Grass seed-March fly

Bibio xanthopus

Pest description and crop damage Adults are dark brown to black flies around 0.45 inches with reddish legs; smallish, dirty-looking, light brown to gray larvae (0.5 inch long when mature in February or March) are quite common in western Oregon fields high in organic matter. They feed primarily on decaying organic matter and only incidentally on grass seedlings.

Heavy fall and winter rains that cause water to puddle force the larvae to the soil surface. Large numbers of these wiggling, squirming larvae roll into seed furrows, "heaving" the seedling plants. If a freeze follows soon afterward, seedling mortality can be high.

Biology and life history In late March and April of some years, flies, bright blue with orange legs, inundate western Oregon. Females scatter eggs randomly over soil surfaces, crop and non-crop alike. The eggs lie dormant through summer and early fall, hatching in unirrigated soils with the onset of fall rains. Larvae develop through February and March of the next year and then pupate. There is one generation per year.

Management-cultural control

The following cultural practices may help in reducing damage.

  • Prepare a fine seedbed and pack well.
  • Close seed row at planting.
  • Plant early. Well established seedlings tolerate more injury than smaller ones.

Management-chemical control

None are registered for this site and use.