Parasitoid wasps are limited in their effectiveness when there are not enough of their prey for them to maintain a population large enough to protect plants. Also, they are not as effective when several pests, not all of which are the wasps' prey, are attacking the same plant.
Work Step by Step
Parasitoid wasps are very specific protectors of plants, so imagine how this specificity could be a problem. A parallel is seen in an earlier chapter on pollinators and specificity.