Jerry Brust, IPM Vegetable Specialist, University of Maryland; jbrust@umd.edu
One odd thing I have seen, which has also been reported to me from a couple of farms, is collapsed plants, usually a broccoli or cabbage transplant, with ants where one would expect to find maggots or cutworms when digging up the plant. On some of these farms the ants had attacked the maggot or cutworm doing the damage, but on many of the farms it was the ants that were doing the damage. I have seen this before; almost always early in the season usually during cool wet periods. It is unclear if the ants are there because a nest was ripped apart during tillage operations and the ants are getting the nest back together or if it is a new nest trying to expand. Normally the ants tend to only disturb a few plants, but on a few farms 20-30% of the plants were damaged. Most of the ant activity is below ground so control is difficult if needed. If cutworms or cabbage maggots are found with the damaged plants the control recommendations in the 2016 Mid-Atlantic Commercial Vegetable Production Recommendations for them can be used and this will help with the ants too.
Organic growers are in a greater bind for control of ants. I have seen controls such as diatomaceous earth, mixtures of garlic and hot pepper, drenches of pyrethrums, boiling water poured onto the soil, and others, but none work very well, if at all. One thing that seems to work, but is labor intensive, is using some sort of ring that goes around the base of each plant. This ring could be some old PVC pipe that is cut to about 2-4 inches in length and is about 1-3 inches in diameter. Some growers use Vaseline at the top of the ring to further hinder the ants from entering the ring. If the ants are fairly deep below ground it will be difficult to get the ring deep enough to keep the ants out without restricting the roots of the growing plant.