Rhododendron, also known as alpine rose, is one of the most popular ornamental trees in the garden. Even after flowering, the evergreen plant is still very attractive. The shock is great when the shiny green rhododendron leaves are eaten. Who is the culprit? How do you get rid of him?
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Feeding damage to rhododendrons comes from the vine weevil (Otiorhynchus). The beetle belongs to the weevil family. It is black in color and reaches a length of up to 14 millimeters. Important identification marks of the beetles are their powerfully developed proboscis and the bulging structures next to the feelers. The back of the vine weevil is corrugated and has yellow dots. The most common representative in Central European regions is the ridged vine weevil (Otorrhynchus sulcatus).
During the day, vine weevils are hardly visible. They hide in the mulch or in the leaves. They only come out of their hiding places in the dark. The beetles are active in the gardens at night from late April to October. Not only do they eat the rhododendron leaves, they also love the leaves of roses, hydrangeas, cherry laurel and ivy. Even potted plants on the terrace are not safe from the pest. In the spring, the beetle enjoys the young plant shoots. Healthy, strong plants can usually survive the damage, even if the eaten leaves are not exactly decorative. However, its larvae pose a great danger. They damage the roots and can lead to wilting and death of the plants.
The animals remain on a single plant for up to three days. They can live two to three years.
Vine weevil or other pests?
Who ate the leaves? Was it weevils or caterpillars? The rhododendron leaves damaged by the vine weevil can be clearly distinguished from damage caused by caterpillars. The conspicuous feeding pattern is also referred to as bay feeding.
This designation is due to the fact that the pest eats bulges in the rhododendron leaves starting at the edge. The beetle never eats holes in the middle of the leaves.
How does the beetle get into the garden?
Larvae usually get into our gardens with purchased plants or potting soil. We recommend thoroughly examining new plants and their substrate for pests.
Controlling the vine weevil is difficult and time-consuming. The measures must always include the elimination of the beetles and the larvae. We present five options:
The tiny roundworms, the nematodes, are the remedy of choice when the vine weevil occurs.
mode of action:
- Nesting of nematodes in vine weevil larvae
- releasing a bacterium
- rapid multiplication of bacteria
- reddish discoloration and death of the larvae
Suitable treatment times are between April and May and between September and October. Choose a day with a cloudy sky or use the evening hours for the treatment. Nematodes do not tolerate strong sunlight and heat. A soil temperature of about 14 degrees Celsius and moist soil are required.
- Buy nematodes from a garden store
- Mix the contents of the pack with 10 liters of water
- pour affected area
Be sure to keep the soil moist for the following days to keep the nematodes active.
Plant broth and liquid manure contribute to the nutrient supply of the garden plants and also help against various pests. Earlier generations already appreciated the effect of wormwood manure against the vine weevil.
- Crush 300 grams of vermouth
- put in a container with 10 liters of water
- Cover and leave for two weeks
- Pour undiluted onto the root area of the rhododendron
An effective way to control otiorhynchus is to set beetle traps. These are prepared boards with nematodes in the grooves on the underside of the board. When the vine weevil retreats under the board at dawn, the nematodes have an easy time. Lay out beetle traps in the root area of the affected plants from May to September. The beetles retreat under the prepared boards during the day.
Nematodes infest the beetles and kill them. The soil must be kept moist for at least two weeks.
Collect the beetles
During the day the beetles stay well hidden in the leaves and the upper layers of the earth. It is hardly possible to observe the pest that has eaten the rhododendron leaves. Outsmart him and take advantage of the darkness. With the help of a flashlight you can remove the large beetle from the leaves.
The seeds of the neem tree can help drive vine weevils out of the garden. In specialist shops you can get pellets or neem press cakes made from the seeds of the neem tree in various concentrations. Every two months, 50 grams of neempress cake is spread on the ground around the rhododendron to control pests.
Tip for prevention
Very simple measures help to prevent the presence of pests such as the vine weevil in your garden. Make your garden as natural as possible, do without chemicals and create a habitat for hedgehogs, birds and small animals. Hedgehogs are among the natural enemies of the vine weevil. Where hedgehogs feel comfortable, the vine weevil has no chance. Small piles of leaves and brushwood are the perfect invitation to the useful animals.