When the edges of the leaves of garden plants are gnawed in the shape of a bay, the thickmouth weevil is active. The beetle should be controlled, because its larvae feed on the roots, the plant may die. Nature provides a perfect remedy for the weevil larvae: small threadworms. The treatment is simple.
The furrowed weevil (Otiorhynchus sulcatus) has become a major pest of many garden plants. Plants at risk include primarily rhododendrons, azaleas, hydrangeas, bergenias, ericas, yews, ivy, cotoneasters, strawberries, and numerous perennials.
The gray-brown colored weevil is about 1 cm in size and usually active only at night. Its wing covers are fused, so it cannot fly and can only move by walking.
The typical damage pattern is bay-shaped feeding spots on the leaf edges or bark feeding on young yew shoots. This damage is ugly, but not life-threatening for the plant. Larvae are different: they live in the soil and feed on the roots of the plants. In case of heavy infestation, the plant dies due to lack of water supply.
Since the larvae can hardly be controlled chemically and the use of these pesticides is increasingly banned with the expansion of water protection areas, other control options were sought as early as the mid-1980s. The scientists found what they were looking for in nature: beneficial nematodes of the species Heterorhabditis bacteriophora were indeed suitable for biological pest control.
A pan-European working group immediately carried out further research. The scientists tested the nematodes in the field and developed methods for cost-effective propagation of the beneficial organisms. It turned out that these nematodes, which are only 0.5 mm in size, are almost 100 percent effective in practical trials, meaning that they are not only more environmentally friendly but also much more efficient than pesticides.
How and when are nematodes used?
Nematodes should be applied when the pest is in the larval stage and soil temperatures are at least 10°C. This results in two application times for field use: in spring, between late April and late May, and in late summer, between mid-August and late September. Both time windows should be used for the first application. Thereafter, one application per year is sufficient, preferably in late summer.
The application is done with the watering can or with the special application device AquMix, which is simply connected to the garden hose and automatically mixes the concentrated nematode solution. The nematodes are supplied as a powdered product that is simply dissolved in water before application (about one teaspoon per watering can).
Treat the soil in the eave area of the plant. Bark or shredded substrate should be removed beforehand to allow the nematodes to penetrate directly into the soil. It can be reapplied after the application. For the mobility of the nematodes in the soil, care should be taken to keep the soil slightly moist for two to three weeks.