Integrated pest management (IPM) of mites

A combination of cultural, biological and conventional chemical control measures will provide the best approach for controlling mite infestations, as part of an integrated pest management programme.

The adoption of a range of cultural control measures will help to minimise potential pest pressure on crops and reduce the need for any regular intervention.

• Always start with clean propagating material and do not take cuttings from any infested mother plants
• Inspect any plant deliveries in a quarantined area and reject or isolate any infested batches of plants
• Sweep up plant debris and use a recommended biocide between crops
• Remove weeds from both cropping and non-cropping areas, as these can act as alternative hosts for many pest species, and maintain weed control during the season. Salix and willowherb are particularly susceptible to attack from two-spotted spider mite
• Promptly and carefully dispose of any crop debris, unmarketable plants and weeds. Place into sealed bags or covered containers, sited as far away from production areas as possible, before disposal

There is no substitute for regular and careful crop walking, carried out by someone who has been trained to recognise the early symptoms of mite damage, the various mite species and their biological control agents. As mites do not fly, sticky traps are ineffective for monitoring mite outbreaks. Focus on known susceptible plant species or cultivars and inspect with a hand lens (at least x10 magnification) to confirm the presence of damage symptoms, mites or eggs. A lens is also useful when introducing biological control agents, to help assess the ratio of predators to prey. Record any monitoring and note pest mite hot spots, as mites like the two-spotted spider mite, broad mite and cyclamen mite can be carried on clothing.

There is a wide range of commercially available predators for use in biological control programmes that are either recommended for the control of pest mite species or will contribute to the control of pest mites when used for the control of other pests, such as thrips. Some of these predators can also occur naturally, particularly if using integrated pest management (IPM) with very limited application of plant protection products.

Choice, combinations, timings and rates of release within an IPM programme should be planned carefully. If necessary, seek advice from the supplier or a specialist IPM consultant. All biological control agents should be released as soon as possible after delivery. If storage is necessary for a short time, keep them in a cool place, out of direct sunlight and avoid any sudden temperature changes, such as refrigeration.

Table 1 lists the commercially available predators.

Several species of predatory mite are available for the control of pest mites. Choice will depend on temperature, humidity, mite pest species and whether control of other pests, such as thrips and whitefly, is also needed.

Phytoseiulus persimilis

Phytoseiulus persimilis is the most commonly used predatory mite for control of two-spotted spider mite, since it is a specialist predator which has coevolved to feed on Tetranychus species. The adult predators are slightly larger than the pest mite and are orange-red and shiny. They have longer legs than spider mites and are very active, running around the leaves searching for prey. Phytoseiulus persimilis will eat both the mobile stages and eggs of mites. Younger predators are smaller and paler than the adults. The eggs are pale pink, oval and about twice the size of those of two-spotted spider mite. These can easily be seen with a hand lens when monitoring, on leaves with spider mite damage, and are a good sign that the predators are establishing. The predators will move readily from plant to plant if they are touching. The predators are supplied in tubes with a bran or vermiculite carrier that is sprinkled on the plants. Products with larger numbers of predators in smaller amounts of carrier leave less carrier on the plants, deliver more accurate numbers of predators and allow more effective use of high doses in spider mite ‘hotspots’.

Phytoseiulus persimilis is most effective when temperatures reach 20°C for a few hours each day and can reproduce twice as fast as two-spotted spider mite between 15°C and 25°C and at over 75% relative humidity. However, as an obligate predator, P. persimilis requires reintroduction if the pest is absent. At temperatures over 30°C, P. persimilis is unable to control two-spotted spider mite. Phytoseiulus persimilis is generally reared on two-spotted spider mite and distributed without a food source in a vermiculite carrier. Therefore, it is important to release the mites as soon as they are delivered so that they can find prey. Koppert has recently developed a new product, ‘Spidex vital’ – a P. persimilis product reared on an alternative food source for quicker establishment in the crop. When first released, the P. persimilis in this product are white, but they quickly turn the characteristic orange-red colour when they start feeding on spider mites.

Month-by-month action points for the control of two-spotted spider can be found in Table 2.

Adult Phytoseiulus persimilis alongside the predator’s egg

Image © ADAS Horticulture.

Several other predatory mites are available for the control of spider mites to supplement control by P. persimilis or for control of other pest mite species.  Some predatory mites primarily used for control of other pests, such as thrips and whitefly, will also contribute to control of some mite species.

