Flea beetles are small predatory insects, belonging to the order of beetles. Small holes, more or less circular, on various plants, attest to their passage and their nuisance. The foliage is said to be riddled. It is in spring, at the time of sowing and the emergence of seeds, that the beetles cause the most damage on crops.
Description of damage to plants
Seedlings in May, in hot and dry weather, are often the object of action by adult insects. The seed, in the process of germinating, has more or less gnawed surface roots.
The first leaves (the cotyledons), then the young shoots have many small circular holes of 1 to 2 millimeters in diameter.
In the case of more developed plants, it is the leaves which are riddled or serrated, then discolored and the subject weakens. Later, in June-July, begins the action of the flea beetle larvae. Circular necrosis appears on the leaves and sometimes we see galleries (mines) hollowed out by larvae between the two epidermis of the limbo (miner action).
If the eggs are laid in the ground, some larvae are born in the soil and feed on roots.
We also know the action ofa winter flea, whose larvae develop in the petiole and in the stem, even in the terminal bud of a brassicaceae, rapeseed. It goes without saying that the predatory action of the larvae can weaken the plants.
The flea beetles and their cycle of beetles
Flea beetles are a few millimeter beetles, dark in color (black to metallic blue), with yellow bands for some species. They have two pairs of wings, the elytra forming the characteristic shell of beetles and the membranous wings, which allow them to fly from plot to plot, and to travel one kilometer to find their food.
Flea beetles are also called "earth fleas" because they have hind limbs suitable for jumping. Their oral apparatus is of the grinder type, which explains the bites of the leaves.
adults, flea beetles hibernate in the ground, protecting themselves in plant residues or under leaves. In spring, around May, the adults "wake up" and feed on rootlets, cotyledons or leaves, before laying eggs, either on the ground at the foot of the plants, or on their leaves. The eggs hatch after one to two weeks. The larvae feed locally on the plant roots or the epidermis of the leaves. They transform around the end of July, and the new adults maintain their nuisance before hibernating.
The plants affected by flea beetles
Flea beetles of the genus Phyllotreta target in particular the plants of the brassicaceae family (crucifers) such as cabbage, cauliflower, rapeseed, watercress, arugula, turnip, radish, horseradish, etc.
Other vegetable plants are victims of flea beetles: beet, artichoke, potato. Field crops are affected such as flax, corn, vines, as well as ornamental plants such as fuchsia, hollyhocks and lavatera.
Prevention and control methods
The fight against flea beetles must focus on the spring attacks, the most damaging to crops.
Several preventive measures can be implemented.
Preferably use treated seeds, or add micro-granules of insecticides in the furrow.
It is possible to place an insect-proof veil, with very fine mesh, on poles covering the seedling, taking care to bury the edges. Knowing that flea beetles hate laying eggs in crusty and damp soils, it will be useful to mulch crops with mowing residue for example, or spread ash and water it regularly.
We can accompany crops of repellents, tansy, white clover or marigold (marigold) which will disturb the smell of flea beetles, making it difficult to spot the most sensitive plants.
In case of strong presence of insects, several solutions can be considered. For example, trap adults with gooey yellow bands, or spray tansy decoctions.
Do not hesitate, if necessary, to treat with pyrethrum or rotenone, as soon as the seeds have emerged. If the treatment takes place much later, it is necessary to wait at least seven days before consuming the vegetables.