pieris rapae although I never tested this. Probably oviposition of unfertilized eggs will not occur within 24 hours. I presume you take newly hatched females?

I would be very surprised if the oviposition preference would change if the eggs are not fertilized but also this I never tested!

If you want to make sure eggs are fertilized, then first you should determine if females are sexually mature (i.e. how long it takes for sexual maturity) and could observe calling behaviour under your test conditions. Once you know females are mature, set up 1 mature female + 2 males in mating cages (either observe for successful matings and only use those females, or wait until you notice consistent oviposition behaviour). I've done this in Trichoplusia ni and it worked really well.

it usually happen when you put the adults in small cages and scarcity of diet. I also face the same problem for p. brassicae. may you tell me the diet composition you used for the feeding of immatures.

Thanks for your answers ! I guess the female I'm using are maybe not all mature, cause they are newly hatched. But I was thinking that they will get mature during the 24hours.. I will now try with 48 hours to see if I get more eggs. Or 24 hours with males, without host for the eggs, then 24 hours with the host. Some more stuff to test before starting my experiment I guess :)

By the way, I have feeders with sugar and water (and dental roll) in all my cages, so they're not suppose to be hungry !

I did some lifetime fecundity work with P. rapae.... It's not uncommon for them to take 2 or even 3 days to become reproductive. If you want to know if they've mated or not then it is possible to dissect them to look for the nuptial gift. It can also give you a good indication on how egg production is going, because you can tease out the whole reporductive system.

http://202.196.80.249/jpkc/zbjiangjingwei/%E8%AF%BE%E7%A8%8B%E8%AE%BA%E6%96%87%E5%8F%82%E8%80%83%E8%B5%84%E6%96%99/%E8%94%AC%E8%8F%9C%E5%AE%B3%E8%99%AB/The%20effect%20of%20host%20acceptability%20on%20oviposition%20and%20egg.pdf

I studied Cnepasia jactatana (Tortricidae) sexual behavior and found that unmated and "old" females may lay many unfertile eggs.....there are not in batches and ill remain white after 8 d of incubation.

I would check the female reproductive system to see if the have mature eggs ready for oviposition. You can try newly hatched, 24, 48, or any time. Dissect the ovary and stain them with carmin red in saline solution (normally 1 minute is enough). Ready to fertilize eggs will remain white or perlish, unmature will absorb the red stain. Hope it helps.

Did you ever determine is p.rapae lays unfertilized eggs?

 No sorry, I hadn't had time to investigate further on this topic, it was a short term project. But all previous answers suggests that they may leave unfertilized eggs, as well as my trials, but I never seriously tried to prove it.