Inula Viscosa, botanically classified as Dittrichia viscosa, is a plant belonging to the Asteraceae family. It’s also known by other common names such as false yellowhead, woody fleabane, sticky fleabane e yellow fleabane, and is widely distributed in Italy, especially in the Central-Southern regions. Due to its colorful and abundant autumn flowering, it certainly doesn’t go unnoticed in the countryside. It’s a robust and wild plant that plays a crucial environmental role, primarily in apiculture.
This species promotes biodiversity and the presence of beneficial insects in the ecosystem and, at the same time, contains natural insecticidal properties. These characteristics are used in the biological control of the olive fruit fly (Bactrocera oleae). Let’s now explore what Inula Viscosa is and its areas of application.
Description of Inula Viscosa
Inula Viscosa is a spontaneous and perennial plant. Biologically, it’s defined as a hemicryptophyte with scapose growth, capable of overwintering through buds located at ground level. It has an erect floral stem that’s often devoid of leaves. This translates to great resilience, which is why it’s often mistakenly considered a weed. Even if cut down to ground level, the plant can sprout new shoots and easily grow within a year.
Aesthetically, it resembles a large bush, formed by vigorous upright and branching stems, sometimes partially reclined, reaching heights of over 1 meter. These stems lignify at the base, have a brown color, and cracked bark. As you move up the stem, towards the central part, the texture becomes more herbaceous, and the stem itself is tough and covered with leaves.
Inula Viscosa leaves are alternate on the stem, very persistent, pubescent, and spiky. They are lanceolate in shape, with an entire or irregularly dentate margin. The size is larger in the lower part (sessile leaves) and gradually reduces as you move up the stem (amplexicaul leaves). Inula’s leaves are known as viscous because they are hairy and contain glands with essential oil. They are rejected by herbivorous animals, probably due to the strong aromatic and resinous odor they emit.
The inflorescence is the distinctive feature of Inula Viscosa. It consists of a terminal panicle in the shape of a pyramid, with numerous showy yellow flowers (capitula) measuring 1-1.5 cm in diameter. The peripheral flowers of the pyramid are female and ligulate, with a long linear ligule; the inner ones are hermaphroditic, with a tubular corolla ending in five teeth. The color of the flowers is a beautiful golden-yellow.
Pollination and Flowering
The pollination of Inula Viscosa is of the entomogamous type, meaning it’s carried out by bees and other insect pollinators. Flowering is persistent and staggered, starting from late summer and usually extending until the end of October. In warm and sunny locations, it’s even been observed flowering in late November. Seed dispersal is carried out by the wind (anemochory), which is another reason for its easy spread.
Fruits and Seeds
The fruit of Inula Viscosa consists of a whitish achene, provided with a simple pappus gathered at the base, which is yellow-grayish in color.
Habitat of Inula Viscosa
Inula Viscosa naturally thrives from sea level to high hills, at around 800-1000 meters in altitude. It’s very common along the coasts, but its presence is not lacking in the countryside either. It prefers uncultivated land, embankments, roadside edges, old dry stone walls, and forest margins. Being a heliophilic plant, it requires light and thus cannot thrive in the innermost understory. Nevertheless, it has no problems growing in very poor, dry, limestone, and clayey soils.
Moreover, it’s famous for being a plant capable of colonizing uncultivated or abandoned fields. This is because it avoids continuous soil cultivation and crop planting. In the South, it’s very common among vineyard rows, well-exposed olive trees, and orchards.
Importance of Inula Viscosa in Apiculture
In apiculture, Inula Viscosa is of strategic importance. It provides abundant reserves of pollen and nectar during a period when other flowering plants are scarce or absent, typically between late summer and autumn. In theory, in high-intensity areas, this plant can be used to produce a monofloral honey, or at least its essences can contribute to the production of a polyfloral autumnal honey.
However, Inula honey is not particularly appreciated in the market, both due to its strong flavor and its tendency to crystallize easily. Furthermore, it’s a very moist honey, which can ferment during storage. For this reason, beekeepers leave bees to keep whatever they produce with Inula Viscosa, without placing hives, and bees store it as essential reserves close to winter.
Inula Viscosa and Biological Control of Olive Fruit Fly
Inula Viscosa represents a perfect example of how environmental biodiversity works and how it can be a winning weapon against crop pests. Olive growers, for instance, contend every year with the damage caused by the olive fly Bactrocera oleae, a phytophagous insect that attacks the drupes while they’re still on the tree, causing abundant fruit drop and affecting the quality of the oil produced from saved olives. Inula Viscosa can be helpful in preventing this problem.
Mechanism of Action
The mechanism of action is as follows: Inula Viscosa is parasitized by the fly Myopites stylatus, a gall-forming tephritid. This insect becomes the overwintering host of the chalcidoid wasp Eupelmus urozonus, a natural antagonist of the olive fruit fly, on which it preys. To simplify, these two insects, Myopites stylatus and Eupelmus urozonus, overwinter on the plant. In spring, the galls formed by Myopites stylatus open, and Eupelmus urozonus emerges, searching for insects to parasitize, including olive fly larvae.
In autumn, the cycle repeats, with Eupelmus urozonus seeking Myopites stylatus to spend the winter on Inula Viscosa. In olive groves (or their edges) where the plant is allowed to grow freely, a reduction in olive fly attacks of over 50% has been observed. Therefore, olive growers would do well to preserve Inula Viscosa in the orchard, recognizing it as a biodiversity stronghold rather than a weed.