PLANT PROTECTION 1 – Pests, Diseases and Weeds

PLANT PROTECTION 1 – Pests, Diseases and Weeds
Fungus gnats
Fungus gnats are becoming an increasingly
widespread and damaging nursery pest.
Scientific name
Fungus gnats (Order Diptera, Family
Mycetophilidae) and black fungus gnats (Family
Sciaridae), eg glasshouse sciarids (Bradysia spp.)
are widespread pests. Do not confuse fungus gnats
with shore flies (Family Ephydridae) and their
larvae which are often minor pests in greenhouses
(Goodwin et al 2000).
Host range
Flies (adults) are short-lived and do not feed on
plants. Maggots (larvae) feed on decaying fungi
and other organic matter in soil and potting media.
They also feed on roots and stems of most
seedlings, cuttings and soft-foliaged mature plants
such as carnation, gerbera, poinsettia and most
hydroponic crops. Shore fly maggots feed on
algae living on media surfaces while the adults
imbibe liquids and leave faecal spots on foliage.
Description & damage
Adult flies are mosquito-like, slender, gray or
black and about 2-5 mm long. They have long
slender dangling legs, long antennae and 1 pair of
wings with a Y-shaped vein at the tip. They are
weak fliers, hover in groups around plants or run
over the surface of seedling and cutting trays and
pots at dusk and can be a nuisance indoors around
potted plants. Larvae (maggots) are mostly small,
thread-like, active, almost transparent (internal
organs can be seen), legless, about 5-8 mm long
with small dark heads (Fig. 46) and can be found
wriggling on or near the surface of soil and potting
mixes. Maggots may leave a tiny slimy glistening
trail on the soil/mix surface. They may gain access
to roots through the base of pots. Pupae are
3-6 mm long, brown, cylindrical and are found in
soil or potting media.
Roots of seedlings, cuttings and young plants
may be damaged by large numbers of maggots.
Roots may be scarred and root hairs eaten off,
causing wilting and secondary attack by disease
organisms. Maggots may also feed on the callus
of cuttings, preventing striking or slowing down
root development. Larger maggots may tunnel into
stems of seedlings and cuttings just below the soil
surface killing them.
General.
Leaves yellow, plants lack vigour. Reduce
plant growth rate and yields – the root damage
reduces nutrient and water uptake.
Spread disease. Feeding maggots can ingest
and spread fungal spores of root rot fungi.
Pythium, Fusarium, Thielaviopsis (Chalara) and
Verticillium are carried in their gut and retained
through to the adult fly to be spread elsewhere.
Adults can also spread grey mould (Botrytis)
which attacks foliage. Shore flies and their larvae
can also transmit Phytophthora and Pythium spp.
Customers complain about flies and maggots,
poor presentation of damaged plants at point of
sale. Shore flies may also reduce marketability.
Flies irritate staff.
Diagnostics.
Fungus gnats are often confused with shore
flies. They are mosquito-like and often found
running on the soil surface, maggots have a
distinct head (Fig. 46). Shore flies look like house
flies, stout, with 5 pale spots on their wings,
maggots have no distinct head (Fig. 47). Adults
can be caught on sticky yellow traps but they can
be difficult to identify. You may need to get
advice. Use a x 10 lens and record counts.
Damage by larvae is often unnoticed because
they can be difficult to find in media or within
plant stems. Reduced growth is hard to quantify.
Pest cycle
There is a complete metamorphosis (egg, larva,
pupa and adult) with several overlapping generations
each season in greenhouses. Development varies with
temperature. At 24 o C egg to adult fly life cycle is
about 3 weeks. Adults mate soon after they emerge
from pupae in the soil and within 2-3 days the female
lays 100-200 small white eggs in cracks on
continually wet soil surfaces, particularly around the
base of plants or in plant debris. These eggs hatch
after 4-6 days into maggot-like larvae which feed for
2-3 weeks on media and plant roots then pupate in
soil or potting media.
‘Overwintering’
Possibly as pupae. Fungus gnats may breed
continuously at temperatures above 24 o C, in
greenhouses and in surrounding drains, etc.
Spread
By adults flying.
By movement of contaminated soil or media in
pots or plant material.
Fig. 46. Fungus gnat (Mycetophilidae).
Left: Adult (fly), 2-5 mm long, mosquito-like.
Right: Larva (maggot), 5-8 mm long, obvious head.
Fig. 47. Shore fly (Ephydridae).
Left: Adult (fly) 3-4 mm long, 5 pale spots on wings.
Right: Larva (maggot), 6-8 mm long, no obvious head.
Insects and allied pests - Diptera (flies) 75