Lixus canescens

Skuhrovec, Jiří & Volovnik, Semyon, 2015, Biology and morphology of immature stages of Lixus canescens (Coleoptera: Curculionidae: Lixinae), Zootaxa 4033 (3), pp. 350-362 : 356-360

Biology of Lixus canescens

Habitats. Lixus canescens was never observed in biotopes where its host plant is not present. Based on Volovnik’s observations, it prefers open sandy areas close to the sea shores. The weevil also inhabits road margins and sandy dykes that have a maximal height of 2.0 m ( Fig. 4 A), almost bare rocky hills near the sea (Kazantip Cape and South Crimea). According to entomological collections (see Material & Methods), this weevil occurs as far from the sea as Khomutovskaya Steppe (Donetzka Province, 47°15'33"N, 38°9'2"E) in areas that are compact and overgrown with vegetation. The weevil occurs on limestone with chernozem.

Adult behaviour. The weevils are diurnal and during sunny weather are present on the leaves or stems of host plants ( Fig. 4 B). In the evening and during cold weather, the adults are motionless and usually are sheltering on the underside of the host plant rosette, near the base of the stem. An infested plant is usually occupied by 1– 2 adults. In case of danger, adults try to hide and go to another side of the stem or to the lower surface of the leaf. When a disturbance is violent, the beetle falls to the ground. Its color pattern makes it barely visible on the sandy soil ( Fig. 4 C). After falling onto its back, the beetle stays motionless for up to 3 minutes (thanatosis). All of the examined adults (22 specimens) were macropterous. They were not observed in flight in their native habitat, but some flew in the laboratory ( Fig. 4 D). Adults can be observed from mid-May onwards. At the end of May, the number of adults in the population reaches a peak, and then decreases. Adults from the new generation can be observed from the end of August up to the 10th of September, but we have never found adults before 10th of August.

Host plant. Both adults and larvae were observed feeding exclusively on Crambe pontica Steven ex Rupr. Adults feed on the leaves of the plant, as well as on the superficial layer of the stems, petioles, and on peduncles with flowers. In choice tests, weevils fed heavily and laid eggs on Crambe only. In no-choice tests, adults fed readily, but never laid eggs on other cruciferous plants, namely: Raphanus sativus L. (stems, leaves, flowers and verdant fruits), Barbarea vulgaris R. Br. (stems and flowers), and Brassica oleracea L. (stems, leaves and petioles).

FIGURE 5. Life cycle of Lixus canescens : A—egg; B—1st instar larva; C—mature larva, and detail of larval head with distinct pattern; D—pupae; E—pupa and a fresh, not fully coloured adult; F—adult in the stem; G—pupation cell. All photos by S. Volovnik.

Life cycle. Lixus canescens is a univoltine species. Mating occurs in the second part of May on the host plants ( Fig. 4 E). Freshly laid eggs are oval, canary yellow, glossy, 0.9–1.6 mm long and 0.5–1.1 mm wide. Usually eggs were laid solitarily in stems (Fig. 5A), but approximately one third of hatches consisted of two eggs, and once we found three eggs in the same hatch. However, we never found two freshly hatched larvae side by side (or egg situated near larva). It`s known that the larvae of Lixus , living in the same stem, will sometimes kill each other ( Romanova 1928; Julien et al. 1999). The process of mating and oviposition has been described in detail previously ( Volovnik 1994, 2007) ( Figs 4 E–G). The site of oviposition is visible as a dark, rounded spot on the green stem ( Fig. 4 H).

