Plant sap which may contain some alkaloids, alcohols, or a sugar and mineral and lipd enriched material will attract leidpteran insects.Mainly lipids staring from C21 to C28 are used by these in insects for synthesis of hormones, while heterocyclic compounds or compound with carbon source are utilized for production of pheromes which are utilized for sex specific communication. 

We know, which substances are released by plants (years ago characterized by their effective carbons only, now much better). This information usually done by gas-chromatographs needs another study, which will tell you which particular species is responding to which substance(-s). This is usually done by observing behavior or measuring responses of particular nerves in the iformation system of insects. It will also help you, if you know the functioning state of trees (e.g., appearance and magnitude of drought stress or hypoxia stress, estimated via sap flow patterns). So a good work for labs as well as open forests.   Jan

Dear Peter,

meybe the attach can help you. some chapters have good point about this subject.

Some aerosols released by plants could play an an important role.

Thank you all for your replies and Fatemeh for the book. Clearly, there is much to be done in this field.

they are FACs: Fatty acids-aminoacids conjugates. fattyacids comes from the source of regurgitate of insect fed on the plants and amino acids from plant sap, if I am not mistaken. Linolenic acid is the most potent fattyacids which conjugates with amino acids are very efficient in emitting the cues for attracting natural enemies. the attachment may help to get a clear idea..