Many years studying defenses in trees that are induced by insects made us realize that such dynamic behaviors are transcriptionally controlled. Lacking genomic tools for working with most trees (at the time) we changed our focus to Arabidopsis, a model with the requisite knowledge base.
The major category of chemical defense in Arabidopsis and its family, Brassicaceae, are glucosinolates, which are synthesized from amino acids and glucose. We (and others) found that concentrations of glucosinolates increase in response to attack by some insects (Mewis et al. 2005). Some classes of glucosinolates are more responsive to certain insects than to others, and some insects fail to elicit any significant change. Hence, biochemical responses to insects can be more-or-less species-specific.
Phenolics such as anthocyanins and flavonoids are also responsive to insects and wounding as well as many other stressors. It is unclear whether these phenolics really have protective value to the plant.
Because defense induction is transcriptionally regulated, we have done several experiments showing that the transcriptome responds differentially to each of four insect species and wounding. So plant responses are specific to attacker at the transcriptional level as well. Changes in transcription factor populations were particularly important to insect-specific responses and we found a few TFs whose expression was specific to responses to insects (Appel et al. 2014; Rehill et al. 2014).
Some of this work on chemical and transcriptional responses attempts to take other factors and constraints into account. We have studied Arabidopsis responses coupled with responses to pathogen attack (Appel et al. 2014; Nguyen et al. 2015) as well as light intensity and wavelength.