Vine weevils

Identification

Weevils 8-12 mm long, black or dark brown, with a short, broad snout, flightless and active at night. Their larvae are white, legless, C-shaped, with a brown head, and live in the soil. Symptoms: the adult beetles notch characteristic semicircular bites into leaf margins at night (box, rhododendron, bergenia, strawberry, houseplants), while the larvae gnaw at roots, bulbs and the base of shoots, causing the plant to wilt suddenly and die. The most common is the black vine weevil (Otiorhynchus sulcatus), more rarely the strawberry root weevil (O. ovatus) and related species. A key factor behind their numbers is parthenogenetic reproduction (virgin birth): populations consist almost exclusively of females, and a single female lays several hundred eggs into the soil without a male, so an outbreak builds up quickly. Life cycle: the beetles overwinter mainly as larvae (some as adults) in the soil; the adults emerge in late spring and early summer and for about 3-4 weeks carry out maturation feeding on the leaves before they begin to lay eggs, while the larvae feed on roots from summer through autumn and again in early spring. The larval stage is the most dangerous - the larvae chew the roots and gnaw the root collar at the base of the shoot (girdling), causing the plant to wilt suddenly despite moist soil and to lift easily out of the substrate; adult feeding (semicircular notches on leaf margins) mainly disfigures the plant and rarely kills it. Plants in containers are particularly at risk, where the limited root ball is quickly destroyed. The beetles are flightless, so they spread by walking across the substrate and - above all - with infested plants and substrate, which makes newly bought potted plants the main route of introduction.

Treatment

Shaking off and collecting the beetles from the plants in the evening, traps made of boards and corrugated cardboard, and - against the larvae in the soil - insect-pathogenic nematodes (Heterorhabditis bacteriophora, Steinernema) applied in warm, moist soil. It is important to inspect newly bought potted plants (a common source of larvae), and as a last resort to use soil treatments. Control relies primarily on biological methods, because the larval stage hides in the soil and is hard to reach with sprays. Insect-pathogenic nematodes are applied at the optimal time against the young, most vulnerable larvae - in late summer and early autumn, and again in spring; Heterorhabditis bacteriophora works in warmer soil (roughly above 12°C), while Steinernema kraussei tolerates lower temperatures and so performs better in the cooler autumn and early spring - in both cases the soil must stay moist. As a supplement, soil-applied entomopathogenic fungi (Metarhizium anisopliae/brunneum, Beauveria bassiana) mixed into the substrate are used, especially in container growing. Against the adults, sticky bands fitted around trunks and pot rims help (they hinder the flightless beetles from climbing), as does clearing leaf litter and hiding places and supporting natural enemies (ground beetles, birds, shrews, hedgehogs). Prevention remains crucial: inspecting and turning over the substrate of newly bought plants breaks the most common route by which larvae are introduced. Chemical control is difficult and limited - contact sprays (a pyrethroid) applied in the evening against feeding beetles have only a stopgap effect owing to the nocturnal habits and hiding places, and do not kill the larvae in the soil; the soil-applied systemic products used in the past were based mainly on neonicotinoids, now largely withdrawn or heavily restricted in the European Union, so they are no longer a standard recommendation.

How to tell it apart from look-alikes

Control — organic and chemical methods

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