Here’s one insect where damage is certainly much more noticeable than the insect itself. The plant’s leaves – especially the youngest ones – are riddled with dozens of small holes and shallow pits, as if they’d been hit with a shotgun. In fact, this type of injury is called shotholing. The pest behind the holes is the flea beetle… and it seems to prefer edible plants.
Cruciferous vegetables such as cabbage, cabbage, radish, turnip and arugula are especially hard hit: they even have not one but several species of flea beetles that are specific just to them. But other vegetables also get hit: plants in Solanaceae family (tomato, potato and eggplant) and chenopods (beetroot, spinach and Swiss chard), for example. Corn leaves as well as those of other cereal crops can also be shotholed.
Among fruits, grape vines are usually the hardest hit, although in certain years, fruit trees sometimes get their share. At my place, the hop flea beetle so damaged my hops each year I finally pulled them out.
Flea beetles are also seen on ornamental plants, but since we don’t eat ornamentals and flea beetles tend to stick to lower leaves (well, at least most species do), the average gardener soon learns to ignore them. They’ll attack annuals and perennials more often than trees and shrubs, but the damage is generally minor and easier to tolerate. Two plants that do suffer major damage and may well need treatment are Virginia creeper and dogwood.
How to Recognize Them
Flea beetles are small, glossy, rather rounded beetles, often black or brown, frequently iridescent, sometimes banded in two colors. They’re only about 1/16 to 3/16 inches (2 to 5 mm) long. There are hundreds of species, some generalists, but most are quite specific as to their host plants.
The insect is less readily seen than the damage it causes, as it flees at the sight of humans. Although it’s perfectly capable of flying, it tends to jump to avoid predators, using its long back legs, hence the name “flea” beetle.
Damage is especially serious on seedlings and young plants, so much so that their growth can be seriously delayed if indeed they aren’t killed outright. It’s the adult beetles who, when they awaken in spring after a winter spent in the ground or in leaf litter, cause the most damage. They become active with the first warm days of the season. They’re more active and cause more damage in dry weather while rain and moisture reduce their feeding.
Adults lay their eggs in early summer, at the base of plants or on lower leaves, and the larvae hatch after one or two weeks, feeding underground on small roots, sometimes of weeds, sometimes of desirable plants, although they cause little damage. When the nymphs emerge from the ground about 10 days later (they’re identical to adults, but smaller), they infest the plants for the rest of the summer, molting regularly until they reach their adult size.
Depending on the local climate and the species, flea beetles can have one to three generations per summer. However, mature plants are generally less affected than young ones, so the damage is less obvious in summer, except in in gardens where you carry out successive sowings.
Prevention is the Best Medicine
As with most insects, flea beetles are easier to prevent than to cure.
For example, you can try planting out young plants started indoors rather than sowing them in the garden (sprouting seedlings suffer much more than more mature ones) or try growing trap crops, which then suffer the bulk of the assault, leaving their neighbors essentially unharmed. For example, radishes and mustard make good trap crops for flea beetles that attack cruciferous vegetables.
Mulching the garden will help keep the soil moist and that discourages flea beetles.
Yellow or white sticky traps can also help catch them. Sometimes the traps become so covered in flea beetles you have to replace them.
If you practice intercropping, where vegetables are grown mixed together rather than in rows, that too can help reduce the damage.
But the best method of prevention is to sow susceptible crops in a different spot than the previous year (in other words, practice crop rotation), then to cover the area with floating row cover as soon as you finish your spring sowing. The famished adult flea beetles that wake up later in the spring won’t be able to penetrate this barrier and will be forced to seek food and shelter elsewhere.
As using a row cover will eliminate the first generation of flea beetles — the one that causes the bulk of the damage —, you can remove it later in the summer when it starts to get dangerously hot underneath the cover, yet you won’t have to worry much about damage. Rarely do enough flea beetles come back in midsummer to seriously harm to plants they missed earlier in the year.
When Flea Beetles are Already at Work
Preventative methods won’t be effective once your plants are under full attack from flea beetles.
To reduce the population, shake infested plants over a container filled with soapy water and adult beetles will fall in and drown. Or go over the plant with a hand vacuum cleaner and suck them up.
You can try spraying plants with neem oil or a natural pyrethrum-based product to reduce the population. Or apply beneficial nematodes to the soil in late spring or early summer. They often control the population quite rapidly.
And don’t forget that birds, toads, frogs and garter snakes are your friends and can help reduce the flea beetle population, as can several smaller, less visible creatures like parasitic wasps and beneficial nematodes. Do whatever you can to encourage beneficial animals to visit your garden and they can help keep flea beetles under control.
Your Tolerance Threshold
Unless you use exclusion methods to keep flea beetles off, such as floating cover, no treatment will be 100% effective … but then, it doesn’t have to be, as most plants continue to grow well in spite of a small amount of damage. Radishes, for example, generally produce abundantly even when their leaves are dotted with holes. And even if vegetables with edible leaves, like cabbage or arugula, have leaves somewhat riddled with them, they’re still edible! It’s when damage to the leaves starts to come close to covering 25% of their surface that the health of the plant begins to be seriously affected and that the need to control the pest becomes more pressing.
Flea beetles: small insects that cause very visible damage, but that don’t necessarily interfere significantly with harvests and bloom. It’s up to you to decide whether you’ll tolerate them or control them.