Ximena, While we don't work with the cabbage aphid, there are several ways in which we reduce or exclude natural enemies of the aphid species we work with (generally in the context of a field experiment where parasitoid abundance can be very high).
The first is a non-woven breathable material used in the shipment of potted plants (we buy ours here: http://www.palmtreepackaging.com/content/breather, but I imagine other vendors exist). We typically secure them to the plant using twist ties to close the open ends. These are usually effective in keeping out even the smallest parasitoids as well as larger predators and are lightweight for delicate plants. A few caveats: 1) wind and water will weigh the bag down and may damage the host plant. We use garden bamboo rods to hold the bag and plant upright. 2) The material will tear and disintegrate under high UV conditions (within a few weeks at our high elevation field sites). 3) If parasitoids or predators do get into the bag, they can really wreck havoc in the enclosed space, so vigilance for their signs (eggs, larvae, aphid mummies) is critical. The second approach is to use a lightweight mesh, such as a polyester organza sewn or glued into sleeves that fit over the plant. This is very similar to the above with the advantage of better water shedding ability. Lee et al. 2005 (doi: 10.1603/0046-225X-34.1.115) suggest a mesh size of 2,500 cells/cm^2. The third approach, which I admit I have not personally used, is to place yellow sticky cards (i.e. those used in greenhouses to control hemipterans). While these wouldn't eliminate your problem, there is literature that shows they reduce enemy abundance when they are placed next to aphids. If you are interested in the survival of alate aphids, this may not be a good option. WP
