Digger Spade, MG Plant Detective
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| Cabbage root maggot damage. Source |
The client came in with pictures of his cabbage and cauliflower transplants. Some looked fine and some, though they had grown well since they were transplanted, were severely wilted - though not quite dead. The clients questions: What caused this? And What can be done about it?
We asked if he had looked at the base of the plant to see if it had been chewed on. He had, it hadn’t. We asked if he had pulled any up to look at the roots, he hadn’t. Has he had this problem in the past? He hasn’t, though he has only grown these crops twice before.
It seemed clear that the problem was due to the robustly growing tops having quite suddenly lost access to water and nutrients. Since there was no sign of damage at ground level, which could be due to cutworms, it seemed most likely that the problem was below ground and the likely candidate the cabbage maggot. We suggested that the client dig up some of the wilted plants to see whether there were any fine roots, or just the main root that is a continuation of the stem. If that were the case, we would be pretty sure that the cabbage maggot was the culprit. And he might find white larvae, though they are only 3/8” long when mature. But we also suggested how he might protect his cabbage family crops if the evidence supported our supposition. We asked if he had seen wireworms in his plot but he had not.
The cabbage maggot (Delia radicum) is the larva of a small fly. The larvae feed on roots and can kill young transplants and damage the roots of older plants including radishes and turnips. The flies pupate in the soil and can have two or three generations in one year. Once an area of the garden has been infested, it should not be planted to crucifers for a year or two. The best defense is to protect transplants using floating row covers or a cage covered with insect screen.
At this stage it would not help to put row cover on the remaining plants since the larvae are in the soil and the next generation would be housed with the remaining plants. There are five insecticides listed on PICOL that target cabbage maggot and are available to homeowners. They kill by contact and ingestion. Since the fly doesn't stick around after laying eggs and the maggot feeds underground it is unlikely that either will make contact with a pesticide sprayed on the plant. So for this year it seems best to feed and water the remaining plants to see if they can escape death by a thousand nibbles and next year protect with row cover on a fly-free site.
The client called after he had examined some of the wilted plants. They had no roots other that the main stem.
Resources
Cabbage Maggot
WSU Extension Fact Sheet FS010E
http://cru.cahe.wsu.edu/CEPublications/FS010E/FS010E.pdf
Cabbage Maggot (WSU)
http://hortsense.cahnrs.wsu.edu/Search/MainMenuWithFactSheet.aspx?CategoryId=5&PlantDefId=44&ProblemId=190
Cabbage Maggot (UMass)
https://ag.umass.edu/fact-sheets/cabbage-root-maggot