Role of plant phenology in mediating interactions between two biological control agents for spotted knapweed

► Examined interaction between two seed insects: fly - Urophora affinis and weevil - Larinus minutus. ► Slower developing knapweed had more flies whereas faster plants had more weevils. ► Weevils attacked seed heads with and without flies, equally. ► Seed heads with multiple flies produced fewer weevils. ► Demonstrates two mechanisms enabling U. affinis to coexist with L. minutus.