Spider mites GWF251
Updated January 2009
Although barely visible, spider mites quickly build up into large colonies. These pests can
ruin the foliage of ornamental plants and reduce yields of fruit and vegetables.
Q What are spider mites? Q Are they something to worry Q Which plants are most
about? at risk?
A Spider mites are tiny wingless
insect-like creatures that have A Although they are extremely A Spider mites have been found
eight legs and a one-piece body. small, spider mites can build up to on over 180 different species of
They are so small that a hand enormous numbers in the right plant. However, some are more
lens is needed to see them conditions, making them one of susceptible than others. Ones
clearly. Greenhouse spider mites the most destructive pests in the likely to be affected include:
(Tetranychus urticae) are also garden. They can ruin foliage, begonia, brugmansia, carnation,
known as two-spotted spider mite spoiling ornamental plants and chrysanthemum, crocosmia,
or red spider mite. Another reducing yields of vegetables, cucumber, dahlia, datura, French
spider mite found indoors is fruit and salad crops. Heavy and runner beans, fuchsias, fruit
the carmine spider mite attacks can even kill plants. trees and bushes, houseplants,
(Tetranychus cinnabarinus). Both impatiens, melon, pelargoniums,
primulas, spruce (Picea),
species feed on plant sap. Q How can I tell if they are strawberries and vines.
present?
Q Can you tell me more about A Look underneath the leaves for
them?
Q What causes outbreaks?
tiny slow-moving mites with two
A Tiny (0.14mm diameter) eggs dark spots on their backs. A hand
A Hot, dry, dusty conditions lead
hatch into white, six-legged lens will help. Another test is to to spider-mite problems. They
larvae. The eggs are easily seen hold a sheet of white paper are often worst in greenhouses,
with a hand lens, but the larvae beneath the plant and shake the conservatories and on
are harder to spot. The adult foliage. The mites will fall on to houseplants. These conditions
female lays 4-6 eggs a day, the paper and can be clearly seen. encourage the mites to feed
starting when they are 36 hours Look out for a fine webbing on the often, causing damage to the
old. It is at this adult stage that underside of the leaves and on plants every time they feed. Even
they take on their characteristic young shoot tips in advanced plants that do not normally suffer
two-spotted appearance. The cases. Check for yellowing from spider mites become at risk
whole life cycle can take as little (chlorosis) and minute speckling in these conditions. Unhealthy
as three days in hot conditions, of leaves. In the later stages, plants are especially vulnerable.
though in cooler weather it may brown falling leaves and needles Cool moist conditions can help
extend to a month. are also seen. Cucumbers and to reduce spider-mite
By September, the shorter melons, in particular, can be populations, perhaps by limiting
days and slower plant growth stop their breeding success.
severely affected by a spider-mite
their reproduction. The remaining attack.
females turn red and find sites to Some strains of spider mites Q Can attacks be avoided?
hibernate, such as in the soil, on cause the plant hypertoxic shock: A Spider mites are less likely to
bark and on greenhouse walls, the leaves become bleached,
attack if you give the plants the
even on pots. The first adults wilted and dried-up.
best possible conditions, misting
seen in spring are also bright red.
foliage regularly. However, take mites present. You have to Suppliers
care not to overdo it, as it can examine your plants closely from
Biowise
encourage fungal disease. April onwards, using a hand lens Hoyle Depot, Graffham,
Get rid of the worst-affected to spot the first few mites. When Petworth, West Sussex GU28 0LR
plants as spider mites spread to you see them, order predators 01798 867574
other plants from them. from the suppliers listed here. You www.biowise-biocontrol.co.uk
Keep new plants separate from might have to make further
others for the first two weeks, in introductions throughout the Defenders Ltd
Occupation Road, Wye,
case they are infested. summer, but often one will be
Ashford, Kent
When you bring plants back enough. The effectiveness of the
TN25 5EN 01233 813121
indoors after a summer outside, biologicial control is reduced if
www.defenders.co.uk
check them carefully for spider the conditions are too hot. If you
mites – it’s best to throw out use shading paint on glass and Green Gardener
infested plants. ventilate the greenhouse well, Brook Hill, Brundall Road,
they should be all right. Blofield NR13 4LB
Q Are there any pesticides You can keep an eye on the 01603 715096
Phytoseiulus by looking with a www.greengardener.co.uk
available for the control of
spider mites? hand lens for bigger, rounder, Harrod Horticultural
redder mites than spider mites. Pinbush Road, Lowestoft, Suffolk
A There are a range of Phytoseiulus won’t survive the NR33 7NL 0845 402 5300
insecticides for control of spider winter, so you have to introduce it www.harrodhorticultural.com
mites. However, bear in mind every year.
that if you have used a pesticide Just Green
If spider mite is a problem Unit 14 Springfield Industrial
and then want to use a biological every year try Amblyseius Estate, Burnham-on-Crouch,
control, you’ll need to wait at andersoni. It works more slowly Essex CM0 8UA
least a month, as residues from than Phytoseiulus, but it can be 01621 785088
spraying may kill the predators introduced before overwintering www.just-green.com
you introduce. By this time the
spider mites re-emerge in mid-
mites will probably be out of Scarletts
late March. This is because it
control. Nayland Road, West Bergholt,
can survive a few weeks without
Colchester, Essex
food and also eats pollen, thrips,
Q Is there a spray that’s springtails and other
CO6 3DH 01206 240466
suitable for organic gardeners? www.scarletts.co.uk
invertebrates. It’s active at
A Yes, we recommend ready-to- temperatures as low as 6°C and The Organic Gardening
use pyrethrum-based sprays is also more tolerant of hot, dry Catalogue
which are suitable for traditional conditions. It can be used Riverdene Business Park, Molesey
or organic gardeners. Spray the outside as well as under Road, Hersham, Surrey
leaf undersides every seven days. protection, provided KT12 4RG 0845 130 1304
Biological control can be used temperatures are suitable. While www.organiccatalog.com
safely 3-4 days after spraying. it can overwinter in the
greenhouse, this probably won’t
Q Tell me more about biological be in sufficient numbers to keep
control. spider mite in check.
A Introduced predatory mites Release all predators
(according to the instructions) on
Phytoseiulus persimilis can
to the problem plants.
eliminate spider mites indoors.
They can eat up to five adults or
60 eggs per day. The drawback is
that they should be introduced
when there are only a few spider
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