Strawberry root borer

Asked October 17, 2020, 8:36 PM EDT

I have discovered small larvae boring into the roots of my strawberry plants right at the junction between the root and the main body of the plant. They consume the root and kill the plant. A photo of the villain is attached. I first started losing plants in July and August but it took until now to discover the cause. I am removing the plants that have been killed including the roots. What else can I do to control the pest? I try to practice organic methods as much as possible but I'm willing to use anything that will work. Thanks for your assistance.

Lane County Oregon

1 Response

This is either the larva of the strawberry crown moth (Synanthedon bibionipennis) or root weevil (Otiorhynchus spp.). The root weevil larva are white with a tan head, and c-shaped. The larva feed on the roots in the fall, winter and spring. With the root weevil you would see chew marks on the leaves in the early summer from the adult weevils. The larvae of the crown moth are also white with a tan head, and 16-25 mm long at full maturity. With the crown moth, when you try to remove the infested plants, the tops may separate from the roots. It is hard to distinguish the two by photo in the larval stage, but the c-shaped body is one clue, as well as feeding marks of the adult weevils. Based on the shape of the larva and feeding location, I am inclined to think this is the crown moth.
For root weevils, adult weevils don't fly but can walk long distances; disking field perimeters can minimize infestation from adjacent areas. Renovation (i.e. pruning off old leaves from the plant) immediately after harvest removes food supply (leaves) for the adults and exposes them to direct sun and increased temperatures. Reduce number of years the strawberries are kept in the ground. Pyrethrin pesticide (organically approved) can be applied at night in the early summer to control the adult weevils.
For the crown moth, removing infested plants is the best approach. I also suggest renovating to remove the eggs, and clean up debris underneath and around the plants. If you start a new strawberry planting, use a new area of your garden. There are no chemical control options for home gardeners for crown moth.
You can find more information on the life cycle of the weevil and crown moth, as well as photos and a few other management options here: https://pnwhandbooks.org/insect/small-fruit/strawberry/strawberry-strawberry-crown-moth, and here: https://pnwhandbooks.org/insect/small-fruit/strawberry/strawberry-root-weevil.