Introduction
Flea beetles are small, often metallic, and dark-colored beetles in the family Chrysomelidae. They are given their common name due to their impressive jumping abilities, afforded by their enlarged hind legs, which serve as a near-foolproof escape from predators. Most flea beetle species are oligophagous herbivores, feeding on several related species of plants. For example, many species in the genus Epitrix feed exclusively on potatoes, tomatoes, eggplant, and other solanaceous crops, while those in the genera Phyllotreta and Psylliodes are specialists on plants in the family Brassicaceae. In North America, these crucifer specialists are economically significant pests of commodity crops like canola, as well as specialty crops such as cabbages, turnips, bok choy, kale, and radishes. In Indiana, there are at least nine different species of flea beetles, which you can learn more about here. The striped flea beetle, Phyllotreta striolata is the most common species on crucifer crops. This beetle, roughly 2mm in size, is marked by two curved yellow stripes, sometimes appearing as four broken marks, on its shiny black elytra (hard wings that cover the abdomen; Figure 1).
Figure 1. The striped flea beetle, Phyllotreta striolata (Photo by John L. Obermeyer).
Life cycle
Flea beetles overwinter as adults, seeking refuge either in soil and leaf litter or on the surface of host plant leaves. Striped flea beetles and other crucifer feeders in the genus Phyllotreta will emerge in spring and often feed first on volunteer Brassicaceous weeds prior to making their way to the crop (Figure 2). Depending on the species, flea beetles lay their eggs either in the soil or on plants throughout the summer, producing 1-3 generations per year. Larvae hatch from the eggs in approximately 14 days and begin to feed on leaves or roots until entering the pupal stage. The larvae pupate in the soil before adult emergence occurs.
Figure 2: Striped flea beetle damage on (A) radish and (B) volunteer Brassicaceous weeds (Photo by Julia Wooby).
Damage
Flea beetles cause damage throughout their larval and adult stages, but the most significant harm typically arises during their adult phase. Varying by species, flea beetle larvae can cause damage to leaves or roots as they mine stem and leaf veins and feed on plant roots. P. striolata larvae are minor root pests during their underground life stage, which can cause stunted growth in severe cases, but this damage is usually insignificant compared to adult foliar feeding. Adult flea beetles are voracious feeders on host plant stems and leaves, resulting in the formation of small “shot-gun” holes (Figure 3). The consequences of flea beetle attacks can be severe, potentially leading to the death of emerging plants, stunted plant growth, and reduced crop yield or marketability. The extent of damage inflicted by flea beetles correlates directly with their population density.
Figure 3. Damage to the leaves of bok choy (Photo by Milena Agila).
Management
Flea beetles are a notoriously difficult pest to control due to their high mobility and small size. When it comes to management, preventative measures and prophylactic insecticide applications to seedlings are often more effective than trying to treat an existing population. Cultural controls play an important role in their management and include manipulating planting times to prevent damage to small transplants and using row covers or fine netting to exclude adult beetles traveling to the crop. Removing weedy areas consisting of early-season hosts for flea beetles can also decrease populations in and around susceptible crops. Flea beetle populations are easily monitored by placing yellow sticky cards at crop height and by carefully observing young brassica crops for “shotgun” damage to determine if and when insecticide applications are necessary. Thresholds vary based on the crop, namely whether the insects are causing direct damage to the marketable portion of the plant (i.e. bok choy, cabbage), or the foliage supporting fruit or root production (i.e. tomato, potato, radish, turnip) (Figure 4). There are multiple insecticides, including some certified for organic use, that are recommended for control of flea beetles. Please refer to the Midwest Vegetable Production Guide for the most recent recommendations. Additionally, trap cropping, the use of implementing a physical barrier of a highly attractive crop to intercept overwintering flea beetles moving into the crop fields, has been used effectively. While the trap crop alone can reduce feeding on the cash crop, combining this approach with insecticide applications to the trap crop can further reduce subsequent generations of beetles in the crop. Trap crops should be selected in consultation with the research literature, so contact your local Extension professional if you are interested in this method. Research into biological control of flea beetles is in its early stages, but several species of commercial entomopathogenic nematodes and fungi have been identified as promising agents of population control for Phyllotreta flea beetles, with the former targeting the soil-dwelling larvae, while the latter can also cause mortality in adult beetles.
Figure 4. Damage to the leaves supporting turnip growth (A) compared to flea beetle damage on the marketable foliage of young bok choy (B) (Photo by Julia Wooby).