Last year I had flea beetles in one of my vegetable garden boxes (thankfully only one) and I couldn’t get rid of them. I tried diatomaceous earth, neem oil, and planting some trap plants to lure them away (radishes). Nothing worked very well, but maybe there were too many by the time I tried to combat them.
I guess I have 2 questions:
1. what is the best way to get rid of them?
2. is the garden box where they lived last year going to be a bastion for them again? or does the winter erase that? Last year, that garden box had kale, lettuce, and arugula, and then the radishes (which turned out quite nice!)
Last year was a very bad year for flea beetles and hopefully this will not be so this summer. In Manitoba, due to the agricultural crop of the Brassica family like canola and mustard that are grown in areas close to where we live they frequently fly into the city. You definitely did the correct procedures to deter them by applying diatomaceous earth and planting trap plants.
The adult flea beetles overwinter in the soil, under garden debris and leaves. They emerge in spring, start feeding on host plants, repeatedly mate and lay their tiny white eggs in the soil in cracks around the host plants. In approximately one week the eggs hatch into larvae, which feed on the roots of host plants until pupating and becoming adults. Adults emerge from mid July to September. This is why dusting with diatomaceous earth could work at this time of their life cycle and when applied in spring. Please note that applying diatomaceous earth may also harm beneficial insects which prey on flea beetles. There is only one life cycle during our summer season.
In addition to what you already did to deter the beetles, try cutting off the food source for the emerging beetles in spring by planting and transplanting one or two weeks later than usual. Or, when you plant trap plants make sure you plant these before you transplant seedlings or your desired plants emerge. Then get rid of the infested plants including the beetles. Use row covers to prevent beetles from moving in to the garden box that was not affected last summer. Create shade by planting densely and water your plants diligently because flea beetles do best when it is hot and dry. Clean up and cultivate your planting boxes in fall.