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Tetranychus urticae Koch

Tetranychidae, ACARINA

Description and life cycle

The two spotted mite (TSM) are very small arthropods, and have an incomplete life cycle. There is no resting stage or pupa in its life cycle and in midsummer this cycle can be as short as seven days. Generally in the greenhouse the life cycle is between 7 to 12 days in summer and 12 - 21 days in winter depending on temperature. TSM loves hot and dry weather, especially between 25 and 30^oC with low humidity. Interestingly high humidity will actually reduce mite numbers.

The mite population consists of males and females with females predominating. Each female produces 15-20eggs per day with a total production of about 100 eggs. The adult female is 0.6mm long, round in shape, and pale yellow to greenish with two characteristic dark green to black spots on each side of the body. The male is smaller, slimmer and has a more diamond shape. The eggs are usually laid on the underside of the leaf under a thin layer of webbing, they are small and translucent becoming whitish as they mature. Nymphs resemble the adults but are much smaller, there are two nymphal stages before becoming an adult.

In late autumn some females can turn an orange red colour (the over wintering female), they stop laying eggs and feeding and search for protected sites to spend the cooler winter months. In most greenhouses small numbers will persist because they provide a suitable climate for TSM all year.

Crops attacked and problems caused

Two-spotted mite (TSM) belongs to a group of mites collectively known as spider mites, because they produce a fine web which can be seen on the surface of the infected leaf. Once the underside of a leaf is overrun with mites they will move onto the top surface where webbing and mite activity will soon become apparent. When numbers are extreme webbing can easily be seen on the whole plant especially on the growing tips.

TSM attack a broad range of plants, from most vegetable crops, ornamentals, tree crops and weeds and frequently occurs in protected environments such as glasshouses.  

These mites feed by first puncturing the cell and then sucking up the juices. Early symptoms of mite damage can be seen as a silvery white flecking or speckling where the mites are feeding, usually along the midrib progressing outward to the leaf edges as the population increases. Growth in affected plants is also reduced with distortion of flower and leaf buds. High mite numbers can remove nearly all the chlorophyll and leaves will turn yellow and dry up.

They prefer dry hot conditions quickly getting out of control in summer months when conditions are suitable.

Host plants and hygiene practices

TSM have a very wide host range with most broadleaf weeds as hosts. It is an important pest of glasshouses as it attacks both ornamental and most vegetable crops. It can also be a pest of outdoor crops, including strawberries, grapevines and most deciduous fruit trees.

There are some simple things that can be done to avoid high mite numbers:

• Control weeds that harbour the pest
• Avoid introducing infested seedlings or other plant material into the crop
• Remove old plants that may be a source of mites for new plantings
• Increase humidity to reduce breeding
• Identify infestations early as the mites are easily moved around the greenhouse by workers

Crop monitoring

MAKE INSPECTIONS ROUTINE AND KEEP GOOD RECORDS !
TSM are unable to fly so they will not be observed on sticky cards, plant inspection is the only way to monitor for mites. Looking for mite damage will indicate which plants to inspect more closely. A hand lens, normally 10-15X is required for monitoring for TSM. TSM will be found on all plant parts including old and new leaves, main and secondary stems, but rarely on the fruit. To be sure about TSM numbers in a crop all these habitats need to be monitored. TSM however do prefer the youngest and most tender growth. Eggs can also be found in these areas.

• Get into the habit of walking right through your crops in a set pattern (a M or Z)
• Check about (about 1%) of your plants very carefully
• Look at leaf and plant surfaces for patches of small feeding spots
• Check underside of leaves for feeding mites
• Keep good records of pest levels and treatments used
  

Note:

• If you can spot isolated hot spots early you may only need to spray a small area!
• Plan to introduce biological control agents as soon as thrips are found
  

Chemical control

In the early days of TSM problems organophosphates (OP's) were used to kill mites and serious resistance problems arose when these chemicals were used continuously. Chemical control of TSM has not usually been difficult but problems are encountered if they become established and 2-3 applications must be timed correctly to fit their life cycle. Recently there is anecdotal evidence of widespread TSM resistance to avermectin based insecticides. This is likely due to near total reliance on this chemical to control mites in many crops instead of rotating chemical groups, and the increasing use of this chemical to control thrips.

Many alternatives are not systemic so may appear ineffective if good coverage is not achieved. All foliage must be well covered with the spray, especially the underside of the leaf especially if the chemical works by contact with no systemic activity.

Application must be made before damaging numbers develop i.e. before webbing can be seen. Small droplet size is much more effective and will make better contact with the pest. Depending on the chemical used repeated application may need to be made, most registered chemicals do not kill the egg stage or have enough residual to kill hatching larvae. Two applications should be used at about 5-7 days apart to kill all stages. If possible try to spot treat infestations instead of treating the whole greenhouse since TSM develops resistance to chemicals very quickly when the same chemical is used. Use chemicals that have different modes of action (i.e. are in a different chemical class and work differently) in rotation to prevent resistance developing.

Botanical oil concentrates have been developed to provide an effective "soft" alternative.

Include a resistance management strategy into your spray program to reduce the chance of resistance. If monitoring indicates the need to spray earlier, then insecticide resistance, inappropriate spray application or inadequate farm hygiene should be suspected and expert advice sought.

Relevant beneficial insects

There are many natural enemies such as predatory mites, lacewings, ladybirds, thrips and pathogenic fungi. These have been investigated and biological control programs using predatory mites have been used. Predatory mites are now available for use in greenhouses and other environments. Predatory mites that can control this pest are now available in Australia at Biological Services at Loxton; and Goodbugs (external links). The suppliers on this page will help you develop an IPM program suitable for your crop and situation. Many also provide IPM monitoring services.   

You can also boost the numbers of wild beneficial insects in your crop naturally by holding back on broad spectrum insecticides, providing safe plant species as habitat near the crop and maintaining higher levels of organic soil carbon.

Download the Two Spotted Mite factsheet (PDF)

Information on other greenhouse pests:
• Western Flower Thrips 
• Whiteflys
• Broad mites
• Aphids