Resource Guide for Organic Insect and Disease ... - Cornell University

method of using Bt is not described on the label. This offlabel
use is permitted by EPA under FIFRA 2ee, but growers
should check with their state pesticide regulators about
specific local regulations. The bait is made by: determining
the application area; mixing the highest concentration
solution of Bt allowed on the label; and then mixing in a bit of
molasses, alfalfa meal, or bran. Dampen the mix if necessary.
Spread the bait along the planted or planned rows in the
evening.
CARROT RUST FLY (Psila rosae)
The carrot rust fly is not a common pest, but when it occurs, it is a major problem because
maggots make carrots unmarketable. Larvae are light yellowish maggots that burrow through
the carrot, leaving rust-colored tunnels (Photo 9.1). The adult is a slender, blackish fly that
emerges in the spring (around the time that 250 base 40°F degree days have accumulated)
from pupae overwintering in the soil. After a short mating period, the females seek out umbelliferous
plants and lay eggs on the soil near the base of the plants. Eggs hatch in 7-10 days,
and the young larvae move downward into the soil, attracted to carbon dioxide emitted by the
plant, and begin feeding on the developing root. When the plants are small, larvae will feed on
young roots, causing misshapen carrots and possibly seedling wilt or death. In the Northeast,
a second generation of rust fly peaks in late July and August (at about 1150 base 40 degree
days), and larvae from this generation tunnel into the tap roots. Damage tends to occur in the
lower two-thirds of the root. Larvae that mature form pupae and overwinter in the soil. Carrots
that are harvested in the fall may have remnant tunnels or immature larvae. Parsley, celery,
parsnips, and wild carrots are also hosts, but carrot is the most economically important crop
infested by the larvae.
Cultural Control:
1. Plan planting and harvesting to avoid peak flights.
2. Crop rotation of a mile or more from other umbelliferous crops and weeds works well
but is often impractical because of the many wild umbelliferous weeds that also serve
as hosts.
3. Row covers are the best management. Install row covers before adults emerge.
Materials Approved for Organic Production:
Several azadirachtin products, PyGanic, garlic products, and a sesame oil product are labeled
for use against carrot rust fly, but efficacy information is lacking.
CARROT WEEVIL (Listronotus oregonensis)
Most umbelliferous crops and wild plants are hosts to the carrot weevil. Some other species
with tap roots are also hosts, such as curly dock and plantain. Though the carrot weevil is
more widespread than the carrot rust fly, it causes damage that is somewhat similar. The
larvae of this beetle are white grubs that tunnel into the root just below the epidermis, leaving
a thin layer of cells above the tunnel. By maturity, this layer collapses and leaves visible,
open channels. Most commonly, the damage is to the upper third of the root. In celery, the
damage is near the base of the petiole and often can be tolerated when trimming removes the
damaged tissue before marketing. In more extreme cases, the tunneling destroys the base of
the plant and may severely injure the roots, causing the plant to wilt and die.
The carrot weevil spends the winter as an adult and can be found in or near carrot fields in crop
debris, weeds, or grassy fields. Often, crops near the boarders of the field suffer more damage
because the adults are entering from nearby overwintering sites.
Cultural Control:
1. Reduce habitat for overwintering adults. Remove debris from crop fields and
surrounding land. Mow grass short in surrounding fields. Kill fall cover crops early in
the spring, and till fields deeply to kill overwintering adults.
Organic Resource Guide 73