Onion Thrips rae agricultural crop pests. Thrips scrape the plant stem or leaf and suck up the sap that flows from the injury. A large infestation can slow a plant's growth and the vegatibles can be poorly formed. Thrips are also known to transmit a virus causing wilt. Onion Thrips are especially injurious to onions, but they feed on carrots, beans, and beans, to name a few. During spring and summer they can produce a new generation every two weeks.