Any ideas on what's up with this broccoli? Last year all the plant had this...

Asked September 21, 2019, 12:08 PM EDT

Any ideas on what's up with this broccoli? Last year all the plant had this right away as heads were forming, in a well-mulched raised bed, regular water and sun. It stunted and twisted them. My friend moved the bed, tried different seed and things were OK until this month.

Ramsey County Minnesota

1 Response

Thanks for your question.

I believe this damage is due to larvae of the Diamondback Moth. Among other things, these larvae can lead to deformed broccoli heads. Your picture showed a large number of black spots right below the broccoli heads. This is characteristic of Diamondback Moth larvae and is their frass. Frass is their excrement. These larvae are quite small (slightly less than 0.5 inches in length) and if agitated, they start to wriggle violently. Unless you were specifically looking for them, they would be difficult to see. These larvae form a cocoon in late summer and adults soon emerge. These adults spend the winter in the soil and plant debris and emerge the following spring to lay eggs. It is possible that if you bought your broccoli at some gardening center, these plants might have had either eggs or larvae on them at the time. Since you had this same problem last year, was the same gardening center involved on both occasions? If so, you should consider a different broccoli source for 2020. As another precaution for 2020, consider planting your broccoli in a different location.

At this point application of a pesticide would probably not do much good as the damage is done. For 2020, consider applying a pesticide as soon as you plant the broccoli. The following will provide more information on these and other points:

For what it is worth, these larvae are the single most destructive pest of broccoli in Minnesota.

Good Luck!!