It happens every year. Your beautiful spring greens and radishes are coming along nicely, and then little round holes start appearing in the leaves. The most likely culprits are tiny flea beetles the size of a pinhead that jump away and hide at the slightest disturbance. Holes made by slugs are larger, with smooth edges, while those caused by diseases feature discolored patches or contrasting margins. Flea beetle holes are round, resembling tiny pits.
Spring Flea Beetles
Flea beetles are so small that identifying them is tricky, though you can certainly catch a few with a yellow or white sticky trap and look at them with a magnifying glass. Of the springtime flea beetles that dine on garden crops, the most common species are crucifer flea beetles (Phyllotreta cruciferae) and striped flea beetles (P. striolata).
These crucifer flea beetles are among the first garden pests to appear in spring, with young seedlings of rocket, pak choi, Chinese cabbage, mustard, radishes and turnips at high risk for damage. Crucifer flea beetles prefer hairy or glossy leaves from plants in the mustard family, though they also can damage young broccoli or cabbage plants. A few flea beetles on fast-growing plants can be ignored, but unprotected seedlings can be so heavily damaged that the leaves appear to have been scorched.
Flea beetles overwinter as adults, which emerge in early spring and lay eggs at the base of host plants, including wild mustard. By early May, garden plants are being threatened by overwintered flea beetles and a new generation, too.
Quite a bit of research has been done in search of reliable organic controls for crucifer flea beetles. Using covers made of horticultural fleece or very fine netting is the best defence, though a few may emerge from the soil under the tightest covers. Do not waste your time applying neem, pyrethrum, or using a vacuum, which don’t work for this pest. Row covers are the best control, followed by the biopesticide spinosad (available in organic formulations) or fungal agents containing Beauveria bassiana and Metarhizium brunneum, which cause flea beetles to dry out and die.
Ready for some good news? Crucifer flea beetle populations fall off in midsummer, with most of them resting in their winter hideouts by September. So, the same leafy greens that are riddled with flea beetle holes in spring may be clean as a whistle when grown as an autumn crop.
Summer Flea Beetles
In American gardens, crucifer flea beetles are replaced in summer by species that feed on solanaceous crops, including potatoes, aubergine, tomatoes and tobacco. Feeding by the potato flea beetle (Epitrix cucumeris) results in potato or tomato leaves that look like they have been shot through with tiny holes amidst numerous smaller brown pinpricks.
Potato flea beetles have numerous natural enemies and seldom reach damaging numbers in a diversified organic garden. Ground beetles consume the tiny larvae, and spiders harvest many adults. Yet unexpected population spikes can occur, with the organic control method of choice being an organic spinosad spray.
Aubergine flea beetles (Epitrix fuscula and E. hirtipennis) are present in most American gardens, ready to descend on eggplant seedlings within days after they are set out. To sidestep trouble, I like to grow eggplant seedlings in containers on my deck for as long as possible, or you could use a high outdoor table; eggplant flea beetles jump fast, but not very high. I install wedding net (tulle) row covers the day the plants go into the ground, which buys them more trouble-free growing time. The plants are so big and healthy by the time the covers are removed to admit pollinators that they can tolerate light flea beetle feeding.
In many cases flea beetle damage is cosmetic. Simply cooking greens with a few holes in them, like the rocket at the top of the page, makes the blemishes disappear. With big, robust potato and tomato plants, flea beetles seldom come close to damaging 30 percent of the plants’ leaf cover, the damage level needed to reduce yields.