Infestations of onion are sporadic from year to year, and no reliable methods have been found for predicting the occurrence of this pest. Black cutworm moths are attracted to fields containing weeds, especially winter or spring annual and perennial weeds such as chickweed, purslane, shepherd's purse, pepper grass and yellow rocket. Therefore, weedy fields tilled shortly before seeding are probable sites for infestation. Moths are to a lesser degree attracted to cereals, such as oat or wheat, that are often used as winter cover crops in many onion growing areas. Eggs of black cutworm are deposited singly or in small clumps on the leaves or stems of attractive hosts.
Neonate larvae settle just below the soil surface to feed.
As larvae consume the below ground portion, they continue to feed and pull the above ground portion into the feeding cell - thus plants appear to be sinking into the soil. A single larva may partially consume a number of onion plants during the seedling to five-leaf stages. Infestations of onion are usually limited to the earliest spring plantings. Management of black cutworm often relies on high volume insecticide sprays directed at the bases of plants. This method is of questionable value because larvae are difficult to contact with the toxicant and they are usually of advanced maturity before detection and treatment.