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Mites of Floriculture Crops – Biology and Control
Mites are not insects. They are arachnids, the same class as spiders.
Several mites attack greenhouse and field floriculture crops. Economically
important species include the two-spotted spider mite, Lewis mite, bulb
mite, cyclamen mite, broad mite and false spider mites.
Two-Spotted Spider Mite
The two-spotted spider mite, Tetranychus urticae, is often called
the “red
spider mite” because of its red colour during the diapause or hibernation
stage. Almost all broadleafed plants are susceptible to attack by
two-spotted mites. They are known to attack over 300 plant species and
thrive under greenhouse conditions. The adult reproductive stage is greenish
with two dark feeding spots on the back and is about 0.5 mm long. Females lay an average of 100 eggs. The round, pearly white eggs are too
small to be seen with the unaided eye. The newly hatched mites go through a
larval and two nymphal stages before becoming adults. (See Figure 1.) The
time from egg to adult is dependent mainly on temperature. Numbers build up
quickly under hot, dry conditions when it can take as little as 8 days at
temperatures of 25 to 35°C. Diapause is induced by decreasing temperatures
and day length, causing the now reddish mites to seek out cracks and
crevices to stay in until more favourable conditions in the spring. This
stage cannot be controlled by pesticides.
Damage
Eggs are laid on the undersurfaces of leaves and all stages feed there.
These mites prefer young, tender leaves where they suck out plant juices. A
chlorotic spot or stipple develops at each feeding site as chloroplasts are
sucked out along with the plant sap. Leaves eventually develop a mottled
appearance. Severe infestations can result in leaves becoming brittle and
parchment-like. Foliage can drop off and plant death may occur. As numbers
build-up, mites form webs of fine silk on plant terminals leading to further
aesthetic injury. There is some evidence that even low levels of mite
feeding causes plant stomatal closure, resulting in decreased CO2 uptake and
reduced transpiration and photosynthesis.
Scouting
Mites won’t be found in sticky traps. Their detection depends upon the
regular scouting of tender, new growth and the underside of leaves. Look for
stippling on the upper leaf surface as signs of mite feeding. Control
efforts are more effective when applied at low infestation levels, so early
detection is crucial. Be especially vigilant under hot, dry conditions that
favour spider mite population explosions. A 10 power hand lens will help
confirm the presence of mites on the lower leaf surfaces. A quick inspection
method suitable for some plants is to place a white sheet of paper under the
foliage and tap the plant. If mites are present some will drop on to the
paper.
Management
Sanitation is the first step in managing mites. Weeds are common host
plants, so ensure that weeds inside and adjacent to greenhouses are
eliminated. New plants should be inspected for mites before placing them in
greenhouses or planting them in fields. Do not have any “pet plants” inside
the greenhouse. Spider mites thrive on plants that are under stress. Be sure
to keep plants watered, give them adequate light, and fertilize properly.
Orient holes in overhead poly tubes so that plants are not under a direct
blast of hot dry air by having the holes point upward. Heavily infested
plants should be discarded. Bag or box plants before carrying them through
the greenhouse.
Biological Control
The most successful biological control agent for two-spotted mites is the
predatory mite Phytoseiulus persimilis. This mite is widely available from
producers of biological control agents. Tropical in origin, this predator is
well adapted to greenhouse conditions, but is not effective outdoors in BC.
Adult P. persimilis are shiny orange, while immature stages are a pale
salmon colour. They can be distinguished from the “red” phase of the
two-spotted mites by the lack of spots, the smooth pear-shaped body, and
their faster movement over leaf surfaces. Each female predatory mite lays
about 50 eggs. At favourable temperatures (20 to 30°C), the life cycle is
about a week, twice as rapid as two-spotted spider mites. P. persimilis
consume 5 to 50 spider mite eggs, nymphs, or adults a day.
Predatory mites are most effective at relative humidities greater than 70%
and temperatures of 20 to 30°C. Very hot, bright conditions favour spider
mites and can result in pest outbreaks. Ideally the plants should be in
close contact, allowing the predators to move easily through the crop. They
are not affected by day length and will remain on the crop as long as pest
mites are present. Unlike some predacious mites, they only feed on mites and
do not eat pollen. They will not survive between crops in an empty
greenhouse and will disappear from the greenhouse if mites are not present
on the crop.
