Let’s start by saying that the black vine weevil (Otiorhynchus) is one of the most feared parasitic insects for our plants. It belongs to a large family of beetles that we will analyze in this article, closely observing the most common species in our country. We will then see how these insects behave and the main damages they cause to horticultural and arboreal crops. By doing this, we will acquire the basics for quick recognition. Finally, we will discuss how to combat this pest with biological control, emphasizing from the outset that this type of defense has a certain level of complexity.
But let’s proceed in order and start with the entomological identification.
The Black Vine Weevil – Entomological Identification
The term “black vine weevil” (Otiorhynchus) refers to a class of insects belonging to the order of beetles, suborder of polyphages, and family of Curculionidae.
This genus includes over 1,500 species, mainly found in the regions of southern Europe, northern Africa, and northern Asia, up to the Himalayas.
Species and Damages
One of the most common species is the otiorhynchus rugosostriatus. This species mainly attacks herbaceous crops of agricultural interest, shrubs, strawberries, and trees found in the woods.
Adults are about 7 mm long, with a blackish color and a robust body, typically curved in the shape of a C.
The larvae are apode, whitish in color, with a reddish-brown head.
This species, like other black vine weevils, is nocturnal. The damage to crops is mainly caused by the trophic activity of the larvae, which penetrate the root system, creating real galleries.
Moreover, to reach adulthood, they feed on the roots until they break them. Plants attacked by this type of black vine weevil show slow growth, withering, and wilting. The adult stages of this species cause mild damage to leaf margins, as their feeding results in leaf erosion.
Another species of significant agronomic interest is the olive black vine weevil, scientifically known as Otiorrhynchus cribricollis. Besides olives, this species attacks plants such as citrus, strawberries, and alfalfa.
The adult insect measures about 8 mm (while the large white larvae reach up to 7 mm). The color is a light brown, and the body is robust. It does not have true wings as the wing covers are fused together.
The olive black vine weevil, when an adult, causes significant leaf erosion on the plant.
In the larval stages, the erosion occurs below the plant collar and on the roots.
If the attacked plants are young, there is a serious risk of them dying.
The last species we want to introduce is known as the vine weevil, Otiorhynchus sulcatus. The plants it prefers to attack are: vines, rhododendrons, azaleas, bay laurel, juniper, and holly. An adult beetle can reach a length of 10 mm, has a shiny body, and can be recognized by its wings with deep grooves and covered with yellow hairs. The larvae, on the other hand, have a light brown color with a reddish head. The damage to plants is mainly caused by the adult insects, which heavily affect the leaves. Finally, the larvae of this species are less aggressive on the roots.
Biological Cycle and Common Traits
Female black vine weevils reproduce through parthenogenesis, meaning they can reproduce without the need for fertilization. The females lay eggs in several phases at the base of affected plants.
The larvae overwinter in the soil and reach maturity the following spring. From the month of May onwards, adult individuals start emerging.
The black vine weevil usually completes only one generation per year. The damages to the plants we described earlier are thus caused: by the adult insects in the spring, and by the larvae in the summer and early autumn (the larvae then overwinter).
It is worth noting that the black vine weevil is a nocturnal insect. Adult specimens come out from underground at dusk to carry out their trophic activity on the leaves. They then return underground at dawn.
Biological Control of the Black Vine Weevil
Due to its characteristics, the black vine weevil is a very difficult insect to combat. The biological remedies that work for other insect species, such as aphids, tomato leafminer, mealybugs, and cabbage butterfly, are not effective against it. Even in traditional agriculture, the use of harmful chemical pesticides does not yield significant results. This type of insect is, in fact, similar to the Asian stink bug, with which it shares a hard external shell that makes intervention difficult.
Fortunately, an effective biological solution has been found to deal with this problem. All that needs to be done is to use specific parasites for this type of insect, namely entomopathogenic nematodes. Let’s find out what they are and how they work.
