Whether rhododendron or cherry laurel, boxwood or roses: A black weevil prefers plants with thicker leaves, but if the selection is too small, it will also gnaw flowers and perennials or even weeds. “The beetle causes typical eating spots on leaves and needles that protrude from the edge into the leaf surface. This so-called bay feeding occurs because the beetle eats into the leaf from the edge of the leaf,” explains the Lower Saxony Chamber of Agriculture, explaining typical signs of an infestation. Because black weevils are nocturnal and extremely shy, they are difficult to control during the day. In addition, their larvae are far more dangerous to crops and ornamental plants.
In addition to the feeding damage described above, which indicates an infestation, you can recognize the shy beetles at dusk at the latest (and certainly at night) by the following features: They are only a few millimeters in size, usually brown or black in color and often have small yellow dots on their ridged shell. They owe their name to the strong trunk on which the antennae are located. The insects mate between spring and autumn – each female is then able to lay up to 1000 eggs. Preferably near the roots so that their larvae have something to eat within a few weeks of hatching. And thus cause great damage to the roots.
Unlike other beetles, black weevils are flightless. If they are in danger or are disturbed while eating, the animals will simply let themselves fall – for this reason you have to be quick if you want to catch the insects. It’s best to use a flashlight and collect the beetles from the infected plants (when it’s dark outside). If you want to save yourself the work, you can use another trick: put wood shavings in an empty flower pot and place it under the infected plant. During the day, black weevils hide in it or fall on it when threatened.
The larvae of the black weevil, which are found in the soil near the roots, are much more difficult to combat. So-called nematodes have proven to be particularly helpful here: These are roundworms that are harmless to humans and (domestic) animals as well as plants. They produce special bacteria that penetrate the larvae and poison them from the inside. The nematodes only live for a few weeks – and disappear on their own, just like the black weevils. However, it is important to know that this watering method is best used between April and May.
You can take preventive measures to prevent black weevils from feeding on your plants in the first place – with neem press cake. Add 50 grams per square meter of this to the soil about every two months so that the active ingredient, which is toxic to pests (beetles and larvae), can penetrate the roots and leaves. However, keep in mind that you should not use pressed neem oil if you have deployed nematodes. Otherwise the toxic oil would also kill the nematodes.
One more note at the end: Black weevils also have enemies, such as shrews and hedgehogs, moles and birds. If you attract the animals to your garden, the beetles will be fought naturally.
Sources: Federal Environment Agency, Lower Saxony Chamber of Agriculture
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