Description of the Pest
Adult asparagus miners are present in May and again in late summer. They are shiny, black, slightly humpbacked flies that are about 0.1 inch (2.5 mm) long. Tiny, whitish eggs (0.001 inch) are laid at the base of the stem and are seldom seen because they are deposited beneath the epidermis of an asparagus stalk. The white-colored larva is about 0.015 inch (0.4 mm) long when it hatches from the egg and grows to about 0.2 inches long. Dark brown, flattened pupae can be seen beneath the epidermis at the end of mines and measure up to 0.17 inch (4 mm) long. Asparagus miner overwinters in the pupal stage either in the stalk or in the soil.
Damage
Asparagus miner larvae occasionally injure asparagus during the fern growth stage but in California are not usually found damaging spears. Larvae mine just beneath the surface of fern stalks. Often the miner feeds upward in a meandering pattern and then turns downward as it continues to feed. It can occasionally girdle the stalk, causing the fern to yellow. Under heavy infestation, multiple mines may be seen in a single fern stalk. A direct association between asparagus miner feeding during the fern stage of plant development and losses in asparagus spear yield has not been conclusively shown.
Management
Asparagus miner populations have been reported to be reduced by several parasitic wasps (Dacnusa rondani, Dacnusa bathyzona, Pleurotropis epigonus, and Sphegigaster spp.), all of which attack pupae. If a heavy infestation is found, burning to destroy the pupae may help reduce the overwintering population (where burning of shredded asparagus ferns is still allowed in California). Spraying insecticides to control asparagus miner is rarely if ever justified.
Organically Acceptable Methods
Encouraging natural enemies.