Nematodes are microscopic cylindrical worms that are not visible to the naked eye but only under a microscope. Most nematodes are harmful parasites for our garden.
However, nature also offers us beneficial species, such as entomopathogenic nematodes. These nematodes can parasitize harmful insects like the black vine weevil and can be profitably used for the biological defense of plants. The most commonly used entomopathogenic nematodes for this purpose belong to the genera Steinernema and Heterorhabditis, particularly Steinenernema feltiae, S. Carpocapsae, S. Kraussei, Heterorahabditis bacteriophora, and H. Megidis.
How They Work
Entomopathogenic nematodes are obligate parasites of larvae and are useful against both beetles (like the black vine weevil) and the larvae of butterflies, flies, and wasps. These insects, in their larval stages, live in the soil or in damp areas, such as the galleries they dig in plants.
Regarding their action on the black vine weevil, it can be summarized as follows. The nematodes are released into the soil, where they search for beetle larvae. Once the larvae are identified and parasitized, the nematodes release symbiotic bacteria inside the larvae, leading to their death within 72 hours. After that, the nematodes can reproduce on the body of the deceased larva, from which they obtain nourishment. When they mature, they search for new larvae.
Advantages of Using Nematodes
First of all, entomopathogenic nematodes are considered macroorganisms. This means that they act through a natural mechanism and do not cause any damage to the ecosystem, especially because the species that are reproduced and commercialized are already present in nature.
They have larvicidal action, which means they only attack the larvae of the target insect, and do not parasitize the adult forms.
They cannot penetrate healthy plant parts but only reach the parasitic insect by exploiting the damage already present on the vegetation.
Moreover, nematodes move, survive, and act only under conditions of high humidity. They can, therefore, reach the black vine weevil larvae in the moist surface layers of the soil, where other products cannot reach.
They do not have withdrawal times and are entirely harmless to humans and animals.
For the biological defense against the black vine weevil, there are two optimal periods for treatment. In spring, acting on the overwintering larvae, and in midsummer, acting on the young larvae, which are even easier to eliminate.
Nematodes are commercialized by specific companies that distribute them in refrigerated polystyrene containers to guarantee the survival of these macroorganisms, which do not withstand extreme temperatures.
The available formulations generally consist of a soft mass dehydrated in an inert substrate (clay, alginate, or gel). This is reactivated by mixing it with water. Simply add the nematodes to the water, shake the solution slowly and steadily, and distribute it on the soil of the plants to be treated.
The treatment should be carried out during the cooler hours of the day, as the nematodes need time to penetrate the soil. Otherwise, heat and sunlight significantly reduce their effectiveness.
By keeping the soil well moist, the nematodes can survive for up to 2 weeks, prolonging their action.
A product based on the entomopathogenic nematode Steinenernema feltiae, ideal for domestic use and with a relatively affordable price (considering the average market price), can be found here.
If you want to search for other entomopathogenic nematodes for the biological defense against the black vine weevil, I suggest also looking into Heterorahabditis bacteriophora.
Here, of course, we have provided general guidelines. For proper and effective use, we always recommend following the specific instructions provided by the manufacturer.
Other Agronomic Techniques to Limit Black Vine Weevil Damage
While for the larval stages, defense with entomopathogenic nematodes is the most effective solution, for the adult stages, the techniques are limited to manual actions and good agronomic management.
Firstly, if there is the presence of adult insects, you can manually remove them in the evening by tapping the leaves. The black vine weevil is quite resilient but has limited mobility, making it easy to capture. Once captured, you can dispose of them safely by burning.
If we had the presence of black vine weevils during the previous season, it is likely that the females have oviposited. Therefore, there is a high probability of the parasite’s presence in subsequent periods. For this reason, it is essential to carefully remove the plant residues from the affected plants, especially the root systems. In the winter, by deep plowing the soil, the overwintering larvae are exposed to intense cold that they cannot survive. Leaving the soil protected and compact in the surface layers favors the natural biological cycle of the black vine weevil.