Synergism of induced plant defenses: Alternative strategies for herbivore pests

Stimulating systemic acquired resistance (SAR) by acibenzolar-S-methyl (ASM) or Actigard 50WG Plant Activator negatively affects anti-herbivore defenses and has been documented in tomato and other crop plants. Negative “cross-talk” between the jasmonate and salicylate response pathways increased levels of disease defenses present in the SAR induced plants at the expense of insect defenses. The focus of the project was to synergize the plant inducible defenses with plant extracts containing compounds known to inhibit insect detoxification enzymes. A series of experiments with the cabbage looper Trichoplusia ni (Lepidoptera: Noctuidae) fed on Actigard-induced or un-induced tomato and cabbage plant leaves compared the addition of quercetin, triphenyltin chloride (TPTC) or verapamil at sublethal concentrations. The combination of 10 ppm quercetin and Actigard-treated cabbage leaves produced a significant negative growth effect with T. ni relative to the Actigard-treated cabbage and the control or quercetin treatments.  Similar effects were observed when Actigard-treated cabbage leaves were sprayed with plant extracts containing quercetin and related phenolic compounds. Actigard activates SAR in both cabbage and tomato, but the synergism noted between quercetin and Actigard was only present in the cabbage plants. Higher levels of peroxidase and PPO occur in the induced tomato relative to cabbage, but the effect may be due to other defense compounds, such as glucosinolates, present in cabbage leaf. No significant differences were observed between T. ni catalase, glutathione reductase or glutathione peroxidase activity after feeding on treated cabbage plants. Quercetin may be inhibiting other detoxification enzymes or protein transport which in turn is not affected by verapamil or TPTC, respectively.