Coleoptera Curculionidae

Natural History

Distribution. This native species is found throughout the eastern United States west to Idaho, Utah, and Texas. In Canada it is known from Ontario. A very similar insect, Lixus mucidus LeConte, may be confused with rhubarb curculio, because it shares the same geographic range, host plants, and biology. It also is native to North America.

Host Plants. These species normally are associated with such weeds as sunflower, Helianthus sp.; dock, Rumex spp.; and perhaps rosinweed, Silphium sp.; but both Lixus concavus and Lixus mucidus attack rhubarb. Interestingly, though the complete life cycle occurs in weeds, eggs and larvae rarely survive in rhubarb, so this vegetable is principally a food host for adults.

Natural Enemies. Population regulation is poorly known in this little-studied insect. The only parasitoid known from rhubarb curculio is Rhaconotus fasciatus (Ashmead) (Hymenoptera: Braconidae). The larvae apparently are cannibalistic, because although it is not uncommon to find several young larvae in the stalk of a host plant, it is rare to have more than one reach maturity in each plant.

Life Cycle and Description. There is a single generation annually. The adults overwinter, emerging in April in the vicinity of Washington D.C. Mating and oviposition occur soon thereafter. Eggs are found in April-mid-June, larvae through July or August, pupation commences in August, and adults begin emergence in September. In Canada, adults do not emerge from overwintering until June, but similarly complete a new generation by September.

Egg. The egg is oval in form and pale yellow, and is deposited in plant tissue at a depth of 2-3 mm. The eggs measure 1.5-1.9 mm long and 1.2-1.3 mm wide. They are deposited principally within the leaf petioles and flower stalk, though sometimes within the larger veins of the leaf. Apparently the weevil deposits the egg within cavities made by feeding, though there are many more feeding sites than oviposition sites. Duration of the egg stage is about eight days. The eggs fare poorly when deposited in rhubarb, apparently suffering from the flow of sap.

Rhubarb curculio larva.

  1. The larva is whitish, but with a brown head that usually bears an inverted Y-shaped mark. The larva's body is curved or C-shaped, highly wrinkled, and lacks thoracic legs. Larvae bore down through the stalk into the root, usually with only a single beetle surviving in each plant. Duration of the larval stage is about 60 days, and then the larva creates a pupal cell in the base of the stem or in the roots.
  2. The pupal cell is 25-40 mm long and 57 mm wide. It is constructed just beneath the soil surface, and 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, though the wings are twisted beneath the ventral surface of the body. The abdominal segments are marked with short spines. Duration of the pupal stage is about eight days, though the adult remains in the pupal chamber for another 7-8 days before emerging.
  3. The adult is 10-14 mm long, and black, but covered with fine gray hairs. Rhubarb curculio is also dusted with yellow powder though this is lacking from L. mucidus, and can be rubbed off. The pronotum and elytra bear punctures. After emerging in September, adults feed briefly on host plants, then seek sheltered locations for overwintering. The adults pass the winter hidden beneath debris near the host plants.

The biology was described by Webster (1889), Chittenden (1900), and Weiss (1912). These species were included in the treatment of eastern beetles by Downie and Arnett (1996) and of Canadian beetles by Campbell et al. (1989).

Damage

The adults of rhubarb curculio may nibble at the edge of leaves in the spring, but damage to rhubarb results principally from gnawing into the leaf stalks. The holes are oval or round, and up to 3 mm deep.

Trichobaris Trinotata
Adult rhubarb curculio.

Sticky sap secretions often exude from the wounds, collecting as glistening drops of gum. Feeding occurs mostly after most rhubarb, an early season crop, has been harvested. Larvae seldom develop in rhubarb. Nevertheless, rhubarb curculio is considered as the most injurious of the insects attacking rhubarb—a plant notably free of insect pests.

Management

Rhubarb curculio is rarely of consequence, though application of insecticides to rhubarb foliage should protect plants from injury. Because its abundance is governed mostly by the presence of favored weed hosts such as dock, elimination of such weeds will eliminate the threat of damage by weevils.

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