Soil-borne pests feed on roots of young vegetable seedlings

Insect pests hidden in the Salinas Valley soil are preventing stand establishment of vegetable crops in selected areas by feeding on the roots of young seedlings.

Cabbage maggots have become widespread in the lighter soils on the east side of Highway 101, and when infestations are heavy while the seedlings are young, they can stunt or kill young broccoli, cauliflower or cabbage plants.

In cool soils with crop or cover-crop residue, springtails, which are usually beneficial in breaking down organic matter in the soil, can become a pest by feeding on the roots of young lettuce seedlings.

Shimat Joseph, a University of California Cooperative Extension entomology farm advisor in Salinas, has spent most of his time this year working to develop management programs for these hidden, root-feeding pests.

The easiest way to monitor for springtails is to leave slices of potatoes or beets on the ground every 50 yards or so for a couple days, and then check these slices for springtails before planting lettuce.

The UC Pest Management Guidelines state there are no materials registered specifically for springtails in lettuce, and there have not been systematic efficacy trials of insecticides on springtails.

But Joseph found this year that insecticides made a difference before replanting one Salinas Valley lettuce field that had been wiped out by the pest.

"I haven't done a comprehensive insecticide trial on springtails; we need a lot of work on insecticide trials. I'm looking forward to springtail insecticide trials next year. But we found some wonderful results with insecticides before replanting an infested field," Joseph said.

Cabbage maggots are even more widespread than springtails in the Salinas Valley, especially on the east side. They are a much less familiar pest, but some details are beginning to emerge about their destructive behavior.

"The key point is the timing and the severity of the infestation. If I stop at a broccoli field, I will find cabbage maggots. The PCAs and growers know they are there. The key is the severity and timing; if the infestation is severe and the plants are young, they won't grow. Cabbage maggots are basically on brassica broccoli, cauliflower and cabbage, while springtails are mainly on lettuce. They are distinctly different problems," Joseph said.

Cabbage maggots can arrive in large numbers in Salinas Valley fields, because the area seems to provide the pest with hospitable weather and plenty of host plants to eat.

Joseph is working to develop a more detailed understanding of how the cabbage maggot life cycle fits with the development of young lettuce seedlings. Answers to these questions about the life cycle should help in developing alternatives to the materials that are currently used to control cabbage maggots.

The UC Pest Management Guidelines advise that materials must be applied at planting if they are to be effective against cabbage maggots.

(Bob Johnson is a reporter in Monterey. He may be contacted at bjohn11135@aol.com.)