The most successful biological control programs are based on
pre-releasing predators monthly or even twice a month on the assumption that
the crop is likely to become infested. Introduction rates range from 2 to 20
per plant, depending on infestation levels and the
crop involved. Suppliers can recommend more precise rates after inspecting
the crop. The most effective and economical introduction system is to apply
P. persimilis at the first sign of spider mites or their feeding (stippling
on leaves) at a rate of two per infested plant and to every fifth plant in
the rest of the crop. Subsequently treat any “hot-spots” that develop.
Releasing some predatory mites into areas outside outbreaks encourages them
to disperse to look for food. At very high spider mite levels it is often
advisable to apply insecticidal soap or other non-residual miticide.
For field crops, Neoseiulus fallacis, a predatory mite of temperate origin,
is promising for two-spotted mite control but has not been fully evaluated.
A naturally occurring small, black ladybug beetle, known as the spider mite
destroyer (Stethorus spp.), is
also being developed as a commercial biological mite control agent for both
greenhouse and outdoor crops.
When using beneficials in a crop, it is often necessary to apply pesticides
for other insects or diseases. Select chemicals that will have the least
negative impact on the biological agents. (Refer to Table 1.5, page 15 in the
2004 Floriculture
Production Guide.) If possible, treat only hot-spots or spray only the part
of the crop that is primarily under attack, such as the upper parts of roses
for aphid control.
Chemical Control
Two-spotted spider mites breed rapidly, so the potential for resistance
development is high. The primary method of minimizing the development of
resistant mite populations is to avoid using the same miticide for more than
three consecutive sprays. Tank-mixing pesticides with different modes of
action is also thought to promote resistance. Because spider mites are very small,
relatively immobile, and occur mainly on leaf undersurfaces, thorough
spray coverage is essential for adequate control. Most pesticides will not
kill the egg stage, so at least two applications are necessary. A
spreader-sticker or wetting agent will improve the effectiveness of most miticides, particularly on waxy leafed plants. However, that may increase
the possibility of phytotoxicity; do a test spray before doing any
large-scale applications. Product labels have information on the use of
adjuvants.
Table 1 lists currently registered products for mite control on commercial
flower crops. It also lists mode of action, residual activity, and the
target life cycle stage for each miticide, three factors to consider when deciding which
miticide to use. Check labels for
specific crop registrations and possible phytotoxicity. (Post this table in
your pesticide mixing area for easy reference.)
Lewis Mite
The Lewis mite, Eotetranychus lewisi, is primarily a pest of poinsettias in
BC. Lewis mites are similar in appearance to two-spotted mites, but are
slightly smaller and have several small spots rather than two large spots
when viewed under a microscope. (See Figure 2.) The damage they inflict on
crops is similar to that of two-spotted spider mites. They feed on the undersurfaces of leaves, piercing the epidermis and removing cell contents.
This results in a stippled appearance on the upper leaf surface. Eventually
the entire leaf becomes bleached and falls off. Heavy infestations will
produce webbing, but not as extensive as that of two-spotted spider mites.
Lewis mites do not enter a diapause or hibernation stage. The developmental time from egg to adult is 12 to 14 days at 21oC.
Females oviposit 2 to 3 eggs per day for about 30 days.
Lewis mite is not a common pest in BC, but poinsettia growers
should be vigilant for it. Plants that appear to loose colour or have
bleached foliage should be closely inspected for Lewis mites. Scouting for
Lewis mites is especially important if you have applied Impower 60 WP (imidacloprid)
because while it controls aphids and white flies, it does not control spider
mites.
Scouting, management, and control methods for Lewis mites are similar to
those for two-spotted mites.
Bulb Mites
Bulb mites, Rhyzoglyphus spp., are slow moving and relatively large
(0.5 to
0.9 mm long). They are pearly-white and smooth, with short reddish legs. From
their origin in Europe, bulb mites have spread throughout the world
presumably via shipments of infested bulbs. Bulb mites occur as large
colonies, not as individuals. Females produce at least 100 eggs that are
deposited near injured or decaying tissue. The time from egg to adult can be
as short as 12 days at 25°C. Factors affecting development time are
temperature, humidity, and host species.