Larvae are endophagous in the host plant stem. A hatching larva mines into the soft, spongy pith (Fig. 5B). The larval tunnel has a circular to oval shape in a cross section of the stem. Making longitudinal twisting tunnels for feeding, the larvae burrow down. Here and there the tunnels are filled with dense, light brownish or greyish, meallike pulp. Colouration of the first instar is light yellow, but a mature larva is whitish (Fig. 5C). A mature larva makes a pupation cell with tiny dense walls. Obviously, these walls are made out of pressed frass and excreta. The inner surface of the walls are smooth, without rough bits of the stem. Such elongated bits (up to 3 mm in length) are usually found on the external layer of the wall (Figs 5G, 6A). Almost all of the pupation cells in this study were situated in the lower quarter of the stem. Some of them were prepared at the base of lateral stems, and any pupation cell can be lower than the root crown. A disturbed pupa quickly wriggles its abdomen and turns around on its longitudinal axis. The body of the pupa and newly emerged adult is oriented vertically, or is slightly inclined (Figs 5D–E). After emergence, a fresh, not fully sclerotised adult remains in its pupation cell for a few days (Figs 5E–F). During this period, the reddish-brown adult becomes its usual whitish colour. Later, the adult gnaws out a round opening in the lateral wall ( Figs 6 C–D) and leaves the pupation cell. Stems of Crambe pontica grows to 75–100 cm and 4.0– 4.5 cm in diameter. The larval tunnel is to 10–15 cm in length and 0.5 cm in diameter. Sometimes two tunnels lay in parallel (Figs 5D–E), and as many as five adults can accomplish development in the same stem.

The exit opening (5–7 mm in diameter) is always situated in the upper part of the pupation cell. From within, the hole is surrounded by a thin-wall piece with irregular boundaries. Usually, the hole is settled asymmetrically with regards to these boundaries ( Fig. 6 B). The longitudinal axis of the hole is twice as long as the weevil’s head (rostrum included). Apparently, the adult operates in something like the following way: when it is ready to emerge from the pupation cell, the adult begins to gnaw the wall in front of itself. The weevil nibbles not at one point, but at the whole piece, wherever it can reach. The moment the mouth parts bite through, it begins to enlarge this opening until the exit is sufficiently large. Adults do not hibernate on the host plants. Most likely, hibernation occurs in the leaf litter, among dry plant debris or in the topsoil.

Biotic interactions. Besides L. canescens , we didn`t find any phytophagous invertebrates on Crambe . Immature stages of L. canescens were found to be parasitised by two wasps (1–2 %), namely: Вrасоn ( Glabrobracon ) chrysostigma Grееsе, 1928 ( Braconidae ) and Exeristes roborator (Fabricius, 1793) (Ichneumonidae) . Larvae, pupae and adults were found infested by parasitic fungi (1–2 %) in the larval tunnels. In August, in such a tunnel, Volovnik found a puparium where a signal fly, Platystoma sp. ( Platystomatidae ) had emerged. Larvae of this fly may devour dead, parasitised or healthy larvae and/or pupae of this weevil. The tunnels of larvae can be inhabited by other small arthropods: mature larvae of the earwig Labidura riparia (Pallas) (Dermaptera) ( Fig. 6 E), larvae of Diptera , ants of Leptothorax sp., Tetramorium caespitum L. and T. impurum (Förs.) (Formicidae) , ground beetles ( Carabidae ), ant-like beetles ( Anthicidae ) and woodlice (Oniscoidea, Crustacea).

Figures

FIGURE 4. Habitats, adults, host plant and life cycle of Lixus canescens: A—habitat with the flowering Crambe pontica; Badult on its host plant; C—weevil is almost invisible on the soil; D—adult before flying; E—mating on host plant; F—female prepares the oviposition site; G—female seeks a hole for oviposition with the apex of her abdomen; H—ovipositional marks. All photos by S. Volovnik.

FIGURE 6. Pupation cells, pupations marks of Lixus canescens, and hosts in free tunnels: A—pupation cell; B—exit opening from within; note, light roundish area was presumably gnawed by adult before emergence; C, D—dry stems with exit openings; small opening (on the right) gnawed out by undefined parasite; E—immature stage of earwig in the tunnel. All photos by S. Volovnik.