Plant growth regulator-mediated anti-herbivore responses of cabbage (Brassica oleracea) against cabbage looper Trichoplusia ni Hübner (Lepidoptera: Noctuidae)

- Cabbage were treated with the plant elicitor, Actigard50WG (BTH), with and without an enzyme inhibitor, quercetin
- Measured increased levels of indole glucosinolates in the salicylic acid (SA) induced cabbage leaves
- Observed reduced growth and glutathione S-transferases (GSTs) activity in cabbage looper larvae after feeding on cabbage
- A combination of BTH and quercetin have a synergistic effect for protection from diseases and herbivores in Brassica crops

Plant elicitors can be biological or chemical-derived stimulators of jasmonic acid (JA) or salicylic acid (SA) pathways shown to prime the defenses in many crops. Examples of chemical elicitors of the JA and SA pathways include methyl-jasmonate and 1,2,3-benzothiadiazole-7-carbothioate (BTH or the commercial plant activator Actigard 50WG, respectively). The use of specific elicitors has been observed to affect the normal interaction between JA and SA pathways causing one to be upregulated and the other to be suppressed, often, but not always, at the expense of the plant's herbivore or pathogen defenses. The objective of this study was to determine whether insects feeding on Brassica crops might be negatively affected by SA inducible defenses combined with an inhibitor of detoxification and anti-oxidant enzymes that regulate the insect response to the plant's defenses. The relative growth rate of cabbage looper Trichoplusia ni Hübner (Lepidoptera: Noctuidae) fed induced cabbage Brassica oleraceae leaves with the inhibitor, quercetin, was significantly less than those fed control cabbage with and without the inhibitor. The reduced growth was related to the reduction of glutathione S-transferases (GSTs) by the combination of quercetin and increased levels of indole glucosinolates in the cabbage treated with BTH at 2.6 × the recommended application rate. These findings may offer a novel combination of elicitor and synergist that can provide protection from plant disease and herbivores in cabbage and other Brassica crops.

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