Damage
Bulb mites are usually considered secondary pests of bulbs, but can be
responsible for serious loses. They invade bulbs at points of mechanical
injury and, once established, they spread rapidly and destroy large areas of
the bulb. In addition to feeding damage, the mites provide access to fungal
root pathogens, such as Pythium, Rhizoctonia, and Fusarium. Infested bulbs
may rot or produce stunted, distorted, or off-colour plants. Host crops
include dahlia, freesia, gladiolus, hyacinth, iris, narcissus, orchid, and
tulip. Vegetable bulbs can also be infested.
Scouting
Bulbs with apparent physical damage or rotted spots should be examined for
bulb mites. They are easily seen with a low power hand lens.
Management
Careful handling of bulbs to avoid the damage that provides access to mites
is important. In storage, maintaining low relative humidities will reduce
the potential for mite infestations. Steam pasteurization will
eliminate bulb mites from soil.
Biological Control
A predacious mite, Cosmolaelopss claviger, readily feeds on bulb mites, but
is not commercially available. There is evidence that Hypoaspis mites,
predatory soil-dwelling mites sold for fungus gnat control, may feed
on bulb mites, but not to the extent that control is achieved.
Chemical Control
Consult with floriculture extension staff for current recommendations.
Cyclamen Mite
The cyclamen mite, Stenotarsonemus pallidus, is tiny (0.13mm) and colourless.
It does best at cooler temperatures and requires high humidity. Under optimal
conditions the time from egg to adult is about 10 to 14 days.
Damage
Symptoms of cyclamen mite feeding often resemble disease symptoms. Cyclamen
mites feed on all parts of the plant, but young foliage is most often
affected. These mites avoid light, so they feed mainly on unopened leaflets
and buds, resulting in wrinkled, deformed leaves and buds that do not open
or produce distorted blooms. New growth is deformed and distorted. Leaves
may be thickened, strap-like, and reduced in size. As leaves unfurl they may
show signs of scarring or bronzing. Favoured host plants include African
violet, azalea, begonia, chrysanthemum, cyclamen, dahlia, delphinium,
exacum, fuchsia, gerbera, geranium, gloxinia, kalanchoe, New Guinea
impatiens, and snapdragon.
Scouting
Recognition of the damage caused by these very small mites is the best way
to determine the presence of an infestation. Especially characteristic are
plants that have ceased blooming and have shortened internodes, distorted
leaves and stems, and blasted buds. Infestations tend to be localized.
Dissecting tissue under a microscope may be necessary to confirm the
presence of cyclamen mites.
Management
If only a few plants are affected, the best way to deal with the problem is
to discard them. Workers should be careful not to spread mites through-out the greenhouse
on their hands or clothing. Cyclamen mites on nonblooming plants can be
killed by immersion, pot and all, for 15 minutes in water heated to 43°C.
Decreasing humidity can also reduce infestation levels.
Biological Control
No reliable biological controls have been identified for cyclamen mites,
although the predatory mite Amblyseius cucumeris is known to feed on
tarsonomid mites and may contribute to their control.
Chemical Control
Two or 3 applications at 5 to 7 day intervals of dicofol (Kelthane) or
endosulfan (Thiodan) will control cyclamen mites. Use sufficient spray
volume and pressure to thoroughly cover all surfaces of the plants. Several
applications at weekly intervals are required because all stages are usually
present and are generally well protected from sprays.
Broad Mites
Broad mites, Plyphagotarsonemus latus, are very small (0.1-0.2mm) and
colorless. Under optimal conditions the time from egg to adult is about 5
days.
Damage:
Broad mites attack a wide range of commercially important plants, including
gerbera, African violets, cyclamen, begonias, impatiens, verbena and
gloxinia. Broad mite damage is usually expressed as distorted and downward
curling leaves resulting from a toxin secreted by the feeding mites. As
well, internodes and petioles of flowers become shortened, and flowers may
fail to open. Severely infested plants become stunted and may die.
Scouting:
Broad mites are too small to be seen with a hand lens. A microscope is
needed to confirm their presence. Broad mites prefer high relative humidity
and therefore hide in growing points, flower buds, and overlapping surfaces
of developing leaves. Although similar in appearance to cyclamen mites,
broad mites can be differentiated by the whitish bumps on their eggs
compared to the smooth eggs of cyclamen mites.
