Parasitoids have successfully been incorporated into biological pest control programs for many years. Plants can influence the effect of parasitoids on herbivores by releasing chemical cues that attract parasitoids and by providing food sources or domatia. Certain parasitoids may be dependent on this plant relationship. Therefore, in agricultural areas where parasitoid presence is desired, ensuring the crops being grown meet all of these requirements is likely to promote higher parasitoid populations and better pest control.
In a sugar beet crop, when only beets were grown, few aphids were parasitized. However, when collard crops were grown next to the sugar beets, parasitism of aphids increased. Collard crops release more VOCs than sugar beets. As a result, the companion collard plants attract more aphid parasitoids, which kill aphids in both the collard and the nearby sugar beets.
In a related study, ethylene and other compounds released by rice plants in response to brown planthopper feeding attracted a facultative parasitoid that parasitizes brown planthopper eggs.
In another study, the presence of plant extrafloral nectaries in cotton crops caused parasitoids to spend more time in the cotton and led to the parasitization of more moth larva than in cotton crops with no nectaries. Since the publication of this study, most farmers have switched to cotton varieties with nectaries. A separate study found that a naturalized cotton variety emitted seven times more VOCs than cultivated cotton varieties when experiencing feeding damage. It is unknown whether this generalizes to other crops; there are cases of other crops that do not show the same trend.
These findings reveal the specific variables a farmer can manipulate to influence parasitoid populations and illustrate the potential impact parasitoid habitat management can have on pest control. In the case of cotton and other similar high-VOC crop scenarios, there is interest in genetically engineering the chemical pathways of cultivated varieties to selectively produce the high VOC's that were observed in the naturalized varieties in order to attract greater natural enemy populations. This presents challenges but could produce promising pest control opportunities.