Pest & Disease Factsheet - Spider mites

Defences for protected and outdoor ornamentals.

Spidermite on rose - image: Dove Associates
Spidermite on rose - image: Dove Associates

Glasshouse red spider mite, also known as the two-spotted spider mite, is a troublesome pest of both protected and outdoor ornamentals.

On outdoor crops, the fruit tree red spider mite (Panonychus ulmi) will attack trees such as Malus, Sorbus and flowering cherries. Panonychus citri will attack crops including choisya and skimmia. Careful and regular monitoring helps to produce an effective integrated pest management programme. Spider mite populations can proliferate under the right conditions and quickly get out of hand. The worst build-ups are from June to September on outdoor and protected crops.

In protected crops, control can be further complicated by hybridisation between the two species of glasshouse red spider mite, Tetranychus urticae and T. cinnabarinus. Strains resistant to some pesticides are also known.

Heavy infestations on protected crops occur in hot, dry conditions and are best controlled by sprays of non-persistent acaricides — pesticides designed to target spider mites and their relatives — before introducing predators. Acaricides that kill the egg stage should be used alongside ones that target the adults and nymphs.

How to recognise them

Colonies of adults, nymphs and eggs are normally found on the lower surfaces of leaves, adjacent to veins, and are best identified with a hand lens. Adults are around 1mm long with four pairs of legs, characteristic of the spider family, compared with the three pairs of legs of insect pests. Despite the name, they tend to be light to dark-green in colour, but are sometimes orange to carmine red (Tetranychus cinnabarinus).

The two-spotted spider mite (Tetranychus urticae) has a pair of dark spots on its back. Overwintering females and those about to produce eggs are orange-red in colour.

Panonychus spp. tend to have longer body hairs and are brown/red in colour. The small, visible red spiders that can be found on foliage are not pests.

Biology

The two key species are Tetranychus urticae and T. cinnabarinus. Breeding occurs from spring to autumn. Females live for a month or more and produce up to 100 eggs, initially transparent but turning white to light-yellow just before hatching. The six-legged larvae feed for a few days before their resting stage.

The full life cycle can be just 12 days at 21°C but can last up to 55 days at 10°C. From August, females go into hibernation until March or April. Favourite hiding places include cracks, leaf litter, debris, cane ends and the corners of Mypex covers.

The fruit tree red spider mite (Panonychus ulmi) lays small, red-brown eggs that overwinter on twigs and branches. They hatch from April to June. Adults feed and breed on the undersides of leaves and four-to-five generations can be produced in a season. As temperatures start to fall in autumn, the females lay overwintering eggs and die. P. citri has a similar life cycle. Mites on webbing (pictured) can be spread by air currents and on clothing.

Symptoms

-Feeding causes tiny yellow spots
on upper surfaces of leaves that converge to form large yellow patches, often associated with "bronzing". Left unchecked, the leaf eventually dies. Heavy infestations can kill the plant.
-Large numbers can distort shoots and leaves as well as desiccating leaves and flowers.
-As plants near death, mites swarm on webbing, often "dripping" from the ends of leaves.

Treatment: biological control

-Predatory mites — Typhlodromus pyri is recommended for fruit-tree red spider mite; Phytoseiulus persimilis (lower temperatures), Amblyseius andersoni and A. californicus (higher temperatures) for glasshouse red spider mite.
-Predatory midges — Feltiella acarisuga) will attack most spider mites.
-Black ladybird (Stethorus), lacewings, anthocorid and Macrolophus bugs will also predate on spider mites.

Treatment: cultural control

-Alternate sprays of different active mechanisms to avoid resistance.
-High humidity suppresses spider mite activity.
-Infestations are most severe on stressed plants. Ensure appropriate irrigation, nutrition and climate are provided for the crop.
-Remove and burn any badly infested material and plant debris.
-Remove debris that may provide overwintering sites and check potential sites, such as Mypex covers, in your autumn clean-up.
-Clean and disinfect between crops if possible.
-Control willow and other weed hosts.

