Repellents

• Some companion plants contain substances that repel particular pest species. Planting tansy near fruit trees, for example, keeps Japanese beetles away from the trees. Chamomile drives away cabbage moths, while parsley discourages asparagus beetles.
  
  

Toxins

• Substances in some plants are toxic to pests that ingest them. Larkspur, for example, kills Japanese beetles. Cornell University's gardening website adds that certain varieties of marigolds destroy nematodes, tiny worms that threaten some plants.
  
  

Considerations

• Some of the species commonly used as companion plants may harm humans even as they repel or kill garden pests. The Home and Garden Site warns, for example, that the repellent plant tansy may kill livestock that ingest it. Four O'Clocks and larkspur may also have toxic effects on humans.