Amblyseius andersoni is native to England and Wales and can be released early in the year, before two-spotted spider mite is present, due to its cold tolerance and omnivorous diet. A. andersoni can be found naturally on many species of trees, shrubs and herbaceous plants and seems to be less susceptible to some plant protection products than some other predatory mites. Adults overwinter in cracks, crevices and leaf litter and become active in early spring. A. andersoni is a generalist predatory mite, which will feed on several mite species, including spider mites, tarsonemid mites and eriophyid mites, along with thrips larvae, pollen, fungal spores and honeydew. Adult mites are straw-coloured and pear-shaped, and, without a microscope and specialist knowledge, indistinguishable from other Amblyseius or Neoseiulus species predatory mites species.

Neoseiulus californicus is considered non-native to the UK so only has a licence for release in permanently protected structures with full enclosure. This species tolerates high temperatures and low humidities better than P. persimilis and, like A. andersoni, is also less susceptible to some plant protection products. N. californicus will feed on spider mites and tarsonemid mites and also pollen. This predator does not feed on as many spider mites as P. persimilis and does not disperse as well or control ‘hot spots’ as well as P. persimilis. Therefore, it is better released early in the season to allow a population to build up and offers useful control when P. persimilis starts to struggle in hot, dry conditions. The adult mite is oval, transparent and about 0.4 mm long. When it has fed on spider mites, N. californicus develops pale orange markings on its back.

Neoseiulus cucumeris is primarily used for thrips control on various crops, including ornamentals, but will also feed on spider mites and tarsonemid mites. This predator is routinely released into strawberry crops for preventive control of tarsonemid mites, as well as western flower thrips. Experience on protected ornamentals has shown that preventive release of N. cucumeris will give good control of broad mite. When cucumber crops used to be grown on straw bales and were attacked by Tyrophagus longior (‘French’ fly), there was some indication that N. cucumeris gave some control of these mites, when used for thrips control.

Amblyseius swirskii, Amblyseius limonicus and Transeius montdorensis are not native to the UK. All these species have a licence for release in permanently protected structures with full enclosure, but check with the supplier about restrictions for release in polythene tunnels. All three species are primarily recommended for control of thrips and whitefly, but they will also feed on pollen and some mite species. Transeius montdorensis can give good reduction of spider mites, eriophyid mites and tarsonemid mites.

Ground-dwelling predatory mites

Stratiolaelaps scimitus is a ground-dwelling predatory mite that is primarily used for control of sciarid fly in growing media but will also feed on other prey, such as thrips pupae and tyrophagus mites. The adult mites are up to 0.8 mm long, off-white, with a pale brown shield covering most of the upper surface of the body.

Amblyseius andersoni (top) alongside a two-spotted spider mite

Image © ADAS Horticulture.

There are various different delivery mechanisms for predatory mites. They can be sprinkled onto plants each week in a bran or vermiculite carrier, either by hand or using an automatic applicator such as the ‘Airobug’ system, which saves on labour time and gives a more even distribution. Alternatively, the predators can be released every 6–8 weeks using controlled-release paper sachets that are hung onto plants, pots or baskets. The sachets generally contain a live food source, which enables the predatory mites to feed and breed within the sachets.

Several products are available from biological control companies for boosting numbers of predatory mites for improved pest control. Cattail (Typha) pollen is available as ‘Nutrimite’ from Biobest, which has been shown to boost numbers of some species of predatory mites, including A. swirskii and T. montdorensis. Two ‘feeder mite’ products are also available, containing prey mites as supplementary food for some species of predatory mites, including all those supplied for thrips control. ‘Nutari’ is available from Koppert Biological Systems and contains Carpoglyphus lactis, and ‘Mitefood’ is available from Bioline AgroSciences and contains Thyreophagus entomophagus. So far, these products have not been used extensively in UK ornamental crops.

The following predators can occur naturally and contribute to the control of various mite species, particularly where IPM is being used with minimal use of plant protection products. Some of them are also commercially available.

Feltiella acarisuga

The predatory midge Feltiella acarisuga is native to the UK and commercially available but can also occur naturally, especially if spider mite numbers are high. The midge is a useful supplement for spider mite control, especially for 'hot spot' treatments. The midge is supplied as pupae in cartons, which should be placed in a shady area near to the spider mite infestation. The adult midges emerge from the pupae and fly out to look for spider mite colonies in which to lay their eggs; this means they can disperse more easily than predatory mites. Adult Feltiella have a reddish body, long legs and antennae and a gnat-like appearance. The eggs are oval, slightly sickle-shaped, translucent to yellowish and are visible with a hand lens. The larvae are yellow or reddish and about 2 mm long and can consume an average of 30 adult spider mites or 80 eggs per day. When fully developed, the larvae spin small white cocoons on the undersides of the leaves, in which they pupate. Optimum conditions are 20–27°C and 80% relative humidity.

