The 'Botanical Triad': The Presence of Insectary Plants Enhances Natural Enemy Abundance on Trap Crop Plants in an Organic Cabbage Agro-Ecosystem.

Habitat manipulation through the incorporation of non-crop plants such as trap crops (to lure pests away from the cash crop) and insectary plants (to provide resources for natural enemies) into agro-ecosystems is an ecological approach to pest management. In a field-scale study, we quantified the effects of integrating the use of trap crops with insectary plants as a novel method to control pest herbivores in an organic cabbage agro-ecosystem. We hypothesized that pests would be concentrated in the trap crop habitat and suppressed by insectary-subsidized natural enemies in situ. We documented arthropod abundance (both adults and immature stages) associated with (1) two insectary plant species (sweet alyssum, , and buckwheat, ) either alone or in combination; (2) a trap crop mixture of mighty mustard (), red Russian kale ( var. acephala), and glossy collards ( var. italica), and (3) cabbage cash crop ( var. capitata). Trap crops were more attractive to pests than the cash crop. On a per-plant basis, densities of the herbivores , , and were 154, 37, and 161× greater on the kale trap crop than on the cabbage cash crop, and 54, 18, and 89× greater on the collards trap crop than on the cash crop. Insectary plants contributed to the consumption of pests that aggregated on the trap crop. Parasitism of by the braconid wasp was significantly increased, and the abundance of eggs and larvae of the predatory coccinellid beetle was greater on the trap crop in the presence of insectary plants compared to trap crops that lacked insectary plants. The 'Botanical Triad' of cash crop, trap crop, and insectary plants represents a new type of agro-ecosystem manipulation that integrates ecosystem service providers (e.g., predators and parasitoids) within the cropping system.