Jerry Brust, IPM Vegetable Specialist, University of Maryland; firstname.lastname@example.org
I visited a few tomato fields this week and found 2-4-week-old tomato plants with some early blight (Alternaria solani) and in some cases bad early blight lesions. This is very early in the season to be seeing this level of early blight. Many of the plants had a few flea beetle adults on the plant and in the areas where the early blight was found also had moderate to high flea beetle feeding (fig. 1). Normally the amount of flea beetle feeding I saw would not have been of much concern, but flea beetles can cause increased infections of Alternaria leaf blight in tomatoes and potatoes and possibly other early blight susceptible crops. I found that there was a strong relationship between the amount of flea beetle feeding and the amount of early blight on tomato plants in different fields on a few farms. If you have moderate flea beetle feeding damage to your Solanaceae plants and you see any early blight starting, you will need to control both the beetle and the disease. Pyrethroids should work well in controlling flea beetles. There is not much organically that will control flea beetles once they are causing economic damage. However, using spinosad (Entrust) before beetles feed heavily on plants is one organic possibility.
Flea beetle adults are generally small and range in size from 0.05 to 0.15 inch. They overwinter as adults on weed hosts surrounding the field, on residues of a previous tomato crop, or in the soil if the previous crop was a flea beetle host. Some flea beetles (Systena blanda–the pale striped flea beetle being one) can feed on amaranths or pigweeds (fig. 2) and will readily move from them over to your crops. Other flea beetles are more host specific (the eggplant, potato, and tobacco flea beetles feed on Solanaceous plants while others prefer broccoli, cabbage and other cole crops). However, all adult flea beetles have similar damage patterns, they chew small round holes in leaves, which make them look as if they have been damaged by fine buckshot, called “shot-holing”. The white larvae feed on underground parts of the plant, but this damage is usually not economically significant. There is normally a second generation during the summer and at times even a third depending on species. Normally foliar damage to larger plants is not considered to be economically important but feeding damage to small plants or seedlings can reduce stand or vigor of the plant. The other exception about flea beetles not being economic pests is when Alternaria is associated with their feeding on smaller tomato plants.
Fig. 1. Tomato leaf with old flea beetle feeding and early blight.
Fig. 2. Pale striped flea beetle feeding on Amaranthus weed.