Cribrate Weevil

The adult cribrate weevil is dark brown to almost black, about 0.33 inch (8 mm) long, and has longitudinal striations on its back. Emergence of adult weevils begins in mid-June and continues through September, with the peak emergence occurring in mid-July. Freshly emerged adults are sexually immature and require 3-4 weeks of intensive host feeding for the egg maturation inside the ovaries. The sexually immature adults are highly mobile and voracious. They hide during the day under dirt clods, under the plant debris, or in the fronds of vegetative shoots.

As the weevils become sexually mature, they become less mobile and tend to stay close to the ground on younger leaves. Eggs are usually deposited in the soil around the roots of the host plant. Eggs hatch in about 18 to 20 days. At first the young grubs feed on feeder roots; as they develop, they begin to attack larger roots. Soft, fleshy roots are generally cut off within a short distance of the crown. Fully grown grubs feed on the cortex of the larger roots, and the roots may be entirely stripped of their bark for several inches or girdled at one or more points. Grubs can feed on the roots up to a depth of 12 inches. Cribrate weevil grubs do better in light soil as they can move around the plant roots. Because of the mild winter temperatures on the central coast of California, the grubs do not overwinter, and the feeding continues through spring. As the grubs complete their development, they begin to move near the soil surface. Pupation generally begins in mid-May, and the pupal period is 4 to 6 weeks.

The cribrate weevil has a broad host range, including artichokes and some of the common weeds such as mustard, cheeseweed, nettle, and oxalis. Although univoltine (one generation per year), adults appear to be active most of the year. Adult weevils are all parthenogenetic females, and males are not known to exist. Also, the adult weevils are flightless because of the fusion of the elytra along the median line. Therefore, the only possible way of introducing this pest to a new location is through bringing in infested plant material or soil.

Damage to the artichoke crop occurs in two different ways: larval damage to the roots and adult damage to the foliage and buds. The damage to the root system results in low plant vigor, which in turn causes a considerable loss of crop yield. In addition, feeding by larvae reduces the root system and infested plants may be pulled out of the ground during the annual cutback operation, resulting in the need to replant.

Adult weevils feed at night on the upper mature leaves of the plant. At moderate infestation levels, artichoke leaves are notched at their margins; when infestations are heavy, leaves are skeletonized down to the petiole. Adult feeding on the foliage causes a definite delay in growth and a loss of fall production. In summer production fields, adult feeding extends to the artichoke buds, and they are rendered unmarketable. In winter chokes, some early buds are damaged by adult weevils.

Cribrate weevil is managed by preventing the introduction of it into the field through infested root cuttings and with summer treatments when monitoring indicates a need.

Although not very cost-effective, crowns can be dipped in a water suspension of entomopathogenic nematode juveniles before planting. This practice is also useful for cleansing infested crowns of artichoke plume moth.

Attempts to control the cribrate weevil grubs in the field with various species of entomopathogenic nematodes have not been successful. A rate as high as 4 billion juveniles of Steinernema carpocapsae per acre applied through sprinkler irrigation failed to give any control of the soil-inhabiting grubs.

When planting a new field, obtain crowns from fields that are known to be free of cribrate weevil infestation to keep the field free of this pest for a longer period. When a field that was heavily infested with cribrate weevil in the past season is to be replanted, first deep plow the field to either kill the grubs and pupae directly or to bring them to the surface where they are exposed to predators. Also, leaving the fields fallow for at least 2 months helps to kill any surviving grubs and adults by starvation. Before replanting in a heavily infested field, fallow for at least 3 to 6 months after destroying the existing artichokes and replant with cribrate-free crowns.

Biological and cultural controls acceptable for use on organically certified crops.

In mid-July through September when most adult weevils emerge, check fields regularly for the distinctive notching at leaf margins caused by adults. Treat at the first sign of feeding injury by adults to the artichoke leaves.