Ceutorhynchus assimilis Paykull Coleoptera Curculionidae

Natural History

Distribution. Cabbage seedpod weevil is native to Europe, and was first discovered in North America in 1931 near Vancouver, British Columbia. It soon spread to Washington, Oregon, and Idaho, becoming a serious pest of cruciferous seed crops. With the increased popularity of rape as a field crop throughout North America, there is potential for cabbage seedpod weevil to spread and assume greater importance as a vegetable pest. For example, Buntin and Raymer (1994) and Boyd and Lentz (1994) reported this insect to be abundant and damaging to oilseed rape (canola) in northern Georgia and in Tennessee, respectively.

Host Plants. This weevil seems to feed exclusively on crucifers. Vegetable crops attacked include broccoli, Brussels sprouts, cabbage, cauliflower, Chinese cabbage, kale, kohlrabi, mustard, radish, rutabaga, and turnip. It also feeds on rape and on cruciferous weeds such as tansymustard, Descurainia spp.; rock-cress, Arabis spp.; shepherdspurse, Capsella bursapastoris; pepperweed, Lepidium spp.; and wild radish, Raphanus spp.

Natural Enemies. Numerous parasitoids are known to attack cabbage seedpod weevil, including Bracon sp. (Hymenoptera: Braconidae); Eupelmella vesicularis (Retzius) (Hymenoptera: Eupelmidae); Eurytoma sp. (Hymenoptera: Eurytomidae); Necremnus duplicatus Gahan and Tetrastichus sp. (both Hymenoptera: Eulophidae); Asaphes californicus Girault, Habrocy-tus sp., Trichomalus fasciatus (Thomson), and Zatropis sp. (all Hymenoptera: Pteromalidae); and Megaspilus sp. (Hymenoptera: Ceraphronidae). The most important species are Trichomalus fasciatus and Necremnus duplicatus, with the former species probably introduced from Europe accidentally at the same time as the weevil (Hanson et al., 1948).

Life Cycle and Description. There is a single generation per year. Adults overwinter in soil or under debris, becoming active in the spring as the air temperatures reach about 15°C. Adults typically feed on nectar and pollen for about a month before beginning egg production. Females denied the opportunity to feed at flowers exhibited poor development of their ovaries (Ni et al., 1990). Females are particularly attracted to volatile emissions of flowers (Evans and Allen-Williams, 1992). The egg development is favored by temperatures of 15° and 20°C, relative to 10° and 25°C (Ni et al., 1990). Flight is limited to periods of relatively low wind speed (less than 0.5 m/s) and warm temperatures (above 22° C) (Kjaer-Pedersen, 1992).

  1. Females deposit eggs singly in the seed pod. The normal distribution of eggs is one per pod. This is critically important in some of the wild hosts, where pods are small and only one larva can survive, but not essential in such crops as rape, where the pods provide excess food (Kozlowski et al., 1983). The egg is white or cream colored, and elliptical. They measure about 0.32-0.35 mm long and 0.20-0.22 mm wide. The eggs hatch in a few days and larvae feed on developing seeds for about 2-3 weeks.
  2. At the time of hatching, the larva is yellow, and measures 3-4 mm long. It soon turns white, with a brown head capsule, and eventually attains a length of about 5-7 mm. At maturity, larvae cut a hole in the seed pod and drop or crawl to the soil to pupate.
  3. The larva digs to a depth of 2-5 cm before creating a pupal chamber. Pupation requires 2-4 weeks. The pupa is white or yellow, and measures 3.2-3.7 mm long.
  4. The adults overwinter at field margins and in woods, becoming active in March or April in Georgia, then commencing oviposition. New adults usually emerge in July, and feed before entering diapause. Adults are small, only 1.75-2.5 mm long. They are black or grayish black. When they first emerge from the pupal stage they often have a fine covering of gray scales, but the gray color diminishes with time.

The biology of cabbage seedpod weevil was provided by Hanson etal. (1948) and Campbell etal. (1989).

Ceutorhynchus Thomsoni
Adult cabbage seedpod weevil.

Damage

Cabbage seedpod weevil larvae feed only on the developing seed pods of crucifers, so their effect on commercial vegetable production is limited mostly to Washington and surrounding areas, where vegetable seed production is an important industry. Each larva damages three seeds on average. The larvae should not pose a threat to home garden production unless seed production is desired. As noted previously, their increased importance in recent years is owing to increased demand for oil-seed rape.

The adult feeding during spring months is similarly restricted mostly to the flower and related tissues. Adult feeding punctures often produce sap-like exu-dates on the pods and stems, and stamens and pistils of the flowers are consumed. Egg laying occurs in the feeding punctures. These wounds also open the pod to invasion by a gall midge, Dasineura brassicae (Winnertz) (Diptera: Cecidomyiidae). Because the midge ovipositor cannot penetrate the seed pod, its abundance and damage are directly related to the weevils. In studies conducted in England, rape showed strong ability to compensate for damage by weevils, but secondary invasion of rape-seed pods by gall midges caused additional damage, thereby increasing the impact of the cabbage seedpod weevil (Free et al., 1983).

Feeding in the summer or autumn by pre-diapause adults occurs on stems, large leaf veins, and any pods or other thick-green plant material that is available.

Management

Insecticides. The presence of cruciferous weeds or previous crucifer crops can predispose an area to problems with seedpod weevil. If high populations are expected or detected, foliar insecticide applications are commonly recommended. To avoid destruction of pollinators, insecticides should be applied pre-bloom or post-bloom, but never during bloom.

Cultural Practices. Availability of overwintering sites may limit the abundance of weevils (Boyd and Lentz, 1994).

Host-Plant Resistance. Doucette (1947) evaluated the relative suitability of different crucifer crops for cabbage seedpod weevil. In general, mustard and radish were less susceptible to attack, whereas Chinese cabbage and turnip were most susceptible. He also documented considerable difference in susceptibility to attack among cabbage cultivars.

Pheromones. An oviposition-deterring phero-mone is deposited by the female following egg-laying, presumably to ensure adequate food for her offspring (Ferguson and Williams, 1991). Eventually this may lead to the ability to deter females from oviposition, thereby reducing damage.

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