Feltiella acarisuga larva feeding on two-spotted spider mite

Image © Nigel Cattlin.

Stethorus punctillum

Stethorus punctillum is a small black ladybird that feeds on spider mites. It is no longer commercially available in the UK but can occur naturally where IPM is being used. Adults are attracted to colonies of spider mites to lay their eggs. Adults and larvae are active predators on all spider mite stages.

Anthocorid bugs

There are several species of anthocorid bugs native to the UK, including commercially available Orius species like Orius laevigatus, which can also occur naturally, and naturally occurring Anthocoris species, such as Anthocoris nemorum. Adults and nymphs are voracious predators of various small prey, including thrips, spider mites, moth eggs and aphids, and they also feed on pollen. The flying adults can disperse easily and are about 3–4 mm long, black with grey and brown chequerboard markings on their backs. Orius laevigatus is primarily used for thrips control and needs temperatures of at least 15°C for egg laying and temperatures of at least 20ºC for good establishment.

Black-kneed capsid bug

The black-kneed capsid bug (Blepharidopterus angulatus) is a common and widespread naturally occurring green bug with a slim, parallel-sided body and black ‘knees’. Adults and nymphs feed on mites, aphids, thrips and other small prey.

Lacewings

The green lacewing Chrysoperla carnea is native to the UK and sometimes occurs naturally in protected crops where IPM is being used, usually together with aphids, which are its preferred prey. The predator is available as eggs or larvae for the control of aphids, but the larvae will feed on other prey, including spider mites and thrips. The adults are not predatory and the predator will not establish in glasshouses, therefore repeated releases of larvae are needed.

Typhlodromus pyri

Typhlodromus pyri is a very common species of predatory mite, found in fruit trees, among a variety of habitats. The mites feed on pollen, fungal spores and mite pest species, particularly the fruit tree red spider mite.

Entomopathogenic fungi are microbiological bioprotectants and therefore need to be approved for use in the UK (unlike predators, parasitoids and nematode biological control agents). Two species of fungal products are approved for use on protected ornamentals in the UK for control of whitefly and they may also give some control of mites: Beauveria bassiana (Botanigard WP and Naturalis-L) and Lecanicillium muscarium (Mycotal).

When using an integrated pest management programme, there may be a requirement to use a plant protection product if the mite population grows too rapidly for the predator(s) to control, or if there is a sudden outbreak of the pest mite and an infestation 'hot spot' occurs. Products may also be needed at the end of the season as a clean-up measure to reduce the population of spider mites carrying over to next season.

Plant protection products should be selected and their application timed carefully, using the least harmful and persistent products where possible. Applications should be planned according to product-label recommendations, life stages of the pest mite controlled, permitted frequency and number of applications and resistance management guidelines.

Table 3 lists both bioprotectant and conventional plant protection products that are currently approved on protected ornamentals that should give some control of mite pests.

Handheld applicator (‘Mini-Airbug’) for biological control agents

Image © AHDB.

As mites can complete many generations each year, there is always the potential of selecting for resistance if the same plant protection product is used too frequently. Two-spotted spider mite is considered to be one of the most resistant pests in the world due to the total number of plant protection products which populations have developed resistance to. In order to reduce the risk of resistance development, it is very important to rotate between different Insecticide Resistance Action Committee (IRAC) groups and to always follow the resistance management strategy outlined on the product label or Extension of Authorisation for Minor Use (EAMU). This will specify the recommended rate and the maximum number of treatments in a year. Integrated pest management also provides a solution to resistance, as using biological control agents will reduce the selection pressure for resistance to plant protection products. Mites are unlikely to develop resistance to physically acting products which work by coating the mites and suffocating or immobilising them.

For further details of compatibility of plant protection products with biological control agents, see the Biobest website and the Koppert Biological Systems website. 

For more information about resistance management, visit the IRAC website.

Authors – Elysia Bartel and Jude Bennison, ADAS Horticulture.

Original author (08/05 ‘The biology and control of two-spotted spider mite in nursery stock’ and 12/09 ‘The biology and control of mites in pot and bedding plants’) – John Buxton, ADAS.

Web page content correct as of June 2021.