Crop Management
Minimize moisture under the benches to reduce breeding of fungus gnats
there. Ample moisture favors development of fungus gnat larvae. However,
existing larvae may attack plant roots if pots are under watered.
Depending on level of fungus gnats, apply either a pesticide or
biocontrol under benches to minimize fungus gnat numbers prior to
starting crop. High numbers of fungus gnat larvae can cause serious
damage to plant roots. Shore fly larvae feed only on algae and can breed
in standing water. It is important to be able to distinguish between the
two insects for control purposes.
Identifying
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Fungus gnat adults are delicate black insects usually less than 1/8
inch long, with long legs and antennae. Their wings are clear,
and have a distinctive Y-shaped vein that distinguishes them
from shoreflies (both are about the size of fruit flies). Fungus
gnat larvae are slender with clear bodies and a black head
capsule.
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Shore fly adults are more stout and usually larger than fungus
gnat adults. They have short legs and antennae, and dark wings
with clear spots on each wing. Larvae are yellowish-brown,
maggot-shaped, and do not have a distinct head capsule. Larvae
of both shore flies and fungus gnats are found in the surface of
the growing medium.
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Monitoring
Yellow Sticky Cards: Count fungus
gnat and shorefly adults weekly on yellow sticky cards. Place some
horizontally a little above soil level; others, vertically at the plant
canopy level. Examine sticky cards weekly and renew them every 1-4
weeks. Your time is an important factor in choosing the number of cards
that will be monitored. A few cards counted per week is infinitely
better than none at all. However, fungus gnat and shorefly populations
vary considerably from place to place in the greenhouse, so place them
thoughtfully.
Potato Slice Inspections: You can monitor the relative number of fungus gnat larvae in the soil by placing a disk of raw potato on the soil surface.
After 7 days, turn the
disk over and count the number of fungus gnat larvae. Mary Harris used
this technique to study fungus gnats and their control at the University
of Georgia. You can assess whether a control measure has killed fungus
gnats by comparing larval numbers before and after treatment. Make disks
of thesame diameter by punching them out of potato slices with a cork
borer or sharpened piece of pipe. Place 10 disks per 1000 square
feet of growing area.
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