Treatment: chemical control

Active ingredient Abamectin
IRAC code 6
Formulations Various including Dynamec (Syngenta)
Action(s) Naturally derived, selective acaricide and insecticide. A short delay
is required prior to the introduction of biological controls.

Active ingredient Beauveria bassiana
Formulations Naturalis-L (Belchim), Botanigard WP (Certis)
Action(s) Primarily for whitefly and thrips control but has some effect on spider mites. Compatible with biological controls.

Active ingredient Clofentezine
IRAC code 10A
Formulation Apollo 50SC* (Adama)
Action(s) Acts on eggs and early nymph stages. On established infestations, use with adult acaricide. Outdoor crops only — compatible with biological controls.

Active ingredient Dimethoate
IRAC code 1B
Formulations Various including Danadim Progress (Headland)
Action(s) Contact, systemic organophosphorus insecticide — incompatible with biological controls.

Active ingredient Etoxazole
IRAC code 10B
Formulation Borneo* (Interfarm)
Action(s) Contact acaricide with moulting-inhibitor abilities on egg, larva and nymph stages. No control effect on adults but will leave females sterile. Protected crops only. Compatible with some biological controls.

Active ingredient Lambda-cyhalothrin
Formulation Hallmark with Zeon Technology* (Syngenta) and others
IRAC code 3
Action(s) Persistent, contact and residual insecticide — incompatible with biological controls.

Active ingredient Maltodextrin
Formulation Eradicoat, Majestik (Certis)
Action(s) Contact-acting with little residual effect. Active on all mobile mite stages. Compatible with some biological controls.

Active ingredient Nutrient-based plus additional natural products
Formulation SB Plant Invigorator (Fargro)
Action(s) Contact insecticide with
physical action. Compatible with some biological controls.

Active ingredient Pyrethrins
IRAC code 3
Formulation Pyrethrum 5EC (Agropharm), Spruzit (Certis)
Action(s) Short-term, knockdown
product. Compatible with biological controls if predators are re-introduced after seven days.

Active ingredient Spirodiclofen
IRAC code 23
Formulation Envidor* (Bayer)
Action(s) Contact-acting acaricide that interferes with development. Not on cordylines. Restrictions on crop location. Compatible with some biological controls.

Active ingredient Tebufenpyrad
IRAC code 21
Formulation Masai (BASF)
Action(s) Acts on eggs and all motile
stages of spider mite. Protected crops
only. Compatible with some biological controls.

Fully updated by Dove Associates

Use plant protection products safely. Always read the label and product information before use.
* EAMU required for use in ornamental plant production outdoors and/or under protection.
Dove Associates shall in no event be liable for the loss of or damage to any crops or biological control agents caused by the use of products mentioned.

SEE HORTICULTURE WEEK'S FULL ARCHIVE OF PEST AND DISEASE FACTSHEETS


Have you registered with us yet?

Register now to enjoy more articles and free email bulletins

Sign up now
Already registered?
Sign in

Before commenting please read our rules for commenting on articles.

If you see a comment you find offensive, you can flag it as inappropriate. In the top right-hand corner of an individual comment, you will see 'flag as inappropriate'. Clicking this prompts us to review the comment. For further information see our rules for commenting on articles.

comments powered by Disqus

Read These Next

Pest & Disease Factsheet - Mealybugs

Pest & Disease Factsheet - Mealybugs

Vines, tomatoes and tropical plants are among those at risk.

Pest & Disease Factsheet - Bacterial and fungal canker

Pest & Disease Factsheet - Bacterial and fungal canker

A wide range of nursery stock can be susceptible to potential damage from various cankers.

Pest & Disease Factsheet - Spider mites

Pest & Disease Factsheet - Spider mites

Defences for protected and outdoor ornamentals.