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Seedcorn beetles

Stenolophus comma (Fabricius) Stenolophus lecontei (Chaudoir)

Slender Seedcorn Beetle

Clivina impressifrons (LeConte) (Coleoptera: Carabidae)

Natural History

Distribution. The seedcorn and slender seedcorn beetles are commonly found in the United States in the midwest and northeast, and occur west through the Great Plains and south to South Carolina. In Canada, they are present in eastern provinces, west to at least Ontario. They are native insects. The early accounts of the Stenolophus spp. are unreliable because the species were often confused; despite early reports suggesting that S. lecontei was the most important of the seedcorn beetles, it now appears that S. comma is the most serious pest.

Host Plants. Carabid beetles are known principally as predators of other insects. Animal matter also is the preferred food source of the seedcorn and slender seedcorn beetles. Eggs, larvae, and probably pupae of soil-dwelling insects are readily consumed. For example, Wyman et al. (1976) documented the reduction of cabbage maggot, Delia radicum (Linnaeus), populations when seedcorn beetles were numerous in crucifer plots.

As suggested by the common names, on occasion these beetles will injure germinating seeds, particularly corn seed. However, beetles also feed on weed seeds. Adults of S. comma feed readily on black nightshade, Solanum nigrum; crabgrass, Digitaria sanguinalis; foxtail, Setaria glauca; lambsquarters, Chenopodium album; and purslane, Portulaca oleraceae, and to a lesser degree on other seeds (Pausch, 1979).

Natural Enemies. The natural enemies of these insects are poorly known. The mite Ovacarus clivinae (Stannard and Vaishampayan) develops inside slender seedcorn beetle, where it is associated with the reproductive system (Stannard and Vaishampayan, 1971).

Life Cycle and Description. These beetles are quite similar in biology. Adults overwinter and can be found throughout the year. Maximum above-ground activity in South Dakota was observed to occur during early summer, followed by almost total absence in late summer, and then reappearance in the autumn. Eggs are most abundant in the spring, larvae and then pupae occur in late spring and early summer, and new adults are produced by mid-summer. Only a single generation is produced annually. Development of the insects from egg to adult requires about 50 days.

Egg. The eggs are laid singly in horizontal tunnels dug about 2 cm deep in the soil. The eggs are white and oval in shape, measuring about 1 mm long and 0.5 mm wide. Mean duration of the egg stage is about 5.4 days (range of 3-12 days), when reared at about 22?C.

Larva. The larvae are yellowish, with a dark-brown head and prothoracic plate. The larvae display three instars, and all instars can be found during the summer months. They are normally found in burrows in the soil at depths of 2-3 cm. Mean duration (range) is reported 10.5 (4-19), 9.3 (5-16), and 12.8 (9-21) days for instars 1-3, respectively, when reared at about 22?C.

Pupa. Pupation occurs within the soil in small cells, each measuring about 13 mm in length and 5 mm in width. The pupa is dark brown or black. Mean duration of the pupal stage is 9.4 days, with a range of 7-13 days, when reared at 22?C.

Adult. The adults are small oblong beetles measuring 5-8 mm long. Adults of the Stenolophus spp. are dark below and yellowish brown or reddish brown above, but with the elytra blackened except for the margin. Slender seedcorn beetle differs in that it is entirely reddish brown and has especially enlarged and flattened front legs suitable for tunneling through soil. Adults are commonly found in moist soil, with a high content of organic matter. In the late autumn, beetles burrow beneath soil, rocks, and logs, where they remain in a small cell until the soil warms to about 5?C in the spring. Adults may fly during the daylight hours in the spring, but are more active in the evening during the summer months. Apparently, copulation occurs below-ground in small chambers constructed by the adults during the spring. Oviposition commences within 14?15 days of emergence, and continues for 5-10 weeks.

The biology of Stenolophus comma was described by Kirk (1975); both S. comma and S. lecontei were considered by Pausch (1979). Slender seedcorn beetle was treated by Phillips (1909) and Pausch and Pausch (1980). These species were included in the keys to adult beetles by Downie and Arnett (1996).

Damage

Damage to germinating seeds caused by seedcorn beetles occurs principally during cool and wet weather in spring. Rarely is an ungerminated seed attacked. The contents of the seed are often consumed, leaving only the hull or seed coat. The result of attack is often a poor stand, as seedlings attacked in this manner may perish. However, if only a small portion of the seed is consumed, the seedling may recover and grow normally.

Management

The adult populations can be sampled with pitfall and blacklight traps. Rows of corn in which plants are missing should be examined carefully because beetles can often be found in association with the poorly germinating seeds. The populations of seed corn beetles can be controlled by application of insecticides to the seed, or in liquid or granular form to the soil at planting or shortly after planting (Daniels, 1977). Resistance to insecticides is evident in some populations (Sechriest et ah, 1971).