|Detail of an egg laying and/or feeding site on a rhubarb (Rheum) stalk|
L. concavus has a distinctive long snout and geniculate antennae with small clubs. The beetle is about half an inch in length, black, covered with yellow dust, and hibernates as an adult. Eggs are a yellow-white colour and oblong in shape; the legless larva is a grub, about 3/4 inch in length, with a brown head that usually bears an inverted Y-shaped mark.
The adult Rhubarb curculio overwinters in leaf litter or other similar sites and appears in mid-May. The adult makes feeding and egg punctures in the crowns, roots and stalks; a jelly-like sap exudes from the wounds as glistening drops of gum, often with extraneous material trapped within. The eggs that are laid in rhubarb do not hatch, but are killed by the sap or crushed by the developing tissues.
Lixus concavus is able to complete its life cycle in the stalks of curly dock, sunflowers and thistles; eggs are laid singly in 1⁄8-inch-deep (3.2 mm) cavities, created by feeding activity, and hatch within a week to ten days. One grub usually develops per plant, the larva having borrowed through the stalk down to ground level, where pupation occurs after around nine weeks; the larva chews an exit hole for the adults before pupating. The pupa is whitish and measures 14–15 mm long. The head bears the long snout of the adult form. The abdominal segments are marked with short spines. The larva takes a few weeks to develop into an adult that feeds for a few weeks before seeking out a site to overwinter. The weevil has one generation per year. The adult weevil feeds upon the margins of the leaves besides puncturing the stalks; damage appears as distinct notches in the plant tissues.
L. concavus are usually present in fairly small numbers and can be handpicked and then destroyed. Removal of the other host plants, such as curly dock, wild dock, sunflower and thistle, during mid-summer, when the L. concavus larvae are still in them, may also help to reduce pest populations. Infested plants should be dug up and burned.
Parasites and cannibalism
The braconid wasp Rhaconotus fasciatus is the only recorded parasitoid of the rhubarb curculio. As stated, only a single larva develops to maturity in hosts other than rhubarb; it is not uncommon however to find several young larvae in the stalk of a host plant, indicating that the larvae may be cannibalistic.
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- Media related to Lixus concavus at Wikimedia Commons
- Data related to Lixus concavus at Wikispecies