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Description
The adult female twospotted spider mite is approximately 0.65 mm long, has 4 pairs of legs and is globular in shape with long spines or hairs on the body and legs. Colour ranges from pale yellow to green with two distinct darker spots on each side of the body. On older mites these spots may give an overall dark colour to the body.
Adult males are smaller, narrower and more pear-shaped. Both males and females have bright red eye spots. Overwintering female mites turn bright orange in autumn and winter.
Eggs are spherical, translucent and about 0.1 mm diameter, and are commonly laid on webbing produced by female mites.
The first stage nymph (often called a larva) has 3 pair legs, and is not much bigger than an egg. The second and third stage nymphs have 4 pairs of legs and are pale yellow or green with two green spots on the abdomen.
Between each nymphal stage there is a moulting phase during which the nymph settles on the leaf and casts the old exoskeleton or skin.
The use of a 10x hand lens is recommended for identifying twospotted spider mites.
Distribution and host range
Twospotted spider mite is found throughout New Zealand on a wide range of fruit, vegetable, ornamental plants, weeds and shelter species.
This mite is an important pest of greenhouse crops including tomato, capsicum, cucumber, melon, pepper, bean, rose, carnation, orchid and chrysanthemum.
Outbreaks of twospotted spider mites commonly occur during hot dry conditions, when plant foliage is covered by dust, or when chemicals are used which disrupt biological control agents.
Signs and symptoms
Spider mites have fine piercing mouthparts that puncture plant cells from which they suck up the cell contents. Both nymphs and adults cause feeding damage to plant leaves.
Twospotted spider mites are normally found on the underside of leaves within the crop. The mites constantly spin fine webbing which is laid over the leaf surface.
Signs and symptoms of twospotted spider mites include:
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Speckling and yellowing of leaves within a crop
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Small mites on the underside of the leaves
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Severely damage leaves become bleached in appearance and fall from the plant
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Loss of plant vigour and production, and death of plants
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Webbing on the tips of leaves in severe infestations, especially on young leaves, and mites 'roping' down
Life history and habits
Twospotted spider mite overwinters as bright orange adult females on old crop debris, greenhouse structures and other plants. In spring they become active and begin laying eggs on newly planted crops.
Eggs hatch in 2-15 days depending on temperature, but development below 12 C is reduced. Developing mites pass through larva, protonymph and deutonymph stages with quiescent or moulting phases between each.
A life cycle may be completed in 2-3 weeks depending on temperature, and many generations per year are possible, especially in greenhouses. Females can lay 2-3 eggs per day or 50-60 eggs in their life time. Therefore, under ideal conditions, huge populations of twospotted spider mites can build up rapidly if left unchecked.
Mites form colonies on the lower leaf surface at first and can expand to the upper leaf surface, and on to flowers and fruit, as populations build up.
When the infestation of mites is high, webbing will be visible on the tips of the leaves and may hang down like a silken thread. This is a dispersal mechanism and helps them move from one plant to the next, and to become airborne on wind currents.
Reproduction and dispersal of twospotted spider mite is greatest under hot, dry conditions. High humidity reduces survival and development.
Twospotted spider mites may be readily moved around crops by the activities of crop workers.
Economic impact
High populations of twospotted spider mite can cause extensive leaf damage and result in reduced crop yield and quality. Because of the speed at which populations can build up under ideal conditions, the application of control measures will normally be warranted.
The susceptibility of crops varies widely, and even cultivars within species will show differing levels of tolerance of this pest.
Twospotted spider mite may be a quarantine pest on some export crops, e.g., cut flowers.
Monitoring
On outdoor crops, such as apples, monitoring the presence of mites on leaves has been a common practice. When 40 leaves out of 50 have 2 or more active mite stages, then application of a control was recommended.
Crop workers should identify and mark 'hot spots' of infestation.
On greenhouse crops, inspection of leaves can also be used to monitor mites. When predatory mites are to be released, it is generally recommended to release one predator mite for every 15-25 twospotted spider mites.
Non-chemical methods
A number of practices are recommended to prevent or minimise the establishment of twospotted spider mites on greenhouse crops. These include:
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Thorough cleaning of greenhouse structures to remove overwintering twospotted spider mites
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Removal and destruction of crop debris at the end of a crop cycle
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Removal of weeds that harbour pest mites
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Remove and destroy heavily infested plants from mite 'hot spots'
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Consider cultivars that are more tolerant of twospotted spider mite
Biological control
A variety of natural predators has been researched to assist with the management of twospotted spider mite. The predatory mite Phytoseiulus persimilis is the most commonly used biological control agent for this pest in greenhouse crops.
Phytoseiulus persimilis is available from BioForce Ltd, who sells the product Mite-E™.
Orius vicinus is available from BioForce Ltd, who sells the product Orius.
Phytoseiulus persimilis and Orius vicinus are most effective when they are used in an Integrated Pest Management programme, when the use of harmful pesticides is avoided and effective crop management practices are adopted.
Chemical control
range of miticides (=acaricides) is claimed to be effective against twospotted spider mites, although few have specific registration claims for use on greenhouse crops.
The nymphs of twospotted spider mite are more susceptible to insecticides than eggs or adults therefore miiticide applications should therefore be targeted at nymphs. Some miticides have activity against eggs and larvae (= ovicidal action) of twospotted spider mite, therefore application of these products should be made when eggs are abundant.
Resistance to miticides by twospotted spider mite is known to occur in New Zealand and elsewhere, and cross-resistance between some miticide groups occurs. Care must be taken to not apply more than the recommended number of applications of miticides from any one chemical in a year.
Apply sprays to crops in a manner to ensure good coverage of the undersides of leaves. Crops recently de-leafed allow better spray penetration. Spot spray 'hot spot' areas to minimise the spread and build of twospotted spider mite in a crop.
Care should be taken to consider the effect of miticides on beneficial insects or mites that may be being used in integrated pest management programmes. Generally, miticides are safer than insecticides to beneficial insects and mites. Consult with BioForce Ltd before applying any miticide when beneficial insects or mites are being used.
If you are uncertain about the identity of any pest in your crop, or need advice on the management of pests contact BioForce Ltd, office@bioforce.net.nz.