Management:
Control of this mite is difficult. Ideally infested plants should be
identified quickly and carefully removed from the greenhouse before the
mites spread. Imported cuttings should be examined to prevent introduction
into a non-infested greenhouse. Growers with broad mite problems one year
should be especially vigilant the next spring for the possibility of a
carry-over population.
Biological control:
No reliable biological control agents have been identified for broad mites,
although the predatory mite Amblysius cucumeris is known to feed on
tarsonemid mites and may contribute to their control.
Chemical control:
Sprays applied to control spider mites will result in broad mite control. A
power sprayer is necessary to achieve adequate penetration. Repeated
applications at five day intervals may be required in warm weather.
False Spider Mite
False spider mites or flat mites (Family Tenuipalpidae) are flattened,
very small, reddish, and slow moving. The main pest species are in the genus
Brevipalpus. Adult false spider mites are about 0.3 mm long. The eggs are
red, somewhat flattened, and take up to 3 weeks to hatch. The immature
stages feed and develop for 5 to 6 weeks before becoming adults. The life
cycle is about 5 times longer than the two-spotted mite life cycle.
Brevipalpus species attack a wide variety of greenhouse plants, but are most
commonly found on orchids.
Damage
False spider mites are not common in BC greenhouses. When present, they feed
first along the midribs of leaves and then disperse outwards. They puncture
the epidermis of the leaf and suck out the plant juices. This results in the
leaves having first a mottled and later a silvery appearance. Under
conditions of severe infestations, plant tissue turns brown and dies. Stems
are also attacked. False spider mites do not produce webbing.
Biological Control
Studies have not been done to evaluate biological control agents for
Brevipalpus mites, but it seems probable that Phytoseiulus persimilis would
feed on them.
Chemical Control
Pesticides that control two-spotted mites will also control false spider
mites. Because they feed on the undersides of the leaves, the pesticide must
be directed upwards for best control.
Table 1. Miticides for Ornamental Crops*
Trade Name |
Active Ingredient |
Registration |
Type of Mite Controlled |
Mode of Action |
Residual Activity** |
Life Cycle Stage Effective Against |
Avermectins |
Avid |
avermectin |
greenhouse |
two-spotted |
contact/ translaminar |
long |
all stages
except egg; takes 3-4 days for kill |
Organophosphates |
D.D.V.P. |
dichlorvos |
greenhouse |
two-spotted |
contact |
knock-down |
all stages except egg |
Dibrom |
naled |
greenhouse & field |
two-spotted |
contact |
knock-down |
all stages except egg |
Lagon |
dimethoate |
field |
all mites |
systemic |
moderate |
all stages except egg |
Malathion |
malathion |
greenhouse & field |
two-spotted |
contact |
knock-down |
all stages except egg |
Orthene |
acephate |
field |
spider mites |
systemic/ contact |
moderate |
all stages except egg |
Organochlorines |
Endosulfan/
Thiodan |
endosulfan |
greenhouse & field |
cyclamen |
contact |
moderate |
all stages except egg |
Kelthane |
dicofol |
greenhouse & field |
all mites |
contact |
high |
adult |
Pyridazinones |
Dyno-mite/
Sanmite |
pyridaben |
greenhouse |
two-spotted |
contact |
moderate |
larval and nymphal |
use only when adult stage is less than 15%
of population; use a quick
knock-down spray first |
Soaps and Oils |
Insecticial
Soap |
potassium salts of fatty acids |
greenhouse & field |
two-spotted |
contact |
knock-down |
all stages except egg |
Other |
Vendex |
fenbutatin oxide |
greenhouse & field |
two-spotted |
contact |
moderate |
all stages except egg |
Floramite |
bifenazate |
greenhouse |
two-spotted, Lewis mite |
contact |
moderate |
all stages except egg |
* check label for specific crop registrations and
for phytotoxicity warnings
** varies with crop and environmental conditions. |
Ministry of Agriculture and Lands
Abbotsford Agriculture Centre
1767 Angus Campbell Road
Abbotsford, BC V3G 2M3
Phone: (604) 556-3001
Revised
October 2006
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